Web Design and Development CS506 - VU LMS

Web Design and Development CS506 - VU LMS
Web Design and Development (CS506) Web Design and Development
CS506
© Copyright Virtual University of Pakistan 1 Web Design and Development (CS506) Lecture 1: Java Features .......................................................................................22 1.1 Design Goals of Java ...................................................................................................... 22 1.1.1 Right Language, Right Time................................................................................... 22 1.1.2 Java - Buzzwords (Vocabulary) .............................................................................. 22 1.1.3 Java -- Language + Libraries .................................................................................. 22 1.1.4 Simple ..................................................................................................................... 23 1.1.5 Object-Oriented....................................................................................................... 23 1.1.6 Distributed / Network Oriented .............................................................................. 23 1.1.7 Robust / Secure / Safe ............................................................................................. 23 1.1.8 Portable ................................................................................................................... 24 1.1.9 Support for Web and Enterprise Web Applications ............................................... 24 1.1.10 High-performance ................................................................................................... 24 1.1.11 Multi-Threaded ....................................................................................................... 24 1.1.12 Dynamic .................................................................................................................. 24 1.1.13 Java Compiler Structure .......................................................................................... 24 1.1.14 Java: Programmer Efficiency .................................................................................. 25 1.1.15 Microsoft vs. Java ................................................................................................... 25 1.1.16 Java Is For Real ....................................................................................................... 25 1.1.17 References ............................................................................................................... 25 Lecture 2: Java Virtual Machine & Runtime Environment .............................26 2.1 Basic Concept ................................................................................................................. 26 2.1.1 Byte code ................................................................................................................ 26 2.1.2 Java Virtual Machine (JVM) .................................................................................. 27 2.1.3 Java Runtime Environment (JRE) .......................................................................... 27 2.1.4 References ............................................................................................................... 28 2.2 Java Program Development and Execution Steps .......................................................... 28 2.2.1 Phase 1: Edit ........................................................................................................... 30 2.2.2 Phase 2: Compile .................................................................................................... 30 2.2.3 Phase 3: Loading ..................................................................................................... 30 2.2.4 Phase 4: Verify ........................................................................................................ 30 2.2.5 Phase 5: Execute ..................................................................................................... 31 © Copyright Virtual University of Pakistan 2 Web Design and Development (CS506) 2.2.6 2.3 References: .............................................................................................................. 31 Installation and Environment Setting ............................................................................... 31 2.3.1 Installation............................................................................................................... 31 2.3.2 Environment Setting ............................................................................................... 31 2.3.2.1 Temporary Path Setting .................................................................................................. 31 2.3.2.2 Permanent Path Setting ................................................................................................... 32 2.3.3 2.4 References ............................................................................................................... 34 First Program in Java....................................................................................................... 34 2.4.1 HelloWorldApp....................................................................................................... 34 2.4.2 HelloWorldApp Described ..................................................................................... 35 2.4.3 Compiling and Running HelloWorldApp ............................................................... 35 2.4.4 Points to Remember ................................................................................................ 36 2.5 An Idiom Explained ....................................................................................................... 36 2.6 References ...................................................................................................................... 37 Lecture 3: Learning Basics ...................................................................................38 3.1 Strings............................................................................................................................. 38 3.1.1 String Concatenation ............................................................................................... 38 3.1.2 Comparing Strings .................................................................................................. 38 3.2 Taking in Command Line Arguments ............................................................................ 39 3.3 Primitives vs. Objects..................................................................................................... 40 3.4 Stack vs. Heap ................................................................................................................ 41 3.5 Wrapper Classes ............................................................................................................. 41 3.5.1 Wrapper Use ........................................................................................................... 42 3.5.2 Converting Strings to Numeric Primitive Data Types ............................................ 42 3.6 Selection & Control Structure ........................................................................................ 44 3.7 Reference:....................................................................................................................... 44 Lecture 4: Object Oriented Programming ..........................................................45 4.1 OOP Vocabulary Review ............................................................................................... 45 4.1.1 Classes .................................................................................................................... 45 4.1.2 Objects .................................................................................................................... 45 4.1.3 Constructor ............................................................................................................. 45 © Copyright Virtual University of Pakistan 3 Web Design and Development (CS506) 4.1.4 Attributes ................................................................................................................ 45 4.1.5 Methods .................................................................................................................. 45 4.2 Defining a Class .............................................................................................................. 45 4.3 Comparison with C++ .................................................................................................... 46 4.4 Task - Defining a Student class ...................................................................................... 47 4.5 Getters / Setters .............................................................................................................. 47 4.6 Using a Class................................................................................................................... 48 4.6.1 4.7 Task - Using Student Class ..................................................................................... 49 More on Classes .............................................................................................................. 50 4.7.1 Static ....................................................................................................................... 50 4.7.2 Garbage Collection & Finalize ............................................................................... 50 4.7.2.1 4.8 Finalize ............................................................................................................................ 51 Reference:....................................................................................................................... 54 Lecture 5: Inheritance ...........................................................................................55 5.1 Comparison with C++ .................................................................................................... 55 5.2 Object - The Root Class ................................................................................................. 58 5.3 Polymorphism ................................................................................................................. 58 5.4 Type Casting .................................................................................................................. 60 5.4.1 Up-casting ............................................................................................................... 60 5.4.2 Down-casting .......................................................................................................... 60 5.5 References: ..................................................................................................................... 60 Lecture 6: Collections ............................................................................................60 6.1 Collections Design ......................................................................................................... 61 6.2 Collection messages ....................................................................................................... 61 6.3 Array List ....................................................................................................................... 61 6.3.1 6.4 Useful Methods ....................................................................................................... 61 HashMap ........................................................................................................................ 63 6.4.1 Useful Methods ....................................................................................................... 63 6.5 References: ..................................................................................................................... 65 6.6 Address Book .................................................................................................................. 65 6.6.1 Problem ................................................................................................................... 65 © Copyright Virtual University of Pakistan 4 Web Design and Development (CS506) 6.6.2 6.7 Approach for Solving Problem ............................................................................... 66 6.6.2.1 Step1 - Make PersonInfo class ........................................................................................ 66 6.6.2.2 Step2 - Make Address Book class................................................................................... 66 6.6.2.3 Step3 - Make Test class (driver program) ....................................................................... 68 Reference ........................................................................................................................ 69 Lecture 7: Intro to Exceptions ..............................................................................69 7.1 Types of Errors ............................................................................................................... 70 7.1.1 Syntax Errors .......................................................................................................... 70 7.1.2 Logic Errors ............................................................................................................ 70 7.1.3 Runtime Errors ........................................................................................................ 70 7.2 What is an Exception? .................................................................................................... 70 7.3 Why handle Exceptions? ................................................................................................ 70 7.4 Exceptions in Java .......................................................................................................... 70 7.5 Exception Hierarchy ....................................................................................................... 71 7.6 Types of Exceptions ....................................................................................................... 71 7.6.1 Unchecked Exceptions ............................................................................................ 71 7.6.2 Checked Exceptions ................................................................................................ 72 7.7 How Java handles Exceptions ........................................................................................ 72 7.7.1 try block .................................................................................................................. 72 7.7.2 Catch block ............................................................................................................. 72 7.7.3 finally block ............................................................................................................ 72 7.7.4 throw ....................................................................................................................... 73 7.7.5 throws...................................................................................................................... 73 7.8 References: ..................................................................................................................... 73 7.9 Code Examples of Exception Handling ........................................................................... 73 7.9.1 Unchecked Exceptions ............................................................................................ 73 7.9.2 Why? ....................................................................................................................... 74 7.9.3 Modify UcException.java ....................................................................................... 74 7.10 Checked Exceptions ....................................................................................................... 75 7.11 The finally block ............................................................................................................ 76 7.12 Multiple catch blocks ..................................................................................................... 77 © Copyright Virtual University of Pakistan 5 Web Design and Development (CS506) 7.13 The throws clause ........................................................................................................... 79 7.14 printStackTrace method ................................................................................................. 80 7.15 Reference ........................................................................................................................ 81 Lecture 8: Streams .................................................................................................82 8.1 The concept of "streams" ................................................................................................. 82 8.2 Stream classification based on Functionality ................................................................. 83 8.3 Stream classification based on data ................................................................................ 84 8.4 Reference ........................................................................................................................ 86 8.5 Modification of Address Book Code ............................................................................... 87 8.5.1 8.6 Adding Persistence Functionality ........................................................................... 87 8.5.1.1 Scenario 1 - Start Up ....................................................................................................... 87 8.5.1.2 Scenario 2 - End/Finish Up ............................................................................................. 90 References ...................................................................................................................... 92 Lecture 9: Abstract Classes and Interfaces .........................................................93 9.1 Problem and Requirements ............................................................................................ 93 9.2 Abstract Classes ............................................................................................................. 93 9.3 Interfaces ........................................................................................................................ 95 9.3.1 Defining an Interface .............................................................................................. 95 9.3.2 Implementing (using) Interface............................................................................... 95 9.4 Interface Characteristics ................................................................................................. 96 9.5 References ...................................................................................................................... 98 Lecture 10: Graphical User Interfaces ................................................................98 10.1 Support for GUI in Java ................................................................................................. 99 10.2 GUI classes vs. Non-GUI Support Classes .................................................................... 99 10.3 java.awt package ............................................................................................................ 99 10.4 javax.swing package....................................................................................................... 99 10.5 A part of the Framework .............................................................................................. 100 10.6 GUI Creation Steps ....................................................................................................... 100 10.6.1 import required packages ...................................................................................... 100 10.6.2 Setup the top level containers ............................................................................... 100 10.6.3 Get the component area of the top level container ............................................... 101 © Copyright Virtual University of Pakistan 6 Web Design and Development (CS506) 10.6.4 Apply layout to component area ........................................................................... 101 10.6.5 Create and Add components ................................................................................. 102 10.6.6 Set size of frame and make it visible .................................................................... 102 10.7 Important Points to Consider........................................................................................ 103 10.8 References: ................................................................................................................... 104 10.9 Graphical User Interfaces - 2 ......................................................................................... 104 10.9.1 Layout Managers ................................................................................................... 104 10.9.1.1 Flow Layout .................................................................................................................. 105 10.9.1.2 Grid Layout ................................................................................................................... 106 10.9.1.3 Border Layout ............................................................................................................... 108 10.10 Making Complex GUIs ................................................................................................. 109 10.10.1 JPanel .................................................................................................................... 110 10.10.2 Solution ................................................................................................................. 110 10.11 Reference:..................................................................................................................... 112 Lecture 11: Event Handling ................................................................................113 11.1 Event Handling Model ................................................................................................. 114 11.2 Event Handling Steps ................................................................................................... 114 11.3 Event Handling Process ............................................................................................... 114 11.3.1 Step 1: Event Generators ...................................................................................... 114 11.3.2 Step 2: Event Handlers/ Event Listener ................................................................ 114 11.3.3 Step 3: Registering Handler with Generator ......................................................... 116 11.4 How Event Handling Participants Interact Behind the Scenes? .................................. 118 11.4.1 Event Generator / Source ...................................................................................... 118 11.4.2 Event Object.......................................................................................................... 118 11.4.3 Event Listener/handler .......................................................................................... 118 11.4.4 JVM ...................................................................................................................... 118 Lecture 12: More Examples of Handling Events ..............................................122 12.1 Handling Mouse Event ................................................................................................. 122 12.1.1 MouseMotionListener interface ............................................................................ 122 12.1.2 MouseListener interface........................................................................................ 122 Lecture 13: Adapter Classes ...............................................................................127 © Copyright Virtual University of Pakistan 7 Web Design and Development (CS506) 13.1 Adapter Classes ............................................................................................................ 127 13.2 Available Adapter classes ............................................................................................ 128 13.2.1 How to use Adapter Classes ................................................................................. 128 13.3 Inner Classes ................................................................................................................ 129 13.4 Anonymous Inner Classes ............................................................................................ 135 13.5 Named vs. Anonymous Objects ................................................................................... 135 13.5.1 Named ................................................................................................................... 135 13.5.2 Anonymous ........................................................................................................... 135 13.6 Summary of Approaches for Handling Events ............................................................ 136 13.7 References .................................................................................................................... 136 Lecture 14: Java Database Connectivity............................................................137 14.1 Introduction .................................................................................................................. 137 14.2 The java.sql package .................................................................................................... 137 14.3 Connecting With Microsoft Access ............................................................................. 137 14.3.1 Create Database .................................................................................................... 137 14.3.2 Setup System DSN................................................................................................ 138 14.4 Basic Steps in Using JDBC ........................................................................................... 139 14.4.1 Import Required Package ...................................................................................... 140 14.4.2 Load Driver ........................................................................................................... 140 14.4.3 Define Connection URL ....................................................................................... 140 14.4.4 Establish Connection With DataBase ................................................................... 140 14.4.5 Create Statement ................................................................................................... 140 14.4.6 Execute a Query .................................................................................................... 141 14.4.7 Process Results of the Query ................................................................................ 141 14.4.8 Close the Connection ............................................................................................ 141 14.5 References:.................................................................................................................... 143 Lecture 15: More On JDBC ................................................................................144 15.1 Useful Statement Methods: ........................................................................................... 144 15.1.1 executeUpdate( ) .................................................................................................... 144 15.1.2 getMaxRows / setMaxRows(int) ............................................................................ 146 15.1.3 getQueryTimeOut / setQueryTimeOut (int) ........................................................... 146 © Copyright Virtual University of Pakistan 8 Web Design and Development (CS506) 15.2 Different Types of Statements ....................................................................................... 146 15.2.1 Statement ............................................................................................................... 146 15.2.2 PreparedStatement ................................................................................................. 147 15.2.3 CallableStatement .................................................................................................. 147 15.2.4 Prepared Statements ............................................................................................... 147 15.3 References:.................................................................................................................... 149 Lecture 16: Result Set ..........................................................................................150 16.1 ResultSet ....................................................................................................................... 150 16.1.1 Default ResultSet ................................................................................................... 150 16.1.2 Useful ResultSet’s Methods ................................................................................... 150 16.1.2.1 next( ) ............................................................................................................................ 150 16.1.2.2 getters ............................................................................................................................ 151 16.1.2.3 close( ) ........................................................................................................................... 151 16.1.2.4 Updatable and/or Scrollable ResultSet ........................................................................... 151 16.1.2.5 Creating Updatable & Scrollable ResultSet .................................................................... 151 16.1.2.6 previous( ) ...................................................................................................................... 152 16.1.2.7 absolute(int) ................................................................................................................... 153 16.1.2.8 updaters (for primitives, String and Object) ................................................................... 153 16.1.2.9 updateRow( ) ................................................................................................................. 154 16.1.2.10 moveToInsertRow(int) ................................................................................................... 155 16.1.2.11 insertRow( ) ................................................................................................................... 156 16.1.2.12 last( ) & first( ) .............................................................................................................. 158 16.1.2.13 getRow( ) ...................................................................................................................... 158 16.1.2.14 deleteRow( ) .................................................................................................................. 158 16.2 References:.................................................................................................................... 161 Lecture 17: Meta Data .........................................................................................162 17.1 ResultSet Meta data ....................................................................................................... 162 17.1.1 Creating ResultSetMetaData object........................................................................ 162 17.1.2 Useful ResultSetMetaData methods ....................................................................... 163 17.1.2.1 getColumnCount ( ) ....................................................................................................... 163 17.1.2.2 getColumnDisplaySize (int) ........................................................................................... 163 17.1.2.3 getColumnName(int) / getColumnLabel (int) ................................................................. 163 © Copyright Virtual University of Pakistan 9 Web Design and Development (CS506) 17.1.2.4 getColumnType (int) ...................................................................................................... 163 17.2 DatabaseMetaData ........................................................................................................ 165 17.2.1 Creating DatabaseMetaData object ........................................................................ 165 17.2.2 Useful ResultSetMetaData methods ....................................................................... 166 17.2.2.1 getDatabaseProductName( ) ........................................................................................... 166 17.2.2.2 getDatabaseProductVersion( ) ........................................................................................ 166 17.2.2.3 getDriverName( ) ........................................................................................................... 166 17.2.2.4 isReadOnly( ) ................................................................................................................. 166 17.3 JDBC Driver Types ....................................................................................................... 167 17.3.1 Type - 1: JDBC - ODBC Bridge ............................................................................ 168 17.3.2 Type - 2: Native - API/partly Java driver ............................................................... 168 17.3.3 Type - 3: Net - protocol/all-Java driver4 ................................................................ 168 17.3.4 Type - 4: Native - protocol / all - java driver .......................................................... 169 17.4 Online Resources .......................................................................................................... 169 17.5 References:.................................................................................................................... 169 Lecture 18: Java Graphics ...................................................................................170 18.1 Painting ......................................................................................................................... 170 18.1.1 How painting works? ............................................................................................. 170 18.1.2 Painting a Swing Component ................................................................................. 172 18.1.2.1 paintComponent( ) ......................................................................................................... 173 18.1.2.2 paintBorder( )................................................................................................................. 173 18.1.2.3 paintChildren( ) .............................................................................................................. 173 Lecture 19: How to Animate? .............................................................................176 19.1 Problem & Solution....................................................................................................... 176 19.2 References ..................................................................................................................... 181 Lecture 20: Applets ..............................................................................................182 20.1 Basic Definition ............................................................................................................ 182 20.2 Applets Support............................................................................................................. 182 20.3 What an Applet is? ........................................................................................................ 182 20.4 The genealogy of Applet ............................................................................................... 182 20.5 Applet Life Cycle Methods ........................................................................................... 184 © Copyright Virtual University of Pakistan 10 Web Design and Development (CS506) 20.5.1 init( ) ...................................................................................................................... 185 20.5.2 start( ) .................................................................................................................... 185 20.5.3 paint( ) ................................................................................................................... 185 20.5.4 stop( ) .................................................................................................................... 185 20.5.5 destroy( )................................................................................................................ 186 20.6 References:.................................................................................................................... 193 Lecture 21: Socket Programming .......................................................................194 21.1 Basic Definition ............................................................................................................ 194 21.2 Socket Dynamics ........................................................................................................... 194 21.3 What is Port? ................................................................................................................. 194 21.4 How Client - Server Communicate ................................................................................ 194 21.5 Steps - To Make a Simple Client ................................................................................... 195 21.5.1 Import required package ........................................................................................ 195 21.5.2 Connect / Open a Socket with Server ..................................................................... 195 Create a client socket (communication socket) ................................................................... 195 21.5.3 Get I/O Streams of Socket...................................................................................... 196 21.5.4 Send / Receive Message ......................................................................................... 196 21.5.5 Close Socket .......................................................................................................... 197 21.6 Steps - To Make a Simple Server .................................................................................. 197 21.6.1 Import required package ........................................................................................ 197 21.6.2 Create a Server Socket ........................................................................................... 197 21.6.3 Wait for Incoming Connections ............................................................................. 197 21.6.4 Get I/O Streams of Socket...................................................................................... 197 21.6.5 Send / Receive Message ......................................................................................... 198 21.6.6 Close Socket .......................................................................................................... 198 21.7 References ..................................................................................................................... 202 Lecture 22: Serialization ......................................................................................203 22.1 Problem ......................................................................................................................... 203 22.1.1 What? .................................................................................................................... 203 22.1.2 Motivation ............................................................................................................. 203 22.1.3 Revisiting AddressBook ....................................................................................... 203 © Copyright Virtual University of Pakistan 11 Web Design and Development (CS506) 22.2 Serialization in Java ...................................................................................................... 204 22.2.1 Serializable Interface ............................................................................................ 204 22.2.2 Automatic Writing ................................................................................................ 204 22.2.3 Automatic Reading ............................................................................................... 204 22.2.4 Serialization: How it works?................................................................................. 204 22.3 Object Serialization & Network .................................................................................... 207 22.4 Preventing Serialization ................................................................................................ 208 22.5 References ..................................................................................................................... 209 Lecture 23: Multithreading .................................................................................210 23.1 Introduction ................................................................................................................... 210 23.2 Sequential Execution vs. Multithreading ....................................................................... 210 23.3 Java Threads.................................................................................................................. 212 23.3.1 Creating Threads in Java ........................................................................................ 212 23.3.1.1 Threads Creation Steps Using Interface .......................................................................... 212 23.3.1.2 Threads Creation Steps Using Inheritance ...................................................................... 213 23.4 Three Loops: Multi-Threaded Execution ....................................................................... 213 23.5 Thread Priorities ............................................................................................................ 215 23.5.1 Thread Priority Scheduling ................................................................................... 216 23.5.2 Problems with Thread Priorities ........................................................................... 218 23.6 References:.................................................................................................................... 218 Lecture 24: More on Multithreading..................................................................219 24.1 Useful Thread Methods ................................................................................................. 221 24.1.1 sleep(int time) method ........................................................................................... 221 24.1.2 yield( ) method....................................................................................................... 224 24.2 Thread States: Life Cycle of a Thread ........................................................................... 226 24.2.1 New state ............................................................................................................... 226 24.2.2 Ready state............................................................................................................. 226 24.2.3 Running state ......................................................................................................... 226 24.2.4 Dead state .............................................................................................................. 227 24.3 Thread’s Joining ............................................................................................................ 227 24.4 References:.................................................................................................................... 228 © Copyright Virtual University of Pakistan 12 Web Design and Development (CS506) Lecture 25: Web Application Development .......................................................229 25.1 Introduction ................................................................................................................... 229 25.2 Web Applications .......................................................................................................... 229 25.3 HTTP Basics ................................................................................................................. 229 25.3.1 Parts of an HTTP request ....................................................................................... 230 25.3.2 Parts of HTTP response ......................................................................................... 230 25.3.3 HTTP Response Codes .......................................................................................... 231 25.4 Server Side Programming.............................................................................................. 233 25.4.1 Why build Pages Dynamically? ............................................................................. 234 25.4.2 Dynamic Web Content Technologies Evolution ..................................................... 236 25.5 Layers & Web Application ............................................................................................ 236 25.5.1 Presentation Layer: ................................................................................................ 237 25.5.2 Business Layer ....................................................................................................... 237 25.5.3 Data Layer ............................................................................................................. 237 25.6 Java - Web Application Technologies ........................................................................... 237 25.7 References:.................................................................................................................... 237 Lecture 26: Java Servlets ....................................................................................238 26.1 What Servlets can do? ................................................................................................... 238 26.2 Servlets vs. other SSP technologies ............................................................................... 238 26.2.1 Convenient ............................................................................................................. 238 26.2.2 Efficient ................................................................................................................. 239 26.2.3 Powerful ................................................................................................................ 239 26.2.4 Portable.................................................................................................................. 239 26.2.5 Inexpensive ............................................................................................................ 239 26.3 Software Requirements ................................................................................................. 239 26.4 Jakarta Servlet Engine (Tomcat) .................................................................................. 239 26.4.1 Environment Setup................................................................................................ 239 26.4.2 Environment Setup Using .zip File ....................................................................... 240 26.4.2.1 Download the Apache Tomcat Server .......................................................................... 240 26.4.2.2 Installing Tomcat using .zip file.................................................................................... 240 26.4.2.3 Set the JAVA_HOME variable ..................................................................................... 241 © Copyright Virtual University of Pakistan 13 Web Design and Development (CS506) 26.4.2.4 Set the CATALINA_HOME variable ........................................................................... 242 26.4.2.5 Set the CLASSPATH variable ...................................................................................... 243 26.4.2.6 Test the server ............................................................................................................... 244 26.4.3 Environment Setup Using .exe File ...................................................................... 244 26.4.3.1 Download the Apache Tomcat Server .......................................................................... 245 26.4.3.2 Installing Tomcat using .exe file ................................................................................... 245 26.4.3.3 Set the JAVA_HOME variable ..................................................................................... 247 26.4.3.4 Set the CATALINA_HOME variable ........................................................................... 247 26.4.3.5 Set the CLASSPATH variable ...................................................................................... 247 26.4.3.6 Test the server ............................................................................................................... 247 26.5 References: ................................................................................................................... 248 Lecture 27: Creating a Simple Web Application in Tomcat ...........................249 27.1 Standard Directory Structure of a J2EE Web Application ........................................... 249 27.2 Writing Servlets ............................................................................................................ 251 27.2.1 Servlet Types ........................................................................................................ 251 27.2.1.1 GenericServlet class ...................................................................................................... 252 27.2.1.2 HttpServlet class ........................................................................................................... 252 27.3 Servlet Class Hierarchy ................................................................................................ 252 27.4 Types of HTTP requests ............................................................................................... 253 27.5 GET & POST, HTTP request types ............................................................................. 253 27.6 Steps for making a Hello World Servlet ...................................................................... 254 27.7 Compiling and Invoking Servlets ................................................................................. 256 27.8 References: ................................................................................................................... 257 Lecture 28: Servlets Lifecycle..............................................................................258 28.1 Stages of Servlet Lifecycle ........................................................................................... 258 28.1.1 Initialize ................................................................................................................ 258 28.1.2 Service................................................................................................................... 259 28.1.3 Destroy .................................................................................................................. 260 28.2 Summary ...................................................................................................................... 260 28.3 Reading HTML Form Data Using Servlets ................................................................... 261 28.3.1 HTML & Servlets ................................................................................................. 261 © Copyright Virtual University of Pakistan 14 Web Design and Development (CS506) 28.3.2 Types of Data send to Web Server ....................................................................... 261 28.3.2.1 Reading HTML Form Data from Servlet ...................................................................... 262 28.4 References: ................................................................................................................... 266 Lecture 29: More on Servlets ..............................................................................267 29.1 Initialization Parameters ............................................................................................... 267 29.1.1 ServletConfig ........................................................................................................ 267 29.1.2 Reading Initialization Parameters ......................................................................... 268 29.1.3 Response Redirection............................................................................................ 270 29.1.4 Sending a standard Redirect.................................................................................. 270 29.1.5 Sending a redirect to an error page ....................................................................... 270 29.2 ServletContext .............................................................................................................. 273 29.3 Request Dispatcher ....................................................................................................... 274 29.4 RequestDispatcher: forward ......................................................................................... 274 29.5 RequestDispatcher: include .......................................................................................... 275 29.6 References: ................................................................................................................... 275 Lecture 30: Dispatching Requests ......................................................................276 30.1 Recap ............................................................................................................................ 276 30.1.1 Sending a standard request: .................................................................................. 276 30.1.2 Redirection to an error page:................................................................................. 276 30.1.3 Forward: ................................................................................................................ 276 30.1.4 Include: ................................................................................................................. 276 30.2 HttpServletRequest Methods........................................................................................ 283 30.2.1 setAttribute(String, Object)................................................................................... 283 30.2.2 getAttribute(String) ............................................................................................... 283 30.2.3 getMethod()........................................................................................................... 283 30.2.4 getRequestURL() .................................................................................................. 284 30.2.5 getProtocol() ......................................................................................................... 284 30.2.6 getHeaderNames() ................................................................................................ 284 30.2.7 getHearderName()................................................................................................. 284 30.3 HttpServletResponse Methods ..................................................................................... 284 30.3.1 setContentType()................................................................................................... 284 © Copyright Virtual University of Pakistan 15 Web Design and Development (CS506) 30.3.2 setContentLength() ............................................................................................... 284 30.3.3 addCookie()........................................................................................................... 284 30.3.4 sendRedirect() ....................................................................................................... 284 30.4 Session Tracking .......................................................................................................... 285 30.4.1 Continuity problem- user’s point of view ............................................................. 285 30.4.2 Continuity problem- Server’s point of view ......................................................... 286 30.5 References: ................................................................................................................... 286 Lecture 31: Session Tracking ..............................................................................287 31.1 Store State Somewhere................................................................................................. 287 31.2 Post-Notes .................................................................................................................... 287 31.3 Three Typical Solutions ............................................................................................... 287 31.3.1 Cookies .................................................................................................................. 288 31.3.1.1 What a cookie is? .......................................................................................................... 288 31.3.1.2 Cookie’s Voyage ........................................................................................................... 288 31.3.2 Potential Uses of Cookies ..................................................................................... 288 31.3.3 Sending Cookies to Browser................................................................................. 289 31.3.4 Reading Cookies from the Client .......................................................................... 289 31.4 References: ................................................................................................................... 298 Lecture 32: Session Tracking 2 ...........................................................................299 32.1 URL Rewriting ............................................................................................................. 299 32.1.1 Disadvantages of URL rewriting .......................................................................... 299 32.2 Hidden Form Fields...................................................................................................... 304 32.3 Java Solution for Session Tracking .............................................................................. 304 32.4 Working with HttpSession ........................................................................................... 305 32.5 HttpSession – Behind the scenes.................................................................................. 308 32.6 Encoding URLs sent to Client...................................................................................... 309 32.7 Difference between encodeURL() and encodeRedirectURL() .................................... 309 32.8 Some Methods of HttpSession ..................................................................................... 312 32.9 References: ................................................................................................................... 313 Lecture 33: Address Book Case Study Using Servlets ......................................314 33.1 Design Process ............................................................................................................. 314 © Copyright Virtual University of Pakistan 16 Web Design and Development (CS506) 33.2 Layers & Web Application .......................................................................................... 314 33.2.1 Step 1 .................................................................................................................... 315 33.2.2 Step 2 .................................................................................................................... 315 33.2.3 Step 3 .................................................................................................................... 316 33.2.4 Step 4 .................................................................................................................... 317 33.2.5 Step 5 .................................................................................................................... 318 33.3 Package......................................................................................................................... 321 33.3.1 What is a package?................................................................................................ 321 33.3.2 How to create a package ....................................................................................... 322 33.3.3 How to use package .............................................................................................. 323 33.4 JavaServer Pages (JSP) ................................................................................................ 323 33.4.1 The Need for JSP .................................................................................................. 323 33.4.2 The JSP Framework .............................................................................................. 323 33.4.3 Advantages of JSP over Competing Technologies ............................................... 324 33.4.4 Setting Up Your Environment .............................................................................. 324 33.5 References: ................................................................................................................... 324 Lecture 34: Java Server Pages ............................................................................325 34.1 First run of a JSP .......................................................................................................... 325 34.1.1 Benefits of JSP ...................................................................................................... 325 34.1.2 JSP vs. Servlet....................................................................................................... 326 34.2 JSP Ingredients .............................................................................................................. 327 34.3 Scripting Elements ....................................................................................................... 328 34.3.1 Comments ............................................................................................................. 328 34.3.2 Expressions ........................................................................................................... 328 34.3.3 Scriptlets ............................................................................................................... 328 34.3.4 Declarations .......................................................................................................... 329 34.4 Writing JSP scripting Elements in XML...................................................................... 330 34.5 References: ................................................................................................................... 331 Lecture 35: JavaServer Pages .............................................................................332 35.1 Implicit Objects ............................................................................................................ 332 35.2 JSP Directives ............................................................................................................... 335 © Copyright Virtual University of Pakistan 17 Web Design and Development (CS506) 35.2.1 Format ................................................................................................................... 336 35.2.2 JSP page Directive ................................................................................................ 336 35.2.3 JSP include Directive ............................................................................................ 337 35.3 JSP Life Cycle Methods ............................................................................................... 339 35.4 References: ................................................................................................................... 339 Lecture 36 .............................................................................................................340 36.1 JavaBeans ..................................................................................................................... 345 36.1.1 JavaBeans Design Conventions ............................................................................ 345 36.2 References: ................................................................................................................... 352 Lecture 37: JSP Action Elements and Scope .....................................................353 37.1 JSP Action Elements .................................................................................................... 353 37.2 Working with JavaBeans using JSP Action Elements ................................................. 354 37.2.1 JSP useBean Action Element ................................................................................ 354 37.2.2 JSP setProperty Action Element ........................................................................... 355 37.2.3 JSP getProperty Action Element ........................................................................... 355 37.3 Sharing Beans & Object Scopes .................................................................................... 359 37.3.1 page ....................................................................................................................... 359 37.3.2 request ................................................................................................................... 360 37.3.3 session ................................................................................................................... 361 37.3.4 Application ............................................................................................................ 362 37.4 Summary of Object’s Scopes ....................................................................................... 363 37.5 More JSP Action Elements .......................................................................................... 365 37.5.1 JSP include action Element ................................................................................... 365 37.5.2 JSP forward action Element .................................................................................. 365 37.6 References: ................................................................................................................... 365 Lecture 38: JSP Custom Tags .............................................................................366 38.1 Motivation .................................................................................................................... 366 38.2 What is a Custom Tag? ................................................................................................ 366 38.3 Why Build Custom Tag? .............................................................................................. 367 38.4 Advantages of using Custom Tags ............................................................................... 367 © Copyright Virtual University of Pakistan 18 Web Design and Development (CS506) 38.5 Types of Tags ............................................................................................................... 367 38.5.1 Simple Tag ............................................................................................................ 367 38.5.2 Tag with Attributes ............................................................................................... 368 38.5.3 Tag with Body....................................................................................................... 368 38.6 Building Custom Tags ................................................................................................... 368 38.6.1 Steps for Building Custom Tags ........................................................................... 368 38.6.2 Develop the Tag Handler class ............................................................................. 369 38.6.3 Write Tag Library Discriptor (.tld) file ................................................................. 369 38.6.4 Deployment ........................................................................................................... 369 38.7 Using Custom Tags ...................................................................................................... 370 38.8 Building tags with attributes ........................................................................................ 372 38.9 References: ................................................................................................................... 380 Lecture 39: MVC + Case Study ..........................................................................381 39.1 Error Page ..................................................................................................................... 381 39.1.1 Defining and Using Error Pages ........................................................................... 381 39.2 Case Study – Address Book ......................................................................................... 382 39.2.1 Ingredients of Address Book ................................................................................ 382 39.3 Model View Controller (MVC).................................................................................... 394 39.3.1 Participants and Responsibilities .......................................................................... 395 39.3.2 Evolution of MVC Architecture ........................................................................... 395 39.3.2.1 MVC Model 1 ............................................................................................................... 395 39.4 References: ................................................................................................................... 396 Lecture 40: MVC Model 2 Architecture............................................................397 40.1 Page-Centric Approach ................................................................................................ 397 40.1.1 Page-with-Bean Approach (MVC Model1) .......................................................... 397 40.2 MVC Model 2 Architecture ......................................................................................... 398 40.3 Case Study: Address Book using MVC Model 2 ......................................................... 399 40.3.1 Introducing a JSP as Controller ............................................................................ 399 40.3.2 How controller differentiates between requests? .................................................. 399 40.4 References: ................................................................................................................... 417 Lecture 41: Layers and Tiers ..............................................................................418 © Copyright Virtual University of Pakistan 19 Web Design and Development (CS506) 41.1 Layers vs. Tiers ............................................................................................................ 418 41.1.1 Layers .................................................................................................................... 418 41.1.1.1 Presentation Layer......................................................................................................... 419 41.1.1.2 Business Layer .............................................................................................................. 419 41.1.1.3 Data Layer ..................................................................................................................... 419 41.1.2 Tiers ...................................................................................................................... 420 41.2 Layers Support in Java ................................................................................................. 421 41.3 J2EE Multi-Tiered Applications .................................................................................. 421 41.4 Case Study: Matrix Multiplication using Layers ......................................................... 422 41.5 References: ................................................................................................................... 432 Lecture 42: Expression Language ......................................................................433 42.1 Overview ...................................................................................................................... 433 42.2 JSP Before and After EL .............................................................................................. 433 42.3 Expression Language Nuggets ..................................................................................... 435 42.3.1 EL Syntax.............................................................................................................. 435 42.3.2 EL Identifiers (cont.) ............................................................................................. 439 42.3.3 EL Accessors ........................................................................................................ 440 42.3.4 EL – Robust Features ............................................................................................ 441 42.3.5 Using Expression Language ................................................................................. 442 42.4 References: ................................................................................................................... 448 Lecture 43: JavaServer Pages Standard Tag Library (JSTL) ........................449 43.1 Introduction .................................................................................................................. 449 43.2 JSTL & EL ................................................................................................................... 449 43.3 Functional Overview .................................................................................................... 449 43.4 Twin Tag Libraries ....................................................................................................... 449 43.5 Using JSTL ................................................................................................................... 450 43.6 Working with Core Actions (tags) ............................................................................... 451 43.7 netBeans 4.1 and JSTL ................................................................................................. 456 Lecture 44: Client Side Validation & JavaServer Faces (JSF) .......................459 44.1 Client Side Validation .................................................................................................. 459 44.1.1 Why is Client Side Validation Good? ................................................................... 459 © Copyright Virtual University of Pakistan 20 Web Design and Development (CS506) 44.2 JavaServer Faces (JSF)................................................................................................. 461 44.2.1 Different existing frameworks .............................................................................. 461 44.2.2 JavaServer Faces ................................................................................................... 461 44.2.3 JSF UI Components .............................................................................................. 462 44.2.4 JSF Events Handling ............................................................................................. 463 44.2.5 JSF Validators ....................................................................................................... 464 44.2.6 JSF – Managed Bean-Intro ................................................................................... 465 44.2.7 JSF – Value Binding ............................................................................................. 465 44.2.8 JSF – Method Binding .......................................................................................... 465 44.2.9 JSF Navigation ...................................................................................................... 465 44.3 References: ................................................................................................................... 466 Lecture 45: JavaServer Faces .............................................................................467 45.1 Web Services ................................................................................................................ 467 45.1.1 Introduction ........................................................................................................... 467 45.1.2 Web service, Definition by W3C .......................................................................... 467 45.1.3 Distributed Computing Evolution ......................................................................... 467 45.1.4 Characteristics of Web services ............................................................................ 468 45.1.5 Why Web services?............................................................................................... 468 45.1.6 Types of Web service............................................................................................ 469 45.2 Web service Architectural Components ....................................................................... 469 45.3 References: ................................................................................................................... 470 45.4 Resources: .................................................................................................................... 471 © Copyright Virtual University of Pakistan 21 Web Design and Development (CS506) Lecture 1: Java Features
This handout is a traditional introduction to any language features. You might not be able to
comprehend some of the features fully at this stage but don’t worry, you’ll get to know about these
as we move on with the course.
1.1
Design Goals of Java
The massive growth of the Internet and the World-Wide Web leads us to a completely new way
of looking at development of software that can run on different platforms like Windows, Linux
and Solaris etc.
1.1.1 Right Language, Right Time
•
•
Java came on the scene in 1995 to immediate popularity.
Before that, C and C++ dominated the software development
o compiled, no robust memory model, no garbage collector causes memory leakages,
not great support of built in libraries
•
Java brings together a great set of "programmer efficient" features
o Putting more work on the CPU to make things easier for the programmer.
1.1.2 Java - Buzzwords (Vocabulary)
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•
From the original Sun Java whitepaper: "Java is a simple, object-oriented, distributed,
interpreted, robust, secure, architecture-neutral, portable, high-performance, multithreaded, and dynamic language."
Here are some original java buzzwords.
1.1.3 Java -- Language + Libraries
•
Java has two parts.
o The core language -- variables, arrays, objects
The Java Virtual Machine (JVM) runs the core language
The core language is simple enough to run on small devices phones, smart
cards, PDAs.
o The libraries
Java includes a large collection of standard library classes to provide "off the
shelf" code. (Useful built-in classes that comes with the language to perform
basic tasks)
Example of these classes is String, ArrayList, HashMap,
StringTokenizer (to break string into substrings), Date ...
Java programmers are more productive in part because they have access to a
large set of standard, well documented library classes.
© Copyright Virtual University of Pakistan 22 Web Design and Development (CS506) 1.1.4 Simple
•
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Very similar C/C++ syntax, operators, etc.
The core language is simpler than C++ -- no operator overloading, no pointers, no
multiple inheritance
The way a java program deals with memory is much simpler than C or C++.
1.1.5 Object-Oriented
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Java is fundamentally based on the OOP notions of classes and objects.
Java uses a formal OOP type system that must be obeyed at compile-time and run-time.
This is helpful for larger projects, where the structure helps keep the various parts
consistent. Contrast to Perl, which has a more anything-goes feel.
1.1.6 Distributed / Network Oriented
•
Java is network friendly -- both in its portable, threaded nature, and because Common
networking operations are built-in to the Java libraries.
1.1.7 Robust / Secure / Safe
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Java is very robust
o Both, vs. unintentional errors and vs. malicious code such as viruses.
o Java has slightly worse performance since it does all this checking. (Or put the
other way, C can be faster since it doesn't check anything.)
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The JVM "verifier" checks the code when it is loaded to verify that it has the correct
Structure -- that it does not use an uninitialized pointer, or mix int and pointer types.
This is one-time "static" analysis -- checking that the code has the correct structure
without running it.
•
The JVM also does "dynamic" checking at runtime for certain operations, such as
pointer and array access, to make sure they are touching only the memory they should.
You will write code that runs into
•
As a result, many common bugs and security problems (e.g. "buffer overflow") are not
possible in java. The checks also make it easier to find many common bugs easy, since
they are caught by the runtime checker.
•
You will generally never write code that fails the verifier, since your compiler is smart
enough to only generate correct code. You will write code that runs into the runtime
checks all the time as you debug -- array out of bounds, null pointer.
Java also has a runtime Security Manager can check which operations a particular piece
of code is allowed to do. As a result, java can run untrusted code in a "sandbox" where,
for example, it can draw to the screen but cannot access the local file system.
•
© Copyright Virtual University of Pakistan 23 Web Design and Development (CS506) 1.1.8 Portable
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"Write Once Run Anywhere", and for the most part this works.
Not even a recompile is required -- a Java executable can work, without change, on any
Java enabled platform.
1.1.9 Support for Web and Enterprise Web Applications
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•
Java provides an extensive support for the development of web and enterprise
applications
Servlets, JSP, Applets, JDBC, RMI, EJBs and JSF etc. are some of the Java
technologies that can be used for the above mentioned purposes.
1.1.10 High-performance
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The first versions of java were pretty slow.
Java performance has gotten a lot better with aggressive just-in-time-compiler (JIT)
techniques.
Java performance is now similar to C -- a little slower in some cases, faster in a few
cases. However memory use and startup time are both worse than C.
Java performance gets better each year as the JVM gets smarter. This works,
because making the JVM smarter does not require any great change to the java
language, source code, etc.
1.1.11 Multi-Threaded
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Java has a notion of concurrency wired right in to the language itself.
This works out more cleanly than languages where concurrency is bolted on after the
fact.
1.1.12 Dynamic
•
Class and type information is kept around at runtime. This enables runtime loading
and inspection of code in a very flexible way.
1.1.13 Java Compiler Structure
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The source code for each class is in a .java file. Compile each class to produce
“.class” file.
Sometimes, multiple .class files are packaged together into a .zip or .jar "archive" file.
On UNIX or windows, the java compiler is called "javac". To compile all the .java files
in a directory use "javac *.java".
© Copyright Virtual University of Pakistan 24 Web Design and Development (CS506) 1.1.14 Java: Programmer Efficiency
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Faster Development
o Building an application in Java takes about 50% less time than in C or C++. So,
Faster time to market
o Java is said to be “Programmer Efficient”.
OOP
o Java is thoroughly OOP language with robust memory system
o Memory errors largely disappear because of the safe pointers and garbage collector.
The lack of memory errors accounts for much of the increased programmer
productivity.
Libraries
o Code re-uses at last -- String, ArrayList, Date, available and documented in a
standard way
1.1.15 Microsoft vs. Java
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Microsoft hates Java, since a Java program (portable) is not tied to any particular
operating system. If Java is popular, then programs written in Java might promote
non-Microsoft operating systems. For basically the same reason, all the nonMicrosoft vendors think Java is a great idea.
Microsoft's C# is very similar to Java, but with some improvements, and some
questionable features added in, and it is not portable in the way Java is. Generally it
is considered that C# will be successful in the way that Visual Basic is: a nice tool
to build Microsoft only software.
Microsoft has used its power to try to derail Java somewhat, but Java remains very
popular on its merits.
1.1.16 Java Is For Real
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•
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Java has a lot of hype, but much of it is deserved. Java is very well matched for
many modern problem
Using more memory and CPU time but less programmer time is an increasingly
appealing tradeoff.
Robustness and portability can be very useful features
A general belief is that Java is going to stay here for the next 10-20 years
1.1.17 References
•
•
Majority of the material in this handout is taken from the first handout of course
cs193j at Stanford.
The Java™ Language Environment, White Paper, by James Gosling & Henry
McGilton
• Java’s Sun site: http://java.sun.com • Java World : www.javaworld.com © Copyright Virtual University of Pakistan 25 Web Design and Development (CS506) Lecture 2: Java Virtual Machine & Runtime Environment
2.1
Basic Concept
When you write a program in C++ it is known as source code. The C++ compiler converts
this source code into the machine code of underlying system (e.g. Windows) If you want to run that
code on Linux you need to recompile it with a Linux based compiler. Due to the difference in
compilers, sometimes you need to modify your code.
Java has introduced the concept of WORA (write once run anywhere). When you write a java
program it is known as the source code of java. The java compiler does not compile this source
code for any underlying hardware system, rather it compiles it for a software system known as
JVM (This compiled code is known as byte code). We have different JVMs for different systems
(such as JVM for Windows, JVM for Linux etc). When we run our program the JVM interprets
(translates) the compiled program into the language understood by the underlying system. So we
write our code once and the JVM runs it everywhere according to the underlying system.
This concept is discussed in detail below
2.1.1 Byte code
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•
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Java programs (Source code) are compiled into a form called Java bytecodes.
The Java compiler reads Java language source (.java) files, translates the source into
Java bytecodes, and places the bytecodes into class (.class) files.
The compiler generates one class file for each class contained in java source file.
© Copyright Virtual University of Pakistan 26 Web Design and Development (CS506) 2.1.2 Java Virtual Machine (JVM)
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•
•
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The central part of java platform is java virtual machine.
Java bytecode executes by special software known as a "virtual machine".
Most programming languages compile source code directly into machine code,
suitable for execution
The difference with Java is that it uses bytecode - a special type of machine code.
The JVM executes Java bytecodes, so Java bytecodes can be thought of as the
machine language of the JVM.
JVM are available for almost all operating systems.
Java byte code is executed by using any operating system’s JVM. Thus achieve
portability.
2.1.3 Java Runtime Environment (JRE)
•
•
•
•
The Java Virtual Machine is a part of a large system i.e. Java Runtime Environment
(JRE).
Each operating system and CPU architecture requires different JRE.
The JRE consists of set of built-in classes, as well as a JVM.
Without an available JRE for a given environment, it is impossible to run Java
software.
© Copyright Virtual University of Pakistan 27 Web Design and Development (CS506) 2.1.4 References
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2.2
Java World: http://www.javaworld.com
Inside Java: http://www.javacoffeebreak.com/articles/inside_java
Java Program Development and Execution Steps
Java program normally go through five phases. These are
•
•
•
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Edit,
Compile,
Load,
Verify and
Execute
We look over all the above mentioned phases in a bit detail. First consider the following figure that
summarizes the all phases of a java program.
© Copyright Virtual University of Pakistan 28 Web Design and Development (CS506) © Copyright Virtual University of Pakistan 29 Web Design and Development (CS506) 2.2.1 Phase 1: Edit
Phase 1 consists of editing a file. This is accomplished with an editor program. The
programmer types a java program using the editor like notepad, and make corrections if
necessary.
When the programmer specifies that the file in the editor should be saved, the program is stored
on a secondary storage device such as a disk. Java program file name ends with a .java
extension.
On Windows platform, notepad is a simple and commonly used editor for the beginners. However
java integrated development environments (IDEs) such as NetBeans, Borland JBuilder, JCreator
and IBM’s Eclipse Java built-in editors that are smoothly integrated into the programming
environment.
2.2.2 Phase 2: Compile
In Phase 2, the programmer gives the command javac to compile the program. The java compiler
translates the java program into byte codes, which is the language understood by the java interpreter.
To compile a program called Welcome.java type javac Welcome.java at the command window
of your system. If the program compiles correctly, a file called Welcome. Class is produced. This
is the file containing the byte codes that will be interpreted during the execution phase.
2.2.3 Phase 3: Loading
In phase 3, the program must first be placed in memory before it can be executed. This is
done by the class loader, which takes the .class file (or files) containing the byte codes
and transfers it to memory. The .class file can be loaded from a disk on your system or
over a network (such as your local university or company network or even the internet).
Applications (Programs) are loaded into memory and executed using the java interpreter
via the command java. When executing a Java application called Welcome, the command
Java Welcome
Invokes the interpreter for the Welcome application and causes the class loader to load
information used in the Welcome program.
2.2.4 Phase 4: Verify
Before the byte codes in an application are executed by the java interpreter, they are verified by the
byte code verifier in Phase 4. This ensures that the byte codes for class that are loaded form the
internet (referred to as downloaded classes) are valid and that they do not violate Java’s security
restrictions. Java enforces strong security because java programs arriving over the network should
© Copyright Virtual University of Pakistan 30 Web Design and Development (CS506) not be able to cause damage to your files and your system (as computer viruses might).
2.2.5 Phase 5: Execute
Finally in phase 5, the computer, under the control of its CPU, interprets the program one byte code
at a time. Thus performing the actions specified by the program.Programs may not work on the first
try. Each of the preceding phases can fail because of various errors. This would cause the java
program to print an error message. The programmer would return to the edit phase, make the
necessary corrections and proceed through the remaining phases again to determine if the
corrections work properly.
2.2.6 References:
•
•
2.3
Java™ How to Program 5th edition by Deitel & Deitel
Sun Java online tutorial: http://java.sun.com/docs/books/tutorial/java/index.html
Installation and Environment Setting
2.3.1 Installation
•
•
•
Download the latest version j2se5.0 (java 2 standard edition) from http://java.sun.com
or get it from any other source like CD.
Note: j2se also called jdk (java development kit). You can also use the previous
versions like jdk 1.4 or 1.3 etc. but it is recommended that you use either jdk1.4 or
jdk5.0
Install j2se5.0 on your system
Note: For the rest of this handout, assume that j2se is installed in C:\Program Files\Java\jdk1.5.0
2.3.2 Environment Setting
Once you successfully installed the j2se, the next step is environment or path setting. You can
accomplish this in either of two ways.
2.3.2.1 Temporary Path Setting
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•
•
•
Open the command prompt from Start Programs Accessories
Command Prompt.
The command prompt screen would be opened in front of you.
Write the command on the command prompt according to the following format
path = < java installation directory\bin >
So, according to handout, the command will look like this
path = C:\Program Files\Java\jdk1.5.0\bin
To Test whether path has been set or not, write javac and press ENTER. If the
list of options displayed as shown in the below figure means that you have
successfully completed the steps of path setting.
© Copyright Virtual University of Pakistan 31 Web Design and Development (CS506) The above procedure is illustrates in the given below picture.
Note: The issue with the temporary path setting is you have to repeat the above explained
procedure again and again each time you open a new command prompt window. To avoid this
overhead, it is better to set your path permanently
2.3.2.2 Permanent Path Setting
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•
In Windows NT (XP, 2000), you can set the permanent environment variable.
Right click on my computer icon click on properties as shown below
© Copyright Virtual University of Pakistan 32 Web Design and Development (CS506) A System Properties frame would appear as shown in the picture.
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Select the advanced tab followed by clicking the Environment Variable
button. The Environment variables frame would be displayed in front of you
Locate the Path variable in the System or user variables, if it is present there,
select it by single click. Press Edit button. The following dialog box would be
appeared.
Write; C:\Program Files\Java\jdk1.5.0\bin at the end of the value field.
Press OK button. Remember to write semicolon (;) before writing the path for
java installation directory as illustrated in the above figure.
If Path variable does not exist, click the New button. Write variable name
“PATH”, variable value C:\Program Files\Java\jdk1.5.0\bin and press OK button.
Now open the command prompt and write javac, press enter button. You see the list of
options would be displayed.
After setting the path permanently, you have no need to set the path for each new
opened command prompt.
© Copyright Virtual University of Pakistan 33 Web Design and Development (CS506) 2.3.3 References
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web Design
and Development and not for any other commercial purpose without the consent of
author.
2.4
First Program in Java
Like any other programming language, the java programming language is used to create
applications. So, we start from building a classical “Hello World” application, which is generally
used as the first program for learning any new language.
2.4.1 HelloWorldApp
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•
Open notepad editor from Start ProgarmFiles
Write the following code into it.
Accessories
Notepad.
Note: Don’t copy paste the given below code. Probably it gives errors and you can’t able to remove
them at the beginning stage.
1./* The HelloWorldApp class implements an application that
2.simply displays "Hello World!" to the standard output.
3.*/
4.
public class HelloWorldApp {
5.public static void main(String[] args) {
6.//Display the string. No global main
7.System.out.println(“Hello World”);
8. }
9. }
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•
To save your program, move to File menu and choose save as option.
Save your program as “HelloWorldApp.java” in some directory. Make sure to
add double quotes around class name while saving your program. For this example
create a folder known as “examples” in D: drive
Note: Name of file must match the name of the public class in the file (at line 4).
Moreover, it is case sensitive. For example, if your class name is MyClasS, than file
name must be MyClasS. Otherwise the Java compiler will refuse to compile the
program.
For the rest of this handout, we assume that program is saved in D:\examples directory.
© Copyright Virtual University of Pakistan 34 Web Design and Development (CS506) 2.4.2 HelloWorldApp Described
•
Lines 1-3
o Like in C++, You can add multiple line comments that are ignored by the compiler.
•
Lines 4
o Line 4 declares the class name as HelloWorldApp. In java, every line of code
must reside inside class. This is also the name of our program
(HelloWorldApp.java).The compiler creates the HelloWorldApp.class if
this program successfully gets compiled.
•
Lines 5
o Line 5 is where the program execution starts. The java interpreter must find this
defined exactly as given or it will refuse to run the program. (However you can
change the name of parameter that is passed to main. i.e. you can write String[]
argv or String[] someParam instead of String[] args)
o Other programming languages, notably C++ also use the main() declaration as the
starting point for execution. However the main function in C++ is global and
resides outside of all classes where as in Java the main function must reside inside a
class. In java there are no global variables or functions. The various parts of this main
function declaration will be covered at the end of this handout.
•
Lines 6
o Again like C++, you can also add single line comment
•
Lines 7
o Line 7 illustrates the method call. The println() method is used to print
something on the console. In this example println() method takes a string
argument and writes it to the standard output i.e. console.
•
Lines 8-9
o Line 8-9 of the program, the two braces, close the method main() and the
classHelloWorldApp respectively.
2.4.3 Compiling and Running HelloWorldApp
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Open the command prompt from Start
Program Files
Accessories. OR
alternatively you can write cmd in the run command window.
Write cd.. to came out from any folder, and cd [folder name] to move inside the specified
directory. To move from one drive to another, use [Drive Letter]: See figure given below
After reaching to the folder or directory that contains your source code, in our case
HelloWorldApp.java.
Use “javac” on the command line to compile the source file ( “.java” file).
• D:\examples> javac HelloWorld.java
© Copyright Virtual University of Pakistan 35 Web Design and Development (CS506) •
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•
If program gets successfully compiled, it will create a new file in the same directory named
HelloWorldApp.class that contains the byte-code.
Use “java” on the command line to run the compiled .class file. Note “.class” would be
added with the file name.
• D:\examples> java HelloWorld
You can see the Hello World would be printed on the console. Hurrah! You are
successful in writing, compiling and executing your first program in java
2.4.4 Points to Remember
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•
2.5
Recompile the class after making any changes
Save your program before compilation
Only run that class using java command that contains the main method, because program
executions always starts form main
An Idiom Explained
•
You will see the following line of code often:
o public static void main(String args[]) { …}
•
About main()
o “main” is the function from which your program starts
o Why public?
o Since main method is called by the JVM that is why it is kept public so that it is
accessible from outside. Remember private methods are only accessible from
within the class
•
Why static?
o Every Java program starts when the JRE (Java Run Time Environment) calls the
© Copyright Virtual University of Pakistan 36 Web Design and Development (CS506) main method of that program. If main is not static then the JRE have to create an
object of the class in which main method is present and call the main method on that
object (In OOP based languages method are called using the name of object if they
are not static). It is made static so that the JRE can call it without creating an object.
o Also to ensure that there is only one copy of the main method per class
•
Why void?
o Indicates that main ( ) does not return anything.
•
What is String args[] ?
o Way of specifying input (often called command-line arguments) at startup of
application. More on it latter
2.6
•
References
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web Design
and Development and not for any other commercial purpose without the consent of
author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 37 Web Design and Development (CS506) Lecture 3: Learning Basics
3.1
Strings
A string is commonly considered to be a sequence of characters stored in memory and accessible
as a unit. Strings in java are represented as objects.
3.1.1 String Concatenation
•
“+” operator is used to concatenate strings
o System.out.println(“Hello” + “World”) will print Hello World on console
•
String concatenated with any other data type such as int will also convert that
datatype to String and the result will be a concatenated String displayed on console.
For example,
o int i = 4;
o int j = 5;
System.out.println (“Hello” + i);// will print Hello 4 on screen
o However
System.out.println(i+j);//will print 9 on the console because both i and j
are of type int.
3.1.2 Comparing Strings
For comparing Strings never use == operator, use equals method of String class.
•
== operator compares addresses (shallow comparison) while equals compares values (deep
comparison)
o E.g. string1.equals(string2)
Example Code: String concatenation and comparison
public class StringTest {
public static void main(String[] args) {
int i = 4;
int j = 5;
System.out.println("Hello" + i); // will print Hello4
System.out.println(i + j); // will print 9
String s1 = new String (“pakistan”);
© Copyright Virtual University of Pakistan 38 Web Design and Development (CS506) String s2 = “pakistan”;
if (s1 == s2) {
System.out.println(“comparing string using == operator”);
}
if (s1.equals( s2) ) {
System.out.println(“comparing string using equal method”);
}
}
}
On execution of the above program, following output will produce
3.2
Taking in Command Line Arguments
In Java, the program can be written to accept command-line-arguments.
Example Code: command-line arguments
/* This Java application illustrates the use of Java command-line
arguments. */
public class CmdLineArgsApp {
public static void main(String[] args){ //main method
System.out.println(”First argument ”
+ args[0]);
System.out.println(”Second argument ” + args[1]);
}//end main
}//End class.
To execute this program, we pass two arguments as shown below:
public void someMethod( ) {
int x; //local variable
System.out.println(x); // compile time error
•
•
These parameters should be separated by space.
The parameters that we pass from the command line are stored as Strings inside the “args”
array. You can see that the type of “args” array is String.
© Copyright Virtual University of Pakistan 39 Web Design and Development (CS506) Example Code: Passing any number of arguments
In java, array knows their size by using the length property. By using, length property we can
determine how many arguments were passed. The following code example can accept any
number of arguments.
/* This Java application illustrates the use of Java
command-line arguments. */
public class AnyArgsApp {
public static void main(String[] args){ //main method
for(int i=0; i < args.length; i++)
System.out.println(“Argument:” + i + “ value ” +args[i]);
}//end main
}//End class.
Output
C:\java AnyArgsApp i can pass any number of arguments
Argument:0 value i
Argument:1 value can
Argument:2 value pass
Argument:3 value any
Argument:4 value number
Argument:5 value of
Argument:6 value arguments
3.3
Primitives vs. Objects
Everything in Java is an “Object”, as every class by default inherits from class “Object”,
except a few primitive data types, which are there for efficiency reasons.
•
Primitive Data Types
o Primitive Data types of java
boolean, byte
char, short
int, float
long, double
•
•
1 byte
2 bytes
4 bytes
8 bytes
Primitive data types are generally used for local variables, parameters and instance variables
(properties of an object)
Primitive data types are located on the stack and we can only access their value,
while objects are located on heap and we have a reference to these objects
© Copyright Virtual University of Pakistan 40 Web Design and Development (CS506) •
Also primitive data types are always passed by value while objects are always
passed by reference in java. There is no C++ like methods
o void someMethod(int &a, int & b ) // not available in java
3.4
Stack vs. Heap
Stack and heap are two important memory areas. Primitives are created on the stack while
objects are created on heap. This will be further clarified by looking at the following diagram
that is taken from Java Lab Course.
3.5
Wrapper Classes
Each primitive data type has a corresponding object (wrapper class). These wrapper
classes provides additional functionality (conversion, size checking etc.), which a primitive
data type cannot provide.
© Copyright Virtual University of Pakistan 41 Web Design and Development (CS506) 3.5.1 Wrapper Use
You can create an object of Wrapper class using a String or a primitive data type
•
•
Integer num = new Integer(4); or
Integer num = new Integer(“4”);
Note: num is an object over here not a primitive data type
You can get a primitive data type from a Wrapper using the corresponding value function
•
int primNum = num.intValue();
3.5.2 Converting Strings to Numeric Primitive Data Types
To convert a string containing digits to a primitive data type, wrapper classes can help.
parseXxx method can be used to convert a String to the corresponding primitive data
type.
•
•
String value = “532”;
int d = Integer.parseInt(value);
String value = “3.14e6”;
double d = Double.parseDouble(value);
The following table summarizes the parser methods available to a java programmer.
Example Code: Taking Input / Output
So far, we learned how to print something on console. Now the time has come to learn how to
print on the GUI. Taking input from console is not as straightforward as in C++. Initially we’ll
study how to take input through GUI (by using JOPtionPane class).
© Copyright Virtual University of Pakistan 42 Web Design and Development (CS506) The following program will take input (a number) through GUI and prints its square on the
console as well on GUI.
1. import javax.swing.*;
2. public class InputOutputTest {
3. public static void main(String[] args) {
4. //takes input through GUI
5. String input = JOptionPane.showInputDialog("Enter number");
6. int number = Integer.parseInt(input);
7. int square = number * number;
8. //Display square on console
9. System.out.println("square:" + square);
10. //Display square on GUI
11. JOptionPane.showMessageDialog(null, "square:"+ square);
12. System.exit(0);
13. }
14. }
On line 1, swing package was imported because it contains the JOptionPane class that will be
used for taking input from GUI and displaying output to GUI. It is similar to header classes of
C++.
On line 5, showInputDialog method is called of JOptionPane class by passing string
argument that will be displayed on GUI (dialog box). This method always returns back a
String regardless of whatever you entered (int, float, double, char) in the input filed.
Our task is to print square of a number on console, so we first convert a string into a
number by calling parseInt method of Integer wrapper class. This is what we done on line
number 6.
Line 11 will display square on GUI (dialog box) by using showMessageDialog method of
JOptionPane class. The first argument passed to this method is null and the second argument
must be a String. Here we use string concatenation.
Line 12 is needed to return the control back to command prompt whenever we use
JoptionPane class.
Compile & Execute
© Copyright Virtual University of Pakistan 43 Web Design and Development (CS506) 3.6
Selection & Control Structure
The if-else and switch selection structures are exactly similar to we have in C++. All
relational operators that we use in C++ to perform comparisons are also available in java
with same behavior. Likewise for, while and do-while control structures are alike to C++.
3.7
•
•
Reference:
Java tutorial: http://www.dickbaldwin.com/java
Example code, their explanations and corresponding figures for this handout are taken from
the book JAVA A Lab Course by Umair Javed. This material is available just for the use of
VU students of the course Web Design and Development and not for any other commercial
purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on“Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 44 Web Design and Development (CS506) Lecture 4: Object Oriented Programming
Java is fundamentally object oriented. Every line of code you write in java must be inside a class
(not counting import directives).OOP fundamental stones Encapsulation, Inheritance and
Polymorphism etc. are all fully supported by java.
4.1
OOP Vocabulary Review
4.1.1 Classes
•
•
•
Definition or a blueprint of a user defined data type
Prototypes for objects
Think of it as a map of the building on a paper
4.1.2 Objects
•
•
•
Nouns, things in the world
Anything we can put a thumb on
Objects are instantiated or created from class
4.1.3 Constructor
•
A special method that is implicitly invoked. Used to create an Object (that is,
an Instance of the Class) and to initialize it.
4.1.4 Attributes
•
Properties an object has
4.1.5 Methods
•
4.2
Actions that an object can do
Defining a Class
© Copyright Virtual University of Pakistan 45 Web Design and Development (CS506) 4.3
Comparison with C++
Some important points to consider when defining a class in java as you probably noticed from the
above given skeleton are
•
•
•
•
•
There are no global variables or functions. Everything resides inside a class. Remember
we wrote our main method inside a class.(For example, in HelloWorldApp
program)
Specify access modifiers (public, private or protected) for each member method or data
members at every line.
o public: accessible anywhere by anyone
o private: Only accessible within this class
o protect: accessible only to the class itself and to its subclasses or other classes in
the same package.
o default: default access if no access modifier is provided. Accessible to all classes in
the same package.
There is no semicolon (;) at the end of class.
All methods (functions) are written inline. There are no separate header and
implementation files.
Automatic initialization of class level data members if you do not initialize them
o Primitives
Numeric (int, float etc) with zero.
Char with null
Boolean with false
o Object References
With null
Note: Remember, the same rule is not applied to local variables (defined inside
method body). Using a local variable without initialization is a compile time error
Public void someMethod( ) {
int x; //local variable
System.out.println(x); // compile time error
}
•
Constructor
o Same name as class name
o Does not have a return type
o No initialization list
© Copyright Virtual University of Pakistan 46 Web Design and Development (CS506) •
o JVM provides a zero argument (default) constructor only if a class doesn’t
define its own constructor
Destructors
o Are not required in java class because memory management is the
responsibility of JVM.
4.4
Task - Defining a Student class
The following example will illustrate how to write a class. We want to write a “Student” class that
•
Should be able to store the following characteristics of student
•
o Roll No
o Name
Provide default, parameterized and copy constructors
o Provide standard getters/setters (discuss shortly) for instance variables
Make sure, roll no has never assigned a negative value i.e. ensuring the
correct state of the object
Provide print method capable of printing student object on console
4.5
Getters / Setters
The attributes of a class are generally taken as private or protected. So to access them outside of
a class, a convention is followed knows as getters & setters. These are generally public
methods. The words set and get are used prior to the name of an attribute. Another
important purpose for writing getter & setters to control the values assigned to an attribute.
Student Class Code
// File Student.java
public class Student {
private String name;
private int rollNo;
// Standard Setters
public void setName (String name) {
this.name = name;
}
// Note the masking of class level variable rollNo
public void setRollNo (int rollNo) {
if (rollNo > 0) {
this.rollNo = rollNo; }else {
this.rollNo = 100;
© Copyright Virtual University of Pakistan 47 Web Design and Development (CS506) }
}
// Standard Getters
public String getName ( ) {
return name;
}
public int getRollNo ( ) {
return rollNo;
}
// Default Constructor
public Student() {
name = “not set”;
rollNo = 100;
}
// parameterized Constructor for a new student
public Student(String name, int rollNo) {
setName(name);//call to setter of name
setRollNo(rollNo); //call to setter of rollNo
}
// Copy Constructor for a new student
public Student(Student s) {
name = s.name;
rollNo = s.rollNo;
}
// method used to display method on console
public void print () {
System.out.print("Student name: " +name);
System.out.println(", roll no: " +rollNo); }
} // end of class
4.6
Using a Class
Objects of a class are always created on heap using the “new” operator followed by
constructor
• Student s = new Student ( ); // no pointer operator “*” between Student and s
• Only String constant is an exception
o String greet = “Hello” ; // No new operator
o However you can also use
• String greet2 = new String(“Hello”);
Members of a class (member variables and methods also known as instance
variables/methods) are accessed using “.” operator. There is no “−>” operator in java
• s.setName(“Ali”);
• s−>setName(“Ali”) is incorrect and will not compile in java
Note: Objects are always passed by reference and primitives are always passed by value
in java.
© Copyright Virtual University of Pakistan 48 Web Design and Development (CS506) 4.6.1 Task - Using Student Class
•
•
Create objects of student class by calling default parameterize and copy constructor
Call student class various methods on these objects.
Student client code
// File Test.java
/* This class create Student class objects and demonstrates
how to call various methods on objects
*/
public class Test{
public static void main (String args[]){
// Make two student obejcts
Student s1 = new Student("ali", 15);
Student s2 = new Student(); //call to default costructor
s1.print(); // display ali and 15
s2.print(); // display not set and 100
s2.setName("usman");
s2.setRollNo(20);
System.out.print("Student name:" + s2.getName());
System.out.println(" rollNo:" + s2.getRollNo());
System.out.println("calling copy constructor");
Student s3 = new Student(s2); //call to copy constructor
s2.print();
s3.print();
s3.setRollNo(-10); //Roll No of s3 would be set to 100
s3.print();
/*NOTE: public vs. private
A statement like "b.rollNo = 10;" will not compile in a client of
the Student class when rollNo is declared
protected or private
*/
} //end of main
} //end of class
© Copyright Virtual University of Pakistan 49 Web Design and Development (CS506) Compile & Execute
Compile both classes using javac commad. Run Test class using java command.
4.7
More on Classes
4.7.1 Static
A class can have static variables and methods. Static variables and methods are
associated with the class itself and are not tied to any particular object. Therefore statics can be
accessed without instantiating an object. Static methods and variables are generally
accessed by class name.
The most important aspect of statics is that they occur as a single copy in the class
regardless of the number of objects. Statics are shared by all objects of a class. Non static methods
and instance variables are not accessible inside a static method because no this reference is
available inside a static method.
We have already used some static variables and methods. Examples are
•
System.out.println(“some text”); ---out is a static variable
•
JOptionPane.showMessageDialog(null,“sometext”);----showMessageDialog is a static method
4.7.2 Garbage Collection & Finalize
Java performs garbage collection and eliminates the need to free objects explicitly. When an
object has no references to it anywhere except in other objects that are also
unreferenced, its space can be reclaimed. Before an object is destroyed, it might be necessary for
the object to perform some action. For example: to close an opened file. In such a case, define a
finalize() method with the actions to be performed before the object is destroyed.
© Copyright Virtual University of Pakistan 50 Web Design and Development (CS506) 4.7.2.1 Finalize
When a finalize method is defined in a class, Java run time calls finalize() whenever it is about to
recycle an object of that class. It is noteworthy that a garbage collector reclaims objects in any order
or never reclaims them. We cannot predict and assure when garbage collector will get back the
memory of unreferenced objects.
The garbage collector can be requested to run by calling System.gc() method. It is not necessary
that it accepts the request and run.
Example Code: using static & finalize ()
We want to count exact number of objects in memory of a Student class the one defined earlier. For
this purpose, we’ll modify Student class.
•
•
•
•
•
Add a static variable countStudents that helps in maintaining the count of student objects.
Write a getter for this static variable. (Remember, the getter also must be static one. Hoping
so, you know the grounds).
In all constructors, write a code that will increment the countStudents by one.
Override finalize() method and decrement the countStudents variable by one.
Override toString() method.
Class Object is a superclass (base or parent) class of all the classes in java by
default. This class has already finalize() and toString() method (used to convert
an object state into string). Therefore we are actually overriding these methods
over here. (We’ll talk more about these in the handout on inheritance).
By making all above modifications, student class will look like
// File Student.java
public class Student {
private String name;
private int rollNo;
private static int countStudents = 0;
// Standard Setters
public void setName (String name) {
this.name = name;
}
// Note the masking of class level variable rollNo
public void setRollNo (int rollNo) {
if (rollNo > 0) {
this.rollNo = rollNo; }else {
this.rollNo = 100;
}
}
© Copyright Virtual University of Pakistan 51 Web Design and Development (CS506) // Standard Getters
public String getName ( ) {
return name;
}
public int getRollNo ( ) {
return rollNo;
}
// gettter of static countStudents variable
public static int getCountStudents(){
return countStudents;
}
// Default Constructor
public Student() {
name = “not set”;
rollNo = 100;
countStudents += 1;
}
// parameterized Constructor for a new student
public Student(String name, int rollNo) {
setName(name);//call to setter of name
setRollNo(rollNo); //call to setter of rollNo
countStudents += 1;
}
// Copy Constructor for a new student
public Student(Student s) {
name = s.name;
rollNo = s.rollNo;
countStudents += 1;
}
// method used to display method on console
public void print () {
System.out.print("Student name: " +name);
System.out.println(", roll no: " +rollNo); }
// overriding toString method of java.lang.Object class
public String toString(){
return “name: ” + name + “ RollNo: ” + rollNo;
}
// overriding finalize method of Object class
public void finalize(){
countStudents -= 1;
}
} // end of class
© Copyright Virtual University of Pakistan 52 Web Design and Development (CS506) Next, we’ll write driver class. After creating two objects of student class, we deliberately
loose object’s reference and requests the JVM to run garbage collector to reclaim the
memory. By printing countStudents value, we can confirm that. Coming up code is of the
Test class.
// File Test.java
public class Test{
public static void main (String args[]){ int numObjs;
// printing current number of objects i.e 0
numObjs = Student.getCountStudents();
System.out.println(“Students Objects” + numObjs);
// Creating first student object & printing its values
Student s1 = new Student("ali", 15);
System.out.println(“Student: ” + s1.toString());
// printing current number of objects i.e. 1
numObjs = Student.getCountStudents();
System.out.println(“Students Objects” + numObjs);
// Creating second student object & printing its values
Student s2 = new Student("usman", 49);
// implicit call to toString() method
System.out.println(“Student: ” + s2);
// printing current number of objects i.e. 2
numObjs = Student.getCountStudents();
System.out.println(“Students Objects” + numObjs);
// loosing object reference
s1 = null;
// requesting JVM to run Garbage collector but there is
// no guarantee that it will run
System.gc();
// printing current number of objects i.e. unpredictable
numObjs = Student.getCountStudents();
System.out.println(“Students Objects” + numObjs);
} //end of main
} //end of class
The compilation and execution of the above program is given below. Note that output may be
different one given here because it all depends whether garbage collector reclaims the
memory or not. Luckily, in my case it does.
© Copyright Virtual University of Pakistan 53 Web Design and Development (CS506) 4.8
•
•
•
•
Reference:
Sun java tutorial: http://java.sun.com/docs/books/tutorial/java
Thinking in java by Bruce Eckle
Beginning Java2 by Ivor Hortan
Example code, their explanations and corresponding execution figures for this
handout are taken from the book JAVA A Lab Course by Umair Javed. This material is
available just for the use of VU students of the course Web Design and Development and
not for any other commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 54 Web Design and Development (CS506) Lecture 5: Inheritance
In general, inheritance is used to implement a “is-a” relationship. Inheritance saves code rewriting
for a client thus promotes reusability.
In java parent or base class is referred as super class while child or derived class is known as sub class.
5.1
•
•
•
•
•
•
Comparison with C++
Java only supports single inheritance. As a result a class can only inherit from one
class at one time.
Keyword extends is used instead of “:” for inheritance.
All functions are virtual by default
All java classes inherit from Object class (more on it later).
To explicitly call the super class constructor, use super keyword. It’s important to
remember that call to super class constructor must be first line.
Keyword super is also used to call overridden methods.
Example Code: using inheritance
We’ll use three classes to get familiar you with inheritance. First one is Employee class. This will act
as super class. Teacher class will inherit from Employee class and Test class is driver class that
contains main method. Let’s look at them one by one
class Employee{
protected int id;
protected String name;
//parameterized constructor
public Employee(int id, String name){
this.id = id;
this.name = name;
}
//default constructor
public Employee(){
// calling parameterized constructor of same (Employee)
// class by using keyword this
this (10, “not set”);
}
//setters
public void setId (int id) {
this.id = id;
}
public void setName (String name) {
this.name = name;
}
© Copyright Virtual University of Pakistan 55 Web Design and Development (CS506) //getters
public int getId () {
return id;
}
public String getName () {
return name;
}
// displaying employee object on console
public void display(){
System.out.println(“in employee display method”);
System.out.println("Employee id:" + id + " name:" + name);
}
//overriding object’s class toString method
public String toString() {
System.out.println(“in employee toString method”);
return "id:" + id + "name:" + name;
}
}//end class
The Teacher class extends from Employee class. Therefore Teacher class is a subclass of
Employee. The teacher class has an additional attribute i.e. qualification.
class Teacher extends Employee{
private String qual;
//default constructor
public Teacher () {
//implicit call to superclass default construct
qual = "";
}
//parameterized constructor
public Teacher(int i, String n, String q){
//call to superclass param const must be first line
super(i,n);
qual = q;
}
//setter
public void setQual (String qual){
this.qual = qual;
}
//getter
public String getQual(){
return qual;
}
//overriding display method of Employee class
public void display(){
System.out.println("in teacher's display method");
© Copyright Virtual University of Pakistan 56 Web Design and Development (CS506) super.display(); //call to superclass display method
System.out.println("Teacher qualification:" + qual);
}
//overriding toString method of Employee class
public String toString() {
System.out.println("in teacher's toString method");
String emp = super.toString();
return emp +" qualification:" + qual;
}
}//end class
Objects of Employee & Teacher class are created inside main method in Test class. Later calls are
made to display and toString method using these objects.
class Test{
public static void main (String args[]){
System.out.println("making object of employee");
Employee e = new Employee(89, "khurram ahmad");
System.out.println("making object of teacher");
Teacher t = new Teacher (91, "ali raza", "phd");
e.display(); //call to Employee class display method
t.display(); //call to Teacher class display method
// calling employee class toString method explicitly
System.out.println("Employee: " +e.toString());
// calling teacher class toString implicitly
System.out.println("Teacher: " + t);
} //end of main
}//end class
Output
© Copyright Virtual University of Pakistan 57 Web Design and Development (CS506) 5.2
Object - The Root Class
The Object class in Java is a superclass for all other classes defined in Java's class
libraries, as well as for user-defined Java classes. For user defined classes, its not
necessary to mention the Object class as a super class, java does it automatically for you.
The class Hierarchy of Employee class is shown below. Object is the super class of Employee
class and Teacher is a subclass of Employee class. We can make another class Manager that can also
extends from Employee class.
5.3
Polymorphism
“Polymorphic” literally means “of multiple shapes” and in the context of OOP,
Polymorphic means “having multiple behaviors”.
• A parent class reference can point to the subclass objects because of is-a relationship.
For example a Employee reference can point to:
•
o Employee Object
o Teacher Object
o Manager Object
A polymorphic method results in different actions depending on the object being
referenced
o Also known as late binding or run-time binding
Example Code: using polymorphism
This Test class is the modification of last example code. Same Employee & Teacher classes are
used. Objects of Employee & Teacher class are created inside main methods and calls are made to
© Copyright Virtual University of Pakistan 58 Web Design and Development (CS506) display and toString method using these objects.
Example Code: using polymorphism
This Test class is the modification of last example code. Same Employee & Teacher classes
are used. Objects of Employee & Teacher class are created inside main methods and calls are
made to display and toString method using these objects.
class Test{
public static void main (String args[]){
// Make employee references
Employee ref1, ref2;
// assign employee object to first employee reference
ref1 = new Employee(89, "khurram ahmad");
// is-a relationship, polymorphism
ref2 = new Teacher (91, "ali raza", "phd");
//call to Employee class display method
ref1.display();
//call to Teacher class display method
ref2.display();
// call to Employee class toString method
System.out.println("Employee: " +ref1.toString());
// call to Teacher class toString method
System.out.println("Teacher: " + ref2.toString());
} //end of main
}//end class
Output
© Copyright Virtual University of Pakistan 59 Web Design and Development (CS506) 5.4
Type Casting
In computer science, type conversion or typecasting refers to changing an entity of one data type
into another. Type casting can be categorized into two types
5.4.1 Up-casting
•
•
•
•
converting a smaller data type into bigger one
Implicit - we don’t have to do something special
No loss of information
Examples of
o Primitives
int a = 10;
double b = a;
o Classes
Employee e = new Teacher( );
5.4.2 Down-casting
•
•
•
•
converting a bigger data type into smaller one
Explicit - need to mention
Possible loss of information
Examples of
o Primitives
double a = 7.65;
int b = (int) a;
o Classes
Employee e = new Teacher( );// up-casting
Teacher t= (Teacher) e;// down-casting
5.5
•
•
•
References:
Java tutorial: http://java.sun.com/docs/books/tutorial/java/javaOO/
Stanford university
Example code, their explanations and corresponding figures for handout 5-1,5-2
are taken from the book JAVA A Lab Course by Umair Javed. This material is
available just for the use of VU students of the course Web Design and
Development and not for any other commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
Lecture 6: Collections
© Copyright Virtual University of Pakistan 60 Web Design and Development (CS506) A collection represents group of objects know as its elements. Java has a built-in support for
collections. Collection classes are similar to STL in C++. An advantage of a collection over
an array is that you don’t need to know the eventual size of the collection in order to add objects to
it. The java.util package provides a set of collection classes that helps a programmer in number of
ways.
6.1
Collections Design
All classes almost provides same methods like get (), size (), isEmpty() etc. These methods will
return the object stored in it, number of objects stored and whether collection contains an
object or not respectively.
Java collections are capable of storing any kind of objects. Collections store references to
objects. This is similar to using a void* in C. therefore down casting is required to get the
actual type. For example, if string in stored in a collection then to get it back, we write
String element = (String)arraylist.get(i);
6.2
Collection messages
Some basic messages (methods) are:
•
•
•
•
•
•
6.3
Constructor
o creates a collection with no elements.
int size()
o returns the number of elements in a collection.
boolean add(Object)
o adds a new element in the collection.
o returns true if the element is added successfully false otherwise.
boolean isEmpty()
o returns true if this collection contains no element false otherwise.
boolean contains(Object)
o returns true if this collection contains the specified element by using iterative
search.
boolean remove(Object)
o removes a single instance of the specified element from this collection, if it is
present .
Array List
It’s like a resizable array. Array List actually comes as a replacement the old “Vector”
collection. As we add or remove elements into or from it, it grows or shrinks over time.
6.3.1 Useful Methods
•
add (Object)
© Copyright Virtual University of Pakistan 61 Web Design and Development (CS506) •
o With the help of this method, any object can be added into Array List because
Object is the superclass of all classes.
o Objects going to add will implicitly up cast.
Object get(int index)
•
o Returns the element at the specified position in the list
o Index ranges from 0 to size()-1
o Must cast to appropriate type
remove (int index)
•
o Removes the element at the specified position in this list.
o Shifts any subsequent elements to the left (subtracts one from their indices).
int size( )
Example Code: Using Array List class
We’ll store Student objects in the Array List. We are using the same student class which we built in
previous lectures/handouts.
We’ll add three student objects and later prints all the student objects after retrieving them from
Array List. Let’s look at the code:
import java.util.*;
public class ArrayListTest {
public static void main(String[] args) {
// creating arrayList object by calling constructor
ArrayList<Student> al= new ArrayList<Student>();
// creating three Student objects
Student s1 = new Student (“ali” , 1);
Student s2 = new Student (“saad” , 2);
Student s3 = new Student (“raza” , 3);
// adding elements (Student objects) into arralylist
al.add(s1);
al.add(s2);
al.add(s3);
// checking whether arraylist is empty or not Boolean
b = al.isEmpty ();
if (b == true) {
System.out.println(“arraylist is empty”);
} else {
int size = al.size();
System.out.println(“arraylist size: ” + size);
© Copyright Virtual University of Pakistan 62 Web Design and Development (CS506) }
// using loop to iterate. Loops starts from 0 to one
// less than size
for (int i=0; i<al.size(); i++ ){
// retrieving object from arraylist
Student s = (Student) al.get(i);
// calling student class print method
s.print();
} // end for loop
} // end main
} // end class
Output
6.4
HashMap
Store elements in the form of key- value pair form. A key is associated with each object that is stored.
This allows fast retrieval of that object. Keys are unique.
6.4.1 Useful Methods
•
put(Object key, Object Value)
•
o Keys & Values are stored in the form of objects (implicit up casting is
performed).
o Associates the specified value with the specified key in this map.
o If the map previously contained a mapping for this key, the old value is
replaced.
Object get(Object key)
© Copyright Virtual University of Pakistan 63 Web Design and Development (CS506) •
o Returns the value to which the specified key is mapped in this identity hash
map, or null if the map contains no mapping for this key.
o Must downcast to appropriate type when used
int size( )
Example Code: using HashMap class
In this example code, we’ll store Student objects as values and their rollnos in the form of strings as
keys. Same Student class is used. The code is:
import java.util.*;
public class HashMapTest {
public static void main(String[] args) {
// creating HashMap object
HashMap h= new HashMap<String, Student> h=new
HashMap<String, Student>();
// creating Student objects
Student s1 = new Student ("ali" , 1);
Student s2 = new Student ("saad" , 2);
Student s3 = new Student ("raza" , 6);
// adding elements (Student objects) where roll nos
// are stored as keys and student objects as values
h.put("one". s1);
h.put("two". S2);
h.put("three". S3);
// checking whether hashmap is empty or not
boolean b = h.isEmpty ();
if (b == true) {
System.out.println(“hashmap is empty”); } else {
int size = h.size();
System.out.println(“hashmap size: ” + size);
}
// retrieving student object against rollno two and
// performing downcasting
Student s = (Student) h.get(“two”);
// calling student’s class print method
s.print();
} // end main
} // end class
Output
© Copyright Virtual University of Pakistan 64 Web Design and Development (CS506) 6.5
•
•
•
•
References:
J2SE 5.0 new features: http://java.sun.com/j2se/1.5.0/docs/relnotes/features.html
Technical Article: http://java.sun.com/developer/technicalArticles/releases/j2se15/
Beginning Java2 by Ivor Horton
Example code, their explainations and corresponding figures for this handout are taken
from the book JAVA A Lab Course by Umair Javed. This material is available just for
the use of VU students of the course Web Design and Development and not for any other
commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
6.6
Address Book
Warning: It is strongly advised that you have to type the code given in this example by yourself.
Do not copy/paste it; most probably you will get unexpected errors that you have never
seen. Some bugs are deliberately introduced as well to avoid copy-pasting. TAs will not
cooperate with you in debugging such errors.
6.6.1 Problem
•
•
We want to build an address book that is capable of storing name, address & phone
number of a person.
Address book provides functionality in the form of a JOptionPane based menu. The
feature list includes
o
o
o
o
Add - to add a new person record
Delete - to delete an existing person record by name
Search - to search a person record by name
Exit - to exit from application
© Copyright Virtual University of Pakistan 65 Web Design and Development (CS506) •
The Address book should also support persistence for person records
6.6.2 Approach for Solving Problem
Building a small address book generally involves 3 steps. Let us briefly discuss each step and write
a solution code for each step
6.6.2.1 Step1 - Make PersonInfo class
•
•
•
First of all you need to store your desired information for each person. For this you
can create a user-defined data type (i.e. a class). Make a class PersonInfo with name,
address and phone number as its attributes.
Write a parameterized constructor for this class.
Write print method in Person class that displays one person record on a message
dialog box.
The code for PersonInfo class is given below.
import javax.swing.*;
class PersonInfo {
String name;
String address;
String phoneNum;
//parameterized constructor
public PersonInfo(String n, String a, String p) {
name = n;
address = a;
phoneNum = p;
}
//method for displaying person record on GUI
public void print( ) {
JOptionPane.showMessageDialog(null, “name: ” + name +
“address:” +address + “phone no:” + phoneNum);
}
}
Note: Not declaring attributes as private is a bad approach but we have done it to
keep things simple here.
6.6.2.2 Step2 - Make Address Book class
© Copyright Virtual University of Pakistan 66 Web Design and Development (CS506) •
•
•
•
•
Take the example of daily life; generally address book is used to store more than one
person records and we don’t know in advance how many records are going to be added
into it.
So, we need some data structure that can help us in storing more than one
PersonInfo objects without concerning about its size.
Array List can be used to achieve the above functionality
Create a class Address Book with an ArrayList as its attribute. This arraylist will be used to
store the information of different persons in the form of PersonInfo Objects.
This class will also provide addPerson, deletePerson & searchPerson methods.
These methods are used for adding new person records, deleting an existing person
record by name and searching among existing person records by name respectively.
Input/Output will be performed through JOptionPane.
The code for AddressBook class is
import javax.swing.*;
import java.util.*;
class AddressBook {
ArrayList<PersonInfo> persons;
//constructor
public AddressBook ( ) {
persons = new ArrayList()<PersonInfo>();
}
//add new person record to arraylist after taking input
public void addPerson( ) {
String name = JOptionPane.showInputDialog(“Enter name”);
String add
= JOptionPane.showInputDialog(“Enter address”);
String pNum = JOptionPane.showInputDialog(“Enter phone no”);
//construct new person object
PersonInfo p = new PersonInfo(name, add, pNum);
//add the above PersonInfo object to arraylist
persons.add(p);
}
//search person record by name by iterating over arraylist
public void searchPerson (String n) {
for (int i=0; i< persons.size(); i++) {
PersonInfo p = (PersonInfo)persons.get(i);
if ( n.equals(p.name) ) {
p.print();
}
} // end for
© Copyright Virtual University of Pakistan 67 Web Design and Development (CS506) } // end searchPerson
//delete person record by name by iterating over arraylist
public void deletePerson (String n) {
for (int i=0; i< persons.size(); i++) {
PersonInfo p = (PersonInfo)persons.get(i);
if ( n.equals(p.name))
{
persons.remove(i);
}
}
}
} // end class
The addperson method first takes input for name, address and phone number and than
construct a PersonInfo object by using the recently taken input values. Then the newly
constructed object is added to the arraylist - persons.
The searchPerson & deletePerson methods are using the same methodology i.e. first they search
the required record by name and then prints his/her detail or delete the record permanently
from the ArrayList.
Both the methods are taking string argument, by using this they can perform their search or
delete operation. We used for loop for iterating the whole ArrayList. By using the size
method of ArrayList, we can control our loop as ArrayList indexes range starts from 0
to one less than size.
Notice that, inside loop we retrieve each PersonInfo object by using down casting
operation. After that we compare each PersonInfo object’s name by the one passed to
these methods using equal method since Strings are always being compared using
equal method.
Inside if block of searchPerson, print method is called using PersonInfo object that will
display person information on GUI. On the other hand, inside if block of deletePerson
method, remove method of ArrayList class is called that is used to delete record from
persons i.e. ArrayList.
6.6.2.3 Step3 - Make Test class (driver program)
•
•
•
This class will contain a main method and an object of AddressBook class.
Build GUI based menu by using switch selection structure
Call appropriate methods of AddressBook class
The code for Test class is
import javax.swing.*;
class Test {
© Copyright Virtual University of Pakistan 68 Web Design and Development (CS506) public static void main (String args[]) {
AddressBook ab = new AddressBook();
String input, s;
int ch;
while (true) {
input = JOptionPane.showInputDialog(“Enter 1 to add ” +
“\n Enter 2 to Search \n Enter 3 to Delete“ +
“\n Enter 4 to Exit”);
ch = Integer.parseInt(input);
switch (ch) {
case 1:
ab.addPerson();
break;
case 2:
s = JOptionPane.showInputDialog(
“Enter name to search ”);
ab.searchPerson(s);
break;
case 3:
s = JOptionPane.showInputDialog(
“Enter name to delete ”);
ab.deletePerson(s);
break;
case 4:
System.exit(0);
}
}//end while
}//end main
}
Note that we use infinite while loop that would never end or stop given that our program should
only exit when user enters 4 i.e. exit option.
Compile & Execute
Compile all three classes and run Test class. Bravo, you successfully completed the all basic
three steps. Enjoy!
6.7
Reference
Entire content for this handout are taken from the book JAVA A Lab Course by Umair Javed.
This material is available just for the use of VU students of the course Web Design and
Development and not for any other commercial purpose.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
Lecture 7: Intro to Exceptions
© Copyright Virtual University of Pakistan 69 Web Design and Development (CS506) 7.1
Types of Errors
Generally, you can come across three types of errors while developing software. These are
Syntax, Logic & Runtime errors.
7.1.1 Syntax Errors
•
Arise because the rules of the language are not followed.
7.1.2 Logic Errors
•
Indicates that logic used for coding doesn’t produce expected output.
7.1.3 Runtime Errors
•
•
•
7.2
•
•
•
Occur because the program tries to perform an operation that is impossible to
complete.
Cause exceptions and may be handled at runtime (while you are running the
program)
For example divide by zero
What is an Exception?
An exception is an event that usually signals an erroneous situation at run time
Exceptions are wrapped up as objects
A program can deal with an exception in one of three ways:
o ignore it
o handle it where it occurs
o handle it an another place in the program
7.3
•
•
•
7.4
•
Why handle Exceptions?
Helps to separate error handling code from main logic (the normal code you write) of
the program.
As different sort/type of exceptions can arise, by handling exceptions we can
distinguish between them and write appropriate handling code for each type for
example we can differently handle exceptions that occur due to division by Zero and
exceptions that occur due to non-availability of a file.
If not handled properly, program might terminate.
Exceptions in Java
An exception in java is represented as an object that’s created when an abnormal
© Copyright Virtual University of Pakistan 70 Web Design and Development (CS506) •
•
7.5
situation arises in the program. Note that an error is also represented as an object in
Java, but usually represents an unrecoverable situation and should not be caught
The exception object stores information about the nature of the problem. For
example, due to network problem or class not found etc.
All exceptions in java are inherited from a class know as Throwable.
Exception Hierarchy
Following diagram is an abridged version of Exception class hierarchy
7.6
Types of Exceptions
Exceptions can be broadly categorized into two types, Unchecked & Checked Exceptions.
7.6.1 Unchecked Exceptions
•
•
•
•
Subclasses of Runtime Exception and Error.
Does not require explicit handling
Run-time errors are internal to your program, so you can get rid of them by
debugging your code
For example, null pointer exception; index out of bounds exception; division by
zero exception.
© Copyright Virtual University of Pakistan 71 Web Design and Development (CS506) 7.6.2 Checked Exceptions
•
•
•
•
•
7.7
Must be caught or declared in a throws clause
Compile will issue an error if not handled appropriately
Subclasses of Exception other than subclasses of Runtime Exception.
Other arrives from external factors, and cannot be solved by debugging
Communication from an external resource - e.g. a file server or database
How Java handles Exceptions
Java handles exceptions via 5 keywords. Try, catch, finally, throw & throws.
7.7.1 try block
•
Write code inside this block which could generate errors
7.7.2 Catch block
•
•
Code inside this block is used for exception handling
When the exception is raised from try block, only than catch block would
execute.
7.7.3 finally block
•
•
This block always executes whether exception occurs or not.
Write clean up code here, like resources (connection with file or database) that
are opened may need to be closed.
The basic structure of using try - catch - finally block is shown in the picture below:
© Copyright Virtual University of Pakistan 72 Web Design and Development (CS506) 7.7.4 throw
•
To manually throw an exception, keyword throw is used. Note: we are not covering throw
clause in this handout
7.7.5 throws
•
•
7.8
•
•
•
•
7.9
If method is not interested in handling the exception than it can throw back the
exception to the caller method using throws keyword.
Any exception that is thrown out of a method must be specified as such by a
throws clause.
References:
Java tutorial by Sun: http://java.sun.com/docs/books/turorial
Beginning Java2 by Ivor Hortan
Thinking in Java by Bruce Eckle
CS193j Stanford University
Code Examples of Exception Handling
7.9.1 Unchecked Exceptions
Example Code: UcException.java
The following program takes one command line argument and prints it on the console
// File UcException.java
public class UcException {
public static void main (String args[ ]) {
System.out.println(args[0]);
}
}
Compile & Execute
If we compile & execute the above program without passing any command line
argument, an ArrayIndexOutOfBoundsException would be thrown. This is shown in
the following picture
© Copyright Virtual University of Pakistan 73 Web Design and Development (CS506) 7.9.2 Why?
Since we have passed no argument, therefore the size of String args[] is zero, and we
have tried to access the first element (first element has index zero) of this array.
From the output window, you can find out, which code line causes the exception to be raised. In
the above example, it is
System.out.println (args[0]);
7.9.3 Modify UcException.java
Though it is not mandatory to handle unchecked exceptions we can still handle
Unchecked Exceptions if we want to. These modifications are shown in bold.
// File UcException.java
public class UcException {
public static void main (String args[ ]) {
try {
System.out.println(args[0]);
catch (IndexOutOfBoundsException ex) {
System.out.println(“You forget to pass command line argument”);
}
}
The possible exception that can be thrown is IndexOutOfBoundsException, so we
handle it in the catch block.
When an exception occurs, such as IndexOutOfBoundsException in this case, then an
object of type IndexOutOfBoundesException is created and it is passed to the
corresponding catch block (i.e. the catch block which is capable of handling this
exception). The catch block receives the exception object inside a variable which is ex in
this case. It can be any name; it is similar to the parameter declared in the method
signature. It receives the object of exception type (IndexOutOfBoundsExceptoin) it is
declared.
Compile & Execute
If we execute the modified program by passing command line argument, the program would
display on console the provided argument. After that if we execute this program again without
passing command line argument, this time information message would be displayed which is
written inside catch block.
© Copyright Virtual University of Pakistan 74 Web Design and Development (CS506) 7.10 Checked Exceptions
Example Code: CException.java
The following program reads a line (hello world) from a file and prints it on the console. The File
reading code is probably new for you. We’ll explain it in the coming handouts (Streams). For now,
assumed that the code written inside the main read one line from a file and prints that to console.
// File CException.java
import java.io.* ;
public class CException {
public static void main (String args[ ]) {
FileReader fr = new FileReader (“input.txt”);
BufferedReader br = new BufferedReader (fr);
//read the line form file
String line = br.readLine();
System.out.println(line); }
}
Compile & Execute
If you try to compile this program, the program will not compile successfully and displays the
message of unreported exception. This happens when there is code that can generate a checked
exception but you have not handled that exception. Remember checked exceptions are detected
by compiler. As we early discussed, without handling Checked exception, out program won’t
compile.
Modify CException.java
As we have discussed earlier, it is mandatory to handle checked exceptions. In order to
compile the code above, we modify the above program so that file reading code is placed
inside a try block. The expected exception (IOException) that can be raised is caught in
catch block.
© Copyright Virtual University of Pakistan 75 Web Design and Development (CS506) // File CException.java
import java.io.* ;
public class CException {
public static void main (String args[ ]) {
try{
FileReader fr = new FileReader (“input.txt”);
BufferedReader br = new BufferedReader (fr);
//read the line form file
String line = br.readLine();
System.out.println(line); }
catch( IOException ex) {
System.out.println(ex);
}
}
}
The code line written inside the catch block will print the exception name on the console if
exception occurs
Compile & Execute
After making changes to your program, it would compile successfully. On executing this program,
hello world would be displayed on the console
Note: Before executing, make sure that a text file named input.txt must be placed in the
same directory where the program is saved. Also write hello world in that file before
saving it.
7.11 The finally block
The finally block always executes regardless of exception is raised or not while as you
remembered the catch block only executes when an exception is raised.
Example Code: FBlockDemo.java
// File FBlockDemo.java
import java.io.* ;
public class FBlockDemo {
public static void main (String args[ ])
© Copyright Virtual University of Pakistan 76 Web Design and Development (CS506) {
try{
FileReader fr = new FileReader (“strings.txt”);
BufferedReader br = new BufferedReader (fr);
//read the line form file
String line = br.readLine();
System.out.println(line); }catch( IOException ex) {
System.out.println(ex);
}
finally {
System.out.println(“finally block always execute”);
}
}
}
Compile & Execute
The program above, will read one line from string.txt file. If string.tx is not present in the same
directory the FileNotFoundException would be raised and catch block would execute
as well as the finally block.
If string.txt is present there, no such exception would be raised but still finally block executes.
This is shown in the following output diagram
7.12 Multiple catch blocks
•
•
Possible to have multiple catch clauses for a single try statement
o Essentially checking for different types of exceptions that may happen
Evaluated in the order of the code
o Bear in mind the Exception hierarchy when writing multiple catch clauses!
o If you catch Exception first and then IOException, the IOException will never be
caught!
© Copyright Virtual University of Pakistan 77 Web Design and Development (CS506) Example code: MCatchDemo.java
The following program would read a number form a file numbers.txt and than prints its square on
the console
// File MCatchDemo.java
import java.io.* ;
public class MCatchDemo {
public static void main (String args[ ]) {
try{
// can throw FileNotFound or IOException
FileReader fr = new FileReader (“numbers.txt”);
BufferedReader br = new BufferedReader (fr);
//read the number form file
String s = br.readLine();
//may throws NumberFormatException, if s is not a no.
int number = Integer.parseInt(s);
System.out.println(number * number); }
catch( NumberFormatException nfEx) {
System.out.println(nfEx);
}
catch( FileNotFoundException fnfEx) {
System.out.println(fnfEx);
}
catch( IOException ioEx) {
System.out.println(ioEx);
}
}
}
We read everything from a file (numbers, floating values or text) as a String. That’s why we first
convert it to number and then print its square on console.
Compile & Execute
If file numbers.txt is not present in the same directory, the FileNotFoundException would
be thrown during execution.
© Copyright Virtual University of Pakistan 78 Web Design and Development (CS506) If numbers.txt present in the same directory and contains a number, than hopefully no exception
would be thrown.
7.13 The throws clause
The following code examples will introduce you with writing & using throws clause.
Example Code: ThrowsDemo.java
The ThrowsDemo.java contains two methods namely method1 & method2 and one main
method. The main method will make call to method1 and than method1 will call method2.
The method2 contains the file reading code. The program looks like one given below
// File ThrowsDemo.java
import java.io.* ;
public class ThrowsDemo {
// contains file reading code
public static void method2( ) {
try{
FileReader fr = new FileReader (“strings.txt”);
BufferedReader br = new BufferedReader (fr);
//read the line form file
String s = br.readLine();
System.out.println(s);
}
catch( IOException ioEx) {
ioEx.printStackTrace(); }
}// end method 2
//only calling method2
© Copyright Virtual University of Pakistan 79 Web Design and Development (CS506) public static void method1( )
{
method2();
}
public static void main (String args[ ]) {
ThrowsDemo.method1();
}
}
7.14 printStackTrace method
•
•
•
Defined in the Throwable class - superclass of Exception & Error classes
Shows you the full method calling history with line numbers.
extremely useful in debugging
Modify: ThrowsDemo.java
•
•
•
Let method2 doesn’t want to handle exception by itself, so it throws the exception
to the caller of method2 i.e. method1
So method1 either have to handle the incoming exception or it can re-throw it to its
caller i.e. main.
Let method1 is handling the exception, so method1& method2 would be
modified as:
// File ThrowsDemo.java
import java.io.* ;
public class ThrowsDemo {
// contains file reading code
public static void method2( ) throws IOException{
FileReader fr = new FileReader (“strings.txt”);
BufferedReader br = new BufferedReader (fr);
//read the line form file
String s = br.readLine();
System.out.println(s);
}// end method 2
// calling method2 & handling incoming exception
public static void method1( ) {
try {
method2();}
catch (IOException ioEx) {
ioEx.printStackTrace(); }
}
© Copyright Virtual University of Pakistan 80 Web Design and Development (CS506) public static void main (String args[ ]) {
ThrowsDemo.method1();
}
}
Compile & Execute
If file strings.txt is not present in the same directory, method2 will throw an exception that
would be caught by method1 and the printStackTrace method will print the full calling
history on console. The above scenario is shown in the output below:
If file strings.txt exist there, than hopefully line would be displayed on the console.
7.15 Reference
•
Example code, their explanations and corresponding figures for this handout are taken
from the book JAVA A Lab Course by Umair Javed. This material is available just for
the use of VU students of the course Web Design and Development and not for any other
commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 81 Web Design and Development (CS506) Lecture 8: Streams
I/O libraries often use the abstraction of a stream, which represents any data source or sink as an
object capable of producing or receiving pieces of data.
The Java library classes for I/O are divided by input and output. You need to import
java.io package to use streams. There is no need to learn all the streams just do it on the need basis.
8.1
•
•
The concept of "streams"
It is an abstraction of a data source/sink
We need abstraction because there are lots of different devices (files, consoles, network,
memory, etc.). We need to talk to the devices in different ways (sequential, random
access, by lines, etc.) Streams make the task easy by acting in the same way for every
device. Though inside handling of devices may be quite different, yet on the surface
everything is similar. You might read from a file, the keyboard, memory or network
connection, different devices may require specialization of the basic stream, but you can
treat them all as just "streams". When you read from a network, you do nothing different
than when you read from a local file or from users typing
//Reading from console
BufferedReader stdin = new BufferedReader(new InputStreamReader(
System.in ));
-------- ( your console)
// Reading from file
BufferedReader br=new BufferedReader(new
FileReader(“input.txt”));
//Reading from network
BufferedReader br = new BufferedReader(new InputStreamReader
(s.getInputStream()));
---- “s” is the socket
•
So you can consider stream as a data path. Data can flow through this path in one
direction between specified terminal points (your program and file, console, Socket etc.)
© Copyright Virtual University of Pakistan 82 Web Design and Development (CS506) 8.2
Stream classification based on Functionality
Based on functionality streams can be categorized as Node Stream and Filter Stream. Node
Streams are those which connect directly with the data source/sick and provide basic
functionality to read/write data from that source/sink
FileReader fr = new FileReader(“input.txt”);
You can see that FileReader is taking a data/source “input.txt” as its argument and hence it is a node
stream.
FilterStreams sit on top of a node stream or chain with other filter stream and provide some
additional functionality e.g. compression, security etc. FilterStreams take other stream as their
input.
BufferedReader bt = new BufferedReader(fr);
BufferedReader makes the IO efficient (enhances the functionality) by buffering the input before
delivering. And as you can see that BufferedReader is sitting on top of a node stream which is
FileReader.
© Copyright Virtual University of Pakistan 83 Web Design and Development (CS506) 8.3
Stream classification based on data
Two types of classes exist:
Classes which contain the word stream in their name are byte oriented and are here since JDK1.0.
These streams can be used to read/write data in the form of bytes. Hence classes with the word
stream in their name are byte-oriented in nature. Examples of byte oriented streams are
FileInputStream, ObjectOutputStream etc.
Classes which contain the word Reader/Writer are character oriented and read and write data in the
form of characters. Readers and Writers came with JDK1.1. Examples of Reader/Writers are
FileReader, PrintWriter etc
© Copyright Virtual University of Pakistan 84 Web Design and Development (CS506) Example Code 8.1: Reading from File
The ReadFileEx.java reads text file line by line and prints them on console. Before we move
on to the code, first create a text file (input.txt) using notepad and write following text lines
inside it.
Text File: input.txt
Hello World
Pakistan is our homeland
Web Design and Development
// File ReadFileEx.java
import java.io.*;
public class ReadFileEx {
public static void main (String args[ ]) {
FileReader fr = null;
BufferedReader br = null;
try {
// attaching node stream with data source
fr = new FileReader(“input.txt”);
// attatching filter stream over node stream
br = new BufferedReader(fr);
// reading first line from file
String line = br.readLine();
// printing and reading remaining lines
while (line != null){
System.out.println(line);
line = br.readLine();
}
// closing streams br.close();
fr.close();
}catch(IOException ioex){
System.out.println(ioex);
}
} // end main
} // end class
© Copyright Virtual University of Pakistan 85 Web Design and Development (CS506) Example Code 8.2: Writing to File
The WriteFileEx.java writes the strings into the text file named “output.txt”. If “output.txt”
file does not exist, the java will create it for you.
// File WriteFileEx.java
import java.io.*;
public class WriteFileEx {
public static void main (String args[ ]) {
FileWriter fw = null;
PrintWriter pw = null;
try {
// attaching node stream with data source
// if file does not exist, it automatically creates it
fw = new FileWriter (“output.txt”);
// attatching filter stream over node stream
pw = new PrintWriter(fw);
String s1 = “Hello World”;
String s2 = “Web Design and Development”;
// writing first string to file
pw.println(s1);
// writing second string to file
pw.println(s2);
// flushing stream
pw.flush();
// closing streams
pw.close();
fw.close();
}catch(IOException ioex){
System.out.println(ioex);
}
} // end main
} // end class
After executing the program, check the output.txt file. Two lines will be written there.
8.4
•
Reference
Example code, their explanations and corresponding figures for this handout are taken
from the book JAVA A Lab Course by Umair Javed. This material is available just for
the use of VU students of the course Web Design and Development and not for any other
commercial purpose without the consent of author.
© Copyright Virtual University of Pakistan 86 Web Design and Development (CS506) 8.5
Modification of Address Book Code
8.5.1 Adding Persistence Functionality
Hopefully, your address book you built previously is giving you the required results except
one i.e. persistence. You might have noticed that after adding some person records in the address
book; if you exit from the program next time on re-executing address book all the previous records
are no more available.
To overcome the above problem, we will modify our program so that on exiting/starting of
address book, all the previously added records are available each time. To achieve this, we have to
provide the persistence functionality. Currently, we will accomplish this task by saving person
records in some text file.
Supporting simple persistence by any application requires handling of two scenarios. These are
•
•
On start up of application - data (person records) must be read from file
On end/finish up of application - data (person records) must be saved in file
To support persistence, we have to handle the above mentioned scenarios
8.5.1.1 Scenario 1 - Start Up
•
•
•
•
•
•
Establish a data channel with a file by using streams
Start reading data (person records) from file line by line
Construct PersonInfo objects from each line you have read
Add those PersonInfo objects in arraylist persons.
Close the stream with the file
Perform these steps while application is loading up
We will read records from a text file named persons.txt. The person records will be present in
the file in the following format.
Ali, defence, 9201211
Usman, gulberg, 5173940
Salman, LUMS, 5272670
persons.txt
As you have seen, each person record is on a separate line. Person’s name, address & phone
number is separated using comma (,).
We will modify our AddressBook.java by adding a new method loadPersons into it.
This method will provide the implementation of all the steps. The method is shown below:
© Copyright Virtual University of Pakistan 87 Web Design and Development (CS506) public void loadPersons ( ){
String tokens[] = null;
String name, add, ph;
try {
FileReader fr = new FileReader("persons.txt");
BufferedReader br = new BufferedReader(fr);
String line = br.readLine();
while ( line != null ) {
tokens = line.split(",");
name = tokens[0];
add = tokens[1];
ph = tokens[2];
PersonInfo p = new PersonInfo(name, add, ph);
persons.add(p);
line = br.readLine();
}
br.close();
fr.close();
}catch(IOException ioEx){
System.out.println(ioEx);
}
}
•
First, we have to connect with the text file in order to read line by line person records
from it. This task is accomplished with the following lines of code:
FileReader fr = new FileReader(“persons.txt”);
BufferedReader br = new BufferedReader(fr);
FileReader is a character based (node) stream that helps us in reading data in the form of
characters. As we are using streams, so we have to import the java.io package in the
AddressBook class.
• We passed the file name persons.txt to the constructor of the FileReader.
Next we add BufferedReader (filter stream) on top of the FileReader because
BufferedReader facilitates reading data line by line. (As you can recall
from the lecture that filter streams are attached on top of node streams). That’s why
the constructor of BufferedReader is receiving the fr - the FileReader
object.
• The next line of code will read line from file by using readLine() method of
© Copyright Virtual University of Pakistan 88 Web Design and Development (CS506) BufferedReader and save it in a string variable called line.
String line = br.readLine();
•
After that while loop starts. The condition of while loop is used to check whether the file is
reached to end (returns null) or not. This loop is used to read whole file till the end. When
end comes (null), this loop will finish.
while (line != null)
•
Inside loop, the first step we performed is tokenizing the string. For this purpose, we have
used split method of String class. This method returns substrings (tokens) according to the
regular expression or delimiter passed to it.
tokens = line.split(“,”);
The return type of this method is array of strings that’s why we have declared tokens as a String
array in the beginning of this method as
String tokens[];
For example, the line contains the following string
Ali,defence,9201211
Now by calling split(“,”) method on this string, this method will return back three substrings
ali defence and 9201211 because the delimiter we have passed to it is comma. The delimiter
itself is not included in the substrings or tokens.
•
The next three lines of code are simple assignments statements. The tokens[0]contains
the name of the person because the name is always in the beginning of the line,
tokens[1] contains address of the person and tokens[2] contains the phone
number of the person.
name = tokens[0];
add= tokens[1];
ph= tokens[2];
The name, add and ph are of type String and are declared in the beginning of this
method.
•
After that we have constructed the object of PersonInfo class by using parameterized
constructor and passed all these strings to it.
PersonInfo p = new PersonInfo(name, add, ph);
•
Afterward the PersonInfo object’s p is added to the arraylist i.e. persons.
persons.add(p);
•
The last step we have done inside loop is that we have again read a line from the file by
© Copyright Virtual University of Pakistan 89 Web Design and Development (CS506) •
•
•
using the readLine() method.
By summarizing the task of while loop we can conclude that it reads the line from a
file,tokenize that line into three substrings followed by constructing the
PersonInfo object by using these tokens. And adding these objects to the arraylist. This
process continues till the file reaches its end.
The last step for reading information from the file is ordinary one - closing the
streams, because files are external resources, so it’s better to close them as soon as possible.
Also observe that we used try/catch block because using streams can result in raising
exceptions that falls under the checked exceptions category - that needs mandatory
handling.
The last important step you have to perform is to call this method while loading up.
The most appropriate place to call this method is from inside the constructor of
AddressBook.java. So the constructor will now look like similar to the one given
below:
………………
public AddressBook () {
Persons = new ArrayList();
loadPersons();
}
………………
AddressBook.java
8.5.1.2 Scenario 2 - End/Finish Up
•
•
•
•
•
•
Establish a datachanel(stream) with a file by using streams
Take out PersonInfo objects from ArrayList (persons)
Build a string for each PersonInfo object by inserting commas (,) between name
& address and address & phone number.
Write the constructed string to the file
Close the connection with file
Perform these steps while exiting from address book.
Add another method savePersons into AddressBook.java. This method will
provide the implementation of all the above mentioned steps. The method is shown
below:
public void savePersons ( ){
try {
PersonInfo p;
String line;
FileWriter fw = new FileWriter("persons.txt");
PrintWriter pw = new PrintWriter(fw);
for(int i=0; i<persons.size(); i++)
{
© Copyright Virtual University of Pakistan 90 Web Design and Development (CS506) p = (PersonInfo)persons.get(i);
line = p.name +","+ p.address +","+ p.phoneNum;
// writes line to file (persons.txt)
pw.println(line);
}
pw.flush();
pw.close();
fw.close();
}catch(IOException ioEx){
System.out.println(ioEx);
}
}
•
•
•
As you can see, that we have opened the same file (persons.txt) again by using a set
of streams.
After that we have started for loop to iterate over arraylist as we did in
searchPerson and deletePerson methods.
Inside for loop body, we have taken out PersonInfo object and after type casting it we
have assigned its reference to a PersonInfo type local variable p. This is achieved by
the help of following line of code
p = (PersonInfo)persons.get(i);
•
Next we build a string and insert commas between the PersonInfo attributes and assign
the newly constructed string to string’s local variable line as shown in the following line
of code.
line = p.name +","+ p.address +","+ p.phoneNum;
Note: Since, we haven’t declare PersonInfo attributes private, therefore we are
able to directly access them inside AddressBook.java.
•
The next step is to write the line representing one PersonInfo object’s information, to
the file. This is done by using println method of PrintWriter as shown below
pw.println(line);
After writing line to the file, the println method will move the cursor/control to the next
line. That’s why each line is going to be written on separate line.
•
The last step for saving information to the file is ordinary one - closing the streams but
before that notice the code line that you have not seen/performed while loading persons
records from file. That is
pw.flush( );
The above line immediately flushes data by writing any buffered output/data to file. This step
© Copyright Virtual University of Pakistan 91 Web Design and Development (CS506) is necessary to perform or otherwise you will most probably lose some data for the reason that
PrintWriter is a Buffered Stream and they have their own internal memory/storage
capacity for efficiency reasons. Buffered Streams do not send the data until their memory is
full.
•
•
Also we have written this code inside try-catch block.
The last important step you have to perform is to call this method before exiting from
the address book. The most appropriate place to call this method is under case4(exit
scenario) in Test.java. So the case
4 will now look like similar to the one given
below:
……………
case 4:
ab.savePersons();
System.exit(0);
………………
Test.java
Compile & Execute
Now again after compiling all the classes, run the Test class. Initially we are assuming
that out persons.txt file is empty, so our arraylist persons will be empty on the
first start up of address book. Now add some records into it, perform search or delete
operations. Exit from the address book by choosing option 4. Check out the persons.txt
file. Don’t get surprised by seeing that it contains all the person records in the format exactly we have
seen above.
Next time you will run the address book; all the records will be available to you. Perform the search or
delete operation to verify that. Finally you have done it!!!
8.6
•
References
Example code, their explanations and corresponding figures for this handout are taken
from the book JAVA A Lab Course by Umair Javed. This material is available just for
the use of VU students of the course Web Design and Development and not for any other
commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 92 Web Design and Development (CS506) Lecture 9: Abstract Classes and Interfaces
9.1
Problem and Requirements
Before moving on to abstract classes, first examine the following class hierarchy shown
below:
•
Suppose that in order to exploit polymorphism, we specify that 2-D objects must be able to
compute their area.
o All 2-D classes must respond to area() message.
•
How do we ensure that?
o Define area method in class Shape
o Force the subclasses of Shape to respond area() message
•
Java’s provides us two solutions to handle such problem
o Abstract Classes
o Interfaces
9.2
Abstract Classes
Abstract classes are used to define only part of an implementation. Because, information is not
complete therefore an abstract class cannot be instantiate. However, like regular classes, they can
also contain instance variables and methods that are fully implemented. The class that inherits
from abstract class is responsible to provide details.
Any class with an abstract method (a method has no implementation similar to pure virtual
function in C++) must be declared abstract, yet you can declare a class abstract that has no
abstract method.
If subclass overrides all abstract methods of the superclass, than it becomes a concrete (a class
whose object can be instantiate) class otherwise we have to declare it as abstract or we cannot
compile it.
© Copyright Virtual University of Pakistan 93 Web Design and Development (CS506) The most important aspect of abstract class is that reference of an abstract class can point to the
object of concrete classes.
Code Example of Abstract Classes
The Shape class contains an abstract method calculateArea() with no definition.
public abstract class Shape{
public abstract void
calculateArea();
}
Class Circle extends from abstract Shape class, therefore to become concrete class it must
provides the definition of calculateArea() method.
public class Circle extends Shape {
private int x, y;
private int radius;
public Circle() {
x = 5;
y = 5;
radius = 10;
}
// providing definition of abstract method
public void calculateArea () {
double area = 3.14 * (radius * radius);
System.out.println(“Area: ” + area);
}
}//end of class
The Test class contains main method. Inside main, a reference s of abstract Shape class is created.
This reference can point to Circle (subclass of abstract class Shape) class object as it is a concrete
class. With the help of reference, method calculateArea() can be invoked of Circle class. This
is all shown in the form of code below:
public class Test {
public static void main(String args[]){
//can only create references of A.C.
Shape s = null;
//Shape s1 = new Shape(); //cannot instantiate
//abstract class reference can point to concrete subclass
s = new Circle();
© Copyright Virtual University of Pakistan 94 Web Design and Development (CS506) s.calculateArea();
}
}//end of class
The compilation and execution of the above program is shown below:
9.3
Interfaces
As we seen one possible java’s solution to problem discussed in start of the tutorial. The second
possible java’s solution is Interfaces.
Interfaces are special java type which contains only a set of method prototypes, but does not provide
the implementation for these prototypes. All the methods inside an interface are abstract by default
thus an interface is tantamount to a pure abstract class - a class with zero implementation. Interface
can also contains static final constants
9.3.1 Defining an Interface
Keyword interface is used instead of class as shown below:
public interface Speaker{
public void speak();
}
9.3.2 Implementing (using) Interface
Classes implement interfaces. Implementing an interface is like signing a contract. A
class that implements an interface will have to provide the definition of all the methods
that are present inside an interface. If the class does not provide definitions of all
methods, the class would not compile. We have to declare it as an abstract class in order
to get it compiled.
© Copyright Virtual University of Pakistan 95 Web Design and Development (CS506) Relationship between a class and interface is equivalent to “responds to” while “is a” relationship
exists in inheritance.
Code Example of Defining & Implementing an Interface The interface Printable contains print()
method.
public interface Printable{
public void print();
}
Class Student is implementing the interface Printable. Note the use of keyword
implements after the class name. Student class has to provide the definition of print method or
we are unable to compile.
The code snippet of student class is given below:
public class Student implements Printable {
private String name;
private String address;
public String toString () {
return "name:"+name +" address:"+address;
}
//providing definition of interface’s print method
public void print() {
System.out.println("Name:" +name+" address"+address);
}
}//end of class
9.4
Interface Characteristics
Similar to abstract class, interfaces imposes a design structure on any class that uses the
interface. Contrary to inheritance, a class can implement more than one interfaces. To do
this separate the interface names with comma. This is java’s way of multiple inheritance.
class Circle implements Drawable , Printable { ………. }
Objects of interfaces also cannot be instantiated.
Speaker s = new Speaker(); // not compile
However, a reference of interface can be created to point any of its implementation class. This is
interface based polymorphism.
© Copyright Virtual University of Pakistan 96 Web Design and Development (CS506) Code Example: Interface based polymorphism
Interface Speaker is implemented by three classes Politician, Coach and Lecturer. Code snippets of
all these three classes are show below:
public class Politician implements Speaker{
public void speak(){
System.out.println(“Politics Talks”);
}
}
public class Coach implements Speaker{
public void speak(){
System.out.println(“Sports Talks”);
}
}
public class Lecturer implements Speaker{
public void speak(){
System.out.println(“Web Design and Development Talks”);
}
}
As usual, Test class contains main method. Inside main, a reference sp is created of Speaker
class. Later, this reference is used to point to the objects of Politician, Coach and Lecturer class.
On calling speak method with the help of sp, will invoke the method of a class to which sp is
pointing.
public class Test{
public static void main (String
Speaker sp = null;
System.out.println("sp pointing
sp = new Politician();
sp.speak();
System.out.println("sp pointing
sp = new Coach();
sp.speak();
System.out.println("sp pointing
sp = new Lecturer();
sp.speak();
}
}
args[ ]) {
to Politician");
to Coach");
to Lecturer");
The compilation and execution of the above program is shown below:
© Copyright Virtual University of Pakistan 97 Web Design and Development (CS506) 9.5
•
References
Example code, their explanations and corresponding figures for this handout are taken
from the book JAVA A Lab Course by Umair Javed. This material is available just for
the use of VU students of the course Web Design and Development and not for any other
commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
Lecture 10: Graphical User Interfaces
© Copyright Virtual University of Pakistan 98 Web Design and Development (CS506) A graphical user interface is a visual interface to a program. GUIs are built from GUI
components (buttons, menus, labels etc). A GUI component is an object with which the user
interacts via the mouse or keyboard.
Together, the appearance and how user interacts with the program are known as the program
look and feel.
10.1 Support for GUI in Java
The classes that are used to create GUI components are part of the “java.awt” or
“javax.swing” package. Both these packages provide rich set of user interface components.
10.2 GUI classes vs. Non-GUI Support Classes
The classes present in the awt and swing packages can be classified into two broad categories.
GUI classes & Non-GUI Support classes.
The GUI classes as the name indicates are visible and user can interact with them. Examples
of these are JButton, JFrame & JRadioButton etc
The Non-GUI support classes provide services and perform necessary functions for GUI classes.
They do not produce any visual output. Examples of these classes are Layout managers (discussed
latter) & Event handling (see handout on it) classes etc.
10.3 java.awt package
AWT stands for “Abstract Windowing Toolkit“ contains original GUI components that
came with the first release of JDK. These components are tied directly to the local
platform’s (Windows, Linux, MAC etc) graphical user interface capabilities. Thus results
in a java program executing on different java platforms (windows, linux, solaris etc) has a
different appearance and sometimes even different user interaction on each platform.
AWT components are often called Heavy Weight Components (HWC) as they rely on the local
platform’s windowing system to determine their functionality and their look and feel. Every time
you create an AWT component it creates a corresponding process on the operating system. As
compared to this SWING components are managed through threads and are known as Light Weight
Components.
This package also provides the classes for robust event handling (see handout on it) and layout
managers.
10.4 javax.swing package
© Copyright Virtual University of Pakistan 99 Web Design and Development (CS506) These are the newest GUI components. Swing components are written, manipulated and
displayed completely in java, therefore also called pure java components. The swing
components allow the programmer to specify a uniform look and feel across all
platforms.
Swing components are often referred to as Light Weight Components as they are
completely written in java. Several swing components are still HWC e.g. JFrame etc.
10.5 A part of the Framework
10.6 GUI Creation Steps
10.6.1 import required packages
•
import java.awt.* and/or javax.swing.* package.
10.6.2 Setup the top level containers
• A container is a collection of related components, which allows other components
to be nested inside it. In application with JFrame, we attach components to the content
pane - a container.
• Two important methods the container class has add and setLayout.
• The add method is used for adding components to the content pane while setLayout
method is used to specify the layout manager.
© Copyright Virtual University of Pakistan 100 Web Design and Development (CS506) • Container are classified into two broad categories that are Top Level
Containers and General Purpose Containers
• Top level containers can contain (add) other containers as well as basic
Components (buttons, labels etc) while general purpose containers are Typically used to
collect basic components and are added to top level containers.
• General purpose containers cannot exist alone they must be added to top level containers
• Examples of top level container are JFrame, Dialog and Applet etc. Our application
uses one of these.
• Examples of general purpose container are JPanel, Toolbar and ScrollPane etc.
• So, take a top level container and create its instance. Consider the following code of line if
JFrame is selected as a top level container
JFrame frame = new JFrame();
10.6.3 Get the component area of the top level container
•
•
•
•
•
•
Review the hierarchy given above, and observe that JFrame is a frame is a
window. So, it can be interpreted as JFrame is a window.
Every window has two areas. System Area & Component Area
The programmer cannot add/remove components to the System Area.
The Component Area often known as Client area is a workable place for the
programmer. Components can be added/removed in this area.
So, to add components, as you guessed right component area of the JFrame is required. It
can be accomplished by the following code of line
Container con = frame.getContentPane();
frame is an instance of JFrame and by calling getContentPane() method on it, it returns the
component area. This component area is of type container and that is why it is stored in a
variable of a Container class. As already discussed, container allows other components to
be added / removed.
10.6.4 Apply layout to component area
•
•
•
•
The layout (size & position etc. How they appear) of components in a container is
usually governed by Layout Managers.
The layout manager is responsible for deciding the layout policy and size of
its components added to the container.
Layout managers are represented in java as classes. (Layout Managers are
going to be discussed in detail later in this handout)
To set the layout, as already discussed use setLayout method and pass object
of layout manager as an argument.
con.setLayout( new FlowLayout( ) );
•
•
We passed an object of FlowLayout to the setLayout method here.
We can also use the following lines of code instead of above.
© Copyright Virtual University of Pakistan 101 Web Design and Development (CS506) FlowLayout layout = new FlowLayout();
con.setLayout(layout);
10.6.5 Create and Add components
•
Create required components by calling their constructor.
JButton button = new JButton ( );
•
•
After creating all components you are interested in, the next task is to add these
components into the component area of your JFrame (i.e ContentPane, the reference to
which is in variable con of type Container)
Use add method of the Container to accomplish this and pass it the component to be added.
con.add(button);
10.6.6 Set size of frame and make it visible
•
A frame must be made visible via a call to setVisible(true) and its size defined via a call
setSize(rows in pixel, columns in pixel) to be displayed on the screen.
frame.setSize(200,300) ;
frame.setVisible(true) ;
Note: By default, all JFrame’s are invisible. To make visible frame visible we have passed true
to the setVisible method.
frame.setVisible(false) ;
Example: Making a Simple GUI
The above figured GUI
contains one text field and a button.
Let’s code it by following the six GUI creation steps we discussed.
Code for Simple GUI:
© Copyright Virtual University of Pakistan 102 Web Design and Development (CS506) // File GUITest.java
//Step 1: import packages
import java.awt.*;
import javax.swing.*;
public class GUITest {
JFrame myFrame ;
//method used for setting layout of GUI
public void initGUI ( ) {
//Step 2: setup the top level container
myFrame = new JFrame();
//Step 3: Get the component area of top-level
container Container c = myFrame.getContentPane();
//Step 4: Apply layouts
c.setLayout( new FlowLayout( ) );
//Step 5: create & add components
JTextField tf = new JTextField(10);
JButton b1 = new JButton("My Button");
c.add(tf);
c.add(b1);
//Step 6: set size of frame and make it visible
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myFrame.setSize(200,150);
myFrame.setVisible(true);
} //end initGUI method
public GUITest () { // default constructor
initGUI ();
}
public static void main (String args[ ]) {
GUITest gui = new GUITest();
}
} // end of class
10.7 Important Points to Consider
© Copyright Virtual University of Pakistan 103 Web Design and Development (CS506) •
•
•
main method (from where program execution starts) is written in the same class. The
main method can be in a separate class instead of writing in the same class its your
choice.
Inside main, an object of GUI test class is created that results in calling of constructor
of the class and from the constructor, initGUI method is called that is responsible for setting
up the GUI.
The following line of code is used to exit the program when you close the window
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
If you delete this line and run your program, the desired GUI would be displayed. However if
you close the window by using (X) button on top left corner of your window, you’ll notice that
the control doesn’t return back to command prompt. The reason for this is that the java process is
still running. However if you put this line in your code, when you exit your prompt will return.
10.8 References:
•
•
•
•
Sun java tutorial: http://java.sun.com/docs/books/tutorial/java
Thinking in java by Bruce Eckle
Beginning Java2 by Ivor Hortan
GUI creation steps are taken from the book Java A Lab Course by Umair Javed
10.9 Graphical User Interfaces - 2
10.9.1 Layout Managers
Layout Managers are used to form the appearance of your GUI. They are concerned with the
arrangement of components of GUI. A general question is “why we cannot place components at
our desired location (may be using the x,y coordinate position?”
The answer is that you can create your GUI without using Layout Managers and you can
also do VB style positioning of components at some x,y co-ordinate in Java, but that is
generally not advisable if you desire to run the same program on different platforms
The appearance of the GUI also depends on the underlying platform and to keep that same the
responsibility of arranging layout is given to the LayoutManagers so they can provide the same
look and feel across different platforms
Commonly used layout managers are
•
•
•
•
•
•
Flow Layout
Grid Layout
Border Layout
Box Layout
Card Layout
GridBag Layout and so on
© Copyright Virtual University of Pakistan 104 Web Design and Development (CS506) Let us discuss the top three in detail one by one with code examples. These top three will meet most
of your basic needs
10.9.1.1 Flow Layout
•
•
Position components on line by line basis. Each time a line is filled, a new line is
started.
The size of the line depends upon the size of your frame. If you stretch your frame while
your program is running, your GUI will be disturbed.
Example Code
// File FlowLayoutTest.java
import java.awt.*;
import javax.swing.*;
public class FlowLayoutTest {
JFrame myFrame ;
JButton b1, b2, b3, b4, b5;
//method used for setting layout of GUI
public void initGUI ( ) {
myFrame = new JFrame(“Flow Layout”);
Container c = myFrame.getContentPane();
c.setLayout( new FlowLayout( ) );
b1 = new JButton(“Next Slide”);
b2 = new JButton(“Previous Slide”);
b3 = new JButton(“Back to Start”);
b4 = new JButton(“Last Slide”);
b5 = new JButton(“Exit”);
c.add(b1);
c.add(b2);
c.add(b3);
c.add(b4);
c.add(b5);
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myFrame.setSize(300,150);
myFrame.setVisible(true);
} //end initGUI method
public FlowLayoutTest () {
// default constructor
initGUI ();
}
public static void main (String args[ ]) {
FlowLayoutTest flTest = new FlowLayoutTest();
© Copyright Virtual University of Pakistan 105 Web Design and Development (CS506) }
} // end of class
Output
10.9.1.2 Grid
•
•
•
•
Layout
Splits the panel/window into a grid(cells) with given number of rows and columns.
Forces the size of each component to occupy the whole cell. Size of each
component is same .
Components are added row wise. When all the columns of the first row are get filled the
components are then added to the next row.
Only one component can be added into each cell.
Example Code
// File GridLayoutTest.java
import java.awt.*;
import javax.swing.*;
public class GridLayoutTest {
JFrame myFrame ;
JButton b1, b2, b3, b4, b5;
//method used for setting layout of GUI
public void initGUI ( ) {
myFrame = new JFrame(“Grid Layout”);
Container c = myFrame.getContentPane();
// rows , cols
c.setLayout( new GridLayout( 3
, 2 ) );
b1 = new JButton(“Next Slide”);
b2 = new JButton(“Previous Slide”);
b3 = new JButton(“Back to Start”);
b4 = new JButton(“Last Slide”);
b5 = new JButton(“Exit”);
c.add(b1);
c.add(b2);
c.add(b3);
© Copyright Virtual University of Pakistan 106 Web Design and Development (CS506) c.add(b4);
c.add(b5);
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myFrame.setSize(300,150);
myFrame.setVisible(true);
} //end initGUI method
public GridLayoutTest () {
// default constructor
initGUI ();
}
public static void main (String args[ ]) {
GridLayoutTest glTest = new GridLayoutTest();
}
} // end of class
Output
Modification
The grid layout also allows the spacing between cells. To achieve spacing between cells, modify the
above program.
Pass additional parameters to the constructor of GridLayout, spaces between rows & spaces between
columns as shown below
c.setLayout( new GridLayout( 3 , 2,10 , 20) );
The output is look similar to one given below.
© Copyright Virtual University of Pakistan 107 Web Design and Development (CS506) 10.9.1.3 Border Layout
•
•
•
•
Divides the area into five regions. North, South, East, West and Center
Components are added to the specified region
If any region not filled, the filled regions will occupy the space but the center region
will still appear as background if it contains no component.
Only one component can be added into each region.
Example Code:
// File BorderLayoutTest.java
import java.awt.*;
import javax.swing.*;
public class BorderLayoutTest {
JFrame myFrame ;
JButton b1, b2, b3, b4, b5;
//method used for setting layout of GUI
public void initGUI ( ) {
myFrame = new JFrame(“Border Layout”);
Container c = myFrame.getContentPane();
c.setLayout( new BorderLayout( ) );
b1 = new JButton(“Next Slide”);
b2 = new JButton(“Previous Slide”);
b3 = new JButton(“Back to Start”);
b4 = new JButton(“Last Slide”);
b5 = new JButton(“Exit”);
© Copyright Virtual University of Pakistan 108 Web Design and Development (CS506) c.add( b1 , BorderLayout.NORTH );
c.add( b2 , BorderLayout.SOUTH );
c.add( b3 , BorderLayout.EAST );
c.add( b4 , BorderLayout.WEST );
c.add( b5 , BorderLayout.CENTER);
myFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
myFrame.setSize(300,150);
myFrame.setVisible(true);
} //end initGUI method
public BorderLayoutTest () { // default constructor
initGUI ();
}
public static void main (String args[ ]) {
BorderLayoutTest glTest = new BorderLayoutTest();
}
} // end of class
Points to Remember
•
•
Revisit the code of adding components, we specify the region in which we want
to add component or otherwise they will not be visible.
Consider the following segment of code: BorderLayout.NORTH,
as you
guessed correctly NORTH is a constant (final) defined in BorderLayout class
public access modifier. Similarly the other ones are defined. Now you understand
why so much emphasis has been made on following the naming conventions.
Output:
10.10 Making Complex GUIs
From the discussion above it seems that the basic Layout Managers may not help us in
constructing complex GUIs, but generally a combination of these basic layouts can do the job. So
let’s try to create the calculator GUI given below
© Copyright Virtual University of Pakistan 109 Web Design and Development (CS506) This GUI has 16 different buttons each of same size and text field on the top and a label ‘my
calculator’ on the bottom.
So, how we can make this GUI? If Border Layout is selected, it has five regions (each region can
have at most one component) but here we have more than five components to add. Lets try Grid
Layout, but all the components in a Grid have same size and the text field at the top and label at
the bottom has different size. Flow Layout cannot be selected because if we stretch our GUI it will
destroy its shape.
Can we make this GUI? Yes, we can. Making of such GUI is a bit tricky business but General
Purpose Containers are there to provide the solution.
10.10.1 JPanel
•
•
It is general purpose container (can’t exist alone, it has to be in some toplevel container)
in which we can put in different components (JButton , JTextField etc even other JPanels)
JPanel has its own layout that can be set while creating JPanel instance
JPanel myPanel = new JPanel ( new FlowLayout( ) );
•
Add components by using add method like shown below.
myPanel.add (button);
•
Must be added to a top level container (like JFrame etc) in order to be visible as
they (general purpose containers) can’t exist alone.
10.10.2 Solution
To make the calculator GUI shown above, take JFrame (top level container) and set its layout to
border. Then take JPanel (general purpose container) and set its layout to Grid with 4 rows and 4
columns.
Add buttons to JPanel as they all have equal size and JPanel layout has been set to
© Copyright Virtual University of Pakistan 110 Web Design and Development (CS506) GridLayout. Afterthat, add text field to the north region, label to the south region and
panel to the center region of the JFrame’s container. The east and west regions are left
blank and the center region will be stretched to cover up these. So, that’s how we can build our
calculator GUI.
Code for Calculator GUI
// File CalculatorGUI.java
import java.awt.*;
import javax.swing.*;
public class CalculatorGUI {
JFrame fCalc;
JButton b1, b2, b3, b4, b5, b6, b7, b8, b9, b0;
JButton bPlus, bMinus, bMul, bPoint, bEqual, bClear;
JPanel pButtons;
JTextField tfAnswer; JLabel lMyCalc;
//method used for setting layout of GUI
public void initGUI ( ) {
fCalc = new JFrame();
b0 = new JButton("0");
b1 = new JButton("1");
b2 = new JButton("2");
b3 = new JButton("3");
b4 = new JButton("4");
b5 = new JButton("5");
b6 = new JButton("6");
b7 = new JButton("7");
b8 = new JButton("8");
b9 = new JButton("9");
bPlus
= new JButton("+");
bMinus
= new JButton("-");
bMul
= new JButton("*");
bPoint
= new JButton(".");
bEqual
= new JButton("=");
bClear
= new JButton("C");
tfAnswer = new JTextField();
lMyCalc = new JLabel("My Clacualator");
//creating panel object and setting its layout
pButtons = new JPanel (new GridLayout(4,4));
//adding components (buttons) to panel
pButtons.add(b1);
pButtons.add(b2);
pButtons.add(b3);
© Copyright Virtual University of Pakistan 111 Web Design and Development (CS506) pButtons.add(bClear);
pButtons.add(b4);
pButtons.add(b5);
pButtons.add(b6);
pButtons.add(bMul);
pButtons.add(b7);
pButtons.add(b8);
pButtons.add(b9);
pButtons.add(bMinus);
pButtons.add(b0);
pButtons.add(bPoint);
pButtons.add(bPlus);
pButtons.add(bEqual);
// getting componenet area of JFrame
Container con = fCalc.getContentPane();
con.setLayout(new BorderLayout());
//adding components to container
con.add(tfAnswer, BorderLayout.NORTH);
con.add(lMyCalc, BorderLayout.SOUTH);
con.add(pButtons, BorderLayout.CENTER);
fCalc.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
fCalc.setSize(300, 300);
fCalc.setVisible(true);
} //end initGUI method
public CalculatorGUI () {
// default constructor
initGUI ();
}
public static void main (String args[ ]) {
CalculatorGUI calGUI = new CalculatorGUI ();
}
} // end of class
10.11 Reference:
•
•
•
•
Sun java tutorial: http://java.sun.com/docs/books/tutorial/java
Thinking in java by Bruce Eckle
Beginning Java2 by Ivor Hortan
Java A Lab Course by Umair Javed
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 112 Web Design and Development (CS506) Lecture 11: Event Handling
One of the most important aspects of most non-trivial applications (especially UI type-apps) is
the ability to respond to events that are generated by the various components of the application,
both in response to user interactions and other system components such as client-server
processing. In this handout we will look at how Java supports event generation and handling and
how to create (and process) custom events.
GUIs generate events when the user interacts with GUI. For example,
•
•
•
Clicking a button
Moving the mouse
Closing Window etc
Both AWT and swing components (not all) generate events
•
•
java.awt.event.*;
javax.swing.event.*;
In java, events are represented by Objects
These objects tell us about event and its source. Examples are:
•
•
ActionEvent (Clicking a button)
WindowEvent (Doing something with window e.g. closing , minimizing)
Some event classes of java.awt.event are shown in diagram below
© Copyright Virtual University of Pakistan 113 Web Design and Development (CS506) 11.1 Event Handling Model
In Java both AWT and Swing components use Event Delegation Model.
•
•
•
In this model processing of an event is delegated to a particular object (handlers) in the
program
It’s a Publish-Subscribe model. That is, event generating component publish an event and
event handling components subscribe for that event. The publisher sends these events to
subscribers. Similar to the way that you subscribe for newspaper and you get the
newspaper at your home from the publisher.
This model separates UI code from program logic, it means that we can create separate
classes for UI components and event handlers and hence business/program
logic is separated from GUI components.
11.2 Event Handling Steps
For a programmer the event Handling is a three step process in terms of code
•
•
•
Step 1: Create components which can generate events (Event Generators)
Step 2: Build component (objects) that can handle events (Event Handlers)
Step 3: Register handlers with generators
11.3 Event Handling Process
11.3.1 Step 1: Event Generators
The first step is that you create an event generator. You have already seen a lot of event generators
like:
• Buttons
• Mouse
• Key
• Window etc
Most of GUI components can be created by calling their constructors. For example
JButton b1 = new JButton(“Hello”);
Now b1 can generate events
Note: We do not create Mouse/Keys etc as they are system components
11.3.2 Step 2: Event Handlers/ Event Listener
The second step is that you build components that can handle events
•
First Technique - By Implementing Listener Interfaces
© Copyright Virtual University of Pakistan 114 Web Design and Development (CS506) o Java defines interfaces for every event type
o If a class needs to handle an event. It needs to implement the corresponding
listener interface
o To handle “ActionEvent” a class needs to implement “ActionListener”
o To handle “KeyEvent” a class needs to implement “KeyListener”
o To handle “MouseEvent” a class needs to implement “MouseListener” and so on
o Package java.awt.event contains different event Listener Interfaces which
are shown in the following figure
Some Example Listeners, the way they are defined in JDK by Sun
public interface ActionListener {
public void actionPerformed(ActionEvent e);
}
public interface ItemListener {
public void itemStateChanged(ItemEvent e);
}
© Copyright Virtual University of Pakistan 115 Web Design and Development (CS506) public
public
public
public
public
}
interface ComponentListener {
void componentHidden(ComponentEvent e);
void componentMoved(ComponentEvent e);
void componentResized(ComponentEvent e);
void componentShown(ComponentEvent e);
o By implementing an interface the class agrees to implement all the methods that
are present in that interface. Implementing an interface is like signing a contract.
o Inside the method the class can do whatever it wants to do with that event
o Event Generator and Event Handler can be the same or different classes
o To handle events generated by Button. A class needs to implement
ActionListener interface and thus needs to provide the definition of
actionPerformed() method which is present in this interface.
public class Test implements ActionListener{
public void actionPerformed(ActionEvent ae) {
// do something
}
}
11.3.3 Step 3: Registering Handler with Generator
•
•
The event generator is told about the object which can handle its events
Event Generators have a method
•
o addXXXListener(_reference to the object of Handler class_)
For example, if b1 is JButton then
o b1.addActionListener(this); // if listener and generator are same class
Event Handling Example
Clicking the “Hello” button will open up a message dialog shown below.
© Copyright Virtual University of Pakistan 116 Web Design and Development (CS506) We will take the simplest approach of creating handler and generator in a single class. Button is
our event generator and to handle that event our class needs to implement ActionListener
Interface and to override its actionPerformed method and in last to do the registration.
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
/* Implementing the interface according to the type of the event,
i.e. creating event handler (first part of step 2 of our process)
*/
4. public class ActionEventTest implements ActionListener{
5. JFrame frame;
6. JButton hello;
// setting layout components
7. public void initGUI ( ) {
8. frame = new JFrame();
9. Container cont = frame.getContentPane();
10. cont.setLayout(new FlowLayout());
//Creating event generator step-1 of our process
11. hello = new JButton("Hello");
/* Registering event handler with event generator.
Since event handler is in same object that contains
button, we have used this to pass the reference.(step
3 of the process) */
12. hello.addActionListener(this);
13. cont.add(hello);
14. frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
15. frame.setSize(150, 150);
16. frame.setVisible(true);
17. }
//constructor
18. public ActionEventTest ( ) {
19. initGUI();
20. }
/* Override actionPerformed method of ActionListener’s
interfacemethod of which will be called when event
takes place (second part of step 2 of our process) */
21. public void actionPerformed(ActionEvent event) {
22. JOptionPane.showMessageDialog(null,"Hello is pressed");
23. }
24. public static void main(String args[]) {
25. ActionEventTest aeTest = new ActionEventTest();
26. }
27.} // end class
© Copyright Virtual University of Pakistan 117 Web Design and Development (CS506) 11.4 How Event Handling Participants Interact Behind the Scenes?
We have already seen that what a programmer needs to do handle events. Let’s see what takes place
behind the scenes, i.e How JVM handles event. Before doing that lets revisit different participants of
Event Handling Process and briefly what they do.
11.4.1 Event Generator / Source
•
•
•
•
Swing and awt components
For example, JButton, JTextField, JFrame etc
Generates an event object
Registers listeners with itself
11.4.2 Event Object
•
•
Encapsulate information about event that occurred and the source of that event
For example, if you click a button, ActionEvent object is created
11.4.3 Event Listener/handler
•
•
•
Receives event objects when notified, then responds
Each event source can have multiple listeners registered on it
Conversely, a single listener can register with multiple event sources
11.4.4 JVM
•
•
•
•
Receives an event whenever one is generated
Looks for the listener/handler of that event
If exist, delegate it for processing
If not, discard it (event).
When button generates an ActionEvent it is sent to JVM which puts it in an event queue.
After that when JVM find it appropriate it de-queue the event object and send it to all the
listeners that are registered with that button. This is all what we shown in the pictorial
form below:
© Copyright Virtual University of Pakistan 118 Web Design and Development (CS506) (Figure from JAVA A Lab Course)
Making Small Calculator
•
•
•
User enters numbers in the provided fields
On pressing “+” button, sum would be displayed in the answer field
On pressing “*” button, product would be displayed in the answer field
© Copyright Virtual University of Pakistan 119 Web Design and Development (CS506) Example Code: Making Small Calculator
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
4. public class SmallCalcApp implements ActionListener{
5. JFrame frame;
6. JLabel firstOperand, secondOperand, answer;
7. JTextField op1, op2, ans;
8. JButton plus, mul;
9. // setting layout
10. public void initGUI ( ) {
11. frame = new JFrame();
12. firstOperand
= new JLabel("First Operand");
13. secondOperand = new JLabel("Second Operand");
14. answer
= new JLabel("Answer");
15. op1 = new JTextField (15);
16. op2 = new JTextField (15);
17. ans = new JTextField (15);
18. plus = new JButton("+");
19. plus.setPreferredSize(new Dimension(70,25));
20. mul = new JButton("*");
21. mul.setPreferredSize(new Dimension(70,25));
22. Container cont = frame.getContentPane();
23. cont.setLayout(new FlowLayout());
24. cont.add(firstOperand);
25. cont.add(op1);
26. cont.add(secondOperand);
27. cont.add(op2);
28. cont.add(plus);
29. cont.add(mul);
30. cont.add(answer);
31. cont.add(ans);
32. plus.addActionListener(this);
33. mul.addActionListener(this);
34.
35.
36.
37.
38.
39.
40.
41.
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200, 220);
frame.setVisible(true);
}
//constructor
public SmallCalcApp ( ) {
initGUI();
}
42. public void actionPerformed(ActionEvent event) {
43. String oper, result;
© Copyright Virtual University of Pakistan 120 Web Design and Development (CS506) 44. int num1, num2, res;
/* All the information regarding an event is contained
inside the event object. Here we are calling the
getSource() method on the event object to figure out
the button that has generated that event.
*/
45. if (event.getSource() == plus) {
46. oper = op1.getText();
47. num1 = Integer.parseInt(oper);
48. oper = op2.getText();
49. num2 = Integer.parseInt (oper);
50. res = num1+num2;
51. result = res+"";
52. ans.setText(result);
53. }
54. else if (event.getSource() == mul) {
55. oper = op1.getText();
56. num1 = Integer.parseInt(oper);
57. oper = op2.getText();
58. num2 = Integer.parseInt (oper);
59. res = num1*num2;
60. result = res+"";
61. ans.setText(result);
62. }
63.
}
64. public static void main(String args[]) {
65. SmallCalcApp scApp = new SmallCalcApp();
66. }
67. }// end class
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 121 Web Design and Development (CS506) Lecture 12: More Examples of Handling Events
12.1 Handling Mouse Event
Mouse events can be trapped for any GUI component that inherits from Component class. For
example, JPanel, JFrame & JButton etc.
To handle Mouse events, two types of listener interfaces are available.
•
•
MouseMotionListener
MouseListener
The class that wants to handle mouse event needs to implement the corresponding interface
and needs to provide the definition of all the methods in that interface.
12.1.1 MouseMotionListener interface
•
•
Used for processing mouse motion events
Mouse motion event is generated when mouse is moved or dragged
A MouseMotionListener interface is defined in JDK as follows:
public interface MouseMotionListener {
public void mouseDragged (MouseEvent me);
public void mouseMoved (MouseEvent me);
}
12.1.2 MouseListener interface
•
Used for processing “interesting” mouse events like when mouse is:
o
o
o
o
o
Pressed
Released
Clicked (pressed & released without moving the cursor)
Enter (mouse cursor enters the bounds of component)
Exit (mouse cursor leaves the bounds of component)
MouseListener interfaces are defined in JDK as follows:
public
public
public
public
public
public
interface MouseListener {
void mousePressed (MouseEvent me);
void mouseClicked (MouseEvent me);
void mouseReleased (MouseEvent me);
void mouseEntered (MouseEvent me);
void mouseExited (MouseEvent me); }
© Copyright Virtual University of Pakistan 122 Web Design and Development (CS506) Example Code: Handling Mouse Events
Example to show Mouse Event Handling .Every time mouse is moved, the coordinates for a new
place is shown in a label.
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
4. public class EventsEx implements MouseMotionListener{
5. JFrame frame;
6. JLabel coordinates;
7. // setting layout
8. public void initGUI ( ) {
9. // creating event generator
10. frame = new JFrame();
11. Container cont = frame.getContentPane();
12. cont.setLayout(new BorderLayout( ) );
13. coordinates = new JLabel ();
14. cont.add(coordinates, BorderLayout.NORTH);
15. // registring mouse event handler with generator
16. frame.addMouseMotionListener(this);
17. frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
18. frame.setSize(350, 350);
19. frame.setVisible(true);
20. } // end initGUI method
21. //default constructor
22.
public EventsEx ( ) {
23. initGUI();
24. }
// MouseMotionListener event hadler handling dragging
25.
public void mouseDragged (MouseEvent me) {
26. int x = me.getX();
27. int y = me.getY();
28. coordinates.setText("Dragged at [" + x + "," + y + "]");
29. }
// MouseMotionListener event handler handling motion
30.
public void mouseMoved (MouseEvent me) {
31. int x = me.getX();
32. int y = me.getY();
33. coordinates.setText("Moved at [" + x + "," + y + "]");
34. }
35.
public static void main(String args[]) {
36. EventsEx ex = new EventsEx();
37. }
38. } // end class
© Copyright Virtual University of Pakistan 123 Web Design and Development (CS506) Another Example: Handling Window Events
Task
We want to handle Window Exit event only
Why?
•
•
When window is closed, control should return back to command prompt.
But we have already achieved this functionality through following line of code
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
•
But, what if we want to display some message (Good Bye) before exiting?
How?
•
•
•
•
To handle window events, we need to implement “WindowListner” interface.
WindowListner” interface contains 7 methods We require only one i.e.
windowClosing
But, We have to provide definitions of all methods to make our class a concrete class
WindowListener interface is defined in the JDK as follows
public
public
public
public
public
public
public
public
}
•
interface WindowListener {
void windowActivated(WindowEvent we);
void windowClosed(WindowEvent we);
void windowClosing(WindowEvent we);
void windowDeactivated(WindowEvent we);
void windowDeiconified(WindowEvent we);
void windowIconified(WindowEvent we);
void windowOpened(WindowEvent we);
public void windowClosing(WindowEvent we) is our required method
© Copyright Virtual University of Pakistan 124 Web Design and Development (CS506) Example Code: WindowExitHandler
This example code is modification of the last code example i.e. EventsEx.java
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
4. public class EventsEx implements MouseMotionListener ,
WindowListener {
5.
6.
//
7.
JFrame frame;
JLabel coordinates;
setting layout
public void initGUI ( ) {
// creating event generator
8. frame = new JFrame();
9. Container cont = frame.getContentPane();
10. cont.setLayout(new BorderLayout( ) );
11. coordinates = new JLabel ();
12. cont.add(coordinates, BorderLayout.NORTH);
// registring mouse event handler with generator
13. frame.addMouseMotionListener(this);
// registering window handler with generator
14. frame.addWindowListener(this);
15. frame.setSize(350, 350);
16. frame.setVisible(true);
17. } // end initGUI method
//default constructor
18. public EventsEx ( ) {
19. initGUI();
20. }
// MouseMotionListener event hadler handling dragging
21. public void mouseDragged (MouseEvent me) {
22. int x = me.getX();
23. int y = me.getY();
24. coordinates.setText("Dragged at [" + x + "," + y + "]");
25. }
// MouseMotionListener event handler handling motion
26. public void mouseMoved (MouseEvent me) {
27. int x = me.getX();
28. int y = me.getY();
29.
30. coordinates.setText("Moved at [" + x + "," + y + "]");
31. }
// window listener event handler
© Copyright Virtual University of Pakistan 125 Web Design and Development (CS506) 32.
33.
34.
35.
36.
37.
38.
39.
40.
41.
public void windowActivated (WindowEvent we) {
public void windowClosed (WindowEvent we) {
}
public void windowClosing (WindowEvent we) {
JOptionPane.showMessageDialog(null, “Good Bye”);
System.exit(0);
}
public void windowDeactivated (WindowEvent we) {
public void windowDeiconified (WindowEvent we) {
public void windowIconified (WindowEvent we) {
public void windowOpened (WindowEvent we) {
}
42.
43.
44.
45.
public static void main(String args[]) {
EventsEx ex = new EventsEx();
}
} // end class
}
}
}
}
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 126 Web Design and Development (CS506) Lecture 13: Adapter Classes
Problem in Last Code Example
Problem
•
•
•
We were interested in windowClosing() method only
But have to provide definitions of all the methods, Why?
Because a class implementing an interface has to provide definitions of all methods
present in that interface.
Solution
•
To avoid giving implementations of all methods of an interface when we are not
using these methods we use Event Adapter classes
13.1 Adapter Classes
•
•
•
•
For listener interfaces containing more than one event handling methods, jdk defines
adapter classes. Examples are
o For WindowListener WindowAdapter
o For MouseMotionListener MouseMotionAdapter
o and many more
Adapter classes provide definitions for all the methods (empty bodies) of their
corresponding Listener interface
It means that WindowAdapter class implements WindowListener interface and
provide the definition of all methods inside that Listener interface
Consider the following example of MouseMotionAdapter and its corresponding
MouseMotionListener interface
public interface MouseMotionListener {
public void mouseDragged (MouseEvent me);
public void mouseMoved (MouseEvent me);
}
public class MouseMotionAdapter implements
MouseMotionListener{
public void mouseDragged (MouseEvent me) {
}
public void mouseMoved (MouseEvent me) {
}
}
© Copyright Virtual University of Pakistan 127 Web Design and Development (CS506) 13.2 Available Adapter classes
13.2.1 How to use Adapter Classes
•
previously handler class need to implement interface
•
Therefore it has to provide definitions of all the methods inside that interface now our
handler class will inherit from adapter class
•
Due to inheritance, all the methods of the adapter class will be available inside our
handler class since adapter classes has already provided definitions with empty bodies.
we do not have to provide implementations of all the methods again
We only need to override our method of interest.
public class EventsEx implements MouseMotionListener{...}
public class EventsEx extends MouseMotionAdapter{...}
•
•
Example Code 13.1: Handling Window Events using Adapter Classes
Here we are modifying the window event code in the last example to show
the use of WindowAdapter instead of WindowListener. Code related to
MouseMotionListener is deleted to avoid cluttering of code.
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
4. public class EventsEx extends WindowAdapter {
5. JFrame frame;
6. JLabel coordinates;
// setting layout
7. public void initGUI ( ) {
// creating event generator
8. frame = new JFrame();
9. Container cont = frame.getContentPane();
10. cont.setLayout(new BorderLayout( ) );
© Copyright Virtual University of Pakistan 128 Web Design and Development (CS506) 11. coordinates = new JLabel ();
12. cont.add(coordinates, BorderLayout.NORTH);
// registering window handler with generator
13. frame.addWindowListener(this);
14. frame.setSize(350, 350);
15. frame.setVisible(true);
16. } // end initGUI method
//default constructor
17. public EventsEx ( ) {
18. initGUI();
19. }
//
//
20.
21.
22.
23.
As you can see that we have only implemented
our required method
public void windowClosing (WindowEvent we) {
JOptionPane.showMessageDialog(null, “Good Bye”);
System.exit(0);
}
24.
25.
26.
27.
public static void main(String args[]) {
EventsEx ex = new EventsEx();
}
} // end class
Problem in Last Code Example
•
•
We have inherited from WindowAdapter
What if we want to use MouseMotionAdpater as well? Or what if our class already
inherited from some other class?
Problem
•
Java allows single inheritance
Solution
•
Use Inner classes
13.3 Inner Classes
•
•
•
A class defined inside another class
Inner class can access the instance variables and members of outer class
It can have constructors, instance variables and methods, just like a regular class
© Copyright Virtual University of Pakistan 129 Web Design and Development (CS506) •
Generally used as a private utility class which does not need to be seen by others
classes
Example Code13.2: Handling Window Event with Inner Class
Here we are modifying the window event code in the last example to show the use of
WindowAdapter as an inner class.
1.
2.
3.
4.
import
import
import
public
java.awt.*;
javax.swing.*;
java.awt.event.*;
class EventEx {
5. JFrame frame;
6. JLabel coordinates;
7. // setting layout
8. public void initGUI ( ) {
9. frame = new JFrame();
10. Container cont = frame.getContentPane();
11. cont.setLayout(new BorderLayout( ));
12. coordinates = new JLabel ();
13. cont.add(coordinates, BorderLayout.NORTH);
/* Creating an object of the class which is handling our
window events and registering it with generator
*/
14. WindowHandler handler = new WindowHandler ();
15. frame.addWindowListener(handler);
16. frame.setSize(350, 350);
17. frame.setVisible(true);
18. } // end initGUI
//default constructor
19. public EventEx ( ) {
© Copyright Virtual University of Pakistan 130 Web Design and Development (CS506) 20. initGUI();
21. }
/* Inner class implementation of window adapter. Outer
class is free to inherit from any other class. */
22. private class WindowHandler extends WindowAdapter {
// Event Handler for WindowListener
23. public void windowClosing (WindowEvent we) {
24. JOptionPane.showMessageDialog(null, “Good Bye”);
25. System.exit(0);
26. }
27. } // end of WindowHandler class
28. public static void main(String args[]) {
29. EventEx e = new EventEx();
30. }
31. } // end class
Example Code 13.3: Handling Window and Mouse Events with Inner
Class
Here we are modifying the window event code of the last example to handle window and
mouse events using inner classes. The diagram given below summarizes the approach.
1.
import java.awt.*;
2.
import javax.swing.*;
3.
import java.awt.event.*;
4.
public class EventEx {
5. JFrame frame;
6. JLabel coordinates;
7. // setting layout
8. public void initGUI ( )
{
9. frame = new JFrame();
10. Container cont = frame.getContentPane();
© Copyright Virtual University of Pakistan 131 Web Design and Development (CS506) 11. cont.setLayout(new BorderLayout( ) );
12. coordinates = new JLabel ();
13. cont.add(coordinates, BorderLayout.NORTH);
/* Creating an object of the class which is handling our
window events and registering it with generator */
14. WindowHandler whandler = new WindowHandler ();
15. frame.addWindowListener(whandler);
/* Creating an object of the class which is handling our
MouseMotion events & registering it with generator */
16. MouseHandler mhandler = new MouseHandler ();
17. frame.addMouseMotionListener(mhandler);
18. frame.setSize(350, 350);
19. frame.setVisible(true);
20. }
//default constructor
21. public EventEx ( ) {
22. initGUI();
23. }
/* Inner class implementation of WindowAdapter. Outer class
is free to inherit from any other class. */
24. private class WindowHandler extends WindowAdapter {
// Event Handler for WindowListener
25. public void windowClosing (WindowEvent we) {
26. JOptionPane.showMessageDialog(null, “Good Bye”);
27. System.exit(0);
28. }
29. } // end of WindowHandler
//Inner class implementation of MouseMotionAdapter
30. private class MouseHandler extends MouseMotionAdapter {
// Event Handler for mouse motion events
31. public void mouseMoved (MouseEvent me) {
32. int x = me.getX();
33. int y = me.getY();
34. coordinates.setText(“Moved at [" + x + "," + y + "]” );
35. }
36. } // end of MouseHandler
37. public static void main(String args[]) {
38. EventEx e = new EventEx();
39. }
40. } // end class
© Copyright Virtual University of Pakistan 132 Web Design and Development (CS506) Example Code: Making Small Calculator using Inner classes
•
•
•
User enters numbers in the provided fields
On pressing “+” button, sum would be displayed in the answer field
On pressing “*” button, product would be displayed in the answer field
1. import java.awt.*;
2. import javax.swing.*;
3. import java.awt.event.*;
4. public class SmallCalcApp{
5. JFrame frame;
6. JLabel firstOperand, secondOperand, answer;
7. JTextField op1, op2, ans;
8. JButton plus, mul;
9. // setting layout
10. public void initGUI ( ) {
11. frame = new JFrame();
12. firstOperand
= new JLabel("First Operand");
13. secondOperand = new JLabel("Second Operand");
14. answer
= new JLabel("Answer");
15. op1 = new JTextField (15);
16. op2 = new JTextField (15);
17. ans = new JTextField (15);
18. plus = new JButton("+");
19. plus.setPreferredSize(new Dimension(70,25));
20. mul = new JButton("*");
21. mul.setPreferredSize(new Dimension(70,25));
22. Container cont = frame.getContentPane();
23. cont.setLayout(new FlowLayout());
24. cont.add(firstOperand);
25. cont.add(op1);
26. cont.add(secondOperand);
27. cont.add(op2);
© Copyright Virtual University of Pakistan 133 Web Design and Development (CS506) 28. cont.add(plus);
29. cont.add(mul);
30. cont.add(answer);
31. cont.add(ans);
/* Creating an object of the class which is handling
button events & registering it with generators */
32. ButtonHandler bHandler = new ButtonHandler();
33. plus.addActionListener(bHandler);
34. mul.addActionListener(bHandler);
35. frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
36. frame.setSize(200, 220);
37. frame.setVisible(true);
38. }
39. //constructor
40. public SmallCalcApp ( ) {
41. initGUI();
42. }
//Inner class implementation of ActionListener
43. private class ButtonHandler implements ActionListener{
44. public void actionPerformed(ActionEvent event) {
45. String oper, result;
46. int num1, num2, res;
47. if (event.getSource() == plus) {
48. oper = op1.getText();
49. num1 = Integer.parseInt(oper);
50. oper = op2.getText();
51. num2 = Integer.parseInt (oper);
52. res = num1+num2;
53. result = res+"";
54. ans.setText(result);
55 }
56. else if (event.getSource() == mul) {
57. oper = op1.getText();
58. num1 = Integer.parseInt(oper);
59. oper = op2.getText();
60. num2 = Integer.parseInt (oper);
61. res = num1*num2;
62. result = res+"";
63. ans.setText(result);
64 }
65. } // end actionPerformed method
66. } // end inner class ButtonHandler
67. public static void main(String args[]) {
68. SmallCalcApp scApp = new SmallCalcApp();
69. }
70. }// end class
© Copyright Virtual University of Pakistan 134 Web Design and Development (CS506) 13.4 Anonymous Inner Classes
•
•
•
•
has no name
same as inner class in capabilities
much shorter
Difficult to understand
13.5 Named vs. Anonymous Objects
13.5.1 Named
•
•
String s = “hello”;
System.out.println(s);
“hello” has a named reference s.
13.5.2 Anonymous
•
System.out.println(“hello”);
We generally use anonymous object when there is just a onetime use of a particular object but
in case of a repeated use we generally used named objects and use that named reference to use that
objects again and again.
Example Code 13.4 Handling Window Event with Anonymous Inner
Class
Here we are modifying the window event code of 13.3 to show the use of anonymous inner
class.
28.
29.
30.
31.
import
import
import
public
java.awt.*;
javax.swing.*;
java.awt.event.*;
class EventsEx extends WindowAdapter {
32. JFrame frame;
33. JLabel coordinates;
// setting layout
34.
public void initGUI ( ) {
// creating event generator
35. frame = new JFrame();
36. Container cont = frame.getContentPane();
37. cont.setLayout(new BorderLayout( ) );
38. coordinates = new JLabel ();
39. cont.add(coordinates, BorderLayout.NORTH);
// registering event handler (anonymous inner class)
© Copyright Virtual University of Pakistan 135 Web Design and Development (CS506) // with generator by using
40. frame.addWindowListener (
41. new WindowAdapter ( ) {
42. public void windowClosing (WindowEvent we) {
43. JOptionPane.showMessageDialog(null, “Good Bye”);
44. System.exit(0);
45. } // end window closing
46. } // end WindowAdapter
47. ); // end of addWindowListener
48. frame.setSize(350, 350);
49. frame.setVisible(true);
50. } // end initGUI method
//default constructor
51. public EventsEx ( ) {
52. initGUI();
53. }
54. public static void main(String args[]) {
55. EventsEx ex = new EventsEx();
56. }
57. } // end class
13.6 Summary of Approaches for Handling Events
•
•
By implementing Interfaces
By extending from Adapter classes
To implement the above two techniques we can use
•
Same class
o putting event handler & generator in one class
•
Separate class
o Outer class
Putting event handlers & generator in two different classes
o Inner classes
o Anonymous Inner classes
13.7 References
Java A Lab Course by Umair Javed
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 136 Web Design and Development (CS506) Lecture 14: Java Database Connectivity
14.1 Introduction
Java Database Connectivity (JDBC) provides a standard library for accessing databases. The
JDBC API contains number of interfaces and classes that are extensively helpful while
communicating with a database.
14.2 The java.sql package
The java.sql package contains basic & most of the interfaces and classes. You automatically get
this package when you download the J2SE™. You have to import this package whenever you
want to interact with a relational database.
14.3 Connecting With Microsoft Access
In this handout, we will learn how to connect & communicate with Microsoft Access Database.
We chooses Access because most of you are familiar with it and if not than it is very easy to
learn.
14.3.1 Create Database
In start create a database “PersonInfo” using Microsoft Access. Create one table named
“Person”. The schema of the table is shown in the picture.
© Copyright Virtual University of Pakistan 137 Web Design and Development (CS506) Add the following records into Person table as shown below:
Save the data base in some folder. (Your database will be saved as an .mdb file)
14.3.2 Setup System DSN
•
•
•
•
After creating database, you have to setup a system Data Source Name (DSN). DSN is a
name through which your system recognizes the underlying data source.
Select Start
Settings
Control Panel
Administrative Tools
Data Sources
(ODBC).
The ODBC Data Source Administrator window would be opened as shown below:
Select System DSN tab. (If you are unable to use System DSN tab due to security
restrictions on your machine, you can use the User DSN tab)
Press Add… button and choose Microsoft Access Driver (*.mdb) from Create New Data
Source window and press Finish button as shown in diagram:
© Copyright Virtual University of Pakistan 138 Web Design and Development (CS506) •
After that, ODBC Microsoft Access Setup window would be opened as shown in
following diagram:
•
Enter the Data Source Name personDSN and select the database by pressing
Select button. The browsing window would be opened, select the desired folder
that contains the database (The database .mdb file you have created in the first
step) Press Ok button.
14.4 Basic Steps in Using JDBC
There are eight (8) basic steps that must be followed in order to successfully
communicate with a database. Let’s take a detail overview of all these one by one.
© Copyright Virtual University of Pakistan 139 Web Design and Development (CS506) 14.4.1 Import Required Package
•
Import the package java.sql.* that contains useful classes and interfaces to
access & work with database.
import java.sql.*;
14.4.2 Load Driver
•
•
•
•
•
•
Need to load suitable driver for underlying database.
Different drivers & types for different databases are available.
For MS Access, load following driver available with j2se.
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
For Oracle, load the following driver. You have to download it explicitly.
Class.forName(“oracle.jdbc.driver.OracleDriver”);
14.4.3 Define Connection URL
•
•
To get a connection, we need to specify the URL of a database (Actually we need
to specify the address of the database which is in the form of URL)
As we are using Microsoft Access database and we have loaded a JDBC-ODBC
driver. Using JDBC-ODBC driver requires a DSN which we have created earlier
and named it personDSN. So the URL of the database will be
String conURL = “jdbc:odbc:personDSN”;
14.4.4 Establish Connection With DataBase
•
•
•
•
Use DriverManager to get the connection object.
The URL of the database is passed to the getConnection method.
Connection con = DriverManager.getConnection(conURL);
If DataBase requires username & password, you can use the overloaded version of
getConnection method as shown below:
String usr = “umair”;
String pwd = “vu”;
Connection con = null;
con = DriverManager.getConnection(conURL, usr, pwd);
14.4.5 Create Statement
•
•
A Statement object is obtained from a Connection object.
Statement stmt = con.createStatement( );
Once you have a statement, you can use it for various kinds of SQL queries.
© Copyright Virtual University of Pakistan 140 Web Design and Development (CS506) 14.4.6 Execute a Query
•
•
The next step is to pass the SQL statements & to execute them.
Two methods are generally used for executing SQL queries. These are:
o executeQuery(sql) method
Used for SQL SELECT queries.
Returns the ResultSET object that contains the results of the query and
can be used to access the query results.
String sql
= “SELECT * from sometable”;
ResultSet rs = stmt.executeQuery(sql);
o executeUpdate(sql) method
This method is used for executing an update statement like INSERT,
UPDATE or DELETE
Returns an Integer value representing the number of rows updated
String sql = “INSERT INTO tablename ” +
“(columnNames) Values (values)” ;
int count = stmt.executeUpdate(sql);
14.4.7 Process Results of the Query
•
•
•
•
•
•
•
The ResultSet provides various getXXX methods that takes a column index
or name and returns the data
The ResultSet maintains the data in the form tables (rows & columns)
First row has index 1, not 0.
The next method of ResultSet returns true or false depending upon
whether the next row is available (exist) or not and moves the cursor
Always remember to call next() method at-least once
To retrieve the data of the column of the current row you need to use the various getters
provided by the ResultSet.
For example, the following code snippet will iterate over the whole
ResultSet and illustrates the usage of getters methods
while ( rs.next() ){
String name = rs.getString(“columnName”); //by using column name
String name = rs.getString(1); // or by using column index }
14.4.8 Close the Connection
•
An opening connection is expensive, postpone this step if additional database operations
are expected
con.close();
© Copyright Virtual University of Pakistan 141 Web Design and Development (CS506) Example Code 14.1: Retrieving Data from ResultSet
The JdbcEx.java demonstrates the usage of all above explained steps. In this code
example, we connect with the PersonInfo database, the one we have created earlier, and
then execute the simple SQL SELECT query on Person table, and then process the query
results.
// File JdbcEx.java
//Step 1: Import package
import java.sql.*;
public class JdbcEx {
public static void main (String args[ ]) {
try {
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
//Step 3: define the connection URL
String url = “jdbc:odbc:personDSN”;
//Step 4: establish the connection
Connection con = DriverManager.getConnection(url);
//Step 5: create Statement
Statement st = con.createStatement();
//Step 6: preapare & execute the query
String sql = “SELECT * FROM Person”;
ResultSet rs = st.executeQuery(sql);
//Step 7: process the results
while(rs.next()){
// The row name is “name” in database “PersonInfo,
// hence specified in the getString() method.
String name = rs.getString(“name”);
String add
= rs.getString(“address”);
String pNum = rs.getString(“phoneNum”);
System.out.println(name + “ ” + add + ” ” + pNum);
}
//Step 8: close the connection
con.close();
}catch(Exception sqlEx){
System.out.println(sqlEx);
}
} // end main
} // end class
The important thing you must notice that we have put all code inside try block and then handle
(in the above example, only printing the name of the exception raised) exception inside catch
block.
Why? Because we are dealing with an external resource (database). If you can recall all IO
related operations involving external resources in java throw exceptions. These exceptions
© Copyright Virtual University of Pakistan 142 Web Design and Development (CS506) are checked exceptions and we must need to handle these exceptions.
Compile & Execute
Since the Person table contains only three records, so the following output would be
produced on executing the above program.
14.5 References:
•
•
•
Java - A Lab Course by Umair Javed
Java tutorial by Sun: http://java.sun.com/docs/books/turorial
Beginning Java2 by Ivor Hortan
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 143 Web Design and Development (CS506) Lecture 15: More On JDBC
In the previous handout, we have discussed how to execute SQL statements. In this handout,
we’ll learn how to execute DML (insert, update, delete) statements as well some useful methods
provided by the JDBC API.
Before jumping on to example, let’s take a brief overview of executeUpdate()
method that is used for executing DML statements.
15.1 Useful Statement Methods:
15.1.1 executeUpdate( )
•
•
•
•
Used to execute for INSERT, UPDATE, or DELETE SQL statements.
This method returns the number of rows that were affected in the database.
Also supports DDL (Data Definition Language) statements CREATE TABLE, DROP
TABLE, and ALERT TABLE etc.
For example,
int num = stmt.executeUpdate(“DELETE from Person WHERE id
= 2” );
Example Code 15.1 : Executing SQL DML Statements
This program will take two command line arguments that are used to update records in the
database. executeUpdate( ) method will be used to achieve the purpose stated above.
// File JdbcDmlEx.java
//step 1:
import package import java.sql.*;
public class JdbcDmlEx {
public static void main (String args[ ]) {
try {
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
//Step 3: define the connection URL
String url = “jdbc:odbc:personDSN”;
//Step 4: establish the connection
Connection con = DriverManager.getConnection(url);
//Step 5: create Statement
Statement st = con.createStatement();
// assigning first command line argument value
String addVar = args[0];
// assigning second command line argument value
© Copyright Virtual University of Pakistan 144 Web Design and Development (CS506) String nameVar = args[1];
// preparing query - nameVar & addVar strings are embedded
// into query within ‘” + string + “’
String sql =
“UPDATE Person SET address = ‘”+addVar+”’” +“ WHERE
name = ‘”+nameVar+”’ ”;
// executing query
int num = st.executeUpdate(sql);
// Step 7: process the results of the query
// printing number of records affected
System.out.println(num + “ records updated”);
//Step 8: close the connection
con.close();
}catch(Exception sqlEx){
System.out.println(sqlEx);
}
} // end main
} // end class
Compile & Execute
The Person table is shown in the following diagram before execution of the program. We want to
update first row i.e. address of the person ali.
The next diagram shows how we have executed our program. We
passed it two arguments. The first one is the address (defence) and later one
is the name (ali) of the person against whom we want to update the address
value.
© Copyright Virtual University of Pakistan 145 Web Design and Development (CS506) The Person table is shown in the following diagram after the execution of the
program. Notice that address of the ali is now changed to defence.
Note
When we execute DML statements (insert, update, delete) we have to commit it in the
database explicitly to make the changes permanent or otherwise we can rollback the
previously executed statements.
But in the above code, you have never seen such a statement. This is due to the fact that java will
implicitly commit the changes. However, we can change this java behavior to manual commit.
We will cover these in some later handout.
15.1.2 getMaxRows / setMaxRows(int)
•
•
Used for determines the number of rows a ResultSet may contain
By default, the number of rows are unlimited (return value is 0), or by using
setMaxRows(int), the number of rows can be specified.
15.1.3 getQueryTimeOut / setQueryTimeOut (int)
•
•
•
Retrieves the number of seconds the driver will wait for a Statement object to
execute.
The current query time out limit in seconds, zero means there is no limit
If the limit is exceeded, a SQLException is thrown
15.2 Different Types of Statements
•
•
As we have discussed in the previous handout that through Statement objects,
SQL queries are sent to the databases.
Three types of Statement objects are available. These are:
15.2.1 Statement
•
The Statement objects are used for executing simple SQL statements.
© Copyright Virtual University of Pakistan 146 Web Design and Development (CS506) •
We have already seen its usage in the code examples.
15.2.2 PreparedStatement
•
•
The PreparedStatement are used for executing precompiled SQL statements and passing in
different parameters to it.
We will talk about it in detail shortly.
15.2.3 CallableStatement
•
•
These are used for executing stored procedures.
We are not covering this topic; See the Java tutorial on it if you are interested in learning it.
15.2.4 Prepared Statements
•
•
•
•
•
•
What if we want to execute same query multiple times by only changing parameters.
PreparedStatement object differs from Statement object as that it is used to
create a statement in standard form that is sent to database for compilation, before
actually being used.
Each time you use it, you simply replace some of the marked parameters (?) using
some setter methods.
We can create PreparedStatement object by using prepareStatement
method of the connection class. The SQL query is passed to this method as an
argument as shown below.
PreparedStatement pStmt = con.prepareStatement (
“UPDATE tableName SET columnName =
? ” + “WHERE columnName = ? ” );
Notices that we used marked parameters (?) in query. We will replace them later on
by using various setter methods.
If we want to replace first ? with String value, we use setString method and to
replace second ? with int value, we use setInt method. This is shown in the
following code snippet:
pStmt.setString (1 , stringValue);
pStmt.setInt
(2 , intValue)
Note: The first market parameter has index 1.
•
Next, we can call executeUpdate (for INSERT, UPDATE or DELETE queries) or
executeQuery (for simple SELECT query) method.
pStmt.executeUpdate();
© Copyright Virtual University of Pakistan 147 Web Design and Development (CS506) Modify Example Code: Executing SQL DML using Prepared
Statements
This example code is modification to the last example code (JdbcDmlEx.java).The
modifications are highlighted as bold face.
// File JdbcDmlEx.java
//step 1: import package
import java.sql.*;
public class JdbcDmlEx {
public static void main (String args[ ]) {
try {
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
//Step 3: define the connection URL
String url = “jdbc:odbc:personDSN”;
//Step 4: establish the connection
Connection con = DriverManager.getConnection(url, ””, ””);
// make query and place ? where values are to
//be inserted later
String sql =
“UPDATE Person SET address = ? “ +
“ WHERE name = ? ”;
// creating statement using Connection object and passing
// sql statement as parameter
PreparedStatement pStmt = con.prepareStatement(sql);
// assigning first command line argument value
String addVar = args[0];
// assigning second command line argument value
String nameVar = args[1];
// setting first marked parameter (?) by using setString()
// method to address.
pStmt.setString(1 , addVar);
// setting second marked parameter(?) by using setString()
// method to name
pStmt.setString(2 , nameVar);
// suppose address is “defence” & name is “ali”
// by setting both marked parameters, the query will look
// like:
//
sql = “UPDATE Person SET address = “defence”
//
WHERE name = “ali” ”
© Copyright Virtual University of Pakistan 148 Web Design and Development (CS506) // executing update statemnt
int num = pStmt.executeUpdate();
// Step 7: process the results of the query
// printing number of records affected
System.out.println(num + “ records updated”);
//Step 8: close the connection
con.close();
}catch(Exception sqlEx){
System.out.println(sqlEx);
}
} // end main
} // end class
Compile & Execute
Execute this code in a similar way as we showed you in execution of the last program. Don’t
forget to pass the address & name values as the command line arguments.
15.3 References:
•
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web
Design and Development and not for any other commercial purpose without the consent
of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 149 Web Design and Development (CS506) Lecture 16: Result Set
This handout will familiarize you with another technique of inserting, updating & deleting
rows. Before moving on, first we look at ResultSet.
16.1 ResultSet
•
A ResultSet contains the results of the SQL query
o
o
o
o
Represented by a table with rows and columns
Maintains a cursor pointing to its current row of data.
Initially the cursor positioned before the row (0).
First row has index 1
16.1.1 Default ResultSet
•
•
•
•
A default ResultSet object is not updatable and has a cursor that moves
forward only.
You can iterate over through it only once and only from the first row to last row.
Until now, we have worked & used it in various examples.
For a quick overview, here how we create a default ResultSet object.
String sql = “SELECT * FROM Person”;
PreparedStatement pStmt = con.prepareStatement(sql);
ResultSet rs = pStmt.executeQuery( );
16.1.2 Useful ResultSet’s Methods
Following methods are used often to work with default ResultSet object. We already seen and
used some of them in code examples.
16.1.2.1 next( )
•
•
Attempts to move to the next row in the ResultSet, if available
The next() method returns true or false depending upon whether the next
row is available (exist) or not.
© Copyright Virtual University of Pakistan 150 Web Design and Development (CS506) •
Before retrieving any data from ResultSet, always remember to call next()at
least once because initially cursor is positioned before first row.
16.1.2.2 getters
•
•
•
•
•
•
To retrieve the data of the column of the current row you need to use the various getters
provided by the ResultSet
These getters return the value from the column by specifying column name or column index.
For example, if the column name is “Name” and this column has index 3 in the ResultSet
object, then we can retrieve the values by using one of the following methods:
String name = rs.getString(“Name”);
String name = rs.getString(3);
These getter methods are also available for other types like getInt(
), getDouble( ) etc.
Consult the Java API documentation for more references.
Note: Remember that first column has an index 1, NOT zero (0).
16.1.2.3 close( )
•
•
Used to release the JDBC and database resources
The ResultSet is implicitly closed when the associated Statement object executes a new
query or closed by method call.
16.1.2.4 Updatable and/or Scrollable ResultSet
•
•
•
It is possible to produce ResultSet objects that are scrollable and/or updatable (since JDK
1.2)
With the help of such ResultSet, it is possible to move forward as well as backward with in
ResultSet object.
Another advantage is, rows can be inserted, updated or deleted by using updatable
ResultSet object.
16.1.2.5 Creating Updatable & Scrollable ResultSet
The following code fragment, illustrates how to make a ResultSet object that is scrollable and
updatable.
String sql = “SELECT * FROM Person”;
PreparedStatement pStmt = con.prepareStatement(sql,
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_UPDATABLE);
ResultSet rs = pStmt.executeQuery( );
Two constants have been used of ResultSet class for producing a ResultSet rs that is
scrollable, will not show changes made by others and will be updatable
© Copyright Virtual University of Pakistan 151 Web Design and Development (CS506) 16.1.2.6 previous( )
•
•
•
Moves the cursor to the previous row in the ResultSet object, if available
Returns true if cursor is on a valid row, false it is off the result set.
Throws exception if result type is TYPE_FORWARD_ONLY.
Example Code 16.1: Use of previous ( ), next( ) & various getters
methods
The ResultSetEx.java shows the use of previous, next and getters methods. We are using
the same Person table of PersonInfo database, the one we had created earlier in this example
and later on.
1.
2.
3.
4.
5.
6.
7.
// File ResultSetEx.java
import java.sql.*;
public class ResultSetEx {
public static void main (String args[ ]) {
try {
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
8. //Step 3: define the connection URL
9. String url = “jdbc:odbc:personDSN”;
10. //Step 4: establish the connection
11. Connection con = DriverManager.getConnection(url);
12.//Step 5: creating PrepareStatement by passing sql and
13.//ResultSet’s constants so that the ResultSet that will
14.//produce as a result of executing query will be
15.//scrollable & updatable
16.String sql =
“SELECT * FROM Person”;
17.PreparedStatement pStmt = con.prepareStatement(sql,
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_UPDATABLE);
20 //Step 6: execute the query
21 ResultSet rs = pStmt.executeQuery();
22 // moving cursor forward i.e. first row
23 rs.next( );
24
25
26
27
// printing column “name” value of current row (first)
System.out.println(“moving cursor forward”);
String name = rs.getString(“Name”);
System.out.println(name);
© Copyright Virtual University of Pakistan 152 Web Design and Development (CS506) 28 // moving cursor forward i.e. on to second row
29 rs.next( );
30 // moving cursor backward i.e to first row
31 rs.previous( );
32.// printing column “name” value of current row (first)
33.System.out.println(“moving cursor forward”);
34.name = rs.getString(“Name”);
35.System.out.println(name);
36.//Step 8: close the connection
37.con.close();
38.}catch(Exception sqlEx){
39.System.out.println(sqlEx);
40.}
41.} // end main
42.} // end class
Compile & Execute:
The sample output is given below:
16.1.2.7 absolute(int)
•
•
•
•
•
Moves the cursor to the given row number in the ResultSet object.
If given row number is positive, moves the cursor forward with respect to beginning of the
result set.
If the given row number is negative, the cursor moves to the absolute row position with
respect to the end of the result set.
For example, calling absolute(-1) positions the cursor on the last row; calling absolute(-2)
moves the cursor to next-to-last row, and so on.
Throws Exception if ResultSet type is TYPE_FORWARD_ONLY
16.1.2.8 updaters (for primitives, String and Object)
•
•
Used to update the column values in the current row or in insert row (discuss later)
Do not update the underlying database
© Copyright Virtual University of Pakistan 153 Web Design and Development (CS506) •
Each update method is overloaded; one that takes column name while other takes column
index. For example String updater are available as:
updateString(String columnName, String value)
updateString(String columnIndex, String value)
16.1.2.9 updateRow( )
•
Updates the underlying database with new contents of the current row of this ResultSet
object
Example Code 16.2: Updating values in existing rows
The following code example updates the Name column in the second row of the
ResultSet object rs and then uses the method updateRow to update the Person table in
database.
This code is the modification of the last one. Changes made are shown in bold face.
1. // File ResultSetEx.java
2. import java.sql.*;
3. public class ResultSetEx {
4. public static void main (String args[ ]) {
5. try {
6. //Step 2: load driver
7. Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
8. //Step 3: define the connection URL
9. String url = “jdbc:odbc:personDSN”;
10. //Step 4: establish the connection
11. Connection con = DriverManager.getConnection(url);
12. //Step 5: create PrepareStatement by passing sql and
13. // ResultSet appropriate fields
14. String sql =
“SELECT * FROM Person”;
15. PreparedStatement pStmt = con.prepareStatement(sql,
16. ResultSet.TYPE_SCROLL_INSENSITIVE,
17. ResultSet.CONCUR_UPDATABLE);
18. //Step 6: execute the query
19. ResultSet rs = pStmt.executeQuery();
20. // moving cursor to second row
21. rs.absolute(2);
22. // update address column of 2nd row in rs
23. rs.updateString(“Address”, “model town”);
24. // update the row in database
25. rs.updateRow( );
26. //Step 8: close the connection
27. con.close();
28. }catch(Exception sqlEx){
29. System.out.println(sqlEx);
30. }
31. } // end main
32. } // end class
© Copyright Virtual University of Pakistan 154 Web Design and Development (CS506) Compile & Execute
Given below are two states of Person table. Notice that address of 2nd row is updated.
Person table: Before execution
Person table: After execution
16.1.2.10moveToInsertRow(int)
•
•
•
•
An updatable resultset object has a special row associate with it i.e. insert row
Insert row - a buffer, where a new row may be constructed by calling updater methods.
Doesn’t insert the row into a result set or into a database.
For example, initially cursor is positioned on the first row as shown in the diagram:
•
By calling moveToInsertRow( ), the cursor is moved to insert row as shown below:
© Copyright Virtual University of Pakistan 155 Web Design and Development (CS506) •
Now, by calling various updaters, we can insert values into the columns of insert row as
shown below.
16.1.2.11 insertRow( )
•
•
•
Inserts the contents of the current row into this ResultSet object and into the database too.
Moves the cursor back to the position where it was before calling
moveToInsertRow()
This is shown in the given below diagram
Note: The cursor must be on the insert row before calling this method or exception
would be raised.
Example Code 16. 3: Inserting new row
The following code example illustrates how to add/insert new row into the
ResultSet as well into the database.
This code is the modification of the last one. Changes made are shown in bold face.
© Copyright Virtual University of Pakistan 156 Web Design and Development (CS506) 1. // File ResultSetEx.java
2. import java.sql.*;
3. public class ResultSetEx {
4. public static void main (String args[ ]) {
5. try {
6. //Step 2: load driver
7. Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
8. //Step 3: define the connection URL
9. String url = “jdbc:odbc:personDSN”;
10.//Step 4: establish the connection
11.Connection con = DriverManager.getConnection(url);
12.//Step 5: create PrepareStatement by passing sql and
13.// ResultSet appropriate fields
14.String sql = “SELECT * FROM Person”;
15.PreparedStatement pStmt = con.prepareStatement(sql,
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_UPDATABLE);
18.//Step 6: execute the query
19.ResultSet rs = pStmt.executeQuery();
20. // moving cursor to insert row
21. rs.moveToInsertRow();
22. // updating values in insert row
23. rs.updateString( “Name”
,
“imitiaz” );
24. rs.updateString( “Address” ,
“cantt”
);
25. rs.updateString( “phoneNum” ,
“9201211” );
26. // inserting row in resultset & into database
27. rs.insertRow( );
28. //Step 8: close the connection
29. con.close();
30. }catch(Exception sqlEx){
31. System.out.println(sqlEx);
32. }
33. } // end main
34. } // end class © Copyright Virtual University of Pakistan 157 Web Design and Development (CS506) Compile & Execute
Given below are two states of Person table. Note that after executing program, a newly added row
is present.
Person table: Before execution
Person table: After execution
16.1.2.12 last( ) & first( )
•
•
Moves the cursor to the last & first row of the ResultSet object respectively.
Throws exception if the ResultSet is TYPE_FORWARD_ONLY
16.1.2.13 getRow( )
•
•
Returns the current row number
As mentioned earlier, the first row has index 1 and so on.
16.1.2.14 deleteRow( )
•
•
Deletes the current row from this ResultSet object and from the underlying database.
Throws exception if the cursor is on the insert row.
© Copyright Virtual University of Pakistan 158 Web Design and Development (CS506) Example Code 16. 4: Deleting existing row
The given below example code shows the usage of last(
deleteRow( ) method.
), getRow( )and
This code is also the modification of the last one. Changes made are shown in bold face.
1.
2.
3.
4.
5.
// File ResultSetEx.java
import java.sql.*;
public class ResultSetEx {
public static void main (String args[ ]) {
try {
6.
7.
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
8.
9.
//Step 3: define the connection URL
String url = “jdbc:odbc:personDSN”;
10.
11.
//Step 4: establish the connection
Connection con = DriverManager.getConnection(url);
12.
//Step 5: create PrepareStatement by passing sql and
13.
14.
//
ResultSet appropriate fields
String sql =
“SELECT * FROM Person”;
15.
PreparedStatement pStmt = con.prepareStatement(sql,
16.
17.
ResultSet.TYPE_SCROLL_INSENSITIVE,
ResultSet.CONCUR_UPDATABLE);
18.
19.
//Step 6: execute the query
ResultSet rs = pStmt.executeQuery();
20.
21.
// moves to last row of the resultset
rs.last();
22.
23.
// retrieving the current row number
int rNo = rs.getRow();
24.
System.out.println(“current row number” + rNo);
25.
26.
27.
28.
// delete current row from rs & db i.e. 4 because
// previously we have called last() method
rs.deleteRow( );
//Step 8: close the connection
© Copyright Virtual University of Pakistan 159 Web Design and Development (CS506) 29.
30.
31.
32.
con.close();
}catch(Exception sqlEx){
System.out.println(sqlEx);
}
33.
} // end main
34.
} // end class
Compile & Execute
The first diagram shows the Person table before execution.
Person table: Before execution
Execution program from command prompt will result in displaying current row number on
console. This can be confirmed from following diagram.
Executing Program from Command Prompt
After execution, the last row (4) is deleted from ResultSet as well as from
database. The Person table is shown after execution
© Copyright Virtual University of Pakistan 160 Web Design and Development (CS506) Person table: After execution
16.2 References:
•
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web
Design and Development and not for any other commercial purpose without the consent
of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 161 Web Design and Development (CS506) Lecture 17: Meta Data
In simple terms, Meta Data is data (information) about data. The actual data has no meaning
without existence of Meta data. To clarify this, let’s look at an example. Given below are listed
some numeric values
1000
2000
4000
What this information about? We cannot state accurately. These values might be
representing some one’s salaries, price, tax payable & utility bill etc. But if we specify Meta data
about this data like shown below:
Salary
1000
2000
4000
Now, just casting a glance on these values, you can conclude that it’s all about some ones
salaries.
17.1 ResultSet Meta data
ResultSet Meta Data will help you in answering such questions
•
•
•
•
•
•
How many columns are in the ResultSet?
What is the name of given column?
Are the column name case sensitive?
What is the data type of a specific column?
What is the maximum character size of a column?
Can you search on a given column?
17.1.1 Creating ResultSetMetaData object
From a
ResultSet (the return type of executeQuery()), derive a
ResultSetMetaData object by calling getMetaData() method as shown in the given code
snippet (here rs is a valid ResultSet object):
ResultSetMetaData rsmd = rs.getMetaData();
Now, rsmd can be used to look up number, names & types of columns.
© Copyright Virtual University of Pakistan 162 Web Design and Development (CS506) 17.1.2 Useful ResultSetMetaData methods
17.1.2.1 getColumnCount ( )
•
Returns the number of columns in the result set
17.1.2.2 getColumnDisplaySize (int)
•
Returns the maximum width of the specified column in characters
17.1.2.3 getColumnName(int) / getColumnLabel (int)
•
•
The getColumnName() method returns the database name of the column
The getColumnLabel() method returns the suggested column label for printouts
17.1.2.4 getColumnType (int)
•
Returns the SQL type for the column to compare against types in java.sql.Types
Example Code 17.1: Using ResultSetMetaData
The MetaDataEx.java will print the column names by using ResultSetMetaData
object and column values on console. This is an excellent example of the scenario where we
have no idea about the column names in advance.
Note: For this example code and for the coming ones, we are using the same database
(PersonInfo) the one we created earlier and repeatedly used. Changes are shown in bold
face
44.
45.
46.
47.
48.
49.
50.
51.
52.
53.
54.
55.
56.
57.
58.
// File MetaDataEx.java
import java.sql.*;
public class MetaDataEx {
public static void main (String args[ ]) {
try {
//Step 2: load driver
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
//Step 3: define the connection URL
String url = “jdbc:odbc:personDSN”;
//Step 4: establish the connection
Connection con = null;
con = DriverManager.getConnection(url, “”, “”);
//Step 5: create PrepareStatement by passing sql and
//
ResultSet appropriate fields
String sql =
“SELECT * FROM Person”;
© Copyright Virtual University of Pakistan 163 Web Design and Development (CS506) 59. PreparedStatement pStmt = con.prepareStatement(sql,
60.
ResultSet.TYPE_SCROLL_INSENSITIVE,
61.
ResultSet.CONCUR_UPDATABLE);
62. //Step 6: execute the query
63. ResultSet rs = pStmt.executeQuery();
64. // get ResultSetMetaData object from rs
65. ResultSetMetaData rsmd = rs.getMetaData( );
66. // printing no. of column contained by rs
67. int numColumns = rsmd.getColumnCount();
68. System.out.println(“Number of Columns:” + numColumns);
69. // printing all column names by using for loop
70. String cName;
71. for(int i=1; i<= numColumns; i++) {
72. cName = rsmd.getColumnName(i);
73. System.out.println(cName);
74. System.out.println(“\t”);
75. }
76. // changing line or printing an empty string
77. System.out.println(“ ”);
78. // printing all values of ResultSet by iterating over it
79. String id, name, add, ph;
80. while( rs.next() )
81. {
82. id
= rs.getString(1);
83. name = rs.getString(2);
84. add = rs.getString(3);
85. ph
= rs.getString(4);
86. System.out.println(id);
87. System.out.println(“\t”);
88. System.out.println(name);
89. System.out.println(“\t”);
90. System.out.println(add);
91. System.out.println(“\t”);
92. System.out.println(ph);
93. System.out.println(“ ”);
94. }
95. //Step 8: close the connection
96. con.close();
97. }catch(Exception sqlEx){
98. System.out.println(sqlEx); }
99. } // end main
100.} // end class
© Copyright Virtual University of Pakistan 164 Web Design and Development (CS506) Compile & Execute:
The database contains the following values at the time of execution of this program. The database
and the output are shown below:
17.2 DatabaseMetaData
Database Meta Data will help you in answering such questions
•
•
•
•
•
What SQL types are supported by DBMS to create table?
What is the name of a database product?
What is the version number of this database product?
What is the name of the JDBC driver that is used?
Is the database in a read-only mode?
17.2.1 Creating DatabaseMetaData object
From a Connection object, a DataBaseMetaData object can be derived. The
following code snippet demonstrates how to get DataBaseMetaData object.
Connection con= DriverManager.getConnection(url, usr, pwd);
DatabaseMetaData dbMetaData = con.getMeataData();
© Copyright Virtual University of Pakistan 165 Web Design and Development (CS506) Now, you can use the dbMetaData to gain information about the database.
17.2.2 Useful ResultSetMetaData methods
17.2.2.1 getDatabaseProductName( )
• Returns the name of the database’s product name
17.2.2.2 getDatabaseProductVersion( )
• Returns the version number of this database product
17.2.2.3 getDriverName( )
• Returns the name of the JDBC driver used to established the connection
17.2.2.4 isReadOnly( )
•
•
Retrieves whether this database is in read-only mode
Returns true if so, false otherwise
Example Code 17.2: using DatabaseMetaData
This code is modification of the example code 17.1. Changes made are shown in bold
face.
102. // File MetaDataEx.java
103. import java.sql.*;
104. public class MetaDataEx {
105.
public static void main (String args[ ]) {
106.
try {
107.
//Step 2: load driver
108.
Class.forName(“sun.jdbc.odbc.JdbcOdbcDriver”);
109.
//Step 3: define the connection URL
110.
String url = “jdbc:odbc:personDSN”;
111.
//Step 4: establish the connection
112.
Connection con = null;
113.
con = DriverManager.getConnection(url, “”, “”);
114.
// getting DatabaseMetaDat object
115.
DatabaseMetaData dbMetaData = con.getMetaData();
116.
117.
118.
119.
120.
121.
122.
123.
124.
// printing database product name
String pName = dbMetaData.getDatabaseProductName();
System.out.println(“DataBase: ” + pName);
// printing database product version
String pVer = dbMetaData.getDatabaseProductVersion();
System.out.println(“Version: ” + pVer);
// printing driver name used to establish connection &
// to retrieve data
String dName = dbMetaData.getDriverName();
© Copyright Virtual University of Pakistan 166 Web Design and Development (CS506) 125.
126.
127.
128.
System.out.println(“Driver: ” + dName);
// printing whether database is read-only or not
boolean rOnly = dbMetaData.isReadOnly();
System.out.println(“Read-Only: ” + rOnly);
129.
130.
131.
132.
133.
134.
135.
136.
137.} //
// you can create & execute statements and can
// process results over here if needed
//Step 8: close the connection
con.close();
}catch(Exception sqlEx){
System.out.println(sqlEx);
}
} // end main
end class
Compile & Execute
On executing the above program, the following output will produce:
17.3 JDBC Driver Types
•
•
•
JDBC Driver Types are divided into four types or levels.
Each type defines a JDBC driver implementation with increasingly higher level of
platform independence, performance, deployment and administration.
The four types are:
o Type - 1: JDBC - ODBC Bridge
o Type 2: Native - API/partly Java driver
o Type 3: Net - protocol/all-Java driver
o Type 4: Native - protocol/all-Java driver
Now, let’s look at each type in more detail
© Copyright Virtual University of Pakistan 167 Web Design and Development (CS506) 17.3.1 Type - 1: JDBC - ODBC Bridge
•
•
•
Translates all JDBC calls into ODBC (Open Database Connectivity) calls and
send them to the ODBC Driver
Generally used for Microsoft database.
Performance is degraded
17.3.2 Type - 2: Native - API/partly Java driver
•
•
•
Converts JDBC calls into database-specific calls such as SQL Server, Informix,
Oracle or Sybase.
Partly-Java drivers communicate with database-specific API (which may be in
C/C++) using the Java Native Interface.
Significantly better Performance than the JDBC-ODBC bridge.
17.3.3 Type - 3: Net - protocol/all-Java driver4
•
•
•
Follows a three-tiered approach whereby the JDBC database requests () are
passed
through the network to the middle-tier server
Pure Java client to server drivers which send requests that are not databasespecific to a server that translates them into a database-specific protocol.
If the middle-tier server is written in java, it can use a type 1or type 2JDBC driver to do
this
© Copyright Virtual University of Pakistan 168 Web Design and Development (CS506) 17.3.4 Type - 4: Native - protocol / all - java driver
•
•
•
Converts JDBC calls into the vendor-specific DBMS protocol so that client
application can communicate directly with the database server
Completely implemented in Java to achieve platform independence and eliminate
deployment issues.
Performance is typically very good
17.4 Online Resources
•
•
•
•
•
Sun’s JDBC Site
http://java.sun.com/products/jdbc/
JDBC Tutorial
http://java.sun.com/docs/books/tutorial/jdbc/
List of available JDBC Drivers
http://industry.java.sun.com/products/jdbc/drivers/
RowSet Tutorial
http://java.sun.com/developer/Books/JDBCTutorial/chapter5.html
JDBC RowSets Implementation Tutorial
http://java.sun.com/developer/onlineTraining/ Database/jdbcrowsets.pdf
17.5 References:
•
•
•
•
Java API documentation 5.0
Java - A Lab Course by Umair Javed
JDBC drivers in the wild
http://www.javaworld.com/javaworld/jw-07-2000/jw-0707-jdbc_p.html
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 169 Web Design and Development (CS506) Lecture 18: Java Graphics
18.1 Painting
Window is like a painter’s canvas. All window paints on the same surface. More
importantly, windows don’t remember what is under them. There is a need to repaint when
portions are newly exposed.
Java components are also able to paint themselves. Most of time, painting is done
automatically. However sometimes you need to do drawing by yourself.
18.1.1 How painting works?
Let’s take windows example. Consider the following diagram in which the blue area is
representing the desktop. The one frame (myApp) is opened in front of desktop with some
custom painting as shown below.
myApp consist of a JPanel. The JPanel contains a JButton. Two rectangles, a circle & a lines are
also drawn on the JPanel.
© Copyright Virtual University of Pakistan 170 Web Design and Development (CS506) After opening notepad and windows explorer window, diagram will look like this:
Lets shuts off the windows explorer, the repaint event is sent to desktop first and then to myApp.
The figure shown below describes the situation after desktop repaint event get executed. Here
you can clearly see that only desktop repaints itself and window explorer remaining part is still
opened in front of myApp.
The following figure shows the situation when myApp’s JPanel calls its repaint method. Notice
that some portion of window explorer is still remains in front of JButton because yet not repaint event
is sent to it.
© Copyright Virtual University of Pakistan 171 Web Design and Development (CS506) Next, JPanel forwards repaint event to JButton that causes the button to be displayed in its
original form.
This is all done automatically and we cannot feel this process cause of stunning speed of modern
computers that performs all these steps in flash of eye.
18.1.2 Painting a Swing Component
Three methods are at the heart of painting a swing component like JPanel etc. For
instance, paint() gets called when it's time to render -- then Swing further factors the paint()
call into three separate methods, which are invoked in the following order:
© Copyright Virtual University of Pakistan 172 Web Design and Development (CS506) protected void paintComponent(Graphics g)
protected void paintBorder(Graphics g)
protected void paintChildren(Graphics g)
Let’s look at these methods in order in which they get executed
18.1.2.1 paintComponent( )
•
•
•
It is a main method for painting
By default, it first paints the background
After that, it performs custom painting (drawing circle, rectangles etc.)
18.1.2.2 paintBorder( )
•
•
Tells the components border (if any) to paint.
It is suggested that you do not override or invoke this method
18.1.2.3 paintChildren( )
•
•
Tells any components contained by this component to paint themselves
It is suggested that you do not override or invoke this method too.
Example: Understanding methods calls
Consider the following figure
The figure above illustrates the order in which each component that inherits from
JComponent paint itself.
Figure 1 to 2 -painting the background and performing custom painting is performed by the
paintComponent method
In Figure 3 - paintBorder is get called
© Copyright Virtual University of Pakistan 173 Web Design and Development (CS506) And finally in figure 4 - paintChildern is called that causes the JButton to render itself.
Note: The important thing to note here is for JButton (since it is a JComponent), all these methods
are also called in the same order.
Your Painting Strategy
•
You must follow the three steps in order to perform painting.
Subclass JPanel
•
•
class MyPanel extends JPanel
Doing so MyPanel also becomes a JPanle due to inheritance
Override the paintComponent(Graphics g) method
•
Inside method using graphics object, do whatever drawing you want to do
Install that JPanel inside a JFrame
•
•
When frame becomes visible through the paintChildren() method your panel become
visible
To become visible your panel will call paintComponent() method which will do your
custom drawing
Example Code 18.1:
Suppose we want to draw one circle & rectangle and a string “Hello World”.
// importing required packages
import javax.swing.*;
import java.awt.*;
// extending class from JPanel
public class MyPanel extends JPanel {
// overriding paintComponent method
public void paintComponent(Graphics g){
// erasing behaviour - this will clear all the
// previous painting
super.paintComponent(g);
© Copyright Virtual University of Pakistan 174 Web Design and Development (CS506) // Down casting Graphics object to Graphics2D
Graphics2D g2 = (Graphics2D)g;
// drawing rectanle
g2.drawRect(20,20,20,20);
// changing the color to blue
g2.setColor(Color.blue);
// drawing filled oval with color i.e. blue
g2.fillOval(50,50,20,20);
// drawing string
g2.drawString("Hello World", 120, 50);
}// end paintComponent
} // end Test class
The Test class that contains the main method as well uses MyPanel (previously built) class is given
below:
// importing required packages
import javax.swing.*;
import java.awt.*;
public class Test {
JFrame f;
// declaring Reference of MyPanel class
MyPanel p;
// parameter less constructor public Test(){
f = new JFrame();
Container c = f.getContentPane();
c.setLayout(new BorderLayout());
// instantiating reference
p = new MyPanel();
// adding MyPanel into container
c.add(p);
f.setSize(400,400);
f.setVisible(true);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
} // end constructor
// main method
public static void main(String args[ ]){
Test t = new Test();
}
} // end Test class
Note: Here we have used only some methods (drawRect( ) & fillOval( ) etc. ) of Graphics class.
For a complete list, see the Java API documentation.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 175 Web Design and Development (CS506) Lecture 19: How to Animate?
If we want to animate something like ball, moving from one place to another, we
constantly need to call paintComponent( ) method and to draw the shape (ball etc.) at new place
means at new coordinates.
Painting is managed by system, so calling paintComponent() directly is not recommended at all.
Similarly calling paint( ) method is also not recommended. Why? Because such code may be
invoked at times when it is not appropriate to paint -- for instance, before the component is
visible or has access to a valid Graphics object.
Java gives us a solution in the form of repaint( ) method. Whenever we need to repaint, we call
this method that in fact makes a call to paint( ) method at appropriate time.
19.1 Problem & Solution
•
•
•
•
•
•
What to do to move the shapes present in example code 18.1 (last example) when a mouse
is dragged
First time painting is what we already have done
When a mouse is clicked find the co-ordinates of that place and paint Rectangle at that
place by requesting, using repaint() call
Here instead of Hard-coding the position of co-ordinates uses some variables. For example
mx , my
o In the last example code, we draw a rectangle by passing hard-coded values like
20
g.drawRect(20,20,20,20);
o Now, we’ll use variables so that change in a variable value causes to display a
rectangle at a new location
g.drawRect(mx,my,20,20);
Similarly, you have seen a tennis game (during lecture). Now, what to do code the paddle
movement.
In the coming up example. We are doing it using mouse, try it using mouse.
Example Code 19.1
The following outputs were produced when mouse is dragged from one location to anther
© Copyright Virtual University of Pakistan 176 Web Design and Development (CS506) First we examine the MyPanel.java class that is drawing a filled rectangle.
import javax.swing.*;
import java.awt.*;
// extending class from JPanel
public class MyPanel extends JPanel {
// variables used to draw rectangles at different locations
int mX = 20;
int mY = 20;
// overriding paintComponent method
public void paintComponent(Graphics g){
// erasing behaviour - this will clear all the previous painting
super.paintComponent(g);
// Down casting Graphics object to Graphics2D
Graphics2D g2 = (Graphics2D)g;
// changing the color to blue
g2.setColor(Color.blue);
// drawing filled oval with color i.e. blue
// using instance variables
g2.fillRect(mX,mY,20,20);
}// end paintComponent
} // end MyPanel class
The Test class is given below. Additionally this class also contains the code for handling mouse
events.
// importing required packages
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class Test {
JFrame f;
// declaring Reference of MyPanel class
MyPanel p;
// parameter less constructor
public Test(){
f = new JFrame();
Container c = f.getContentPane();
c.setLayout(new BorderLayout());
// instantiating reference
p = new MyPanel();
// adding MyPanel into container
c.add(p);
f.setSize(400,400);
f.setVisible(true);
© Copyright Virtual University of Pakistan 177 Web Design and Development (CS506) f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// creating inner class object
Handler h = new Handler();
// registering MyPanel to handle events
p.addMouseMotionListener(h);
} // end constructor
// inner class used for handling events
public class Handler extends MouseMotionAdapter{
// capturing mouse dagged events
public void mouseDragged(MouseEvent me){
// getting the X-Position of mouse and assigning
// value to instance variable mX of MyPanel class
p.mX = me.getX();
// getting the Y-Position of mouse and assigning
// value to instance variable mX of MyPanel class
p.mY = me.getY();
// call to repaint causes rectangle to be drawn on
// new location
p.repaint() ;
} // end mouseDragged
} // end Handler class
// main method
public static void main(String args[ ]){
Test t = new Test();
}
} // end MyPanel class
On executing this program, when you drag mouse from one location to another, rectangle
is also in sync with the movement of mouse. Notice that previously drawn rectangle is
erased first.
If we exclude or comment out the following line from MyPanel class
super.paintComponent(g);
Dragging a mouse will produce a similar kind of output shown next
© Copyright Virtual University of Pakistan 178 Web Design and Development (CS506) Example Code 19.2: Ball Animation
The ball is continuously moving freely inside the corner of the frames. The sample outputs
are shown below:
First we examine the MyPanel.java class that is drawing a filled oval.
import javax.swing.*;
import java.awt.*;
// extending class from JPanel
public class MyPanel extends JPanel {
// variables used to draw oval at different locations
int mX = 200;
int mY = 0;
// overriding paintComponent method
public void paintComponent(Graphics g){
// erasing behaviour - this will clear all the
// previous painting
super.paintComponent(g);
// Down casting Graphics object to Graphics2D
Graphics2D g2 = (Graphics2D)g;
// changing the color to blue
g2.setColor(Color.blue);
// drawing filled oval with blue color
// using instance variables
g2.fillOval(mX,mY,20,20);
}// end paintComponent
} // end MyPanel class
The Test class is given below. Additionally this class also contains the code for handling mouse
events.
// importing required packages
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class AnimTest implements ActionListener {
JFrame f;
© Copyright Virtual University of Pakistan 179 Web Design and Development (CS506) MyPanel p;
// used to control the direction of ball
int x, y;
public AnimTest(){
f = new JFrame();
Container c = f.getContentPane();
c.setLayout(new BorderLayout());
x = 5;
y = 3;
p = new MyPanel(); c.add(p);
f.setSize(400,400);
f.setVisible(true);
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
// creating a Timer class object, used for firing
// one or more action events after a specified delay
// Timer class constructor requires time in
// milliseconds and object of class that handles
// action events
Timer t = new Timer (5, this);
// starts the timer, causing it to start sending
// action events to listeners
t.start();
} // end constructor
// event handler method
public void actionPerformed(ActionEvent ae){
// if ball reached to maximum width of frame minus
// 40 since diameter of ball is 40 then change the
// X-direction of ball
if (f.getWidth()-40 == p.mX)
x = -5;
// if ball reached to maximum height of frame
// minus 40 then change the Y-direction of ball
if (f.getHeight()-40 == p.mY)
y = -3;
// if ball reached to min. of width of frame,
// change the X-direction of ball
if (p.mX == 0 )
x = 5;
// if ball reached to min. of height of frame,
// change the Y-direction of ball
if (p.mY == 0 )
y = 3;
// Assign x,y direction to MyPanel’s mX & mY
p.mX += x;
p.mY += y;
// call to repaint() method so that ball is drawn on
© Copyright Virtual University of Pakistan 180 Web Design and Development (CS506) // new locations
p.repaint();
} // end actionPerformed() method
// main method
public static void main(String args[ ]){
AnimTest at = new AnimTest();
}
} // end AnimTest class
19.2 References
•
Java, A Lab Course by Umair Javed
•
Painting in AWT & Swing
http://java.sun.com/products/jfc/tsc/articles/painting/index.html
•
Performing Custom Painting
http://java.sun.com/docs/books/tutorial/uiswing/14painting/index.html
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 181 Web Design and Development (CS506) Lecture 20: Applets
20.1 Basic Definition
•
•
•
•
•
•
A small program written in Java and included in a HTML page.
It is independent of the operating system on which it runs
An applet is a Panel that allows interaction with a Java program
A applet is typically embedded in a Web page and can be run from a browser
You need special HTML in the Web page to tell the browser about the applet
For security reasons, applets run in a sandbox: they have no access to the client’s file
system
20.2 Applets Support
•
•
•
Most modern browsers support Java 1.4 if they have the appropriate plugin
Sun provides an application appletviewer to view applets without using browser.
In general you should try to write applets that can be run with any browser
20.3 What an Applet is?
•
•
•
•
You write an applet by extending the class Applet or JApplet
Applet is just a class like any other; you can even use it in applications if you want
When you write an applet, you are only writing part of a program
The browser supplies the main method
20.4 The genealogy of Applet
The following figure shows the inheritance hierarchy of the JApplet class. This hierarchy
determines much of what an applet can do and how, as you'll see on the next few pages.
© Copyright Virtual University of Pakistan 182 Web Design and Development (CS506) Example Code 20.1: Writing a Simple Applet
Below is the source code for an applet called HelloApplet. This displays a “Hello World”
string. Note that no main method has been provided.
// File HelloApplet.java
//step 1: importing required packages
import java.awt.*;
import javax.swing.*;
// extending class from JApplet so that our class also becomes an
//applet
public class HelloApplet extends JApplet {
// overriding paint method
public void paint(Graphics g) {
// write code here u want to display & draw by using
// Graphics object
g.drawString(“Hello World”, 30 , 30);
}
} // end class
After defining the HelloApplet.java, the next step is to write .html file. Below is
the source code of Test.html file. The Test.html contains the ordinary html code except
one.
<html>
<head>
<title> Simple Applet </title> </head>
<body>
<!-- providing the class name of applet with width &
height
-->
<applet code="HelloApplet.class”
width=150 height=100>
</applet>
</body>
</html>
© Copyright Virtual University of Pakistan 183 Web Design and Development (CS506) Compile & Execute
By simply double clicking on Test.html file, you can view the applet in your browser.
However, you can also use the appletviewer java program for executing or running
applets.
The
applet
viewer
is
invoked
from
the
command
line
by
the
command
appletviewer htmlfile
where htmlfile is the name of the file that contains the html document. For our example, the
command looks like this:
appletviewer Test.html
As a result, you will see the following output
20.5 Applet Life Cycle Methods
When an applet is loaded, an instance of the applet's controlling class (an Applet
subclass) is created. After that an applet passes through some stages or methods, each of them are
build for specific purpose.
An applet can react to major events in the following ways:
•
•
•
•
It can initialize itself.
It can start running.
It can stop running.
It can perform a final cleanup, in preparation for being unloaded
The applet’s life cycle methods are called in the specific order shown below. Not every applet
needs to override every one of these methods.
© Copyright Virtual University of Pakistan 184 Web Design and Development (CS506) Let’s take a look on each method in detail and find out what they do:
20.5.1 init( )
•
•
•
Is called only once.
The purpose of init( ) is to initialize the applet each time it's loaded (or reloaded).
You can think of it as a constructor
20.5.2 start( )
•
•
•
To start the applet's execution
For example, when the applet's loaded or when the user revisits a page that contains the
applet
start( ) is also called whenever the browser is maximized
20.5.3 paint( )
•
•
•
paint( ) is called for the first time when the applet becomes visible
Whenever applet needs to be repainted, paint( ) is called again
Do all your painting in paint( ), or in a method that is called from paint( )
20.5.4 stop( )
•
•
To stop the applet's execution, such as when the user leaves the applet's page or quits the
browser.
stop( ) is also called whenever the browser is minimized
© Copyright Virtual University of Pakistan 185 Web Design and Development (CS506) 20.5.5 destroy( )
•
•
Is called only once.
To perform a final cleanup in preparation for unloading
Example Code 20.2: Understanding Applet Life Cycle Methods
The following code example helps you in understanding the calling sequence of applet’s
life cycle methods. These methods are only displaying debugging statements on the
console.
// File AppletDemo.java
//step 1: importing required packages
import java.awt.*;
import javax.swing.*;
// extending class from JApplet so that our class also becomes an
//applet
public class AppletDemo extends JApplet {
// overriding init method
public void init ( ) {
System.out.println("init() called");
}
// overriding start method
public void start ( ){
System.out.println("start() called");
}
// overriding paint method
public void paint(Graphics g){
System.out.println("paint() called");
}
// overriding stop method
public void stop(){
System.out.println("stop() called");
}
// overriding destroy method
public void destroy(){
System.out.println("destroy() called");
}
} // end class
The DemoTest.html file is using this applet. The code snippet of it given below:
<html>
<head>
<title> Applet Life Cycle Methods </title> </head>
<body>
© Copyright Virtual University of Pakistan 186 Web Design and Development (CS506) <!-- providing the class name of applet with width &
height
-->
<applet code="AppletDemo.class”
width=150 height=100>
</applet>
</body>
</html>
Compile & Execute
To understand the calling sequence of applet life cycle methods, you have to execute it by using
appletviewer command. Do experiments like maximizing, minimizing the applet,
bringing another window in front of applet and keep an eye on console output.
Example Code 20.3: Animated Java Word
Sample Output
The browser output of the program is given below:
Design Process
• The Program in a single call of paint method
o Draws string “java” on 40 random locations
o For every drawing, it selects random font out of 4 different fonts
o For every drawing, it selects random color out of 256 * 256 * 256 RGB colors
© Copyright Virtual University of Pakistan 187 Web Design and Development (CS506) •
•
Repaint is called after every 1000 ms.
After 10 calls to repaint, screen is cleared
Generating Random Numbers
•
Use static method random of Math class
o Math.random() ;
•
•
•
Returns positive double value greater than or equal to 0.0 or less than 1.0.
Multiply the number with appropriate scaling factor to increase the range and type cast it,
if needed.
o int i = (int)( Math.random() * 5 ); // will generate random numbers between 0 & 4.
Program’s Modules
The program is build using many custom methods. Let’s discuss each of them one
by one that will help in understanding the overall logic of the program.
•
drawJava( )
As name indicates, this method will be used to write String “java” on random
locations. The code is given below:
// method drawJava
public void drawJava(Graphics2D g2) {
// generate first number randomly. The panel width is 1000
int x = (int) (Math.random() * 1000);
// generate second number randomly. The panel height is 700
int y = (int) (Math.random() * 700);
// draw String on these randomly selected numbers
g2.drawString("java", x, y);
}
•
chooseColor( )
This method will choose color randomly out of 256 * 256 * 256 possible colors. The code
snippet is given below:
// method chooseColor
public Color chooseColor() {
© Copyright Virtual University of Pakistan 188 Web Design and Development (CS506) // choosing red color value randomly
int r = (int) (Math.random() * 255);
// choosing green color value randomly
int g = (int) (Math.random() * 255);
// choosing blue color value randomly
int b = (int) (Math.random() * 255);
// constructing a color by providing R-G-B values
Color c = new Color(r, g, b);
// returning color
return c;
}
•
chooseFont( )
This method will choose a Font for text (java) to be displayed out of 4 available fonts. The code
snippet is given below:
// method chooseFont
public Font chooseFont() {
// generating a random value that helps in choosing a font
int fontChoice = (int) (Math.random() * 4) + 1;
// declaring font reference
Font f = null;
// using switch based logic for selecting font
switch (fontChoice) {
case 1:
f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);
break;
case 2:
f = new Font("SansSerif", Font.PLAIN, 17);
break;
case 3:
f = new Font("Monospaced", Font.ITALIC, 23);
break;
case 4:
f = new Font("Dialog", Font.ITALIC, 30);
© Copyright Virtual University of Pakistan 189 Web Design and Development (CS506) break;
} // end switch
// returns Font object
return f;
} //end chooseFont
•
paint( )
The last method to be discussed here is paint(). By overriding this method, we will print string
“java” on 40 random locations. For every drawing, it selects random font out of 4 different
fonts & random color out of 256 * 256 * 256 RGB colors.
Let’s see, how it happens:
// overriding method paint
public void paint(Graphics g) {
// incrementing clear counter variable.
clearCounter++;
// printing 40 “java” strings on different locations by
// selcting random font & color
for (int i = 1; i <= 40; i++) {
// choosing random color by calling chooseColor() method
Color c = chooseColor();
// setting color
g2.setColor(c);
// choosing random Font by calling chooseColor() method
Font f = chooseFont();
g2.setFont(f);
// drawing string “java” by calling drawJava() method
drawJava(g2);
}
// end for loop
Graphics2D g2 = (Graphics2D) g;
// checking if paint is called 10 times then clears the
// screen and set counter again to zero
if (clearCounter == 10) {
g2.clearRect(0, 0, 1000, 700);
clearCounter = 0;
}
} // end paint method
© Copyright Virtual University of Pakistan 190 Web Design and Development (CS506) Merging Pieces
By inserting all method inside JavaAnim.java class, the program will look like one given below.
Notice that it contains methods discussed above with some extra code with which you are already
familiar.
// File JavaAnim.java
//step 1: importing required packages
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
public class JavaAnim extends JApplet implements ActionListener {
// used to count how many times paint is called
int clearCounter;
// declaring Timer reference
Timer t;
// overriding init method, used to initialize variables
public void init() {
setBackground(Color.black);
clearCounter = 0;
Timer t = new Timer(1000, this);
t.start();
}
// overriding paint method - discussed above
public void paint(Graphics g) {
clearCounter++;
Graphics2D g2 = (Graphics2D) g;
if (clearCounter == 10) {
g2.clearRect(0, 0, 1000, 700); clearCounter = 0;
}
for (int i = 1; i <= 40; i++) {
Color c = chooseColor(); g2.setColor(c);
Font f = chooseFont(); g2.setFont(f);
drawJava(g2);
}
}
// overriding actionPerformed()of ActionListener interface
// called by Timer object
public void actionPerformed(ActionEvent ae) {
© Copyright Virtual University of Pakistan 191 Web Design and Development (CS506) repaint();
}
// chooseColor method - discussed above
public Color chooseColor() {
int r = (int) (Math.random() * 255);
int g = (int) (Math.random() * 255);
int b = (int) (Math.random() * 255);
Color c = new Color(r, g, b);
return c;
} // chooseFont method - discussed above
public Font chooseFont() {
int fontChoice = (int) (Math.random() * 4) + 1;
Font f = null;
switch (fontChoice) {
case 1:
f = new Font("Serif", Font.BOLD + Font.ITALIC, 20);
break;
case 2:
f = new Font("SansSerif", Font.PLAIN, 17);
break;
case 3:
f = new Font("Monospaced", Font.ITALIC, 23);
break;
case 4:
f = new Font("Dialog", Font.ITALIC, 30);
break;
}
return f;
}
// drawJava() method - discussed above
public void drawJava(Graphics2D g2) {
int x = (int) (Math.random() * 1000);
int y = (int) (Math.random() * 700);
g2.drawString("java", x, y);
}
} // end class
The AnimTest.html file is using this applet. The code snippet of it given below:
<html>
<head>
<title> Animated Java Word </title> </head>
© Copyright Virtual University of Pakistan 192 Web Design and Development (CS506) <body>
<applet code="JavaAnim.class" width=1000 height=700> </applet>
</body>
</html>
Compile & Execute
You can execute it directly using browser or by using appletviewer application. For
having fun, you can use “your name” instead of “java” and watch it in different colors.
20.6 References:
•
•
Java, A Lab Course by Umair Javed
Writing Applets
o http://java.sun.com/docs/books/tutorial/applet/
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 193 Web Design and Development (CS506) Lecture 21: Socket Programming
21.1 Basic Definition
•
•
A socket is one endpoint of a two-way communication link between two programs
running generally on a network.
A socket is a bi-directional communication channel between hosts. A computer on a
network often termed as host.
21.2 Socket Dynamics
•
•
•
•
As you have already worked with files, you know that file is an abstraction of your
hard drive. Similarly you can think of a socket as an abstraction of the network.
Each end has input stream (to send data) and output stream (to receive data) wired up
to the other host.
You store and retrieve data through files from hard drive, without knowing the actual
dynamics of the hard drive. Similarly you send and receive data to and from network
through socket, without actually going into underlying mechanics.
You read and write data from/to a file using streams. To read and write data to socket,
you will also use streams.
21.3 What is Port?
•
•
•
•
It is a transport address to which processes can listen for connections request.
There are different protocols available to communicate such as TCP and UDP. We
will use TCP for programming in this handout
There are 64k ports available for TCP sockets and 64k ports available for UDP, so at
least theoretically we can open 128k simultaneous connections.
There are well-known ports which are
o below 1024
o provides standard services
o Some well-known ports are:
FTP works on port 21
HTTP works on port 80
TELNET works on port 23 etc.
21.4 How Client - Server Communicate
•
•
•
Normally, a server runs on a specific computer and has a socket that is bound to a
specific port number.
The server just waits, listening to the socket for a client to make a connection request.
On the client side: The client knows the hostname of the machine on which the server
is running and the port number to which the server is connected.
© Copyright Virtual University of Pakistan 194 Web Design and Development (CS506) •
•
•
•
•
•
•
•
As soon as client creates a socket that socket attempts to connect to the specified
server.
The server listens through a special kind of socket, which is named as server socket.
The sole purpose of the server socket is to listen for incoming request; it is not used
for communication.
If everything goes well, the server accepts the connection. Upon acceptance, the
server gets a new socket, a communication socket, bound to a different port number.
The server needs a new socket (and consequently a different port number) so that it
can continue to listen through the original server socket for connection requests while
tending to the needs of the connected client. This scheme is helpful when two or more
clients try to connect to a server simultaneously (a very common scenario).
On the server side, if the connection is accepted, a socket is successfully created and
the client can use the socket to communicate with the server.
Note that the socket on the client side is not bound to the port number used to make
contact with the server. Rather, the client is assigned a port number local to the
machine on which the client is running.
The client and server can now communicate by writing to or reading from their
sockets.
21.5 Steps - To Make a Simple Client
To make a client, process can be split into 5 steps. These are:
21.5.1 Import required package
You have to import two packages
•
•
java.net.*;
java.io.*;
21.5.2 Connect / Open a Socket with Server
Create a client socket (communication socket)
Socket s
= new Socket(“serverName”, serverPort) ;
© Copyright Virtual University of Pakistan 195 Web Design and Development (CS506) •
•
serverName: Name or address of the server you wanted to connect such as
http://www.google.com or 172.2.4.98 etc. For testing if you are running client and server
on the same machine then you can specify “localhost” as the name of server
serverPort : Port number you want to connect to
The scheme is very similar to our home address and then phone number.
21.5.3 Get I/O Streams of Socket
Get input & output streams connected to your socket
•
For reading data from socket
As stated above, a socket has input stream attached to it.
InputStream is = s.getInputStream();
// now to convert byte oriented stream into character oriented buffered reader
// we use intermediary stream that helps in achieving above stated purpose
InputStreamReader isr= new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
•
For writing data to socket
A socket has also output stream attached to it. Therefore,
OutputStream os = s.getOutputStream();
// now to convert byte oriented stream into character oriented print writer
// here we will not use any intermediary stream because PrintWriter constructor
// directly accepts an object of OutputStream
PrintWriter pw = new PrintWriter(os, true);
Here notice that true is also passed to so that output buffer will flush.
21.5.4 Send / Receive Message
Once you have the streams, sending or receiving messages isn’t a big task. It’s very much similar to
the way you did with files
•
To send messages
pw.println(“hello world”);
•
To read messages
String recMsg = br.readLine();
© Copyright Virtual University of Pakistan 196 Web Design and Development (CS506) 21.5.5 Close Socket
Don’t forget to close the socket, when you finished your work
s.close();
21.6 Steps - To Make a Simple Server
To make a server, process can be split into 7 steps. Most of these are similar to
steps used in making a client. These are:
21.6.1 Import required package
You need the similar set of packages you have used in making of client
•
•
java.net.*;
java.io.*;
21.6.2 Create a Server Socket
In order to create a server socket, you will need to specify port no eventually on which server will
listen for client requests.
ServerSocket ss = new ServerSocket(serverPort) ;
•
serverPort: port local to the server i.e. a free port on the server machine. This is the same
port number that is given in the client socket constructor
21.6.3 Wait for Incoming Connections
The job of the server socket is to listen for the incoming connections. This listening part is done
through the accept method.
Socket s = ss.accept();
The server program blocks ( stops ) at the accept method and waits for the incoming client
connection when a request for connection comes it opens a new communication socket (s) and use
this socket to communicate with the client.
21.6.4 Get I/O Streams of Socket
Once you have the communication socket, getting I/O streams from communication socket is
similar to the way did in making a client
© Copyright Virtual University of Pakistan 197 Web Design and Development (CS506) •
For reading data from socket
InputStream is = s.getInputStream();
InputStreamReader
isr=
new
InputStreamReader(is);
BufferedReader
br = new BufferedReader(isr);
•
For writing data to socket
OutputStream os = s.getOutputStream();
PrintWriter pw = new PrintWriter(os, true);
21.6.5 Send / Receive Message
Sending and receiving messages is very similar as discussed in making of client
•
To send messages:
pw.println(“hello world”);
•
To read messages
String recMsg = br.readLine();
21.6.6 Close Socket
s.close();
Example Code 21.1: Echo Server & Echo Client
The client will send its name to the server and server will append “hello” with the name send by
the client. After that, server will send back the name with appended “hello”.
EchoServer.java
Let’s first see the code for the server
// step 1: importing required package
import java.net.*;
import java.io.*;
import javax.swing.*;
public class EchoServer{
public static void main(String args[]){
try {
© Copyright Virtual University of Pakistan 198 Web Design and Development (CS506) //step 2: create a server socket
ServerSocket ss = new ServerSocket(2222);
System.out.println("Server started...");
/* Loop back to the accept method of the server
socket and wait for a new connection request. So
server will continuously listen for requests
*/
while(true) {
// step 3: wait for incoming connection
Socket s = ss.accept();
System.out.println("connection request recieved");
// step 4: Get I/O streams
InputStream is = s.getInputStream();
InputStreamReader isr= new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
OutputStream os = s.getOutputStream();
PrintWriter pw = new PrintWriter(os,true);
// step 5: Send / Receive message
// reading name sent by client
String name = br.readLine();
// appending “hello” with the received name
String msg = "Hello " + name + " from Server";
// sending back to client
pw.println(msg);
// closing communication sockey
s.close();
} // end while
}catch(Exception ex){
System.out.println(ex);
}
}
} // end class
EchoClient.java
The code of the client is given below
// step 1: importing required package
import java.net.*;
import java.io.*;
import javax.swing.*;
public class EchoClient{
© Copyright Virtual University of Pakistan 199 Web Design and Development (CS506) public static void main(String args[]){
try {
//step 2: create a communication socket
/* if your server will run on the same machine then you can pass
“localhost” as server address.Notice that port no is similar to
one passed while creating server socket */
Socket s = new Socket(“localhost”, 2222);
// step 3: Get I/O streams
InputStream is = s.getInputStream();
InputStreamReader isr= new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
OutputStream os = s.getOutputStream();
PrintWriter pw = new PrintWriter(os,true);
// step 4: Send / Receive message
// asking user to enter his/her name
String msg = JOptionPane.showInputDialog( "Enter your name");
// sending name to server
pw.println(msg);
// reading message (name appended with hello) from
// server
msg = br.readLine();
// displaying received message
JOptionPane.showMessageDialog(null , msg);
// closing communication socket
s.close();
}catch(Exception ex){
System.out.println(ex);
}
}
} // end class
Compile & Execute
After compiling both files, run EchoServer.java first, from the command prompt window. You’ll see
a message of “server started” as shown in the figure below. Also notice that cursor is continuously
blinking since server is waiting for client request
© Copyright Virtual University of Pakistan 200 Web Design and Development (CS506) Now, open another command prompt window and run EchoClient.java from it. Look at
EchoServer window; you’ll see the message of “request received”. Sooner, the
EchoClient program will ask you to enter name in input dialog box. After entering name
press ok button, with in no time, a message dialog box will pop up containing your name
with appended “hello” from server. This whole process is illustrated below in pictorial
form:
Sending name to server
© Copyright Virtual University of Pakistan 201 Web Design and Development (CS506) Response from server
Notice that server is still running, you can run again EchoClient.java as many times untill server is
running.
To have more fun, run the server on a different computer and client on a different. But
before doing that find the IP of the computer machine on which your EchoServer will
eventually run. Replace “localhost” with the new IP and start conversion over network
21.7 References
•
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web
Design and Development and not for any other commercial purpose without the consent
of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 202 Web Design and Development (CS506) Lecture 22: Serialization
22.1 Problem
22.1.1 What?
•
You want to send an object to a stream.
22.1.2 Motivation
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•
A lot of code involves boring conversion from a file to memory
o As you might recall that AddressBook program reads data from file and then parses
it
This is a common problem
22.1.3 Revisiting AddressBook
We read record from a text file named persons.txt. The person record was present in the file in the
following format:
Ali,defence,9201211
Usman,gulberg,5173940
Salman,LUMS,5272670
persons.txt
The code that was used to construct Person objects after reading information from the file is given
below. Here only the part of code is shown, for complete listing, see AddressBook code in your
earlier handout.
………………………
FileReader fr = new FileReader("persons.txt");
BufferedReader br = new BufferedReader(fr);
String line = br.readLine();
while ( line != null ) {
tokens = line.split(",");
name = tokens[0];
add
= tokens[1];
ph
= tokens[2];
PersonInfo p = new PersonInfo(name, add, ph);
// you can add p into arraylist, if needed
line = br.readLine();
}
© Copyright Virtual University of Pakistan 203 Web Design and Development (CS506) As you have seen a lot of parsing code is required for converting a line into PersonInfo
objects. Serialization mechanism eases developer’s life by achieving all above in a very
simple way.
22.2 Serialization in Java
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•
•
Java provides an extensive support for serialization
Object knows how to read or write themselves to streams
Problem:
o As you know, objects get created on heap and have some values therefore
Objects have some state in memory
o You need to save and restore that state.
o The good news is that java serialization takes care of it automatically
22.2.1 Serializable Interface
•
•
•
By implementing this interface a class declares that it is willing to be read/written by
automatic serialization machinery
Found in java.io package
Tagging interface - has no methods and serves only to identify the semantics of being
serializable
22.2.2 Automatic Writing
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•
System knows how to recursively write out the state of an object to stream
If an object has the reference of another object, the java serialization mechanism takes
care of it and writes it too.
22.2.3 Automatic Reading
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•
System knows how to read the data from Stream and re-create object in memory
The recreated object is of type “Object” therefore Down-casting is required to convert
it into actual type.
22.2.4 Serialization: How it works?
•
To write an object of PersonInfo, ObejctOutputStream and its method writeObject( ) will
be used
PersonInfo p = new PersonInfo( );
ObejctOutputStream out;
// writing PersonInfo’s object p
out.writeObject(p);
•
To read that
will be used
object
back,
© Copyright Virtual University of Pakistan ObejctInputStream
and
its
method
readObject()
204 Web Design and Development (CS506) ObejctInputStream in;
// reading PersonInfo’s object. Remember type
casting // is required
PersonInfo obj = (PersonInfo)in.readObject( );
Example Code 22.1: Reading / Writing PersonInfo objects
We want to send PersonInfo object to stream. You have already seen this class number of times
before. Here it will also implement serializable interface.
PersonInfo.java
import javax.swing.*;
import java.io.* ;
class PersonInfo implements Serializable{
String name;
String address;
String phoneNum;
//parameterized constructor
public PersonInfo(String n, String a, String p) {
name = n;
address = a;
phoneNum = p;
}
//method for displaying person record on GUI
public void printPersonInfo( ) {
JOptionPane.showMessageDialog(null ,
“name: ” + name + “address:” +address + “phone no:” +
phoneNum);
}
} // end class
WriteEx.java
The following class will serialize PersonInfo object to a file
import java.io.*;
public class WriteEx{
public static void main(String args[ ]){
PersonInfo pWrite = new PersonInfo("ali", "defence",
"9201211");
try {
// attaching FileOutput stream with “ali.dat”
FileOutputStream fos = new FileOutputStream("ali.dat");
// attaching ObjectOutput stream over FileOutput stream
ObjectOutputStream out = new ObjectOutputStream(fos);
//serialization
© Copyright Virtual University of Pakistan 205 Web Design and Development (CS506) // writing object to ‘ali.dat’
out.writeObject(pWrite);
// closing streams
out.close();
fos.close();
} catch (Exception ex){
System.out.println(ex);
}
}
}
// end class
ReadEx.java
The following class will read serialized object of PersonInfo from
file i.e “ali.data”
import java.io.*;
public class ReadEx{
public static void main(String args[ ]){
try {
// attaching FileInput stream with “ali.dat”
FileInputStream fis = new FileInputStream("ali.dat");
// attaching FileInput stream over ObjectInput stream
ObjectInputStream in = new ObjectInputStream(fis);
//de-serialization
// reading object from ‘ali.dat’
PersonInfo pRead = (PersonInfo)in.readObject( );
// calling printPersonInfo method to confirm that
// object contains same set of values before
// serializatoion
pRead.printPersonInfo();
// closing streams
in.close();
fis.close();
} catch (Exception ex){
System.out.println(ex); }
}//end main function
} // end class
Compile & Execute
After compilation, first run the WriteEx.java file and visit the “ali.dat” file. Then run
ReadEx.java from different command or same command prompt.
© Copyright Virtual University of Pakistan 206 Web Design and Development (CS506) 22.3 Object Serialization & Network
•
You can read / write to a network using sockets.
•
All you need to do is attach your stream with socket rather than file.
•
The class version should be same on both sides (client & network) of the network .
Example Code 22.2: Sending/Reading Objects to/from Network
We are going to use same PersonInfo class listed in example code 22.1. An object of
PersonInfo class will be sent by client on network using sockets and then be read by server
from network.
Sending Objects over Network
The following class ClientWriteNetEx.java will send an object on network
import java.net.*;
import java.io.*;
import javax.swing.*;
public class ClientWriteNetEx{
public static void main(String args[]){
try {
PersonInfo p = new PersonInfo(“ali”, “defence”, “9201211”);
// create a communication socket
Socket s = new Socket(“localhost”, 2222);
// Get I/O streams
OutputStream is = s.getOutputStream();
// attaching ObjectOutput stream over Input stream
ObjectOutputStream oos= new ObjectOutputStream(is);
// writing object to network
oos.writeObject(p);
// closing communication socket
s.close();
}catch(Exception ex){
System.out.println(ex); }
}} // end class
Reading Objects over Network
The following class ServerReadNetEx.java will read an object of PersonInfo sent by client.
import
import
import
public
java.net.*;
java.io.*;
javax.swing.*;
class ServerReadNetEx{
© Copyright Virtual University of Pakistan 207 Web Design and Development (CS506) public static void main(String args[]){
try {
// create a server socket
ServerSocket ss = new ServerSocket(2222);
System.out.println("Server started...");
/* Loop back to the accept method of the server
socket and wait for a new connection request. So
server will continuously listen for requests
*/
while(true) {
// wait for incoming connection
Socket s = ss.accept();
System.out.println("connection request recieved");
// Get I/O streams
InputStream is = s.getInputStream();
// attaching ObjectOutput stream over Input stream
ObjectInputStream ois = new ObjectInputStream(is);
// read PersonInfo object from network
PersonInfo p = (PersonInfo)ois.readObject( );
p.printPersonInfo();
// closing communication socket
s.close();
} // end while
}catch(Exception ex){
System.out.println(ex); }
}} // end class
Compile & Execute
After compiling both files, run ServerReadNetEx.java first, from the command prompt
window. Open another command prompt window and run ClientWriteNetEx.java
will send an Object of PersonInfo to
from it. The ClientWriteNetEx.java
ServerReadNetEx.java that displays that object values in dialog box after reading it from
network.
22.4 Preventing Serialization
•
•
•
•
Often there is no need to serialize sockets, streams & DB connections etc because they do
not represent the state of object, rather connections to external resources
To do so, transient keyword is used to mark a field that should not be serialized
So we can mark them as,
o transient Socket s;
o transient OutputStream os;
o transient Connection con;
Transient fields are returned as null on reading
© Copyright Virtual University of Pakistan 208 Web Design and Development (CS506) Example Code 22. 3: transient
Assume that we do not want to serialize phoneNum attribute of PersonInfo class, this can be
done as shown below
PersonInfo.java
import javax.swing.*;
import java.io.*
class PersonInfo implements Serializable{
String name;
String address;
transient String phoneNum;
public PresonInfo(String n, String a, String p) {
name = n;
address = a;
phoneNm = p;
}
public void printPersonInfo( ) {
JOptionPane.showMessageDialog(null ,
“name: ” + name + “address:” +address + “phone no:” +
phoneNum); }
} // end class
22.5 References
Entire material for this handout is taken from the book JAVA A Lab Course by Umair Javed.
This material is available just for the use of VU students of the course Web Design and
Development and not for any other commercial purpose without the consent of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 209 Web Design and Development (CS506) Lecture 23: Multithreading
23.1 Introduction
Multithreading is the ability to do multiple things at once within the same application. It provides
finer granularity of concurrency. A thread — sometimes called an execution context or a
lightweight process — is a single sequential flow of control within a program.
Threads are light weight as compared to processes because they take fewer resources then a process.
A thread is easy to create and destroy. Threads share the same address space i.e. multiple threads
can share the memory variables directly, and therefore may require more complex synchronization
logic to avoid deadlocks and starvation.
23.2 Sequential Execution vs. Multithreading
Every program has atleast one thread. Programs without multithreading executes
sequentially. That is, after executing one instruction the next instruction in sequence is executed.
If a function is called then until the completion of the function the next instruction is not
executed. Similarly if there is a loop then instructions after loop only gets executed when the loop
gets completed. Consider the following java program having three loops in it.
// File ThreeLoopTest.java
public class ThreeLoopTest {
public static void main (String args[ ]) {
//first loop
for (int i=1; i<= 5; i++)
System.out.println(“first ” +i);
// second loop
for (int j=1; j<= 5; j++)
System.out.println(“second ” + j);
// third loop
for (int k=1; k<= 5; k++)
System.out.println(“third ” + k);
} // end main
} // end class
When the program executes, the loops are executed sequentially, one after the other. It generates the
following output.
© Copyright Virtual University of Pakistan 210 Web Design and Development (CS506) Note: Each loop has 5 iterations in the ThreeLoopTest program.
However, if we use multithreading — with one thread per loop — the program may generate
the following output.
Note: Each loop has 10 iterations in the ThreadTest program. Your output can be different from the
one given above.
Notice the difference between the outputs of the two programs. In ThreeLoopTest each loop
generated a sequential output while in ThreadTest the output of the loops got intermingled
i.e. concurrency took place and loops executed simultaneously.
© Copyright Virtual University of Pakistan 211 Web Design and Development (CS506) Let us code our first multithreaded program and try to learn how Java supports
multithreading.
23.3 Java Threads
Java includes built-in support for threading. While other languages have threads bolted on to an
existing structure i.e. threads were not the part of the original language but latter came into existence
as the need arose.
All well known operating systems these days support multithreading. JVM transparently maps Java
Threads to their counter-parts in the operating system i.e. OS Threads. JVM allows threads in
Java to take advantage of hardware and operating system level advancements. It keeps track
of threads and schedules them to get CPU time. Scheduling may be pre-emptive or cooperative. So
it is the job of JVM to manage different tasks of thread. Let’s see how we can create threads?
23.3.1 Creating Threads in Java
There are two approaches to create threads in Java.
•
•
Using Interface
Using Inheritance
Following are the steps to create threads by using Interface:
•
•
•
•
Create a class where you want to put some code that can run in parallel with some other
code and let that class implement the Runnable interface.
Runnable interface has the run() method therefore provide the implementation for the
run() method and put your code that you want to run in parallel here.
Instantiate Thread class object by passing Runnable object in constructor
Start thread by calling start() method
Following are the steps to create threads by using Inheritance:
•
•
•
•
Inherit a class from java.lang.Thread class
Override the run() method in the subclass
Instantiate the object of the subclass
Start thread by calling start() method
23.3.1.1 Threads Creation Steps Using Interface
To write a multithreaded program using Runnable interface, follow these steps:
•
Step 1 - Implement the Runnable Interface
class Worker implements Runnable
© Copyright Virtual University of Pakistan 212 Web Design and Development (CS506) •
Step 2 - Provide an Implementation of run() method
public void run( ){
// write thread behavior
// code that will be executed by the thread
•
Step 3 - Instantiate Thread class object by passing Runnable object in the
constructor
Worker w = new Worker (“first”);
Thread t = new Thread (w);
•
Step 4 - Start thread by calling start() method
t.start();
23.3.1.2 Threads Creation Steps Using Inheritance
To write a multithreaded program using inheritance from Thread class, follow these steps:
•
Step 1 - Inherit from Thread Class
class Worker extends Thread
•
Step 2 - Override run() method
public void run( ){
// write thread behavior
// code that will execute by thread
•
Step 3 - Instantiate subclass object
Worker w = new Worker(“first”);
• Step 4 - Start thread by calling start() method
w.start();
23.4 Three Loops: Multi-Threaded Execution
So far we have explored:
•
•
•
What is multithreading?
What are Java Threads?
Two ways to write multithreaded Java programs
Now we will re-write the ThreeLoopTest program by using Java Threads. At first we will use the
Interface approach and then we will use Inheritance.
© Copyright Virtual University of Pakistan 213 Web Design and Development (CS506) Code Example using Interface
// File Worker.java
public class Worker implements Runnable {
private String job ;
//Constructor of Worker class
public Worker (String j ){
job = j;
}
//Implement run() method of Runnable interface
public void run ( ) {
for(int i=1; i<= 10; i++)
System.out.println(job + " = " + i);
}
} // end class
// File ThreadTest.java
public class ThreadTest{
public static void main (String args[ ]){
//instantiate three objects
Worker first = new Worker (“first job”);
Worker second = new Worker (“second job”);
Worker third = new Worker (“third job”);
//create three objects of Thread class & passing worker
//(runnable) to them
Thread t1 = new Thread (first );
Thread t2 = new Thread (second);
Thread t3 = new Thread (third);
//start threads to execute
t1.start();
t2.start();
t3.start();
}//end main
} // end class
Code Example using Inheritance
Following code is similar to the code given above, but uses Inheritance instead of
interface:
// File Worker.java
public class Worker extends Thread{
private String job ;
//Constructor of Worker class
public Worker (String j ){
© Copyright Virtual University of Pakistan 214 Web Design and Development (CS506) job = j;
}
//Override run() method of Thread class
public void run ( ) {
for(int i=1; i<= 10; i++)
System.out.println(job + " = " + i);
}
} // end class
// File ThreadTest.java
public class ThreadTest{
public static void main (String args[ ]) {
//instantiate three objects of Worker (Worker class is now
//becomes a Thread because it is inheriting from it)class
Worker first = new Worker (“first job”);
Worker second = new Worker (“second job”);
Worker third = new Worker (“third job”);
//start threads to execute
first.start();
second.start();
third.start();
}//end main
} // end class
23.5 Thread Priorities
Threads provide a way to write concurrent programs. But on a single CPU, all the threads do not run
simultaneously. JVM assigns threads to the CPU based on thread priorities. Threads with higher
priority are executed in preference to threads with lower priority. A thread’s default priority is same
as that of the creating thread i.e. parent thread.
A Thread’s priority can be any integer between 1 and 10. We can also use the following predefined
constants to assign priorities.
Thread.MAX_PRIORITY (typically 10)
Thread.NORM_PRIORITY (typically 5)
Thread.MIN_PRIORITY (typically 1)
© Copyright Virtual University of Pakistan 215 Web Design and Development (CS506) To change the priority of a thread, we can use the following method
setPriority(int priority)
It changes the priority of this thread to integer value that is passed. It throws an
IllegalArgumentException if the priority is not in the range MIN_PRIORITY to
MAX_PRIORITY i.e. (1-10).
For example, we can write the following code to change a thread’s priority.
Thread t = new Thread (RunnableObject);
// by using predefined constant
t.setPriority (Thread.MAX_PRIORITY);
// by using integer constant
t.setPriority (7);
23.5.1 Thread Priority Scheduling
The Java runtime environment supports a very simple, deterministic scheduling algorithm called
fixed-priority scheduling. This algorithm schedules threads on the basis of their priority relative to
other Runnable threads.
At any given time, when multiple threads are ready to be executed, the runtime system chooses
for execution the Runnable thread that has the highest priority. Only when that thread stops,
yields (will be explained later), or becomes Not Runnable will a lowerpriority thread start
executing. If two threads of the same priority are waiting for the CPU, the scheduler arbitrarily
chooses one of them to run. The chosen thread runs until one of the following conditions becomes
true:
•
•
•
A higher priority thread becomes Runnable.
It yields, or its run() method exits.
On systems that support time-slicing, its time allotment has expired.
Then the second thread is given a chance to run, and so on, until the interpreter exits.
Consider the following figure in which threads of various priorities are represented by capital
alphabets A, B, …, K. A and B have same priority (highest in this case). J and K have same
priority (lowest in this case). JVM start executing with A and B, and divides CPU time between
these two threads arbitrarily. When both A and B comes to an end, it chooses the next thread C to
execute.
© Copyright Virtual University of Pakistan 216 Web Design and Development (CS506) Code Example: Thread Priorities
Try following example to understand how JVM executes threads based on their priorities.
// File PriorityEx.java
public class PriorityEx{
public static void main (String args[ ]){
//instantiate two objects
Worker first = new Worker (“first job”);
Worker second = new Worker (“second job”);
//create two objects
Thread t1 = new Thread (first );
Thread t2 = new Thread (second);
//set thread priorities
t1.setPriority (Thread.MIN_PRIORITY);
t2.setPriority (Thread.MAX_PRIORITY);
//start threads to execute
t1.start();
t2.start();
}//end main
} // end class
© Copyright Virtual University of Pakistan 217 Web Design and Development (CS506) Output
23.5.2 Problems with Thread Priorities
However, when using priorities with Java Threads, remember the following two issues:
First a Java thread priority may map differently to the thread priorities of the underlying
OS. It is because of difference in priority levels of JVM and underlying OS.
For example
•
•
Solaris has 232-1 priority levels
Windows NT has only 7 user priority levels
Second, starvation can occur for lower-priority threads if the higher-priority threads
never terminate, sleep, or wait for I/O indefinitely.
23.6 References:
•
•
•
Java, A Practical Guide by Umair Javed.
Java How to Program by Deitel and Deitel.
CS193j handouts on Stanford.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 218 Web Design and Development (CS506) Lecture 24: More on Multithreading
In this handout, we’ll cover different aspects of multithreading. Some examples are given to make
you understand the topic of multithreading. First we will start with an example that reads data
from two text files simultaneously.
Example Code: Reading Two Files Simultaneously
The task is to read data from file “first.txt” & “second.txt” simultaneously. Suppose that files
contain the following data as shown below:
first 1
first 2
first 3
first 4
first 5
first 6
first 7
first 8
first 9
first 10
first.txt
second 1
second 2
second 3
second 4
second 5
second 6
second 7
second 8
second 9
second 10
second.txt
Following is the code of ReadFile.java that implements Runnable interface. The file reading
code will be written inside run ( ) method
// File ReadFile.java
import java.io.*;
public class ReadFile implements Runnable{
//attribute used for name of file
String fileName;
// param constructor
public ReadFile(String fn){
fileName = fn;
© Copyright Virtual University of Pakistan 219 Web Design and Development (CS506) }
// overriding run method
// this method contains the code for file reading
public void run ( ){
try
{
// connecting FileReader with attribute fileName
FileReader fr = new FileReader(fileName);
BufferedReader br = new BufferedReader(fr);
String line = "";
// reading line by line data from file
// and displaying it on console
line = br.readLine();
while(line != null) {
System.out.println(line);
line = br.readLine();
}
fr.close();
br.close();
}catch (Exception e){
System.out.println(e);
}
} // end run() method
}
Next, look at the Test.java class that contains the main() method.
// File Test.java
public class Test {
public static void main (String args[]){
// creating ReadFile objects by passing file names to them
ReadFile first = new ReadFile("first.txt");
ReadFile second = new ReadFile("second.txt");
// Instantiating thread objects and passing
// runnable (ReadFile) objects to them
Thread t1 = new Thread(first);
Thread t2 = new Thread(second);
// starting threads that cause threads to read data from
// two different files simultaneously
t1.start();
t2.start();
}
}
© Copyright Virtual University of Pakistan 220 Web Design and Development (CS506) Output
On executing Test class, following kind output would be generated:
24.1 Useful Thread Methods
Now let’s discuss some useful thread class methods.
24.1.1 sleep(int time) method
•
•
•
•
•
•
•
Causes the currently executing thread to wait for the time (milliseconds) specified
Waiting is efficient equivalent to non-busy. The waiting thread will not occupy
the processor
Threads come out of the sleep when the specified time interval expires or when
interrupted by some other thread
Thread coming out of sleep may go to the running or ready state depending upon
the availability of the processor. The different states of threads will be discussed
later
High priority threads should execute sleep method after some time to give low
priority threads a chance to run otherwise starvation may occur
sleep()
method can be used for delay purpose i.e. anyone can call
Thread.sleep()method
Note that sleep()
method can throw InterruptedException. So, you
need try-catch block
© Copyright Virtual University of Pakistan 221 Web Design and Development (CS506) Example Code: Demonstrating sleep ( ) usage
Below the modified code of Worker.java is given that we used in the
previous handout.
// File Worker.java
public class Worker implements Runnable {
private String job ;
//Constructor of Worker class
public Worker (String j ){
job = j;
}
//Implement run() method of Runnable interface
public void run ( ) {
for(int i=1; i<= 10; i++) {
try {
Thread.sleep(100);
// go to sleep for 100 ms
}catch (Exception ex){
System.out.println(ex);
}
System.out.println(job + " = " + i); } // end for
} // end run
} // end class
Below is the code of SleepEx.java that contains the main() method. It will use the
Worker class created above.
// File SleepEx.java
public class SleepEx {
public static void main (String args[ ]){
// Creating Worker objects
Worker first
= new Worker (“first job”);
Worker second = new Worker (“second job”);
// Instantiating thread class objects
Thread t1 = new Thread (first );
Thread t2 = new Thread (second);
// starting thread
t1.start();
t2.start();
}
} // end class
© Copyright Virtual University of Pakistan 222 Web Design and Development (CS506) Output
On executing SleepEx.java, the output will be produced with exact alternations between first
thread & second thread. On starting threads, first thread will go to sleep for 100 ms. It gives a
chance to second thread to execute. Later this thread will also go to sleep for 100 ms. In the
mean time the first thread will come out of sleep and got a chance on processor. It will print job
on console and again enters into sleep state and this cycle goes on until both threads finished the
run() method.
Example Code: Using sleep ( ) for delay purpose
Before jumping on to example code, lets reveal another aspect about main() method. When
you run a Java program, the VM creates a new thread and then sends the
main(String[] args) message to the class to be run! Therefore, there is always at least one
running thread in existence. However, we can create more threads which can run concurrently
with the existing default thread.
sleep()method can be used for delay purpose. This is demonstrated in the
DelayEx.java given below:
// File DelayEx.java
public class DelayEx {
public static void main (String args[ ]){
System.out.println(“main thread going to sleep”);
try {
// the main thread will go to sleep causing delay
Thread.sleep(100);
}catch (Exception ex){
System.out.println(ex);
}
System.out.println(“main thread coming out of sleep”); }
© Copyright Virtual University of Pakistan 223 Web Design and Development (CS506) // end main()
} // end class
Output
On executing DelayEx class, you will experience a delay after the first statement
displayed. The second statement will print when the time interval expired. This has been show
below in the following two diagrams:
24.1.2 yield( ) method
•
•
•
Allows any other threads of the same priority to execute (moves itself to the end
of the priority queue)
If all waiting threads have a lower priority, then the yielding thread resumes
execution on the CPU
Generally used in cooperative scheduling schemes
Example Code: Demonstrating yield ( ) usage
Below the modified code of Worker.java is given
// File Worker.java
public class Worker implements Runnable {
private String job ;
//Constructor of Worker class
public Worker (String j ){
job = j;
}
//Implement run() method of Runnable interface
public void run ( )
{
for(int i=1; i<= 10; i++) {
// giving chance to a thread to execute of same priority
© Copyright Virtual University of Pakistan 224 Web Design and Development (CS506) Thread.yield( );
System.out.println(job + " = " + i);
} // end for
} // end run
} // end class
Below is the code of YieldEx.java that contains the main () method. It will use the
Worker class created above.
// File YieldEx.java
public class YieldEx {
public static void main (String args[ ]){
// Creating Worker objects
Worker first = new Worker (“first job”);
Worker second = new Worker (“second job”);
// Instantiating thread class objects
Thread t1 = new Thread (first );
Thread t2 = new Thread (second);
// starting thread
t1.start();
t2.start();
}
} // end class
Output
Since both threads have the same priority (until we change the priority of some thread explicitly).
Therefore both threads will execute on alternate basis. This can be confirmed from the
output given below:
© Copyright Virtual University of Pakistan 225 Web Design and Development (CS506) 24.2 Thread States: Life Cycle of a Thread
A thread can be in different states during its lifecycle as shown in the figure.
Some Important states are:
24.2.1 New state
•
When a thread is just created
24.2.2 Ready state
•
•
•
Thread’s start() method invoked
Thread can now execute
Put it into the Ready Queue of the scheduler
24.2.3 Running state
•
Thread is assigned a processor and now is running
© Copyright Virtual University of Pakistan 226 Web Design and Development (CS506) 24.2.4 Dead state
•
•
Thread has completed or exited
Eventually disposed of by system
24.3 Thread’s Joining
•
•
•
Used when a thread wants to wait for another thread to complete its run() method
For example, if thread2 sent the thread2.join() message, it causes the currently executing
thread to block efficiently until thread2 finishes its run() method
Calling join method can throw InterruptedException, so you must use try-catch block to
handle it
Example Code: Demonstrating join( ) usage
Below the modified code of Worker.java is given. It only prints the job of the worker
// File Worker.java
public class Worker implements Runnable {
private String job ;
public Worker (String j ){
job = j;
}
public void run ( ) {
for(int i=1; i<= 10; i++) {
System.out.println(job + " = " + i); } // end for
} // end run
} // end class
The class JoinEx will demonstrate how current running (main) blocks until the remaining
threads finished their run ()
// File JoinEx.java
public class JoinEx {
public static void main (String args[ ]){
Worker first = new Worker ("first job");
Worker second = new Worker ("second job");
Thread t1 = new Thread (first );
Thread t2 = new Thread (second);
System.out.println("Starting...");
// starting threads
t1.start();
t2.start();
// The current running thread (main) blocks until both
//workers have finished
© Copyright Virtual University of Pakistan 227 Web Design and Development (CS506) try {
t1.join();
t2.join();
}
catch (Exception ex) {
System.out.println(ex);
}
System.out.println("All done ");
} // end main
}
Output
On executing JoinEx, notice that “Starting” is printed first followed by printing workers
jobs. Since main thread does not finish until both threads have finished their run (). Therefore
“All done” will be print on last.
24.4 References:
•
•
•
Java, A Practical Guide by Umair Javed
Java tutorial by Sun: http://java.sun.com/docs/books/tutorial/
CS193j handouts on Stanford
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 228 Web Design and Development (CS506) Lecture 25: Web Application Development
25.1 Introduction
Because of the wide spread use of internet, web based applications are becoming vital part of
IT infrastructure of large organizations. For example web based employee performance
management systems are used by organizations for weekly or monthly reviews of
employees. On the other hand online course registration and examination systems can allow
students to study while staying at their homes.
25.2 Web Applications
In general a web application is a piece of code running at the server which facilitates a
remote user connected to web server through HTTP protocol. HTTP protocol follows
stateless Request-Response communication model. Client (usually a web-browser) sends a request
to Server, which sends back appropriate response or error message.
A Typical Web Application
A web server is software which provides users, access to the services that are present on
the internet. These servers can provide support for many protocols used over internet or
intranet like HTTP, FTP, telnet etc
25.3 HTTP Basics
A protocol defines the method and way of communication between two parties. For example
when we talk to our teacher we use a certain way which is different from the way that we adopt
with our friends or parents. Similarly there are many different protocols used by computers to
communicate with each other depending on applications.
For example an Echo Server only listens to incoming name messages and sends back hello
message, while HTTP protocol uses various types of request-response messages.
© Copyright Virtual University of Pakistan 229 Web Design and Development (CS506) HTTP Communication Model
25.3.1 Parts of an HTTP request
•
•
•
Request Method: It tells the server the type of action that a client wants to perform
URI: Uniform Resource Indicator specifies the address of required document or resource
Header Fields: Optional headers can be used by client to tell server extra
information about request e.g. client software and content type that it understands.
HTTP Request Example
•
•
•
Body: Contains data sent by client to the server
Other request headers like FROM (email of the person responsible for request) and VIA
(used by gateways and proxies to show intermediate sites the request passes) can also be
used.
Request Parameters
o Request can also contain addition information in form of request parameters
In URL as query string e.g.
http://www.gmail.com/register?name=ali&state=punjab
As part of request body (see Figure 3)
25.3.2 Parts of HTTP response
•
•
Result Code: A numeric status code and its description.
Header Fields: Servers use these fields to tell client about server information like
configurations and software etc.
© Copyright Virtual University of Pakistan 230 Web Design and Development (CS506) •
Body: Data sent by server as part of response that is finally seen by the user.
Figure 4: HTTP Response Example
25.3.3 HTTP Response Codes
•
An HTTP Response code tell the client about the state of the response i.e. whether it’s a
valid response or some error has occurred etc. HTTP Response codes fall into five general
categories
o 100-199
Codes in the 100s are informational, indicating that the client should
respond with some other action.
100: Continue with partial request.
o 200-299
Values in the 200s signify that the request was successful.
200: Means everything is fine.
o 300-399
Values in the 300s are used for files that have moved and usually include
a Location header indicating the new address.
300: Document requested can be found several places; they'll be listed in
the returned document.
o 400-499
Values in the 400s indicate an error by the client.
404: Indicates that the requested resource is not available.
401: Indicates that the request requires HTTP authentication.
403: Indicates that access to the requested resource has been denied.
© Copyright Virtual University of Pakistan 231 Web Design and Development (CS506) o 500-599
Codes in the 500s signify an error by the server.
503: Indicates that the HTTP server is temporarily overloaded and unable
to handle the request.
404: Indicates That The Requested Resource Is Not Available
401: Indicates That Request Requires HTTP Authentication
© Copyright Virtual University of Pakistan 232 Web Design and Development (CS506) 25.4 Server Side Programming
Web server pages can be either static pages or dynamic pages. A static web page is a simple
HTML (Hyper Text Transfer Language) file. When a client requests an HTML page the server
simple sends back response with the required page.
Static Web Page Request And Response
An example of static web page is given below
While in case of dynamic web page s server executes an application which generates HTML
web pages according to specific requests coming from client. These dynamically generated web
pages are sent back to client with the response.
© Copyright Virtual University of Pakistan 233 Web Design and Development (CS506) 25.4.1 Why build Pages Dynamically?
We need to create dynamic web pages when the content of site changes frequently and client specific
response is required. Some of the scenarios are listed below
•
The web page is based on data submitted by the user e.g. results page from search engines
and order confirmation pages at on line stores.
© Copyright Virtual University of Pakistan 234 Web Design and Development (CS506) •
The Web page is derived from data that changes frequently e.g. a weather report or news
headlines page.
•
The Web page uses information from databases or other server-side resources e.g. an ecommerce site could use a servlet to build a Web page that lists the current price and
availability of each item that is for sale.
© Copyright Virtual University of Pakistan 235 Web Design and Development (CS506) Server side programming involves
•
•
Using technologies for developing web pages that include dynamic content.
Developing web based applications which can produce web pages that contain
information that is connection-dependent or time-dependent.
25.4.2 Dynamic Web Content Technologies Evolution
Dynamic web content development technologies have evolved through time in speed, security,
ease of use and complexity. Initially C based CGI programs were on the server. Then template
based technologies like ASP and PHP were then introduced which allowed ease of use for
designing complex web pages. Sun Java introduced Servlets and JSP that provided more speed and
security as well as better tools for web page creation.
Dynamic Web Content Technologies Evolution
25.5 Layers & Web Application
Normally web applications are partitioned into logical layers. Each layer performs a specific
functionality which should not be mixed with other layers. Layers are isolated from each other to
reduce coupling between them but they provide interfaces to communicate with each other.
Simplified View Of A Web Application And Its Layers
© Copyright Virtual University of Pakistan 236 Web Design and Development (CS506) 25.5.1 Presentation Layer:
•
Provides a user interface for client to interact with application. This is the only part of
application visible to client.
25.5.2 Business Layer
•
The business or service layer implements the actual business logic or functionality of the
application. For example in case of online shopping systems this layer handles transaction
management.
25.5.3 Data Layer
•
This layer consists of objects that represent real-world business objects such as an Order,
OrderLineItem, Product, and so on.
25.6 Java - Web Application Technologies
There are several Java technologies available for web application development which includes Java
Servlets, JavaServer Pages, and JavaServer Faces etc.
Java Web Application Technologies (Presentation/Web Tier)
25.7 References:
•
•
Java, A Practical Guide by Umair Javed.
Java tutorial by Sun: http://java.sun.com/docs/books/tutorial/ .
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 237 Web Design and Development (CS506) Lecture 26: Java Servlets
Servlets are java technology’s answer to CGI programming. CGI was widely used for generating
dynamic content before Servlets arrived. They were programs written mostly in C, C++ that run on
a web server and used to build web pages.
As you can see in the figure below, a client sends a request to web server, server forwards that
request to a servlet, servlet generates dynamic content, mostly in the form of HTML pages, and
returns it back to the server, which sends it back to the client. Hence we can say that servlet is
extending the functionality of the webserver (The job of the earlier servers was to respond only
to request, by may be sending the required html file back to the client, and generally no processing
was performed on the server)
26.1 What Servlets can do?
•
•
•
Servlets can do anything that a java class can do. For example, connecting with
database, reading/writing data to/from file etc.
Handles requests sent by the user (clients) and generates response dynamically
(normally HTML pages).
The dynamically generated content is send back to the user through a webserver
(client)
26.2 Servlets vs. other SSP technologies
The java’s servlet technology has following advantage over their counter parts:
26.2.1 Convenient
Servlets can use the whole java API e.g. JDBC. So if you already know java, why learn
Perl or C. Servlets have an extensive infrastructure for automatically parsing and decoding HTML
form data, reading and sending HTTP headers, handling cookies and tracking session etc and many
more utilities.
© Copyright Virtual University of Pakistan 238 Web Design and Development (CS506) 26.2.2 Efficient
With traditional CGI, a new process is started for each request while with servlets each request is
handled by a lightweight java thread, not a heavy weight operating system process. (more on this
later)
26.2.3 Powerful
Java servlets let you easily do several things that are difficult or impossible with regular CGI. For
example, servlets can also share data among each other
26.2.4 Portable
Since java is portable and servlets is a java based technology therefore they are generally portable
across web servers
26.2.5 Inexpensive
There are numbers of free or inexpensive web servers available that are good for personal use or low
volume web sites. For example Apache is a commercial grade webserver that is absolutely free.
However some very high end web and application servers are quite expensive e.g. BEA weblogic.
We’ll also use Apache in this course
26.3 Software Requirements
To use java servlets will be needed
•
•
•
J2SE
Additional J2EE based libraries for servlets such as servlet-api.jar and jspapi.jar. Since these libraries are not part of J2SE, you can download these APIs
separately. However these APIs are also available with the web server you’ll be using.
A capable servlet web engine (webserver)
26.4 Jakarta Servlet Engine (Tomcat)
Jakarta is an Apache project and tomcat is one of its subprojects. Apache Tomcat is an open
source web server, which is used as an official reference implementation of Java Servlets and
Java Server Pages technologies.
Tomcat is developed in an open and participatory environment and released under the Apache
software license
26.4.1 Environment Setup
To work with servlets and JSP technologies, you first need to set up the environment. Tomcat
© Copyright Virtual University of Pakistan 239 Web Design and Development (CS506) installation can be performed in two different ways (a) using .zip file (b) using .exe file.
This setup process is broken down into the following steps:
1.
2.
3.
4.
5.
6.
Download the Apache Tomcat Server
Install Tomcat
Set the JAVA_HOME variable
Set the CATALINA_HOME variable
Set the CLASSPATH variable
Test the Server
26.4.2 Environment Setup Using .zip File
Let’s take a detail look on each step and get some hands on experience of environment
setup.
26.4.2.1 Download the Apache Tomcat Server
From the http://tomcat.apache.org, download the zip file for the current release (e.g.
jakarta-tomcat-5.5.9.zip or any latest version) on your C:\ drive. There are different
releases available on site. Select to download .zip file from the Binary Distributions
core section.
Note: J2SE 5.0 must be installed prior to use the 5.5.9 version of tomcat.
26.4.2.2 Installing Tomcat using .zip file
•
•
•
Unzip the file into a location (e.g. C:\).
(Rightclick on the zip file and select
unziphere option )
When the zip file will unzipped a directory structure will be created on your computer
such as:
The C:\jakarta-tomcat-5.5.9 folder is generally referred as root directory or
CATALINA_HOME
Note: After extraction, make sure C:\jakarta-tomcat-5.5.9 contains a bin
subdirectory. Sometimes students create their own directory and unzip the file there such as
C:\jakarta-tomcat-5.5.9\jakarta-tomcat-5.5.9.This causes problems while giving path information
© Copyright Virtual University of Pakistan 240 Web Design and Development (CS506) 26.4.2.3 Set the JAVA_HOME variable
•
•
•
JAVA_HOME indicates the root directory of your jdk. Set the JAVA_HOME
environment variable to tell Tomcat, where to find java
This variable should list the base JDK installation directory, not the bin
subdirectory
To set it, right click on My Computer icon. Select the advanced tab, a System
Properties window will appear in front of you like shown below. Select the
Environment Variables button to proceed.
•
On clicking Environment Variable button, the Environment Variables window
will open as shown next
© Copyright Virtual University of Pakistan 241 Web Design and Development (CS506) •
•
•
Create a new User variable by clicking New button as shown above, the New
User Variable window will appear
Set name of variable JAVA_HOME
The value is the installation directory of JDK
(for example C:\Program
Files\j2sdk_nb\j2sdk1.4.2). This is shown below in the picture. Please note
that bin folder is not included in the path.
•
Press Ok button to finish
26.4.2.4 Set the CATALINA_HOME variable
CATALINA_HOME is used to tell the system about the root directory of the TOMCAT.
There are various files (classes, exe etc) needed by the system to run.
© Copyright Virtual University of Pakistan 242 Web Design and Development (CS506) CATALINA_HOME is used to tell your system (in this case your web server Tomcat) where
the required files are.
•
•
•
To Set the CATALINA_HOME environment variable, create another User Variable.
Type CATALINA_HOME as the name of the environment variable.
Its value should be the path till your top-level Tomcat directory. If you have
unzipped the Tomcat in C drive. It should be C:\jakarta-tomcat-5.5.9.
This is shown below:
•
Press Ok button to finish
Note: To run Tomcat (web server) you need to set only the two environment variables
and these are JAVA_HOME & CATALINA_HOME
26.4.2.5 Set the CLASSPATH variable
Since servlets and JSP are not part of the Java 2 platform, standard edition, you have to identify
the servlet classes to the compiler. The server already knows about the servlet classes, but the
compiler (i.e., javac) you use for compiling source files of servlet does not. So if you don't
set your CLASSPATH, any attempt to compile servlets, tag libraries, or other classes that
use the servlet API will fail with error messages about unknown classes.
•
•
•
To Set the CLASSPATH environment variable, create another User Variable.
Type CLASSPATH as the name of the environment variable.
Its value should be the path for servlet-api.jar and jsp-api.jar.
These file can be found on following path:
•
Press OK button to finish the setting of CLASSPATH variable
© Copyright Virtual University of Pakistan 243 Web Design and Development (CS506) 26.4.2.6 Test the server
Before making your own servlets and JSP, verify that the server is working
properly. Follow these steps in order to do that:
•
•
•
Open the C:\jakarta-tomcat-5.5.9\bin folder and locate the startup.batfile.
Double clicking on this file will open up a DOS window, which will disappear, and
another DOS window will appear, the second window will stay there. If it does not your
paths are not correctly set.
Now to check whether your server is workig or not, open up a browser window and type
http://localhost:8080. This should open the default page of omcat as shown in next
diagram:
Note: If default page doesn’t displayed, open up an internet explorer window, move on to Tools
Internet Options Connections LAN Settings. Make sure that option of “Bypass proxy server
for local addresses” is unchecked.
There is another easier way to carry out the environment setup using .exe file. However, it is
strongly recommended that you must complete the environment setup using .zip file to know the
essential fundamentals.
26.4.3 Environment Setup Using .exe File
Let’s look at the steps involved to accomplish the environment setup using .exe file.
© Copyright Virtual University of Pakistan 244 Web Design and Development (CS506) 26.4.3.1 Download the Apache Tomcat Server
From the http://tomcat.apache.org, download the .exe file for the current release (e.g.
jakarta-tomcat-5.5.9.zip) on your C:\ drive. There are different releases available on site.
Select to download Windows executable (.exe) file from Binary Distributions
Core
section.
Note: J2SE 5.0 must be installed to use the 5.5.9 version of tomcat.
26.4.3.2 Installing Tomcat using .exe file
•
•
Run the .exe file by double clicking on it.
Moving forward in setup, you will reach to the following window
•
•
•
Select install type “Full” and press Next button to proceed.
Choose the folder in which you want to install Apache Tomcat and press Next to proceed.
The configuration window will be opened. Leave the port unchanged (since by default web
servers run on port 8080, you can change it if you really want
to). Specify the user name & password in the specified fields and press Next
button to move forward. This is also shown in the diagram coming next:
© Copyright Virtual University of Pakistan 245 Web Design and Development (CS506) •
•
•
The setup will automatically select the Java Virtual Machine path. Click
Install button to move ahead.
Finish the setup with the Run Apache Tomcat option selected. It will cause the tomcat
server to run in quick launch bar as shown in diagram below. The Apache Tomcat
shortcuts will also added to Programs menu.
•
Double clicking on this button will open up Apache Tomcat Properties window. From here
you
can
start
or
stop
your
web
server.
You
can
also
configure many options if you want to. This properties window is shown
below:
© Copyright Virtual University of Pakistan 246 Web Design and Development (CS506) 26.4.3.3 Set the JAVA_HOME variable
Choosing .exe mode does not require completing this step.
26.4.3.4 Set the CATALINA_HOME variable
Choosing .exe mode does not require completing this step.
26.4.3.5 Set the CLASSPATH variable
Same as step 5 of .zip installation mode
26.4.3.6 Test the server
If tomcat installation is made using .exe file, follow these steps
•
•
Open the Apache Tomcat properties window by clicking on the Apache Tomcat button
from Quick Launch.
Start the tomcat server if it is not running by clicking on Start button.
© Copyright Virtual University of Pakistan 247 Web Design and Development (CS506) •
Open up a browser window and type http://localhost:8080. This should open the default
page of Tomcat as shown in the next diagram:
Note: If default page doesn’t displayed, open up an internet explorer window, move on to Tools
Internet Options Connections LAN Settings. Make sure that option of “Bypass proxy
server for local addresses” is unchecked.
26.5 References:
•
•
Java, A Lab Course by Umair Javed.
Java Servlet & JSP tutotrial http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/ .
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 248 Web Design and Development (CS506) Lecture 27: Creating a Simple Web Application in Tomcat
In this handout, we’ll discuss the standard tomcat directory structure, a pre-requisite for
building any web application. Different nuts and bolts of Servlets will also be discussed.
In the later part of this handout, we’ll also learn how to make a simple web application
using servlet.
27.1 Standard Directory Structure of a J2EE Web Application
A web application is defined as a hierarchy of directories and files in a standard layout. Such
hierarchies can be used in two forms
•
Unpack
o Where each directory & file exists in the file system separately
o Used mostly during development
•
Pack
o Known as Web Archive (WAR) file
o Mostly used to deploy web applications
The webapps folder is the top-level Tomcat directory that contains all the web
applications deployed on the server. Each application is deployed in a separate folder often
referred as “context”.
To make a new application e.g myapp in tomcat you need a specific folder hierarchy.
•
Create a folder named myapp in C:\jakarta-tomcat-5.5.9\webapps
folder. This name will also appear in the URL for your application. For example
http://localhost:8080/myapp/index.html
© Copyright Virtual University of Pakistan 249 Web Design and Development (CS506) •
•
•
•
•
All JSP and html files will be kept in main application folder (C:\jakartatomcat-5.5.9\webapps\myapp)
Create another folder inside myapp
folder and change its name to WEB-INF.
Remember WEB-INF is case sensitive and it is not WEB_INF
Configuration files such as web.xml will go in WEB-INF
folder
(C:\jakarta-tomcat-5.5.9\webapps\myapp\WEB-INF)
Create another folder inside WEB-INF folder and change its name to classes.
Remember classes name is also case sensitive.
Servlets and Java Beans will go in classes folder (C:\jakarta-tomcat5.5.9\webapps\myapp\WEB-INF\classes)
That’s the minimum directory structure required in order to get started. This is also shown in
the figure below:
•
•
•
To test application hierarchy, make a simple html file e.g. index.html
Write some basic HTML code into it and save it in main application directory i.e.
C:\jakarta-tomcat-5.5.9\webapps\myapp\
Restart the
server and
access it
by using
the
URL
http://localhost:8080/myapp/index.html
A more detailed view of the Tomcat standard directory structure is given below.
file.
© Copyright Virtual University of Pakistan 250 Web Design and Development (CS506) •
•
•
Here you can see some other folders like lib & tags under the WEB-INF.
The lib folder is required if you want to use some achieve files (.jar). For
example an API in jar format that can help generating .pdf files.
Similarly tags folder is helpful for building custom tags or for using .tag
files.
Note: Restart Tomcat every time you create a new directory structure, a servlet or a java bean so
that it can recognize it. For JSP and html files you don’t have to restart the server.
27.2 Writing Servlets
27.2.1 Servlet Types
•
•
•
Servlet related classes are included in two main packages javax.servlet
and javax.servlet.http.
Every servlet must implement the javax.servlet.Servlet
interface, it
contains the servlet’s life cycle methods etc. (Life cycle methods will be discussed in
next handout)
In order to write your own servlet, you can subclass from GernericServlet
or HttpServlet
© Copyright Virtual University of Pakistan 251 Web Design and Development (CS506) 27.2.1.1 GenericServlet class
•
•
Available in javax.servlet package
Implements javax.servlet.Servlet
Extend your class from this class if you are interested in writing protocol independent
servlets
27.2.1.2 HttpServlet class
•
•
•
•
Available in javax.servlet.http package
Extends from GenericServlet class
Adds functionality for writing HTTP specific servlets as compared
GernericServlet
Extend your class from HttpServlet, if you want to write HTTP based servlets
to
27.3 Servlet Class Hierarchy
The Servlet class hierarchy is given below. Like all java classes GenericServlet also
inherits from Object class. Apart from GenericServlet and HttpServlet classes,
ServletRequest,
HttpServletRequest,
ServeltResponse and
HttpServletResponse are also helpful in writing a servlet.
As you can guess ServletRequest & ServletResponse are used in conjunction
with GenericServlet. These classes are used for processing protocol independent requests
and generating protocol independent responses respectively.
© Copyright Virtual University of Pakistan 252 Web Design and Development (CS506) HttpServletRequest & HttpServletRespose are used for processing HTTP protocol
specific requests and generating HTTP specific response. Obviously these classes will be
used in conjunction with HttpServet class, which means you are making a HTTP protocol
specific servlet.
27.4 Types of HTTP requests
HTTP supports different types of request to be sent over to server. Each request has some
specific purpose. The most important ones are get & post. Given below a brief overview
of each request type is given. You can refer to RFC of HTTP for further details.
•
•
•
•
•
•
GET: Requests a page from the server. This is the normal request used when
browsing web pages.
POST: This request is used to pass information to the server. Its most common
use is with HTML forms.
PUT: Used to put a new web page on a server.
DELETE: Used to delete a web page from the server.
OPTIONS: Intended for use with the web server, listing the supported options.
TRACE: Used to trace servers
27.5 GET & POST, HTTP request types
Some details on GET and POST HTTP request types are given below.
•
GET
o Attribute-Value pair is attached with requested URL after ‘?’.
o For example if attribute is ‘name’ and value is ‘ali’ then the request will be
http://www.gmail.com/register?name=ali
o For HTTP based servlet, override doGet () methods of HttpServlet class
to handle these type of requests.
•
POST
o Attribute-Value pair attached within the request body. For your reference HTTP
request diagram is given below again:
© Copyright Virtual University of Pakistan 253 Web Design and Development (CS506) o Override doPost()method of HttpServlet class to handle POST type requests.
27.6 Steps for making a Hello World Servlet
To get started we will make a customary “HelloWorldServlet”. Let’s see what
are the steps involved in writing a servlet that will produce “Hello World”
1. Create a directory structure for your application (i.e. helloapp). This is a one time process
for any application
2. Create a HelloWorldServlet
source file by extending this class from
HttpServlet and overriding your desired method. For example doGet() or
doPost().
3. Compile it (If get error of not having required packages, check your class path)
4. Place the class file of HelloWorldServlet in the classes folder of your
web application (i.e. myapp).
a. Note: If you are using packages then create a complete structure under classes
folder
5. Create a deployment descriptor (web.xml) and put it inside WEB-INF folder
6. Restart your server if already running
7. Access it using Web browser
Example Code: HelloWorldServlet.java //File HelloWorldServlet.java
// importing required packages
import java.io.*;
import javax.servlet.*;
© Copyright Virtual University of Pakistan 254 Web Design and Development (CS506) import javax.servlet.http.*;
// extending class from HttpServelt
public class HelloWorldServlet extends HttpServlet {
/* overriding doGet() method because writing a URL in the browser
by default generate request of GET type As you can see,
HttpServletRequest and HttpServletResponse are passed to this
method. These objects will help in processing of HTTP request and
generating response for HTTP This method can throw
ServletException or IOException, so we mention these exception
types after method signature
*/
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
/* getting output stream i.e PrintWriter from response object by
calling getWriter method on it As mentioned, for generating
response, we will use HttpServletResponse object
*/
PrintWriter out = response.getWriter();
/* printing Hello World in the browser using PrintWriter
object. You can also write HTML like
out.println(“<h1> Hello World </h1>”)
*/
out.println(“Hello World! ”);
} // end doGet()
} // end HelloWorldServlet
Example Code: web.xml
eXtensible Markup Language (xml) contains custom defined tags which
information about the content. To learn more about XML visit http://ww.w3schools.com.
convey
Inside web.xml, the <web-app> is the root tag representing the web application. All other
tags come inside of it.
<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app>
<servlet>
<servlet-name> HelloWorldServlet </servlet-name>
<servlet-class> HelloWorldServlet </servlet-class>
© Copyright Virtual University of Pakistan </servlet>
255 Web Design and Development (CS506) <servlet-mapping>
<servlet-name> HelloWorldServlet </servlet-name> <url-pattern>
/myfirstservlet </url-pattern>
</servlet-mapping>
</web-app>
The <servlet> tag represents one’s servlet name and its class. To specify the name of
servlet, <servlet-name> tag is used. Similarly to specify the class name of servlet (it
is the same name you used for making a servlet), <servlet-class> tag is used.
Note: It is important to note here that you can specify any name for a servlet inside
<servlet-name> tag. This name is used for referring to servlet in later part of
web.xml. You can think of it as your id assigned to you by your university while you have
actually different name (like <servlet-class>).
Next we will define the servlet mapping. By defining servlet mapping we are specifying URL to
access a servlet. <servlet-mapping> tag is used for this purpose.
Inside <servlet-mapping> tag, first you will write the name of the servlet for which
you want to specify the URL mapping using <servlet-name> tag and then you will define the
URL pattern using <url-pattern> tag. Notice the forward slash (/ ) is used before specifying
the url. You can specify any name of URL. The forward slash indicates the root of your
application.
<url-pattern> /myfirstservlet </url-pattern>
Now you can access HelloWorldServelt (if it is placed in myapp application) by giving
the following url in the browser
http://localhost:8080/myapp/myfirstservlet
Note: Save this web.xml file by placing double quotes(“web.xml”) around it as you did to save
.java files.
27.7 Compiling and Invoking Servlets
•
•
•
•
Compile HelloWorldServlet.java using javac command.
Put
HelloWorldServlet.class
in
C:\jakarta-tomcat5.5.9/webapps/myapp/WEB-INF/classes folder
Put
web.xml
file
in
C:\jakarta-tomcat5.5.9/webapps/myapp/WEB-INF folder
Invoke your servlet by writing following URL in web browser. Don’t forget to
restart your tomcat server if already running
http://localhost:8080/myapp/myfirstservlet
© Copyright Virtual University of Pakistan 256 Web Design and Development (CS506) Note: By using IDEs like netBeans® 4.1, you don’t have to write web.xml by
yourself or even to worry about creating directory structure and to copy files inappropriate
locations. However manually undergoing this process will strengthen your concepts and will help
you to understand the underlying mechanics.
27.8 References:
•
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web
Design and Development and not for any other commercial purpose without the consent
of author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 257 Web Design and Development (CS506) Lecture 28: Servlets Lifecycle
In the last handout, we have seen how to write a simple servlet. In this handout we will look
more specifically on how servlets get created and destroyed. What different set of method are
invoked during the lifecycle of a typical servlet.
The second part consists on reading HTML form data through servlet technology. This will be
explored in detail using code example
28.1 Stages of Servlet Lifecycle
A servlet passes through the following stages in its life.
•
•
•
Initialize
Service
Destroy
As you can conclude from the diagram below, that with the passage of time a
servlet passes through these stages one after another.
28.1.1 Initialize
When the servlet is first created, it is in the initialization stage. The webserver invokes he
init() method of the servlet in this stage. It should be noted here that init() is only called
once and is not called for each request. Since there is no constructor available in Servlet so this
urges its use for one time initialization (loading of resources, setting of parameters etc) just as
the init() method of applet.
Initialize stage has the following characteristics and usage
•
•
•
Executed once, when the servlet gets loaded for the first time
Not called for each client request
The above two points make it an ideal place to perform the startup tasks which are
done in constructor in a normal class.
© Copyright Virtual University of Pakistan 258 Web Design and Development (CS506) 28.1.2 Service
The service() method is the engine of the servlet, which actually processes the client’s request.
On every request from the client, the server spawns a new thread and calls the service()
method as shown in the figure below. This makes it more efficient as compared to the
technologies that use single thread to respond to requests.
The figure below show both versions of the implementation of service cycle. In the upper
part of diagram, we assume that servlet is made by sub-classing from GenericServlet.
(Remember, GenericServlet is used for constructing protocol independent servlets.).
To provide the desired functionality, service() method is overridden. The client sends a
request to the web server; a new thread is created to serve this request followed by calling the
service() method. Finally a response is prepared and sent back to the user according to
the request.
© Copyright Virtual University of Pakistan 259 Web Design and Development (CS506) The second part of the figure illustrates a situation in which servlet is made using
HttpServlet class. Now, this servlet can only serves the HTTP type requests. In these
servlets doGet() and doPost() are overridden to provide desired behaviors. When a
request is sent to the web server, the web server after creating a thread, passes on this request
to service() method. The service() method checks the HTTP requests type (GET,
POST etc) and calls the doGet() or doPost() method depending on how the request is
originally sent. After forming the response by doGet()
or doPost()
method, the
response is sent back to the service() method that is finally sent to the user by the web
server.
28.1.3 Destroy
The web server may decide to remove a previously loaded servlet instance, perhaps because it is
explicitly asked to do so by the server administrator, or perhaps servlet container shuts down or
the servlet is idle for a long time, or may be the server is overloaded. Before it does, however it
calls the servlets destroy() method. This makes it a perfect spot for releasing the acquired
resources.
28.2 Summary
•
A Servlet is constructed and initialized. The initialization can be performed inside of
init() method.
• Servlet services zero or more requests by calling service() method that may
decide to call further methods depending upon the Servlet type (Generic or HTTP
specific)
• Server shuts down, Servlet is destroyed and garbage is collected
The following figure can help to summarize the life cycle of the Servlet
© Copyright Virtual University of Pakistan 260 Web Design and Development (CS506) The web sever creates a servlet instance. After successful creation, the servlet enters into
initialization phase. Here, init() method is invoked for once. In case web server fails in
previous two stages, the servlet instance is unloaded from the server.
After initialization stage, the Servlet becomes available to serve the clients requests and to
generate response accordingly. Finally, the servlet is destroyed and unloaded from web
server.
28.3 Reading HTML Form Data Using Servlets
In the second part, the required concepts and servlet technology is explored in order to read
HTML form data. To begin with, let’s first identify in how many ways a client can send data
28.3.1 HTML & Servlets
Generally HTML is used as a Graphics User Interface for a Servlet. In the figure below,
HTML form is being used as a GUI interface for MyServlet. The data entered by the user in
HTML form is transmitted to the MyServlet that can process this data once it read out.
Response may be generated to fulfil the application requirements.
28.3.2 Types of Data send to Web Server
When a user submits a browser request to a web server, it sends two categories of data:
•
Form Data
Data that the user explicitly type into an HTML form. For example: registration
information provided for creating a new email account.
© Copyright Virtual University of Pakistan 261 Web Design and Development (CS506) •
HTTP Request Header Data
Data, which is automatically, appended to the HTTP Request from the client for
example, cookies, browser type, and browser IP address.
Based on our understanding of HTML, we now know how to create user forms. We also
know how to gather user data via all the form controls: text, password, select, checkbox,
radio buttons, etc. Now, the question arises: if I submit form data to a Servlet, how do I
extract this form data from servlet? Figuring this out, provides the basis for creating
interactive web applications that respond to user requests.
28.3.2.1 Reading HTML Form Data from Servlet
Now let see how we can read data from “HTML form” using Servlet. The
HttpServletRequest object contains three main methods for extractingform data submitted by
the user:
•
getParameter(String name)
o Used to retrieve a single form parameter and returns String corresponding to name
specified.
o Empty String is returned in the case when user does not enter any thing in the
specified form field.
o If the name specified to retrieve the value does not exist, it returns null.
Note: You should only use this method when you are sure that the parameter has only one
value. If the parameter might have more than one value, use getParamterValues().
•
getParameterValues(String name)
o Returns an array of Strings objects containing all of the given values of the given
request parameter.
o If the name specified does not exist, null is returned
•
getParameterNames()
o If you are unsure about the parameter names, this method will be helpful
o It returns Enumeration of String objects containing the names of the parameters
that
come with the request.
o If the request has no parameters, the method returns an empty Enumeration.
Note: All these methods discussed above work the same way regardless of the request
type(GET or POST). Also remember that form elements are case sensitive for example,
“userName” is not the same as the “username.”
© Copyright Virtual University of Pakistan 262 Web Design and Development (CS506) Example Code: Reading Form Data using Servlet
This example consists of one HTML page (index.html), one servlet (MyServlet.java) and one
xml file (web.xml) file. The HTML page contains two form parameters: firstName and
surName. The Servlet extracts these specific parameters and echoes them back to the browser
after appending “Hello”.
Note: The example given below and examples later in coming handouts are built using
netBeans®4.1. It’s important to note that tomcat server bundled with netBeans® runs on 8084
port by default.
index.html
Let’s have a look on the HTML code used to construct the above page. <html>
<head>
<title> Reading Two Parameters </title> </head>
<body>
<H2> Please fill out this form: </H2>
<FORM METHOD="GET"
ACTION="http://localhost:8084/paramapp/formservlet"
NAME="myform" >
<BR> Firstname:
<INPUT TYPE = “text” NAME="firstName">
<BR> Surname:
<INPUT TYPE = “text” NAME="surName">
<BR>
© Copyright Virtual University of Pakistan 263 Web Design and Development (CS506) <INPUT TYPE="submit" value="Submit Form">
<INPUT TYPE="reset" value="Reset">
</FORM>
</body>
</html>
Let’s discuss the code of above HTML form. As you can see in the <FORM> tag, the attribute
METHOD is set to “GET”. The possible values for this attribute can be GET and POST. Now what
do these values mean?
•
•
Setting the method attribite to “GET” means that we want to send the HTTP request
using the GET method which will evantually activate the doGet() method of the
servlet. In the GET method the information in the input fields entered by the user, merges
with the URL as the query string and are visible to the user.
Setting METHOD value to “POST” hides the entered information from the user as this
information becomes the part of request body and activates doPost() method of the
servlet.
Attribute ACTION of<FROM>tag is set to http://localhost:8084/paramapp/formservlet.
The form data will be transmitted to this URL. paramapp is the name of web application
created using netBeans. formservlet
is the value of <url-pattern> defined in the web.xml. The code of web.xml is
given at the end.
The NAME attribute is set to “myform” that helps when the same page has more than one forms.
However, here it is used only for demonstration purpose.
To create the text fields where user can enter data, following lines of code come into play
<INPUT TYPE = “text” NAME="firstName">
<INPUT TYPE = “text” NAME="surName">
Each text field is distinguished on the basis of name assigned to them. Later these
names also help in extracting the values entered into these text fields.
MyServlet.java
Now let’s take a look at the servlet code to which HTML form data is submitted.
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class MyServlet extends HttpServlet
{
© Copyright Virtual University of Pakistan 264 Web Design and Development (CS506) public void doGet(HttpServletRequest req,
HttpServletResponse res) throws ServletException, IOException
{
// reading first name parameter/textfield
String fName = req.getParameter(“firstName”);
// reading surname parameter/textfield
String sName = req.getParameter(“surName”);
// gettting stream from HttpServletResponse object
PrintWriter out = res.getWriter();
out.println("Hello: " + fName + " " + sName);
out.close();
}
}// end FormServlet
We started the code with importing three packages.
import java.io.*,
import javax.servlet.*;
import javax.servlet.http.*;
These packages are imported to have the access on PrintWriter, HttpServlet,
HttpServletRequest,
HttpServletResponse,
ServletException
and
IOException classes.
The class MySevlet
extends from HttpServlet
to inherit the HTTP specific
functionality. If you recall HTML code (index.html) discussed above, the value of
mehtod attribute was set to “GET”. So in this case, we only need to override doGet()
Method.
Entering inside doGet() method brings the crux of the code. These are:
String fName = req.getParameter(“firstName”);
String sName = req.getParameter(“surName”);
Two String variables fName and sName are declared that receive String values returned by
getParameter() method. As discussed earlier, this method returns String
corresponds to the form parameter. Note that the values of name attributes of input tags used in
index.html have same case with the ones passed to getParameter() methods as
parameters. The part of HTML code is reproduced over here again:
<INPUT TYPE = “text” NAME="firstName">
<INPUT TYPE = “text” NAME="surName">
In the last part of the code, we get the object of PrintWriter stream from the object of
© Copyright Virtual University of Pakistan 265 Web Design and Development (CS506) HttpServletResponse. This object will be used to send data back the response. Using
PrintWriter object (out), the names are printed with appended “Hello” that becomes visible
in the browser.
web.xml
<?xml version="1.0" encoding="ISO-8859-1"?> <web-app>
<servlet>
<servlet-name> FormServlet </servlet-name>
<servlet-class> MyServlet </servlet-class> </servlet>
<servlet-mapping>
<servlet-name> FormServlet </servlet-name>
<url-pattern> /formservlet </url-pattern> </servlet-mapping>
</web-app>
The <servlet-mapping>
tag contains two tags <servlet-name>
and
<urlpatteren> containing name and pattern of the URL respectively. Recall the value of
action attribute of the <form> element in the HTML page. You can see it is exactly the same
as mentioned in <url-pattern> tag.
http://localhost:8084/paramapp/formservlet
28.4 References:
•
•
•
JAVA a Lab Course by Umair Javed
Java API documentation
Core Servlets and JSP by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 266 Web Design and Development (CS506) Lecture 29: More on Servlets
The objective of this handout is to learn about the use and implementation of initialization
parameters for a Servlet. Moreover different ways of redirecting response and forwarding or
including requests also discussed in detail.
29.1 Initialization Parameters
Some times at the time of starting up the application we need to provide some initial
information e,g, name of the file where server can store logging information, DSN for
database etc. Initial configuration can be defined for a Servlet by defining some string
parameters in web.xml. This allows a Servlet to have initial parameters from outside. This
is similar to providing command line parameters to a standard console based application.
Example: setting init parameters in web.xml
Let’s have a look on the way of defining these parameters in web.xml
<init-param> //defining param 1
<param-name> param1 </param-name>
<param-value> value1 </param-value>
</init-param>
<init-param> //defining param 2
<param-name> param2 </param-name>
<param-value> value2 </param-value> </init-param>
In the above code, it is shown that for each parameter we need to define separate
<initparam> tag that have two sub tags <param-name> and <param-value>, which
contain the name and values of the parameter respectively.
29.1.1 ServletConfig
Every Servlet has an object called ServletConfig associated with it as shown in the fig.
below. It contains relevant information about the Servlet like initialization parameters
defined in web.xml
© Copyright Virtual University of Pakistan 267 Web Design and Development (CS506) 29.1.2 Reading Initialization Parameters
Now let’s see, how we can access init parameters inside the Servlet. The method
getInitParameter()of ServletConfig is usually used to access init parameters. It
takes a String as parameter, matches it with <param-name> tag under all
<init-param> tags and returns <param-value> from the web.xml
One way is to override init()
method as shown in the code below. The
ServletConfig object can then be used to read initialization parameter.
public void init(ServletConfig config) throws ServletException {
String name = config.getInitParameter(“paramName”);
}
Another way to read initialization parameters out side the init () method is
•
•
Call getServletConfig() to obtain the ServletConfig object
Use getInitParameter()
of ServletConfig
to read
parameters
initialization
public void anyMethod() // defined inside servlet
{
ServletConfig config = getServletConfig();
String name = config.getInitParameter(“param_name");
}
Example Code: Reading init parameters
MyServlet.java will read the init parameter (log file name) defined inside web.xml. The
code is given below:
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class MyServlet extends HttpServlet {
// attribute used to store init-parameter value
String fileName;
// overriding init() method
public void init(ServletConfig config) throws ServletException{
super.init(config);
// reading init-parameter “logfilename” stored in web.xml
fileName = config.getInitParameter("logfilename");
© Copyright Virtual University of Pakistan 268 Web Design and Development (CS506) }
/*
Both doGet() & doPost() methods are override over here.
processRequest() is called from both these methods. This makes
possible for a servlet to handle both POST and GET requests
identically.
*/
// Handles the HTTP GET request type
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP POST request type
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
PrintWriter out = response.getWriter();
// writing init-parameter value that is store in fileName
out.println(fileName);
out.close();
}
} // end MyServlet
web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app>
<servlet>
<servlet-name> MyServlet </servlet-name>
<servlet-class> MyServlet </servlet-class>
<init-param>
<param-name> logfilename </param-name>
<param-value> logoutput.txt </param-value> </init-param>
</servlet>
© Copyright Virtual University of Pakistan 269 Web Design and Development (CS506) <servlet-mapping>
<servlet-name> MyServlet </servlet-name>
<url-pattern> /myservlet </url-pattern>
</web-app>
</servlet-mapping>
29.1.3 Response Redirection
We can redirect the response of the Servlet to another application resource (another Servlet, an
HTML page or a JSP) but the resource (URL) must be available to the calling Servlet, in the same
Servlet context (discussed later).
There are two forms of response redirection that can be possible:
•
•
Sending a standard redirect
Sending a redirect to an error page
29.1.4 Sending a standard Redirect
•
•
•
Using response.sendRedirect(“myHtml.html”)
method, a new request
is generated which redirects the user to the specified URL.
If the URL is of another Servlet, that second Servlet will not have access to the
original request object. For example, if the request is redirected from servlet1 to
servlet2, then servlet2 would not be able to access the request object of servlet1.
To have access to the original request object, you must use the request dispatching
technique (discussed later) instead of redirect.
29.1.5 Sending a redirect to an error page
Instead of using response.sendRedirect (), wecan useresponse.sendEorror () to show user an error
page. This method takes two parameters, first the error number that is a predefined constant of the
response class (listed below) and seconds the appropriate error message. The steps to redirect the
user to an error page are:
•
•
•
An error code is sent as a parameter of response.sendError (int,
msg)
method
The error page is displayed with the msg passed to method
The error numbers are predefined constants of the HttpServletResponse class.
For example:
o SC_NOT_FOUND (404)
o SC_NO_CONTENT (204)
o SC_REQUEST_TIMEOUT (408)
© Copyright Virtual University of Pakistan 270 Web Design and Development (CS506) Example Code: Response Redirection
The example given below demonstrates a typical sign on example in which a user is asked to
provide login/password, providing correct information leads to welcome page or otherwise to a
registration page. This example consists of login.html, welcome.html, register.html and
MyServlet.java files. Let’s examine these one after another.
login.html
This page contains two text fields; one for entering username and another for password. The data
from this page is submitted to MyServlet.java.
<html>
<body>
<h2> Please provide login details</h2>
<FORM METHOD="POST"
ACTION="http://localhost:8084/redirectionex/myservlet"
NAME="myForm" >
<BR> User Id:
<INPUT TYPE="text" name="userid"/>
<BR> Password:
<INPUT TYPE="password" name="pwd"/>
<BR> <BR>
<input type="submit" value="Submit Form"/>
</form>
</body>
</html>
welcome.html
The user is directed to this page only if user provides correct login / password. This page only
displays a successfully logged-in message to the user.
<html>
<body>
<h2> You have successfully logged in </h2> </body>
</html>
register.html
The user is redirected to this page in case of providing incorrect login/password information.
The user can enter user id, address and phone number here to register.
Note: The code given below will only show fields to the user. It does not register user as no
such functionality is added into this small example.
© Copyright Virtual University of Pakistan 271 Web Design and Development (CS506) <html>
<body>
<h2>Your login is incorrect. Please register yourself</h2>
<FORM METHOD="POST" ACTION="" NAME="myForm">
<BR> Name:
<INPUT TYPE="text" NAME="userid"/>
<BR> Address:
<INPUT TYPE="text" NAME="address"/>
<BR> Phone No:
<INPUT TYPE="text" NAME="phoneno"/>
<BR> <BR>
<input type="submit" value="Register"/>
</FORM>
</body>
</html>
MyServlet.java
MyServlet.java accepts requests from login.html and redirects the user to welcome.html or
register.html based on the verification of username & password provided. Username &
password are compared with fix values in this example, however you can verify these from
database or from a text file etc.
import
import
import
import
public
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
class MyServlet extends HttpServlet {
// Handles the HTTP GET request type
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP POST request type
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
© Copyright Virtual University of Pakistan 272 Web Design and Development (CS506) String id = request.getParameter("userid");
String pwd = request.getParameter("pwd");
// comparing id & password with fix values
if(id.equals("ali") && pwd.equals("vu")) {
// redirectign user to welcome.html
response.sendRedirect("welcome.html");
} else {
// redirecting user to register.html
response.sendRedirect("register.html");
/* if you want to display an error message to the
user, you can use the following method
response.sendError(
response.SC_PROXY_AUTHENTICATION_REQUIRED, "Send Error Demo" );
*/
} // end else
}
29.2 ServletContext
ServletContext belongs to one web application. Therefore it can be used for sharing
resources among servlets in the same web application.
As initialization parameters, for a single servlet are stored in ServletConfig,
ServetContext can store initialization parameters for the entire web application. These
parameters are also called context attributes and exist for the lifetime of the application.
The following figure illustrates the sharing of context attributes among all the servlets
of a web application.
Note:
•
•
There is a single ServletContext per web application
Different Sevlets will get the same ServletContext object, when calling
getServletContext() during different sessions
© Copyright Virtual University of Pakistan 273 Web Design and Development (CS506) 29.3 Request Dispatcher
RequestDispatcher provides a way to forward or include data from another source. The method
getRequestDispatcher(String path) of ServletContext returns a RequestDispatcher object
associated with the resource at the given path passed as a parameter.
Two important methods of RequestDispatcher are:
•
•
forward(ServletRequest req, ServletResponse resp)
include(ServletRequest req, ServletResponse resp)
29.4 RequestDispatcher: forward
Characteristics of forward methods are:
•
•
•
•
It allows a Servlet to forward the request to another resource (Servlet, JSP or
HTML file) in the same Servlet context.
Forwarding remains
transparent to
the
client unlike
res.sendRedirect(String location). You can not see the changes in the URL.
Request Object is available to the called resource. In other words, it remains in
scope.
Before forwarding request to another source, headers or status codes can be set, but
output content cannot be added.
To clarify the concepts, lets take the help from following figure. User initates the request to
servlet1. servlet1 forwards the request to servlet2 by calling forward(request,
response). Finally a response is returned back to the user by servlet2.
© Copyright Virtual University of Pakistan 274 Web Design and Development (CS506) 29.5 RequestDispatcher: include
It allows a Servlet to include the results of another resource in its response. The two major
differences from forward are:
•
•
Data can be written to the response before an include
The first Servlet which receive the request, is the one which finishes the response
It will be more cleared from the following figure. User sends a HTTPRequest to Servlet1.
Serlet2 is called by Servlet1 by using include(request, response) method. The response
generated by Servlet2 sends back to Servlet1. Servlet1 can also add its own response content and
finally send it back to user.
29.6 References:
•
•
•
Java A Lab Course by Umair Javed
Core Servlets and JSP by Marty Hall
Java API documentation
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 275 Web Design and Development (CS506) Lecture 30: Dispatching Requests
In this handout we will start with request dispatching techniques and give some examples related
to that. Further more some methods of HttpResponse and HttpRequest will also be discussed.
Finally, this handout will be concluded by discussing the importance of session racking. Before
starting, let’s take a look at the summery of the previous lecture.
30.1 Recap
In the previous lecture we had some discussion about Response Redirection and Request
Dispatcher. We said that Response Redirection was used to redirect response of the Servlet to
another application resource. This resource might be another Servlet or any JSP page.
Two forms of Response redirection were discussed. These were:
30.1.1 Sending a standard request:
Using response.sendRedirect(“path of resource”) method, a new request is generated which
redirects the user to the given URL. If the URL is of another servlet, that second servlet will not
be able to access the original request object.
30.1.2 Redirection to an error page:
An error code is passed as a parameter along with message to response.sendError(int, msg)
method. This method redirects the user to the particular error page in case of occurrence of
specified error.
Similarly request dispatching provides us the facility to forward the request processing to another
servlet, or to include the output of another resource (servlet, JSP or HTML etc) in the response.
Unlike Response Redirection, request object of calling resource is available to called resource.
The two ways of Request Dispatching are:
30.1.3 Forward:
Forwards the responsibility of request processing to another resource.
30.1.4 Include:
Allows a servlet to include the results of another resource in its response. So unlike forward, the
first servlet to receive the request is the one which finishes the response.
Example Code: Request Dispatching - include
Lets start with the example of include. We will see how a Servlet includes the output of another
resource in its response. The following example includes a calling Servlet MyServlet and
Servlet IncludeServlet, who’s output will be included in the calling Servlet.
© Copyright Virtual University of Pakistan 276 Web Design and Development (CS506) The code of MyServlet.java servlet is given below.
MyServlet.java
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class MyServlet extends HttpServlet {
/* this method is being called by both doGet() and doPost().We
usually follow this practice, when we are not sure about the
type of incoming request to the servlet. So the actual
processing is being done in the processRequest().
*/
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<body>");
out.println("<h1>Start of include request </h1>");
out.flush();
// getting the object of ServletContext, that will be used to
// obtain the object of RequestDispacther
ServletContext context = getServletContext();
// getting the object of RequestDispatcher by passing the path
// of included resource as a parameter
RequestDispatcher rd =
context.getRequestDispatcher("/includeservlet");
// calling include method of RequestDispatcher by passing
// request and response objects as parameters. This will execute
//the second servlet and include its output in the first servlet
rd.include(request, response);
/* the statements below will be executed after including the
output of the /includeservlet */
out.println("<h1>End of include request </h1>");
out.println("</body>");
out.println("</html>");
© Copyright Virtual University of Pakistan 277 Web Design and Development (CS506) // closing PrintWriter stream
out.close();
}
// This method only calls processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// This method only calls processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
} // end MyServlet
Include Servlet
Now let’s take a look at the code of IncludeServlet.java
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class IncludeServlet extends HttpServlet {
// this method is being called by both doGet() and doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// Obtaining the object of PrintWriter, this will return the
// same PrintWriter object we have in MyServlet
PrintWriter out = response.getWriter();
// Including a HTML tag using PrintWriter
out.println("<h1> <marquee>I am included </marquee></h1>");
}
protected void doGet(HttpServletRequest request,
© Copyright Virtual University of Pakistan 278 Web Design and Development (CS506) HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
} // end IncludeServlet
In the processRequest(), firstly we get the PrintWriter stream from the
HttpServletResponse object. Then we include an HTML tag to the output of the calling
servlet. One thing that must be considered is that PrintWriter stream is not closed in the
end, because it is the same stream that is being used in the calling servlet and this stream may
also be used in the calling servlet again. So, if it is closed over here, it can not be used again in
the calling servlet.
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name>MyServlet</servlet-name>
<servlet-class>MyServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>IncludeServlet</servlet-name>
<servlet-class>IncludeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>MyServlet</servlet-name>
<url-pattern>/myservlet</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>IncludeServlet</servlet-name>
<url-pattern>/includeservlet</url-pattern>
</servlet-mapping>
</web-app>
© Copyright Virtual University of Pakistan 279 Web Design and Development (CS506) Code Example: Request Dispatcher – forward
As discussed earlier, we can forward the request processing to another resource using forward
method of request dispatcher. In this example, the user enters his/her name and salary on the
index.html and submits the form to FirstServlet, which calculates the tax on salary
and forwards the request to another servlet for further processing i.e. SecondServlet.
index.html
<html>
<body>
<form method="POST" ACTION = “firstservlet" NAME="myForm">
<h2> Enter your name</h2>
<INPUT TYPE="text" name="name"/>
<br/>
<h2> Salary</h2>
<INPUT TYPE="text" name="salary"/>
<BR/><BR/>
<INPUT type="submit" value="Submit"/>
</form>
</body>
</html>
FirstServlet.java
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class FirstServlet extends HttpServlet {
// this method is being called by both doGet() and doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// getting value of salary text filed of the HTML form
String salary = request.getParameter("salary");
// converting it to the integer.
int sal = Integer.parseInt(salary);
// calculating 15% tax
© Copyright Virtual University of Pakistan 280 Web Design and Development (CS506) int tax = (int)(sal * 0.15);
// converting tax into string
String taxValue = tax + "";
// request object can store values in key-value form, later it
// can be retrieved by using getAttribute() method
request.setAttribute("tax", taxValue);
// getting object of servletContext
ServletContext sContext = getServletContext();
// getting object of request dispatcher
RequestDispatcher rd =
sContext.getRequestDispatcher("/secondservlet");
// calling forward method of request dispatcher
rd.forward(request, response);
}
// This method is calling processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// This method is calling processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
Note: It the case of Forward, it is illegal to make the reference of PrintWriter stream in
the calling Servlet. Only the called resource can use PrintWriter stream to generate
response
SecondServlet.java
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class SecondServlet extends HttpServlet {
© Copyright Virtual University of Pakistan 281 Web Design and Development (CS506) // this method is being called by both doGet() and doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
// obtaining values of name and salary text fields of index.html
String name = request.getParameter("name");
String salary = request.getParameter("salary");
/* getting attribute value that has been set by the calling
servlet i.e. FirstServlet */
String tax = (String)request.getAttribute("tax");
// generating HTML tags using PrintWriter
out.println("<html>");
out.println("<head>");
out.println("<title>SecondServlet</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1> Welcome " + name+ "</h1>");
out.println("<h3> Salary " + salary+ "</h3>");
out.println("<h3> Tax " + tax+ "</h3>");
out.println("</body>");
out.println("</html>");
out.close();
}
// This method is calling processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// This method is calling processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
© Copyright Virtual University of Pakistan 282 Web Design and Development (CS506) web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name>FirstServlet</servlet-name>
<servlet-class>FirstServlet</servlet-class>
</servlet>
<servlet>
<servlet-name>SecondServlet</servlet-name>
<servlet-class>SecondServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>FirstServlet</servlet-name>
<url-pattern>/firstservlet</url-pattern>
</servlet-mapping>
<servlet-mapping>
<servlet-name>SecondServlet</servlet-name>
<url-pattern>/secondservlet</url-pattern>
</servlet-mapping>
</web-app>
30.2 HttpServletRequest Methods
Let’s discuss some methods of HttpServletRequest class
30.2.1 setAttribute(String, Object)
We can put any object to the context using setAttribute() method in the key-value pair form..
These attributes are also set or reset between requests. These are often used in conjunction with
Request Dispatcher. This has also been illustrated in the above example. These attributes are
available every where in the same web application so that any other Servlet or JSP resource can
access them by using getAttribute() method.
30.2.2 getAttribute(String)
The objects set by the setAttribute() method can be accessed using getAttribute() method.
Passing the key in the form of string as a parameter to this method will return the object
associated with that particular key in the context. Cast the object into its appropriate type.
30.2.3 getMethod()
This method returns the name of HTTP method which was used to send the request. The
two possible returning values could be, get or post.
© Copyright Virtual University of Pakistan 283 Web Design and Development (CS506) 30.2.4 getRequestURL()
It can be used to track the source of Request. It returns the part of the request’s URL with
out query string.
30.2.5 getProtocol()
It returns the name and version of the protocol used.
30.2.6 getHeaderNames()
It returns the enumeration of all available header names that are contained in the request.
30.2.7 getHearderName()
It takes a String parameter that represents the header name and returns that appropriate header.
Null value is returned if there is no header exists with the specified name.
30.3 HttpServletResponse Methods
Let’s discuss some methods of HttpServletResponse class
30.3.1 setContentType()
Almost every Servlet uses this header. It is used before getting the PrintWriter Stream. It is used
to set the Content Type that the PrintWriter is going to use. Usually we set “text/html”, when we
want to send text output or generate HTML tags on the client’s browser.
30.3.2 setContentLength()
This method is used to set the content length. It takes length as an integer parameter.
30.3.3 addCookie()
This method is used to add a value to the Set-Cookie header. It takes a Cookie object as a
parameter and adds it to the Cookie-header. We will talk more about Cookies in the session
tracking part.
30.3.4 sendRedirect()
This method redirects the user to the specific URL. This method also accepts the relative URL. It
takes URL string as parameter and redirects the user to that resource.
© Copyright Virtual University of Pakistan 284 Web Design and Development (CS506) 30.4 Session Tracking
Many applications require a series of requests from the same client to be associated withone
another. For example, any online shopping application saves the state of a user's shopping cart
across multiple requests. Web-based applications are responsible for maintaining such state,
because HTTP protocol is stateless. To support applications that need to maintain state, Java
Servlet technology provides an API for managing sessions and allows several mechanisms for
implementing sessions.
Before looking inside the session tracking mechanism lets see the limitation of HTTP protocol to
get the real picture of problems that can happen with out maintaining thesession.
30.4.1 Continuity problem- user’s point of view
Suppose a user logs on to the online bookshop, selects some books and adds them to his cart. He
enters his billing address and finally submits the order. HTTP cannot track session as it is
stateless in nature and user thinks that the choices made on page1 are remembered on page3.
© Copyright Virtual University of Pakistan 285 Web Design and Development (CS506) 30.4.2 Continuity problem- Server’s point of view
The server has a very different point of view. It considers each request independent from other
even if the requests are made by the same client.
30.5 References:
•
•
Java A Lab Course by Umair Javed
Core Servlet and JSP by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 286 Web Design and Development (CS506) Lecture 31: Session Tracking
We have discussed the importance of session tracking in the previous handout. Now, we’ll
discover the basic techniques used for session tracking. Cookies are one of these techniques and
remain our focus in this handout. Cookies can be used to put small information on the client’s
machine and can be used for various other purposes besides session tracking. An example of
simple “Online Book Store”, using cookies, will also be surveyed.
As mentioned elsewhere, HTTP is a stateless protocol. Every request is considered
independent of every other request. But many applications need to maintain a conversational
state with the client. A shopping cart is a classical example of such conversational state.
31.1 Store State Somewhere
To maintain the conversational state, the straightforward approach is to store the state. But
where? These states either can be stored on server or on client. However, both options have their
merits and demerits. Let’s cast a glance on these options:
Storing state on server side makes server really complicated as states needed to be stored for
each client. Some one can imagine how much space and processing is required in this
scenario as some web servers are hit more than hundred times in a second. E.g. Google,
Yahoo etc.
What if states are stored on client side in order to maintain a conversation? Do all the clients permit
you doing that? What if client (user) wiped out these states from the machine?
Concluding this discussion, state is stored neither completely on server side nor on client.
States are maintained by the mutual cooperation of both client & server. Generally modern
servers give the capability to store state on the server side and some information (e.g. client
ID/state ID) passed from the client will relate each client with its corresponding state.
31.2 Post-Notes
In order to maintain the conversational state, server puts little notes (some text, values etc) on the
client slide. When client submits the next form, it also unknowingly submits these little notes.
Server reads these notes and able to recall who the client is.
31.3 Three Typical Solutions
Three typical solutions come across to accomplish session tracking. These are:
1. Cookies
2. URL Rewriting
3. Hidden Fields
© Copyright Virtual University of Pakistan 287 Web Design and Development (CS506) 31.3.1 Cookies
31.3.1.1 What a cookie is?
Don’t be tempted? These are not, what you might be thinking off. In fact, in computer
terminology, “a cookie is a piece of text that a web server can store on a client’s(user)hard disk”.
Cookies allow the web sites to store information on a client machine and later retrieve it. The pieces
of information are stored as name-value pair on the client. Later while reconnecting to the same site
(or same domain depending upon the cookie settings), client returns the same name-value pair to
the server.
31.3.1.2 Cookie’s Voyage
To reveal the mechanism of cookies, let’s take an example. We are assuming here
that the web application we are using will set some cookies
• If you type URL of a Web site into your browser, your browser sends a request for that web
page
o For example, when you type www.amazon.com a request is send to the Amazon’s
server
• Before sending a request, browser looks for cookie files that amazon has set
o If browser finds one or more cookie files related to amazon, it will send it along
with the request
o If not, no cookie data will be sent with the request
• Amazaon web server receives the request and examines the request for cookies
If cookies are received, amazon can use them
o If no cookie is received, amazon knows that you have not visited before or the
cookies that were previously set got expired.
o Server creates a new cookie and send to your browser in the header of HTTP
Response so that it can be saved on the client machine.
31.3.2 Potential Uses of Cookies
Whether cookies have more pros or cons is arguable. However, cookies are helpful in the following
situations
•
•
•
•
identifying a user during an e-commerce session. For example, this book is added into
shopping cart by this client.
Avoiding username and password as cookies are saved on your machine
customizing a site. For example, you might like email-inbox in a different look form
others. This sort of information can be stored in the form of cookies on your machine and
latter can be used to format inbox according to your choice.
Focused Advertising. For example, a web site can store information in the form of
cookies about the kinds of books, you mostly hunt for.
© Copyright Virtual University of Pakistan 288 Web Design and Development (CS506) 31.3.3 Sending Cookies to Browser
Following are some basic steps to send a cookie to a browser (client).
1. Create a Cookie Object
A cookie object can be created by calling the Cookie constructor, which takes two strings: the
cookie name and the cookie value.
Cookie c = new Cookie (“name”, “value”);
2. Setting Cookie Attributes
Before adding the cookie to outgoing headers (response), various characteristics of the cookie
can be set. For example, whether a cookie persists on the disk or not. If yes then how long.
A cookies by default, lasts only for the current user session (i.e. until the user quits the session)
and will not be stored on the disk.
Using setMaxAge(int lifetime) method indicates how much time (in seconds) should
elapse before the cookie expires.
c.setMaxAge(60); // expired after one hour
3. Place the Cookie into HTTP response
After making changes to cookie attributes, the most important and unforgettable step is to add this
currently created cookie into response. If you forget this step, no cookie will be sent to the browser.
response.addCookie(c);
31.3.4 Reading Cookies from the Client
To read the cookies that come back from the client, following steps are generally followed.
1. Reading incoming cookies
To read incoming cookies, get them from
HttpServeltRequest by calling following method
the request
object
of
the
Cookie cookies[] = request.getCookies();
This call returns an array of Cookies object corresponding to the name & values that came in
the HTT P request header.
© Copyright Virtual University of Pakistan 289 Web Design and Development (CS506) 2. Looping down Cookies Array
Once you have an array of cookies, you can iterate over it. Two important methods of Cookie
class are getName() & getValue(). These are used to retrieve cookie name and value
respectively.
// looping down the whole cookies array
for(int i=0; i<cookies.length; i++) {
// getting each cookie from the array
Cookie c = cookies[i];
// in search for particular cookie
if( c.getName().equals(“someName”) {
/* if found, you can do something with cookie
or with the help of cookie.
If don’t want to process further, loop can also be stopped using
break statement
*/
}
} // end for
Example Code1: Repeat Visitor
In the example below, servlet checks for a unique cookie, named “repeat”. If the cookie is present,
servlet displays “Welcome Back”. Absence of cookie indicates that the user is visiting this site
for the first time thus servlet displays a message “Welcome Aboard”.
This example contains only one servlet “RepeatVisitorServlet.java” and its code is
given below. A code snippet of web.xml is also accompanied.
Note: As a reminder, all these examples are built using netBeans4.1. This IDE will write
web.xml for you. However, here it is given for your reference purpose only, or for those which
are not using any IDE to strengthen their concepts
RepeatVisitorServlet.java
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class RepeatVisitorServlet extends HttpServlet {
// Handles the HTTP <code>GET</code> method.
protected void doGet(HttpServletRequest request,
© Copyright Virtual University of Pakistan 290 Web Design and Development (CS506) HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP <code>POST</code> method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from both doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
// writing html
out.println("<html>");
out.println("<body>");
out.println("<h2>Cookie Example </h2>");
String msg = "";
boolean repeatVisitor = false;
// reading cookies
Cookie[] cookies = request.getCookies();
// if cookies are returned from request object
if (cookies != null) {
//search for cookie -- repeat
for (int i = 0; i < cookies.length; i++) {
// retrieving one cookie out of array
Cookie c = cookies[i];
// retrieving name & value of the cookie
String name = c.getName();
String val = c.getValue();
// confirming if cookie name equals “repeat” and
// value equals “yes”
if( name.equals("repeat") && val.equals("yes"))
{
msg= "Welcome Back";
repeatVisitor = true;
break;
© Copyright Virtual University of Pakistan 291 Web Design and Development (CS506) }
} // end for
} // end if
// if no cookie with name “repeat” is found
if (repeatVisitor == false)
{
// create a new cookie
Cookie c1 = new Cookie("repeat", "yes");
// setting time after which cookies expires
c1.setMaxAge(60);
// adding cookie to the response
response.addCookie(c1);
msg = "Welcome Aboard";
}
// displaying message value
out.println("<h2>" + msg + "</h2>");
out.println("</body>");
out.println("</html>");
out.close();
}
}// end RepeatVisitorServlet
web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app>
<servlet>
<servlet-name> RepeatVisitorServlet </servlet-name>
<servlet-class> RepeatVisitorServlet </servlet-class> </servlet>
<servlet-mapping>
<servlet-name> RepeatVisitorServlet </servlet-name>
<url-pattern> /repeatexample </url-pattern>
</servlet-mapping>
</web-app>
Output
On first time visiting this URL, an output similar to the one given below would be displayed
© Copyright Virtual University of Pakistan 292 Web Design and Development (CS506) On refreshing this page or revisiting it within an hour (since the age of cookie was set to 60 mins),
following output should be expected.
Example Code2: Online Book Store using cookies
A scale down version of online book store is going to be built using cookies. For the first time,
cookies will be used to maintain the session.
Three books will be displayed to the user along with check boxes. User can select any check box
to add the book in the shopping cart. The heart of the application is, it remembers the books
previously selected by the user.
The following figure will help you understand the theme of this example. Books displayed
under the heading of “You have selected the following books” were added to cart one after
another. The important thing is server that remembers the previously added books by the
same user and thus maintains the session. Session management is accomplished using
cookies.
© Copyright Virtual University of Pakistan 293 Web Design and Development (CS506) Online Book Store example revolves around one ShoppingCartServlet.java. This
Servlet has one global HashMap (globalMap) in which HashMap of individual user
(sessionInfo) are going to be stored. This (sessionInfo) HashMap stores the books selected by
the user.
What’s the part of cookies? Cookie (named JSESSIONID, with unique value) is used to keep
the unique sessionID associated with each user. This sessionID is passed back and forth
between user and the server and is used to retrieve the HashMap (sessionInfo) of the user
from the global HashMap at the server. It should be noted here that, HashMaps of individual
users are stored in a global HashMap against a sessionID.
ShoppingCartServlet.java
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
public class ShoppingCartServlet extends HttpServlet {
// used to generate a unique value which is
// used as a cookie value
public static int S_ID = 1;
// used to store HashMaps of indiviual users
public static HashMap<String, HashMap> globalMap =
<String, HashMap> new HashMap();
// Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP <code>POST</code> method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from both doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
© Copyright Virtual University of Pakistan 294 Web Design and Development (CS506) // declaring user's HashMap
HashMap<String, String> sessionInfo = null;
String sID = "";
// method findCookie is used to determine whether browser
// has send any cookie named "JSESSIONID"
Cookie c = findCookie(request);
// if no cookies named "JSESSIONID" is recieved, means that
// user is visiting the site for the first time.
if (c == null) {
// make a unique string
sID = makeUniqueString();
// creating a HashMap where books selected by the
// user will be stored
sessionInfo = new HashMap<String, String>();
// add the user's HashMap (sessionInfo) into the
// globalMap against unique string i.e. sID
globalMap.put(sID, sessionInfo);
// create a cookie named "JSESSIONID" alongwith
// value of sID i.e. unique string
Cookie sessionCookie = new Cookie("JSESSIONID", sID);
// add the cookie to the response
response.addCookie(sessionCookie);
} else {
// if cookie is found named "JSESSIONID",
// retrieve a HashMap from the globalMap against
// cookie value i.e. unique string which is your
//sessionID
sessionInfo = (HashMap<String, String>) globalMap.get(
c.getValue() );
}
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Shooping Cart Example</title>");
out.println("</head>");
© Copyright Virtual University of Pakistan 295 Web Design and Development (CS506) out.println("<body>");
out.println("<h1>Online Book Store</h1>");
String url =
"http://localhost:8084/cookiesessionex/shoppingcartex";
// user will submit the from to the same servlet
out.println("<form action=" + url +">" +
"<h3><input type=checkbox name=firstCB value=firstCB />" +
" java core servlts</h3>" +
"<br>"+
“<h3><input type=checkbox name=secondCB value=secondCB />" +
" java how to program</h3>" +
"<br>"+
"<h3><input type=checkbox name=thirdCB value=thirdCB />" +
" java complete reference</h3>" +
"<br>"+
"<input type=submit value=\"Add to Cart\" />" +
"</from>"
);
out.println("<br/>");
out.println("<h1>You have selected followig books</h1>");
out.println("<br/>");
//reteriving
String fBook
String sBook
String tBook
params of check boxes
= request.getParameter("firstCB");
= request.getParameter("secondCB");
= request.getParameter("thirdCB");
// if first book is selected then add it to
// user's HashMap i.e. sessionInfo
if ( fBook != null && fBook.equals("firstCB") ) {
sessionInfo.put("firstCB", "java core servlets");
}
// if second book is selected then add it to
// user's HashMap i.e. sessionInfo
if (sBook != null && sBook.equals("secondCB")){
sessionInfo.put("secondCB", "java how to program");
}
// if third book is selected then add it to
© Copyright Virtual University of Pakistan 296 Web Design and Development (CS506) // user's HashMap i.e. sessionInfo
if (tBook != null && tBook.equals("thirdCB")){
sessionInfo.put("thirdCB", "java complete reference");
}
// used to display the books currently stored in
// the user's HashMap i.e. sessionInfo
printSessionInfo(out, sessionInfo);
out.println("</body>");
out.println("</html>");
out.close();
}
// end processRequest()
// method used to generate a unique string
public String makeUniqueString(){
return "ABC" + S_ID++;
}
// returns a reference global HashMap.
public static HashMap findTableStoringSessions(){
return globalMap;
}
// method used to find a cookie named "JSESSIONID"
public Cookie findCookie(HttpServletRequest request){
Cookie[] cookies = request.getCookies();
if (cookies != null) {
for(int i=0; i<cookies.length; i++) {
Cookie c = cookies[i];
if (c.getName().equals("JSESSIONID")){
// doSomethingWith cookie
return c;
}
}
}
return null;
}
// used to print the books currently stored in
// user's HashMap. i.e. sessionInfo
public void printSessionInfo(PrintWriter out,
HashMap sessionInfo)
{
String title = "";
title= (String)sessionInfo.get("firstCB");
if (title != null){
© Copyright Virtual University of Pakistan 297 Web Design and Development (CS506) out.println("<h3> "+ title +"</h3>");
}
title= (String)sessionInfo.get("secondCB");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
title= (String)sessionInfo.get("thirdCB");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
}
} // end ShoppingCartServlet
web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app>
<servlet>
<servlet-name> ShoppingCart </servlet-name>
<servlet-class> ShoppingCartServlet </servlet-class> </servlet>
<servlet-mapping>
<servlet-name> ShoppingCart </servlet-name>
<url-pattern> /shoppingcartex </url-pattern> </servletmapping>
</web-app>
31.4 References:
•
•
•
•
Java A Lab Course by Umair Javed
Core Servlets and JSP by Marty Hall
Stanford Course - Internet Technologies
Java API documentation
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 298 Web Design and Development (CS506) Lecture 32: Session Tracking 2
In the last handout we have discussed the solutions for session tracking and talked about
one important mechanism cookies in detail. We said cookies allow the server to store
information on a client machine and later retrieve it. Now we will see two more
mechanisms that provide us facility to maintain a session between user’s requests. These
are URL Rewriting and Hidden Form Fields. After that we will discuss a session tracking
API provided by java.
32.1 URL Rewriting
URL rewriting provides another way for session tracking. With URL rewriting, the parameter
that we want to pass back and forth between the server and client is appended to the URL. This
appended information can be retrieve by parsing the URL. This information can be in the form
of:
•
•
•
Extra path information,
Added parameters, or
Some custom, server-specific URL change
Note: Due to limited space available in rewriting a URL, the extra information is usually limited to a
unique session ID.
The following URLs have been rewritten to pass the session ID 123
•
•
•
•
Original -http://server: port/servlet /rewrite
Extra path information -http://server: port/servlet/rewrite/123
Added parameters -http://server: port/servlet/rewrite?id=123
Custom change -http://server: port/servlet/rewrite;$id$123
32.1.1 Disadvantages of URL rewriting
The following Disadvantages of URL rewriting, are considerable: •
•
What if the user bookmarks the page and the problem get worse if server is not
assigning a unique session id.
Every URL on a page, which needs the session information, must be rewritten
each time page is served, which can cause
o Computationally expensive
o Can increase communication overhead
•
•
unlike cookies, state information stored in the URL is not persistent
this mechanism limits the client interaction with the server to HTTP GET request.
© Copyright Virtual University of Pakistan 299 Web Design and Development (CS506) Example Code: Online Bookstore using URL Rewriting
This is the modified version of online book store (selling two books only, however you can add in
on your own) that is built using cookies in the last handout. Another important difference is books
are displayed in the form of hyperlink instead of check boxes. URL rewriting mechanism is used to
maintain session information.
How to make Query String
Before jumping on to example, one important technique is needed to be learned i.e. making
on query string. If you ever noticed the URL of a servlet in a browser that is receiving some
HTML form values, also contains the HTML fields name with values entered/selected by the
user.
Now, if you want to pass some attribute and values along with URL, you can use the
technique of query string. Attribute names and values are written in pair form after the ?.
For example, if you want to send attribute “name” and its value “ali”, the URL will look
like
Original URL
http://server:port/servletex /register
After adding parameters
http://server:port/servletex/register ?name=ali
If you want to add more than one parameter, all subsequent parameters are separated by & sign.
For example
Adding two parameters http://server:port/servletex/register ?name=ali&address=gulberg
URLRewriteServlet.java :
import
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
java.util.*;
public class URLRewriteServlet extends HttpServlet {
// used to generate a unique value which is
// used as a cookie value
public static int S_ID = 1;
© Copyright Virtual University of Pakistan 300 Web Design and Development (CS506) // used to store HashMaps of indiviual users
public static HashMap<String, HashMap> globalMap = new
HashMap<String, HashMap>();
// Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP <code>POST</code> method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from both doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// declaring user's HashMap
HashMap<String, String> sessionInfo = null;
// reading sessionId
String sID = request.getParameter(“JSESSIONID”);
/* if parameter JSESSIONID is received, means that user is
visiting the site for the first time. */
if (sID == null)
{
// make a unique string
sID = makeUniqueString();
// creating a HashMap where books selected by the
// user will be stored
sessionInfo = new HashMap<String, String>();
// add the user's HashMap (sessionInfo) into the
// globalMap against unique string i.e. sID
globalMap.put(sID, sessionInfo);
}else {
// if parameter "JSESSIONID" has some value
© Copyright Virtual University of Pakistan 301 Web Design and Development (CS506) // retrieve a HashMap from the globalMap against
// sID i.e. unique string which is your sessionID
sessionInfo = (HashMap<String, String>) globalMap.get(sID);
}
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Shopping Cart Example</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Online Book Store</h1>");
// Making three URLS by using query string mechanism
// The attributes/parameters are JSSESSIONID and book name (like
// firstCB) along with values sID and book name respectively
String firsturl =
"http://localhost:8084/urlbookstore/urlrewriteservlet?JSESSIONID=
" + sID + "&firstCB=firstCB";
String secondurl =
"http://localhost:8084/urlbookstore/urlrewriteservlet?JSESSIONID=
" + sID + "&secondCB=secondCB";
out.println("<h3><a href=" + firsturl + ">" +
" java core servlts </a> </h3>" +
"<br>"
"<h3><a href=" + secondurl + ">" +
" java how to program </a> </h3>" +
"<br>"
);
out.println("<br/>");
out.println("<h1>You have selected following books</h1>");
out.println("<br/>");
//retrieving params that are emebded in URLs
String fBook = request.getParameter("firstCB");
String sBook = request.getParameter("secondCB");
// if first book is selected then add it to
// user's HashMap i.e. sessionInfo
if ( fBook != null && fBook.equals("firstCB") ) {
sessionInfo.put("firstCB", "java core servlets");
}
// if second book is selected then add it to
// user's HashMap i.e. sessionInfo
© Copyright Virtual University of Pakistan 302 Web Design and Development (CS506) if (sBook != null && sBook.equals("secondCB")){
sessionInfo.put("secondCB", "java how to program");
}
// used to display the books currently stored in
// the user's HashMap i.e. sessionInfo
printSessionInfo(out, sessionInfo);
out.println("</body>");
out.println("</html>");
out.close();
} // end processRequest()
// method used to generate a unique string
public String makeUniqueString(){
return "ABC" + S_ID++;
}
// returns a reference global HashMap.
public static HashMap findTableStoringSessions(){
return globalMap;
}
// used to print the books currently stored in
// user's HashMap. i.e. sessionInfo
public void printSessionInfo(PrintWriter out,
HashMap sessionInfo)
{
String title = "";
title= (String)sessionInfo.get("firstCB");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
title= (String)sessionInfo.get("secondCB");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
}
} // end URLRewriteServlet
© Copyright Virtual University of Pakistan 303 Web Design and Development (CS506) web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app>
<servlet>
<servlet-name> URLRewriteServlet </servlet-name>
<servlet-class> URLRewriteServlet </servlet-class>
<servlet-mapping>
<servlet-name> URLRewriteServlet </servlet-name>
<url-pattern> /urlrewriteservlet </url-pattern>
</servlet-mapping>
</web-app>
</servlet>
32.2 Hidden Form Fields
HTML forms can have an element that looks like the following:
<INPUT TYPE="HIDDEN" NAME="sessionid" VALUE="123" />
Hidden Forms Fields do not affect the appearance of HTML page. They actually
contain the information that is needed to send to the server. Thus, hidden fields
can also be used to store information (like sessionid) in order to maintain session.
In the above figure you can see the use of Hidden form fields for storing particular
information.
32.3 Java Solution for Session Tracking
Java provides an excellent solution to all the problems that occurred in tracking a session. The
Servlet API provides several methods and classes specifically designed to handle session
tracking. In other words, servlets have built in session tracking.
© Copyright Virtual University of Pakistan 304 Web Design and Development (CS506) Sessions are represented by an HttpSession object. HttpSession tacking API
built on top of URL rewriting and cookies. All cookies and URL rewriting mechanism is
hidden and most application server uses cookies but automatically revert to URL
rewriting when cookies are unsupported or explicitly disabled. Using HttpSession
API in servlets is straightforward and involves looking up the session object associated with the
current request, creating new session object when necessary, looking up information
associated with a session, storing information in a session, and discarding completed or
abandoned sessions.
32.4 Working with HttpSession
Let’s have a look on HttpSession working step by step.
1. Getting the user’s session object
To get the user’s session object, we call the getSession() method of
HttpServeltRequest that returns the object of HttpSession
HttpSession sess = request.getSession(true);
If true is passed to the getSession() method, this method returns the current
session associated with this request, or, if the request does not have a session, it
creates a new one. We can confirm whether this session object (sess) is newly
created or returned by using isNew() method of HttpSession. In case of passing
false, null is returned if the session doesn’t exist.
2. Storing information in a Session
To store information in Session object (sess), we use setAttribute() method
of HttpSession class. Session object works like a HashMap, so it is able to store
any java object against key. So you can store number of keys and their values in pair form. For
example,
sess.setAttribute(“sessionid”, ”123”);
3. Looking up information associated with a Session
To retrieve back the stored information from session object, getAttribute()method of
HttpSession class is used. For example,
String sid=(String)sess.getAttribute(“sessionid”);
Note: - getAttribute() method returns Object type, so typecast is required.
© Copyright Virtual University of Pakistan 305 Web Design and Development (CS506) 4. Terminating a Session
After the amount of time, session gets terminated automatically. We can see its
maximum activation time by using getMaxInactiveInterval()
method of
HttpSession
class.
However, we can also terminate any existing session
manually. For this, we need to call invalidate () method of HttpSession
class as shown below.
sess.invalidate()
Example Code: Showing Session Information
To understand HttpSession API properly we need to have a look on an example. In
this example, we will get the session object and check whether it is a new user or not. If
the user is visiting for the first time, we will print “Welcome” and if we find the old one,
we’ll print “Welcome Back”. Moreover, we will print the session information and count
the number of accesses for every user
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class ShowSessionServlet extends HttpServlet {
// Handles the HTTP <code>GET</code> method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP <code>POST</code> method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from both doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
© Copyright Virtual University of Pakistan 306 Web Design and Development (CS506) {
// used for displaying message (like Welcomem, Newcomer) to
// user
String heading;
response.setContentType("text/html");
// Getting session object
HttpSession session = request.getSession(true);
/* Getting stored information using getAttribute() method */
Integer accessCount =
(Integer)session.getAttribute("sessionCount");
/* If user comes for the first time, accessCount will be
assigned null, so we can guess easily that this a new user */
if (accessCount == null)
{
accessCount = new Integer(1);
heading = "Welcome, Newcomer";
} else
{
heading = "Welcome Back";
// Incrementing the value
accessCount = new Integer(accessCount.intValue() + 1);
}
/* Storing the new value of accessCount in the session using
setAttribute() method */
session.setAttribute("sessionCount", accessCount);
// Getting the PrintWriter
PrintWriter out = response.getWriter();
/*Generating HTML tags using PrintWriter to print session info
and no of times this user has accessed this page */
out.println("<HTML>" +
" <BODY>" +
" <h1>Session Tracking Example</h1>" +
" <H2>Information on Your Session:</H2>\n" +
" <H3> Session ID: " + session.getId() + "</H3>" +
" <H3>Number of Previous Accesses: " + accessCount +
© Copyright Virtual University of Pakistan 307 Web Design and Development (CS506) " </H3>" +
" </BODY>" +
" </HTML>"
);
//Closing the PrintWriter stream
out.close();
} // end processRequest
} // end ShowSessionServlet class
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name> ShowSession
</servlet-name>
<servlet-class> ShowSessionServlet </servlet-class>
</servlet>
<servlet-mapping>
<servlet-name> ShowSession </servlet-name>
<url-pattern> /showsession </url-pattern>
</servlet-mapping>
</web-app>
32.5 HttpSession – Behind the scenes
When we call getSession() method, there is a lot going on behind the scenes. For
every user, a unique session ID is assigned automatically. As the server deals with lot of
users at a time, this ID is used to distinguish one user from another. Now here is the
question, how this ID sends to the user? Answer is, there are two options
Option 1: If the browser supports cookies, the Servlet will automatically creates a
session cookie and store the session ID within that cookie.
Option 2: If the first option fails because of browser that does not support cookies
then the Servlet will try to extract the session ID from the URL
© Copyright Virtual University of Pakistan 308 Web Design and Development (CS506) 32.6 Encoding URLs sent to Client
Servlet will automatically switch to URL rewriting when cookies are not supported or
disabled by the client. When Session Tracking is based on URL rewriting, it requires
additional help from the Servlets. For a Servlet to support session tracking via URL
rewriting, it has to rewrite (encode) every local URL before sending it to the client. Now see how
this encoding works
HttpServletResponse provides two methods to perform encoding
•
•
String encodeURL(String URL)
String encodeRedirectURL(String URL)
If Cookies are disabled, both methods encode (rewrite) the specific URL to include the session
ID and returns the new URL. However, if cookies are enabled, the URL is returned
unchanged.
32.7 Difference between encodeURL() and encodeRedirectURL()
encodeURL() is used for URLs that are embedded in the webpage, that the servlet generates.
For example,
String URL = ”/servlet/sessiontracker”;
String eURL = response.encodeURL(URL);
out.println(“<A HREF=\” ” + eURL + ”\ ”> …… </A>”);
Whereas encodeRedirectURL() is used for URLs that refers yours site is in
sendRedirect() call. For example,
String URL = ”/servlet/sessiontracker”;
String
eURL
=
Response.sendRedirect(eURL);
response.encodeRedirectURL(URL);
Example Code: OnlineBookStore using HttpSession
This book store is modified version of last one, which is built using URL rewriting
mechanism. Here, HttpSession will be used to maintain session.
ShoppingCartServlet.java
import
import
import
import
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class ShoppingCartServlet extends HttpServlet {
© Copyright Virtual University of Pakistan 309 Web Design and Development (CS506) // Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// Handles the HTTP <code>POST</code> method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// called from both doGet() & doPost()
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
HttpSession session = request.getSession(true);
PrintWriter out = response.getWriter();
out.println("<html>");
out.println("<head>");
out.println("<title>Shopping Cart Example</title>");
out.println("</head>");
out.println("<body>");
out.println("<h1>Online Book Store</h1>");
// First URL built using query string, representing first book
String firstURL =
"http://localhost:8084/urlrewritebookstore/shoppingcart?book=firs
t";
// Second URL built using query string, representing second book
// Note that parameter name is still book, so that later we need
// to read only this parameter
String secondURL =
"http://localhost:8084/urlrewritebookstore/shoppingcart?book=seco
nd";
// Encoding URLs
String eURL1 = response.encodeURL( firstURL );
String eURL2 = response.encodeURL( secondURL );
© Copyright Virtual University of Pakistan 310 Web Design and Development (CS506) out.println(
"<h3><a href=" + eURL1 + ">" +
" java core servlets </a> </h3>" + "<br>"+
"<h3><a href=" + eURL2 + ">" +
" java How to Program </a> </h3>"
);
out.println("<br/>");
out.println("<h1>You have selected following books</h1>");
out.println("<br/>");
//retrieving params that are emebded in URLs
String fBook = request.getParameter("firstCB");
String sBook = request.getParameter("secondCB");
out.println("<br/>");
out.println("<h1>You have selected following books</h1>");
out.println("<br/>");
//retrieving param that is embedded into URL
String book = request.getParameter("book");
if (book != null){
// if firstURL, value of first hyperlink is clicked
// then storing the book into session object against fBook
if (book.equals("first")){
session.setAttribute("fBook", "java core servlets");
}
// if secondURL, value of second hyperlink is clicked
// then storing the book into session object against sBook
else if(book.equals("second")){
session.setAttribute("sBook", "java how to program");
}
}//outer if ends
// used to display the books currently stored in
// the HttpSession object i.e. session
printSessionInfo(out, session);
out.println("</body>");
out.println("</html>"); out.close();
} // end processRequest()
// used to display values stored in HttpSession object
public void printSessionInfo(PrintWriter out,
HttpSession session)
{
String title = "";
© Copyright Virtual University of Pakistan 311 Web Design and Development (CS506) // reading value against key fBook from session,
// if exist displays it
title= (String)session.getAttribute("fBook");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
// reading value against key sBook from session,
// if exist displays it
title= (String)session.getAttribute("sBook");
if (title != null){
out.println("<h3> "+ title +"</h3>");
}
} // end printSessionInfo
} // end ShoppingCartServlet
web.xml
<?xml version="1.0" encoding="UTF-8"?> <web-app>
<servlet>
<servlet-name> ShoppingCartServlet </servlet-name>
<servlet-class> ShoppingCartServlet </servlet-class>
</servlet>
<servlet-mapping>
<servlet-name> ShoppingCartServlet </servlet-name>
<url-pattern> /shoppingcart </url-pattern>
</servlet-mapping>
</web-app>
32.8 Some Methods of HttpSession
Now let’s explore some methods of HttpSession class
•
setAttribute(String, Object)
o This method associates a value with a name.
•
getAttribute(String)
o Extracts previously stored value from a session object. It returns null if no value
is associated with the given name
© Copyright Virtual University of Pakistan 312 Web Design and Development (CS506) •
removeAttribute(String)
o This method removes values associated with the name
•
getId( )
o This method returns the unique identifier of this session
•
getCreationTime( )
o This method returns time at which session was first created
•
getMaxInactiveInterval( ) , setMaxInactiveInterval(int)
o To get or set the amount of time session should go without access before being
invalidated.
32.9 References:
•
•
•
•
•
Java A Lab Course by Umair Javed
Core Servlets and JSP by Marty Hall
Stanford Course - Internet Technologies
Java Tutorial on Servlets
o http://java.sun.com/j2ee/tutorial/1_3-fcs/doc/Servlets11.html
Java API documentation
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 313 Web Design and Development (CS506) Lecture 33: Address Book Case Study Using Servlets
33.1 Design Process
In this handout, we will discuss the design process of a simple address book. A step by step
procedure of creating a simple address book is given below.
33.2 Layers & Web Application
As discussed previously, normally web applications are partitioned into logical layers.
Each layer performs a specific functionality which should not be mixed with other layers.
For example data access layer is used to interact with database and we do not make any
direct calls to database from the presentation layer. Layers are isolated from each other to
reduce coupling between them but they provide interfaces to communicate with each
other.
Simplified view of a web application and its layers
•
Presentation Layer
o Provides a user interface for client to interact with application. This is the only
o Part of application visible to client.
•
Business Layer
o The business or service layer implements the actual business logic or functionality
of the application. For example in case of online shopping systems this layer
Handles transaction management.
•
Data Layer
o This layer consists of objects that represent real-world business objects such as an
© Copyright Virtual University of Pakistan 314 Web Design and Development (CS506) Order, OrderLineItem, Product, and so on. It also encapsulates classes which are
used to interact with the data providing services such as databases, other web
services etc.
In our case study of address book, we will also try to make use of the layered
architecture. We will create a separate layer for working with data, and our presentation
and business logic will be merged into servlets. It means that we will not have separate
layers for presentation and business rather one layer (formed by servlets)
will do the job
of both presentation and business logic. The extent to which you divide your application
into layers depends upon the size of the application and some other factors such as
scalability, portability etc.
33.2.1 Step 1
•
•
Create a database (AddressBook)
Make a table named Person according to the figure shown below. It has columns name,
address, phomeNum
33.2.2 Step 2
The next step is to create a class that can hold the information of a single person.
Remember we have stored the information in the database, now when we extract this
information from the database as a result of some search, we will require some object to
store the data for that particular person. The PersonInfo
class will be used at that
point to store the retrieved data and transport it to presentation layer. Also we extend this
application and add the functionality of “AddingNewContacts” in the database. The
PersonInfo class can be used to transport data from front end to the database.
•
Make a PersonInfo class with the following consideration
It has three three attributes: name, address, ph. No.
It has a parameterized constructor which takes in the above mentioned parameters Override the
toString() method:
© Copyright Virtual University of Pakistan 315 Web Design and Development (CS506) //File: PersonInfo.java
public class PersonInfo {
String name;
String address;
String phoneNum;
public PersonInfo(String n, String a, String pn) {
name = n;
address = a;
phoneNum = pn;
}
public String toString( ){
return "Name: " + name + " Address: " + address +
" Phone No: " + phoneNum;
}
}// end class PersonInfo
Note: To keep the code simple, attributes (name, address & phoneNum) are not declared
as private, which is indeed not a good programming approach.
33.2.3 Step 3
Now we will create a class that will be used to interact with the database for the search, insert,
update and delete operations. We will call it PersonDAO where DAO stands for the “data access
object”. The PersonDAO along with the PersonInfo class forms the data layer of our
application. As you can see that these two classes do not contain any code related to presentation
or business logic (There is not much of business logic in this application anyway). So
PersonDAO along with PersonInfo is used to retrieve and store data in this application. If at
some stage we choose to use some other way of storing data (e.g. files) only the PersonDAO
class will change and nothing else, which is a sign of better design as compared to a design in
which we put everything in a single class.
So, Make a PersonDAO class which contains:
A searchPerson(String name) method that first establishes a connection
to the database and returns PersonInfo object after searching the information of
the specified person from the database.
//File: PersonDAO.java
import java.sql.*;
public class PersonDAO {
// method searchPerson
public PersonInfo searchPerson(String sName){
© Copyright Virtual University of Pakistan 316 Web Design and Development (CS506) PersonInfo person = null;
try {
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String url = "jdbc:odbc:AddressBookDSN";
Connection con = DriverManager.getConnection(url);
String sql = "SELECT * FROM Person WHERE name = ?";
PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString(1, sName);
ResultSet rs = pStmt.executeQuery();
if (rs.next( ) ) {
String name = rs.getString("name");
String add = rs.getString("address");
String pNum = rs.getString("phoneNum");
person = new PersonInfo(name, add, pNum);
con.close();
}catch(Exception ex){
System.out.println(ex);
}
return person;
}// end method
}
33.2.4 Step 4
To find what user wants to search, we need to give user an interface through which he/she can
enter the input. The SearchPesonServlet.java will do this job for us, It will collect the
data from the user and submit that data to another class. The SearchPersonServlet forms
the part of our presentation layer. As you can see that it is being used to present a form to the user
and collect input.
Write SearchPersonServlet.java
Will take input for name to search in address book
Submits the request to ShowPersonServlet
//File: SearchPersonServlet.java
import java.io.*;
© Copyright Virtual University of Pakistan 317 Web Design and Development (CS506) import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class SearchPersonServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println(
"<html>" +
"<body>" +
"<h1> Address Book </h1>" +
"<form action=showperson >" +
// showperson is alias or
// url pattern of
// ShowPersonServlet
"<h2> Enter name to search </h2> <br/>" +
"<input type=text name=pName /> <br/>" +
"<input type=submit value=Search Person />" +
"</form>" +
"</body>" +
"</html>"
);
out.close();
}
// Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// Handles the HTTP POST method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
33.2.5 Step 5
The data submitted by the SearchPersonServlet will be submitted to another servlet i.e.
ShowPersonServlet, which will interact with the DataLayer(Business logic processing)
collects the output and show it to the user. The ShowPersonServlet forms the part of our
presentation layer and business layer. As you can see that it is being used to do processing on the
© Copyright Virtual University of Pakistan 318 Web Design and Development (CS506) incoming data and giving it to data layer (business layer) and present data/output to the user
(presentation layer)
Write ShowPersonServlet.java
Receives request from SearchPersonServlet
Instantiate objects of PersonInfo and PersonDAO class
Call searchPerson() method of PersonDAO class Show results
//File : ShowPersonServlet.java
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ShowPersonServlet extends HttpServlet {
protected void
processRequest(HttpServletRequestrequest,
HttpServletResponse response)
throws ServletException, IOException{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String name = request.getParameter("pName");
// creating PersonDAO object, and calling searchPerson() method
PersonDAO personDAO = new PersonDAO();
PersonInfo person = personDAO.searchPerson(name);
out.println("<html>");
out.println("<body>");
out.println("<h1>Search Results</h1>");
if (person != null){
out.println("<h3>"+ person.toString() +"</h3>" );
}
else{
out.println("<h3>Sorry! No records found</h3>" );
}
out.println("</body>");
out.println("</html>");
out.close();
}
// Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
} // Handles the HTTP POST method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);}} // end ShowPersonServlet
© Copyright Virtual University of Pakistan 319 Web Design and Development (CS506) Sequence Diagram: Address Book (search use case)
© Copyright Virtual University of Pakistan 320 Web Design and Development (CS506) 33.3 Package
Many times when we get a chance to work on a small project, one thing we intend to do is to put
all java files into one single directory (folder). It is quick, easy and harmless. However if our small
project gets bigger, and the number of files is increasing, putting all these files into the same
directory would be a nightmare for us. In java we can avoid this sort of problem by using
Packages.
33.3.1 What is a package?
In simple terms, a set of Java classes organized for convenience in the same directory to
avoid the name collisions. Packages are nothing more than the way we organize files into
different directories according to their functionality, usability as well as category they
should belong to. An obvious example of packaging is the JDK package from SUN
(java.xxx.yyy) as shown below:
Figure. Basic structure of JDK package
Basically, files in one directory (or package) would have different functionality from
those of another directory. For example, files in java.io package do something related to
I/O, but files in java.net package give us the way to deal with the Network.
Packaging also helps us to avoid class name collision when we use the same class name
as that of others. For example, if we have a class name called "ArrayList", its name
would crash with the ArrayList class from JDK. However, this never happens because
JDK use java.util as a package name for the ArrayList class (java.util.ArrayList).
So our ArrayList class can be named as "ArrayList" or we can put it into another
package like com.mycompany.ArrayList without fighting with anyone. The benefits of
© Copyright Virtual University of Pakistan 321 Web Design and Development (CS506) using package reflect the ease of maintenance, organization, and increase collaboration
among developers. Understanding the concept of package will also help us manage and
use files stored in jar files in more efficient ways.
33.3.2 How to create a package
Suppose we have a file called HelloWorld.java, and we want to put this file in a
package world. First thing we have to do is to specify the keyword package with the
name of the package we want to use (world in our case) on top of our source file, before
the code that defines the real classes in the package, as shown in our HelloWorld class
below:
// only comment can be here
package world;
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello World");
}
One thing you must do after creating a package for the class is to create nested subdirectories to
represent package hierarchy of the class. In our case, we have the worldpackage, which requires
only one directory. So, we create a directory (folder) world and put our HelloWorld.java into it.
© Copyright Virtual University of Pakistan 322 Web Design and Development (CS506) 33.3.3 How to use package
By using "import" keyword, all class files reside only in that package can be imported.
For example,
// we can use any public classes inside world package
import world.*;
// import all public classes from java.util package
import java.util.*;
// import only ArrayList class (not all classes in
// java.util package)
import java.util.ArrayList;
Note: While working with IDEs, You don’t have to create folders (packages) and to place
classes at right locations. Many IDEs (like netBeans® 4.1) performs this job on your behalf.
33.4 JavaServer Pages (JSP)
Like Servlets, JSP is also a specification. JSP technology enables Web developers and designers
to rapidly develop and easily maintain, information-rich, dynamic Web pages that leverage
existing business systems. As part of the Java technology family, JSP technology enables rapid
development of Web-based applications that are platform independent. JSP technology separates
the user interface from content generation, enabling designers to change the overall page layout
without altering the underlying dynamic content.
33.4.1 The Need for JSP
With servlets, it is easy to
•
•
•
•
•
•
•
Read form data
Read HTTP request headers
Set HTTP status codes and response headers
Use cookies and session tracking
Share data among servlets
Remember data between requests
Get fun, high-paying jobs
But, it sure is a pain to
•
•
Use those println() statements to generate HTML
Maintain that HTML
33.4.2 The JSP Framework
•
•
•
Use regular HTML for most of the pages
Mark servlet code with special tags
Entire JSP page gets translated into a servlet (once), and servlet is what actually
© Copyright Virtual University of Pakistan 323 Web Design and Development (CS506) •
gets invoked (for each request)
The Java Server Pages technology combine with Java code and HTML tags in the
same document to produce a JSP file.
33.4.3 Advantages of JSP over Competing Technologies
•
•
•
Versus ASP or ColdFusion
o JSPs offer better language for dynamic part i.e. java
o JSPs are portable to multiple servers and operating systems
Versus PHP
o JSPs offer better language for dynamic part
o JSPs offer better tool support
Versus pure servlets
o JSPs provide more convenient way to create HTML
o JSPs can use standard front end tools (e.g., UltraDev)
o JSPs divide and conquer the problem of presentation and business logic.
33.4.4 Setting Up Your Environment
In order to create a web-application that entirely consists of JSP pages and Html based
pages, the setup is fairly simple as compared to a servlet based web application.
• Set your CLASSPATH.
No.
• Compile your code.
No.
• Use packages to avoid name conflicts.
No.
• Put JSP page in special directory, like WEB-INF for servlets
No.
o tomcat_install_dir/webapps/ROOT
o jrun_install_dir/servers/default/default-app
• Use special URL to invoke JSP page.
No
• However
o If you want to use java based classes in an application along with JSPs, Previous
rules about CLASSPATH, install dirs, etc, still apply to regular classes used by JSP
33.5 References:
•
•
•
Java A Lab Course by Umair Javed
Java Package Tutorial by Patrick Bouklee http://jarticles.com/package/package_eng.html
JavaServer Pages Overview http://java.sun.com/products/jsp/overview.html
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 324 Web Design and Development (CS506) Lecture 34: Java Server Pages
As we concluded in our discussion on JSP, JSP is a text based document capable of returning
either static or dynamic content to a client’s browser. Static content and dynamic content can
be intermixed. The examples of static content are HTML, XML & Text etc. Java code, displaying
properties of JavaBeans and invoking business logic defined in custom tags are all examples of
dynamic content.
34.1 First run of a JSP
Figure below shows what phases a JSP passed through before displaying result.
The web browser makes a request to JSP source code. This code is bifurcated into HTML
and java code by the JSP parser. The java source code is compiled by the Java compiler
resulting in producing a servlet equivalent code of a JSP.
The servlet code is intermixed
with HTML and displayed to the user. It is important to note that a JSP only passes
through all these phases when it is invoked for the first time or when the changes have
been made to JSP. Any later call to JSP does not undergo of compilation phase.
34.1.1 Benefits of JSP
•
•
Convenient
o we already know java and HTML. So nothing new to be learned to work with
JSP.
o Like servlets (as seen, ultimately a JSP gets converted into a servlet), provides an
extensive infrastructure for
Tracking sessions
Reading and sending HTML headers
Parsing and decoding HTML form data
Efficient
o Every request for a JSP is handled by a simple JSP java thread as JSP gets
© Copyright Virtual University of Pakistan 325 Web Design and Development (CS506) •
•
converted into a servlet. Hence, the time to execute a JSP document is not
dominated by starting a process.
Portable
o Like Servlets, JSP is also a specification and follows a well standardized API.The
JVM which is used to execute a JSP file is supported on many architectures and
operating systems.
Inexpensive
o There are number of free or inexpensive Web Servers that are good for
commercial quality websites.
34.1.2 JSP vs. Servlet
Let’s compare JSP and Servlet technology by taking an example that simply plays current
date.
First have a look on JSP that is displaying a current date. This page more looks like a
HTML page except of two strangely written lines of codes. Also there are no signs of
doGet(), doPost().
<%@ page import=“java.util.*” %>
<html>
<body>
<h3>
Current Date is:<%= new Date()%>
</h3>
</body>
</html>
Now, compare the JSP code above with the Servlet code given below that is also displaying the
current date.
//File: SearchPersonServlet.java
import
import
import
import
import
public
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
java.util.*;
class SearchPersonServlet extends HttpServlet {
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html");
© Copyright Virtual University of Pakistan 326 Web Design and Development (CS506) PrintWriter out = response.getWriter();
out.println(
“<html>” +
“<body>” +
“<h3>” +
“Current Date is:“ + new Date() +
“</h3>” +
“</body>” +
“</html>”
);
out.close();
}
// Handles the HTTP GET method.
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
// Handles the HTTP POST method.
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
}
Clearly, a lot of code is needed to be written in the case of servlet example to perform a
basic job.
34.2 JSP Ingredients
Besides HTML, a JSP may contain the following elements.
• Directive Elements
o Provides global control of JSP ……..……………..<%@%>
• Scripting Elements
o JSP comments ……………………………………...<%----%>
o declarations……………………………………...<%! %>
o Used to declare instance variables & methods
expressions……………………………………...<%=%>
o A java code fragment which returns String
scriptlets……………………………………...<%%>
o Blocks of java code
© Copyright Virtual University of Pakistan 327 Web Design and Development (CS506) •
Action Elements
o Special JSP tags ……..……………………………..<jsp: .…./>
We’ll discuss in detail all the ingredients of JSP. This handout will cover only scripting elements,
remaining ones will be discussed in next handouts.
34.3 Scripting Elements
34.3.1 Comments
Comments are ignored by JSP-to-servlet translator. Two types of comments are possibly used in
JSP.
•
HTML comment:
These comments are shown in browser, means on taking view source of the web
page; these sorts of comments can be read. Format of HTML comments is like to:
<!-- comment text-->
•
JSP comment:
These comments are not displayed in browser and have format like:
<%-- comment text --%>
34.3.2 Expressions
The format of writing a Java expression is:
<%= Java expression %>
These expressions are evaluated, after converted to strings placed into HTML page at the place it
occurred in JSP page
Examples of writing Expressions are:
• <h2> Time: <% new java.util.Date() %> </h2>
will print current data & time after converting it to String
• <h2> Welcome: <% request.getParameter(“name”)%> </h2>
will print the name attribute
34.3.3 Scriptlets
The format of writing a scriptlet is: <% Java code %>
After opening up the scriptlet tag, any kind of java code can be written inside it. This code is
inserted verbatim into corresponding servlet.
© Copyright Virtual University of Pakistan 328 Web Design and Development (CS506) Example of writing a scriptlet is:
• <%String n = request.getParameter(“name”);
out.println(“welcome ” + n);
%>
The above scriptlet reads the name attribute and prints it after appending “welcome”
34.3.4 Declarations
The format of writing a declaration tag is: <%! Java code %>
This tag is used to declare variables and methods at class level. The code written inside this tag is
inserted verbatim into servlet’s class definition.
Example of declaring a class level (attribute) variable is:
•
<%!
private int someField = 5; %>
%>
Example of declaring a class level method is:
•
<%!
public void someMethod ( …… ) {
…………….
}
%>
Code Example: Using scripting elements
The next example code consists of two JSP pages namely first.jsp and
second.jsp. The user will enter two numbers on the first.jsp and after pressing
the calculate sum button, able to see the sum of entered numbers on second.jsp
first.jsp
This page only displays the two text fields to enter numbers along with a button.
<html>
<body>
<h2>Enter two numbers to see their sum</h1>
<!—the form values will be posted to second.jsp -->
<form name = "myForm" action="second.jsp" >
<h3> First Number </h3>
<input type="text" name="num1" />
<h3> Second Number </h3>
© Copyright Virtual University of Pakistan 329 Web Design and Development (CS506) <input type="text" name="num2" /> <br/><br/>
<input type="submit" value="Calculate Sum" /> </form>
</body>
</html>
second.jsp
This page retrieves the values posted by first.jsp. After converting the numbers into integers,
displays their sum.
<html>
<body>
<!-- JSP to sum two numbers -->
<%-- Declaration--%>
<%!
// declaring a variable to store sum int res;
// method helps in calculating the sum
public int sum(int op1, int op2) {
return op1 + op2;
}
%>
<%-- Scripltet--%>
<%
String op1 = request.getParameter("num1");
String op2 = request.getParameter("num2");
int firstNum = Integer.parseInt(op1);
int secondNum = Integer.parseInt(op2);
// calling method sum(), declared above in declartion tag
res = sum(firstNum, secondNum);
%>
<%-- expression used to display sum --%>
<h3>Sum is: <%=res%> </h3>
</body>
</html>
34.4 Writing JSP scripting Elements in XML
Now days, the preferred way for composing a JSP pages is using XML. Although writing JSP
pages in old style is still heavily used as we had shown you in the last example. Equivalent
XML tags for writing scripting elements are given below:
Comments: No equivalent tag is defined
© Copyright Virtual University of Pakistan 330 Web Design and Development (CS506) Declaration:<jsp:declartion> </jsp:declaration>
Expression:<jsp:expression> </jsp:expression>
Scriptlet:<jsp:scriptlet>
</jsp:scriptlet>
It’s important to note that every opening tag also have a closing tag too. The second.jsp of last
example is given below in XML style.
<?xml version="1.0" encoding="UTF-8"?>
<jsp:root xmlns:jsp="http://java.sun.com/JSP/Page" version="2.0">
<!-- to change the content type or response encoding change the
following line
-->
<jsp:directive.page contentType="text/xml;charset=UTF-8"/>
<!-- any content can be specified here, e.g.: -->
<jsp:element name="text">
<jsp:body>
<jsp:declaration>
int res;
public int sum(int op1, int op2) {
return op1 + op2;
}
</jsp:declaration>
<jsp:scriptlet>
String op1 = request.getParameter("num1");
String op2 = request.getParameter("num2");
int firstNum = Integer.parseInt(op1);
int secondNum = Integer.parseInt(op2);
res = sum(firstNum, secondNum);
</jsp:scriptlet>
<jsp:text> Sum is: </jsp:text>
<jsp:expression> res </jsp:expression>
</jsp:body>
</jsp:element> </jsp:root>
34.5 References:
•
•
Java A Lab Course by Umair Javed
Core Servlets and JSP by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 331 Web Design and Development (CS506) Lecture 35: JavaServer Pages
We have started JSP journey in the last handout and thoroughly discussed the JSP
scripting elements. JSP directive elements and implicit objects will be discussed in this
handout. Let’s review JSP journey again to find out what part we have already covered.
We start our discussion from implicit objects. Let’s find out what these are?
35.1 Implicit Objects
To simplify code in JSP expressions and scriptlets, you are supplied with eight
automatically defined variables, sometimes called implicit objects. The three most
important variables are request, response
& out. Details of these are given
below:
•
request
This variable is of type HttpServletRequest, associated with the request. It
gives you access to the request parameters, the request type (e.g. GET or POST),
and the incoming HTTP request headers (e.g. cookies etc).
•
response
This variable is of type HttpServletResponse, associated with the response
to client. By using it, you can set HTTP status codes, content type and response
headers etc.
© Copyright Virtual University of Pakistan 332 Web Design and Development (CS506) •
out
This is the object of JspWriter used to send output to the client.
Code Example: Use of Implicit Objects
The following example constitutes of 4 JSP pages. These are index.jsp,
controller.jsp, web.jsp and java.jsp. The user will select either the option
of “java” or “web” from index.jsp, displayed in the form of radio buttons and
submits the request to controller.jsp. Based on the selection made by the user,
controller.jsp will redirect the user to respective pages (web.jsp or java.jsp).
The flow of the example is shown below in the pictorial form.
The code of these entire pages is given below.
index.jsp
<html>
<body>
<h2>Select the page you want to visit</h2>
<form name="myForm" action="controller.jsp" >
<h3>
<input type="radio" name = "page" value="web"/>
Web Design & Develoment
</h3>
<br>
<h3>
<input type="radio" name = "page" value="java"/>
Java
</h3>
© Copyright Virtual University of Pakistan 333 Web Design and Development (CS506) <br>
<input type="submit" value="Submit" />
</form>
</body>
</html>
controller.jsp
<html>
<body>
<!-- scriptlet -->
<%
// reading parameter “page”, name of radio button using
// implicit object request
String pageName = request.getParameter("page");
// deciding which page to move on based on “page” value
// redirecting user by using response implicit object
if (pageName.equals("web")) {
response.sendRedirect("web.jsp");
} else if (pageName.equals("java") )
{
response.sendRedirect("java.jsp");
}
%>
</body>
</html>
web.jsp
<html>
<body>
// use of out implicit object, to generate HTML
<%
out.println( "<h2>" +
"Welcome to Web Design & Development Page" + "</h2>"
);
%>
</body>
</html>
© Copyright Virtual University of Pakistan 334 Web Design and Development (CS506) java.jsp
<html>
<body>
// use of out implicit object, to generate HTML
<%
out.println( "<h2>" +
"Welcome to Java Page" + "</h2>"
);
%>
</body>
</html>
The details of remaining 5 implicit objects are given below:
•
session
This variable is of type HttpSession, used to work with session object.
•
application
This variable is of type ServletContext. Allows to store values in key-value pair form that
are shared by all servlets in same web application/
•
config
This variable is of type ServletConfig. Represents the JSP configuration options e.g.
init-parameters etc.
•
pageContext
This variable is of type javax.servlet.jsp.PageContext, to give a single point of
access to many of the page attributes. This object is used to stores the object values associated
with this object.
•
exception
This variable is of type java.lang.Throwable. Represents the exception
that is passed to JSP error page.
•
page
This variable is of type java.lang.Object. It is synonym for this.
35.2 JSP Directives
JSP directives are used to convey special processing information about the page to JSP
container. It affects the overall structure of the servlet that results from the JSP page. It enables
programmer to:
© Copyright Virtual University of Pakistan 335 Web Design and Development (CS506) •
•
•
Specify page settings
To include content from other resources
To specify custom-tag libraries
35.2.1 Format
<%@ directive {attribute=”val”}* %>
In JSP, there are three types of directives: page, include & taglib. The formats
of using these are:
•
•
•
page:<%@ page{attribute=”val”}*%>
include:<%@ include{attribute=”val”}*%>
taglib:<%@ taglib{attribute=”val”}*%>
35.2.2 JSP page Directive
Give high level information about servlet that will result from JSP page. It can be used
anywhere in the document. It can control
•
•
•
•
•
•
Which classes are imported
What class the servlet extends
What MIME type is generated
How multithreading is handled
If the participates in session
Which page handles unexpected errors etc.
The lists of attributes that can be used with page directive are:
•
•
•
•
•
•
•
•
•
language = “java”
extends = “package.class”
import = “package.*,package.class”
session = “true | false”
info= “text”
contentType = “mimeType”
isThreadSafe = “true | false”
errorPage= “relativeURL”
isErrorPage = “true | false”
Some example uses are:
•
•
•
To import package like java.util
<%@page import=“java.util.*” info=“using util package” %>
To declare this page as an error page
<%@ page isErrorPage = “true” %>
To generate the excel spread sheet
<%@ page contentType = “application/vnd.ms-excel” %>
© Copyright Virtual University of Pakistan 336 Web Design and Development (CS506) 35.2.3 JSP include Directive
Lets you include (reuse) navigation bars, tables and other elements in JSP page. You can include
files at
•
•
Translation Time (by using include directive)
Request Time (by using Action elements, discussed in next handouts)
Format
<%@include file=“relativeURL”%>
Purpose
To include a file in a JSP document at the time document is translated into a servlet. It
may contain JSP code that affects the main page such as response page header settings
etc.
Example Code: using include directive
This example contains three JSP pages. These are index.jsp, header.jsp &
footer.jsp. The header.jsp will display the text of “web design and development” along
with current date. The footer.jsp will display only “virtual university”. The outputs of both
these pages will be included in index.jsp by using JSP include directive.
header.jsp
<%@page import="java.util.*"%>
<html>
<body>
<marquee>
<h3> Web Desing & Development </h3>
<h3><%=new Date()%></h3>
</marquee>
</body>
</html>
footer.jsp
<html>
<body>
<marquee>
<h3> Virtual University </h3>
</marquee>
</body>
</html>
© Copyright Virtual University of Pakistan 337 Web Design and Development (CS506) index.jsp
<html>
<body>
// includes the output of header.jsp
<%@include file="header.jsp" %>
<TABLE BORDER=1>
<TR><TH></TH><TH>Apples<TH>Oranges
<TR><TH>First Quarter<TD>2307<TD>4706
<TR><TH>Second Quarter<TD>2982<TD>5104
<TR><TH>Third Quarter<TD>3011<TD>5220
<TR><TH>Fourth Quarter<TD>3055<TD>5287 </TABLE>
// includes the output of footer.jsp
<%@include file="footer.jsp" %>
</body>
</html>
Example Code: setting content type to generate excel spread sheet
In this example, index.jsp is modified to generate excel spread sheet of the last example.
The change is shown in bold face.
index.jsp
// setting content type to generate excel sheet using page
directive
<%@page contentType="application/vnd.ms-excel"
%>
<html>
<body>
// includes the output of header.jsp
<%@include file="header.jsp" %>
<TABLE BORDER=1>
<TR><TH></TH><TH>Apples<TH>Oranges
<TR><TH>First Quarter<TD>2307<TD>4706
<TR><TH>Second Quarter<TD>2982<TD>5104
<TR><TH>Third Quarter<TD>3011<TD>5220
<TR><TH>Fourth Quarter<TD>3055<TD>5287 </TABLE>
// includes the output of footer.jsp
<%@include file="footer.jsp" %>
</body>
</html>
© Copyright Virtual University of Pakistan 338 Web Design and Development (CS506) 35.3 JSP Life Cycle Methods
The life cycle methods of JSP are jspInit(), _jspService() and
jspDesroy(). On receiving each request, _jspService() method is invoked that
generates the response as well.
35.4 References:
•
•
Java A Lab Course by Umair Javed
Core Servlets and JSP by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 339 Web Design and Development (CS506) Lecture 36
In the last handout, we learned how to work with JSP directives and the use of implicit objects.
In this handout, we’ll learn about JavaBeans and what affect they produce. Before learning
JavaBeans, let’s start with an example that helps us to understand the impact of using JavaBeans.
Code Example: Displaying Course Outline
This example is actually the modification of the last one we had discussed in previous handout.
User will select either course “web design and development” or “java”. On submitting request,
course outline would be displayed of the selected course in tabular format. This course outline
actually loaded from database. The schema of the database used for this example is given below:
The flow of this example is shown below:
index.jsp
This page is used to display the course options to the user in the radio button form.
<html>
<body>
<h2>Select the page you want to visit</h2>
<form name="myForm" action="controller.jsp" >
<h3>
<input type="radio" name = "page" value="web"/>
Web Design & Develoment
</h3> <br>
<h3>
<input type="radio" name = "page" value="java"/>
Java
</h3><br>
© Copyright Virtual University of Pakistan 340 Web Design and Development (CS506) <input type="submit" value="Submit" />
</form>
</body>
</html>
controller.jsp
Based upon the selection made by the user, this page will redirect the user to respective
pages. Those are web.jsp and java.jsp
<html>
<body>
<!-- scriptlet -->
<%
// reading parameter named page
String pageName = request.getParameter("page");
// redirecting user based on selection made
if (pageName.equals("web")) {
response.sendRedirect("web.jsp");
} else if (pageName.equals("java") )
response.sendRedirect("java.jsp");
}
%>
/body>
</html>
{
web.jsp
This page is used to display course outline of “web design and development” in a tabular
format after reading them from database. The code is:
// importing java.sql package using page directive, to work with
// database
<%@page import="java.sql.*"%>
<html>
<body>
<center>
<h2> Welcome to Web Design & Development Page </h2>
<h3> Course Outline</h3>
<TABLE BORDER="1" >
<TR>
© Copyright Virtual University of Pakistan 341 Web Design and Development (CS506) <TH>Session No.</TH>
<TH>Topics</TH>
<TH>Assignments</TH>
</TR>
<%-- start of scriptlet --%>
<%
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:CourseDSN";
Connection con = DriverManager.getConnection(conUrl);
// preparing query using join statements
String sql = " SELECT sessionNo, topic, assignment " +
" FROM Course, SessionDetail" +
" WHERE courseName = ? " +
" AND Course.courseId = SessionDetail.courseID";
PreparedStatement pStmt = con.prepareStatement(sql);
// setting parameter value ”web”.
pStmt.setString( 1 , "web");
ResultSet rs = pStmt.executeQuery();
String sessionNo;
String topic;
String assignment;
// iterating over resultset
while (rs.next()) {
sessionNo = rs.getString("sessionNo");
topic = rs.getString("topic");
assignment = rs.getString("assignment");
if (assignment == null){
assignment = "";
}
%>
<%-- end of scriptlet --%>
<%-- The values are displayed in tabular format using
expressions, however it can also be done using
© Copyright Virtual University of Pakistan 342 Web Design and Development (CS506) out.println(sessionNo) like statements
--%>
<TR>
<TD> <%=sessionNo%> </TD>
<TD> <%=topic%> </TD>
<TD> <%=assignment%> </TD>
</TR>
<%
} // end while
%>
</TABLE >
</center>
</body>
</html>
java.jsp
The code of this page is very much alike of “web.jsp”. The only change is in making of
query. Here the value is set “java” instead of “web”
// importing java.sql package using page directive, to work with
// database
<%@page import="java.sql.*"%>
<html>
<body>
<center>
<h2> Welcome to Java Page </h2>
<h3> Course Outline</h3>
<TABLE BORDER="1" >
<TR>
<TH>Session No.</TH>
<TH>Topics</TH>
<TH>Assignments</TH>
</TR>
<%-- start of scriptlet --%>
<%
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:CourseDSN";
© Copyright Virtual University of Pakistan 343 Web Design and Development (CS506) Connection con = DriverManager.getConnection(conUrl);
// preparing query using join statements
String sql = " SELECT sessionNo, topic, assignment " +
" FROM Course, SessionDetail" +
" WHERE courseName = ? " +
" AND Course.courseId = SessionDetail.courseID";
PreparedStatement pStmt = con.prepareStatement(sql);
// setting parameter value ”web”.
pStmt.setString( 1 , "java");
ResultSet rs = pStmt.executeQuery();
String sessionNo;
String topic;
String assignment;
// iterating over resultset
while (rs.next()) {
sessionNo = rs.getString("sessionNo");
topic = rs.getString("topic");
assignment = rs.getString("assignment");
if (assignment == null){
assignment = "";
}
%>
<%-- end of scriptlet --%>
<%-- The values are displayed in tabular format using
expressions, however it can also be done using
out.println(sessionNo) like statements
--%>
<TR>
<TD> <%=sessionNo%> </TD>
<TD> <%=topic%> </TD>
<TD> <%=assignment%> </TD>
</TR>
<%
} // end while
%>
</TABLE >
</center>
</body>
</html>
© Copyright Virtual University of Pakistan 344 Web Design and Development (CS506) Issues with Last Example
Too much cluttered code in web.jsp and java.jsp. This makes it very
difficult to understand (probably you experienced it by yourself)
and to make changes/enhancements.
A single page is doing everything that is really a bad approach while making
of web applications. The tasks performed by web.jsp or java.jsp are:
- Displaying contents (Presentation logic)
- Connecting with database (DB connectivity logic)
- Results Processing (Business Logic)
Can we simplify it? Yes, the answer lies in the use of JavaBeans technology.
36.1 JavaBeans
•
•
A java class that can be easily reused and composed together in an application.
Any java class that follows certain design conventions can be a JavaBean.
36.1.1 JavaBeans Design Conventions
These conventions are:
• A bean class must have a zero argument constructor
• A bean class should not have any public instance variables/attributes (fields)
• Private values should be accessed through setters/getters
o For boolean data types, use boolean isXXX( ) & setXXX(boolean)
• A bean class must be serializable
A Sample JavaBean
The code snippet of very basic JavaBean is given below that satisfies all the conventions
described above. The MyBean.java class has only one instance variable.
public class MyBean implements Serializable {
private String name;
// zero argument constructor
public MyBean( ){
name = “”;
}
// standard setter
public void setName(String n) {
name = n;
© Copyright Virtual University of Pakistan 345 Web Design and Development (CS506) }
// standard getter
public String getName( ) {
return name;
}
// any other method
public void print( ) {
System.out.println(“Name is: ” + name);
}
} // end Bean class
Example Code: Displaying course outline by incorporating
JavaBeans
This example is made by making more enhancements to the last one. Two JavaBeans are
included in this example code. These are CourseOutlineBean & CourseDAO.
The CourseOutlineBean is used to represent one row of the table. It contains the
following attributes:
•
•
•
sessionNo
topic
assignment
The CourseDAO (where DAO stands of Data Acess Object) bean encapsulates database
connectivity and result processing logic.
The web.jsp and java.jsp will use both these JavaBeans. The code of these and the JSPs
used in this example are given below.
CourseOutlineBean.java
package vu;
import java.io.*;
public class CourseOutlineBean implements Serializable{
private int sessionNo;
private String topic;
private String assignment;
© Copyright Virtual University of Pakistan 346 Web Design and Development (CS506) // no argument constructor
public CourseOutlineBean() {
sessionNo = 0;
topic = "";
assignment = "";
}
// setters
public void setSessionNo(int s){
sessionNo = s;
}
public void setTopic(String t){
topic = t;
}
public void setAssignment(String a){
assignment = a;
}
// getters
public int getSessionNo( ){
return sessionNo;
}
public String getTopic( ){
return topic;
}
public String getAssignment( ){
return assignment;
}
} // end class
CourseDAO.java
package vu;
import java.io.*;
import java.sql.*;
import java.util.*;
public class CourseDAO implements Serializable{
private Connection con;
public CourseDAO() {
establishConnection();
}
© Copyright Virtual University of Pakistan 347 Web Design and Development (CS506) //********** establishConnection method ********************
// method used to make connection with database
private void establishConnection(){
try{
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:CourseDSN";
con = DriverManager.getConnection(conUrl);
}catch(Exception ex){
System.out.println(ex);
}
//*********** retrieveCourseList method ********************
public ArrayList retrieveCourseList(String cName){
ArrayList courseList = new ArrayList();
try{
}
String sql = " SELECT sessionNo, topic, assignment " +
" FROM Course, SessionDetail" +
" WHERE courseName = ? " +
" AND Course.courseId = SessionDetail.courseID ";
PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString(1, cName);
ResultSet rs = pStmt.executeQuery();
int sNo;
String topic;
String assignment;
while ( rs.next() ) {
sNo
= rs.getInt("sessionNo");
topic = rs.getString("topic");
assignment = rs.getString("assignment");
if (assignment == null){
assignment = "";
}
// creating a CourseOutlineBean object
CourseOutlineBean cBean = new CourseOutlineBean();
cBean.setSessionNo(sNo);
cBean.setTopic(topic);
© Copyright Virtual University of Pakistan 348 Web Design and Development (CS506) cBean.setAssignment(assignment);
// adding a bean to arraylist
courseList.add(cBean);
}
}catch(Exception ex){
System.out.println(ex);
} finally {
// to close connection
releaseResources();
}
// returning ArrayList object
return courseList;
} // end retrieveCourseOutline
//********** releaseResources method ********************
private void releaseResources(){
try{
if(con != null){
con.close();
}
}catch(Exception ex){
System.out.println();
}
}
} // end releaseResources
}// end CourseDAO
index.jsp
This page is used to display the course options to the user in the radio button form.
<html>
<body>
<h2>Select the page you want to visit</h2>
<form name="myForm" action="controller.jsp" >
<h3>
<input type="radio" name = "page" value="web"/>
Web Design & Develoment
</h3> <br>
<h3>
© Copyright Virtual University of Pakistan 349 Web Design and Development (CS506) <input type="radio" name = "page" value="java"/>
Java
</h3><br>
<input type="submit" value="Submit" />
</form>
</body>
</html>
controller.jsp
Based on user selection, redirects the user to desired page.
<html>
<body>
<!-- scriptlet -->
<%
String pageName = request.getParameter("page");
if (pageName.equals("web")) {
response.sendRedirect("web.jsp");
} else if (pageName.equals("java") )
response.sendRedirect("java.jsp");
}
%>
{
</body>
</html>
web.jsp
This page is used to display course outline of “web design and development” in a tabular
format after reading them from database. Moreover, this page also uses the JavaBeans
(CourseOutlineBean & CourseDAO).
<%@page import="java.util.*" %>
<%-- importing vu package that contains the JavaBeans--%>
<%@page import="vu.*" %>
<html>
<body>
<center>
<h2> Welcome to Web Design & Development Course </h2>
© Copyright Virtual University of Pakistan 350 Web Design and Development (CS506) <h3> Course Outline</h3>
<TABLE BORDER="1" >
<TR>
<TH>Session No.</TH>
<TH>Topics</TH>
<TH>Assignments</TH>
</TR>
<%-- start of scriptlet --%>
<%
// creating CourseDAO object
CourseDAO courseDAO = new CourseDAO();
// calling retrieveCourseList() of CourseDAO class and
// passing “web” as value. This method returns ArrayList
ArrayList courseList = courseDAO.retrieveCourseList("web");
CourseOutlineBean webBean = null;
// iterating over ArrayList to display course outline
for(int i=0; i<courseList.size(); i++){
webBean = (CourseOutlineBean)courseList.get(i);
%>
<%-- end of scriptlet --%>
<TR>
<TD> <%= webBean.getSessionNo()%> </TD>
<TD> <%= webBean.getTopic()%> </TD>
<TD> <%= webBean.getAssignment()%> </TD>
</TR>
<%
} // end for
%>
</TABLE >
</center>
</body>
</html>
java.jsp
The code contains by this page is almost same of web.jsp. Here, “java” is passed to
retieveCourseList( ) method. This is shown in boldface.
<%@page import="java.util.*" %>
<%-- importing vu package that contains the JavaBeans--%>
<%@page import="vu.*" %>
<html>
<body>
<center>
<h2> Welcome to Java Course </h2>
<h3> Course Outline</h3>
© Copyright Virtual University of Pakistan 351 Web Design and Development (CS506) <TABLE BORDER="1" >
<TR>
<TH>Session No.</TH>
<TH>Topics</TH>
<TH>Assignments</TH>
</TR>
<%-- start of scriptlet --%>
<%
// creating CourseDAO object
CourseDAO courseDAO = new CourseDAO();
// calling retrieveCourseList() of CourseDAO class and
// passing “java” as value. This method returns ArrayList
ArrayList courseList = courseDAO.retrieveCourseList("java");
CourseOutlineBean javaBean = null;
// iterating over ArrayList to display course outline
for(int i=0; i<courseList.size(); i++){
javaBean = (CourseOutlineBean)courseList.get(i);
%>
<%-- end of scriptlet --%>
<TR>
<TD> <%= javaBean.getSessionNo()%> </TD>
<TD> <%= javaBean.getTopic()%> </TD>
<TD> <%= javaBean.getAssignment()%> </TD>
</TR>
<%
} // end for
%>
</TABLE >
</center>
</body>
</html>
36.2 References:
•
Entire material for this handout is taken from the book JAVA A Lab Course by Umair
Javed. This material is available just for the use of VU students of the course Web
Design and Development and not for any other commercial purpose without the consent of
author.
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 352 Web Design and Development (CS506) Lecture 37: JSP Action Elements and Scope
The journey we had started of JSP is very much covered except of JSP action elements. In this
handout, we’ll study the use of JSP action elements. Further also learn how and where to store
JavaBean objects that can be shared among JSP pages.
Let’s first quickly look on the JSP journey to find out where we have reached.
37.1 JSP Action Elements
JSP action elements allow us to work with JavaBeans, to include pages at request time and to
forward requests to other resources etc.
Format
Expressed using XML syntax
•
•
•
Opening tag<jsp:actionElement attribute=”value” ….. >
Body body
Closing tag</jsp:actionElement>
Empty tags (without body) can also be used like
<jsp:actionElement attribute=”value” ….. >
© Copyright Virtual University of Pakistan 353 Web Design and Development (CS506) Some JSP Action Elements
•
To work with JavaBeans
-<jsp:useBean />
-<jsp:setProperty />
-<jsp:getProperty />
•
To include resources at request time
-<jsp:include />
•
To forward request to another JSP or Servlet
-<jsp:forward />
•
To work with applets
-<jsp:plugin />
37.2 Working with JavaBeans using JSP Action Elements
The three action elements are used to work with JavaBeans. These are discussed in detail
below:
37.2.1 JSP useBean Action Element
It is used to obtain a reference to an existing JavaBean object by specifying id(name of
object) and scope in which bean is stored. If a reference is not found, the bean is
instantiated.
The format of this action element is:
<jsp:useBean
id = “name”
scope = “page|request|session|application” class=“package.Class ”
/>
The id attribute specifies the name of the JavaBean object that is also used for later
references. The scope attribute can have one possible value out of page, request, session and
application. If this attribute is omitted, the default value of scope attribute is page. We’ll discuss in
detail about scope shortly.
The class attribute specifies the type of object is going to be created.
jsp:useBean is being equivalent to building an object in scriptlet. For example to build an
object of MyBean using scriptlet is:
<%
MyBean m = new MyBean( );
%>
© Copyright Virtual University of Pakistan 354 Web Design and Development (CS506) Achieving above functionality using
this:
jsp:useBean action element will look like
<jsp:useBean
id = “m”
scope = “page”
class=“vu.MyBean”
/>
In the above code snippet, we are assuming that MyBean lies in vu package.
37.2.2 JSP setProperty Action Element
To set or change the property value of the specified bean. String values are converted to types of
properties by using the related conversion methods.
The format of this action element is:
<jsp:setProperty name = “beanName or id” property = “name” value
=“value”/>
The name attribute should match the id given in jsp:useBean. The property
attribute specifies the name of the property to change and the value attribute specifies the new
value.
jsp:setProperty is being equivalent to following code of scriptlet. For example to change
the name property of m (instance of MyBean) using scriptlet is:
<%
m.setProperty(“ali”);
%>
Achieving above functionality using jsp:setProperty action element will look like
this:
<jsp:setProperty name = “m” property = “name” value = “ali” />
37.2.3 JSP getProperty Action Element
Use to retrieves the value of property, converts it to String and writes it to output stream.
The format of this action element is:
<jsp:getProperty name = “beanName or id”property = “name”/>
jsp:getProperty is being equivalent to following code of scriptlet. For example to
retrieve the name property of m (instance of MyBean) followed by writing it to output stream,
scriptlet code will look like:
© Copyright Virtual University of Pakistan 355 Web Design and Development (CS506) <%
String name = m.getName( ); out.println(name);
%>
Achieving above functionality using jsp:getProperty action element will look like
this:
<jsp:getProperty name = “m”property = “name” />
Example Code: Calculating sum of two numbers by using action
elements and JavaBean
This example contains index.jsp and result.jsp and one JavaBean i.e.
SumBean. User will enter two numbers on index.jsp and their sum will be
displayed on result.jsp. Let’s examine these one after another
SumBean.java
The SumBean has following attributes
•
•
•
firstNumber
secondNumber
sum
The firstNumber and secondNumbers are “write-only” properties means for these only
setters would be defined. Whereas sum is a “read-only” property as only getter would be
defined for it.
The SumBean also contain one additional method for calculating sum i.e.
calulateSum(). After performing addition of firstNumber with secondNumber, this
method will assign the result to sum attribute.
package vu;
import java.io.*;
public class SumBean implements Serializable{
private int firstNumber;
private int secondNumber;
private int sum;
// no argument constructor
public SumBean() {
firstNumber = 0;
secondNumber = 0;
sum = 0;
© Copyright Virtual University of Pakistan 356 Web Design and Development (CS506) }
// firstNumber & secondNumber are writeonly properties
// setters
public void setFirstNumber(int n){
firstNumber = n;
}
public void setSecondNumber(int n){
secondNumber = n;
}
// no setter for sum
// sum is a read only property
public int getSum( ){
return sum;
}
// method to calculate sum
public void calculateSum() {
sum = firstNumber + secondNumber;
}
}
index.jsp
This page will display two text fields to enter number into them.
<html>
<body>
<h2>Enter two numbers to calculate their sum</h2>
<form name="myForm" action="result.jsp">
<h3>
Enter first number
<input type="text" name="num1" />
<br/>
Enter second number
<input type="text" name="num2" />
<br/>
<input type="submit" value="Calculate Sum" />
</h3>
</form>
</body>
</html>
result.jsp
This page will calculate the sum of two entered numbers by the user and displays the sum
back to user. The addition is performed using SumBean
<%-- importing vu package that contains the SumBean --%>
<%@page import="vu.*"%>
© Copyright Virtual University of Pakistan 357 Web Design and Development (CS506) <html>
<body>
<h2>The sum is:
<%-- instantiating bean using action element -- %>
<%-//Servlet equivalent code of useBean
SumBean sBean = new SumBean();
--%>
<jsp:useBean id="sBean" class="vu.SumBean" scope="page"/>
<%-- setting firstNumber property of sBean
using action elements
-- %>
<%-- implicit conversion from string to int as num1 is of type
String and firstNumber is of type int
--%>
<%-//Servlet equivalent code of setProperty for num1
int no = Integer.parseInt(request.getParameter("num1"));
sBean.setFirstNumber(no);
--%>
<jsp:setProperty name="sBean"
property="firstNumber" param="num1" />
<%-//Servlet equivalent code of setProperty for num2
int no = Integer.parseInt(request.getParameter("num2"));
sBean.setSecondNumber(no);
--%>
//Servlet equivalent code of setProperty for num2
int no = Integer.parseInt(request.getParameter("num2"));
sBean.setSecondNumber(no);
<jsp:setProperty name="sBean"
property="secondNumber" param="num2" />
<%
// calling calculateSum() method that will set the value of
// sum attribute
sBean.calculateSum();
%>
<%-// servlet equivalent code of displaying sum
© Copyright Virtual University of Pakistan 358 Web Design and Development (CS506) int res = sBean.getSum();
out.println(res);
--%>
<jsp:getProperty name="sBean" property="sum" />
</h2>
</body>
</html>
37.3 Sharing Beans & Object Scopes
So far, we have learned the following techniques to create objects.
•
•
•
Implicitly through JSP directives
Explicitly through actions
Directly using scripting code
Although the beans are indeed bound to local variables, that is not the only behavior. They are
also stored in four different locations, depending on the value of the optional scope attribute of
jsp:useBean. The scope attribute has the following possible values: page, request,
session and application.
Let’s discover what impact these scopes can produce on JavaBeans objects which are stored in one
of these scopes.
37.3.1 page
This is the default value of scope attribute, if omitted. It indicates, in addition to being
bound to local variable, the bean object should be placed in the pageContext
object. The bean’s values are only available and persist on JSP in which bean is
created.
In practice, beans created with page scope are always accessed (their values) by
jsp:getProperty, jsp:setProperty, scriptlets or expressions later in the same page. This
will be more cleared with the help of following diagram:
© Copyright Virtual University of Pakistan 359 Web Design and Development (CS506) In the diagram above, first.jsp generates a request “request 1” that is submitted to
second.jsp. Now, second.jsp creates an object m of MyBean by calling its default
constructor and stores a value “ali” for the name property by making a call to appropriate setter
method. Since, the scope specified in this example is “page” when the object of MyBean
is instantiated using jsp:useBean action element. Therefore, object (m) of MyBean is stored
in PageContext.
Whether, second.jsp forwards the same request (request 1) to third.jsp or generates a
new request (request 2), at third.jsp, values (e.g. ali) stored in MyBean object m, are not
available. Hence, specifying scope “page” results in using the object on the same page where they
are created.
37.3.2 request
This value signifies that, in addition to being bound to local variable, the bean object
should be placed in ServletRequest object for the duration of the current
request. In other words, until you continue to forward the request to another
JSP/servlet, the beans values are available. This has been illustrated in the following
diagram.
© Copyright Virtual University of Pakistan 360 Web Design and Development (CS506) In the diagram above, MyBean is instantiated by specifying scope =“request”
that results in storing object in ServletRequest. A value “ali” is also stored
in m using setter method.
second.jsp forwards the same request (request 1) to third.jsp, since scope of m (object of
MyBean) is request, as a result third.jsp can access the values(e.g. ali) stored in
m. According to the figure, third.jsp generates a new request (request 2) and submits it to
fourth.jsp. Since a new request is generated therefore values stored in object m (e.g. ali) are
not available to fourth.jsp.
37.3.3 session
This value means that, in addition to being bound to local variable, the bean object
will be stored in the HttpSession object associated with the current request. As
you already know, object’s value stored in HttpSession persists for whole user’s
session. The figure below helps in understanding this concept.
© Copyright Virtual University of Pakistan 361 Web Design and Development (CS506) In the diagram above, MyBean is instantiated by specifying scope = “session” that results
in storing object in HttpSession. A value “ali” is also stored in m using setter method.
Irrespective of request forwarding or new request generation from second.jsp to
other resources, the values stored in HttpSession remains available until user’s
session is ended.
37.3.4 Application
This very useful value means that, in addition to being bound to local variable, the bean object
will be stored in ServletContext. The bean objects stored in ServletContext is
shared by all JSPs/servlets in the same web application. The diagram given below illustrates this
scenario:
© Copyright Virtual University of Pakistan 362 Web Design and Development (CS506) 37.4 Summary of Object’s Scopes
Let’s take another view of session, request & page scopes in the next figure that helps us to
understand the under beneath things.
© Copyright Virtual University of Pakistan 363 Web Design and Development (CS506) The figure shows four JavaServer Pages. Each page has its own page scope. Therefore
objects stored in page scope are only available to same pages on which they are created.
Suppose page1 forwards the request to page2. Objects stored in request scope remains available
to page1 as well to page 2. Similar case is true for page 3 & page 4.
If user makes a visit to all these pages in one session, object’s values stored in session scope
remains available on all these pages.
To understand the difference between sessions & application scope, consider the
following figure:
As you can conclude from the figure, for each user (client), objects are stored in different
sessions. However, in the case of application scope, all users stores objects in single
place.
© Copyright Virtual University of Pakistan 364 Web Design and Development (CS506) 37.5 More JSP Action Elements
Let’s talk about two important action elements. These are include & forward.
37.5.1 JSP include action Element
It is used to include files at request time. For example, to reuse HTML, JSP or plain text content. It’s
important to note that JSP content cannot affect main page (in which output is included); only output
of included JSP is used. It also allows updating of the included content without changing the main JSP.
The jsp:include action element requires two attributes: page & flush.
•
•
page: a relative URL of the file to be included.
flush: must have the value “true”
<jsp:include page = “relative URL”
flush = “true” />
jsp:include is being equivalent to following code of scriptlet. For example to include
the output of one.jsp , scriptlet code will look like:
<%
RequestDispatcher rd =request.getRequestDispatcher(“one.jsp”);
rd.include(request, response);
%>
Achieving above functionality using jsp:include action element will look like this:
<jsp:include page = “one.jsp”
flush = “true” />
37.5.2 JSP forward action Element
It is used to forward request to another resource. The format of
is:
<jsp:forward page = “one.jsp” />
jsp:forward action
jsp:forward is being equivalent to following code of scriptlet. For example to forward
the request to one.jsp , scriptlet code will look like:
<%
RequestDispatcher rd = request.getRequestDispatcher(“one.jsp”);
rd.forward(request, response);
%>
37.6 References:
•
•
Java A Lab Course by Umair Javed.
Core Servlets and JavaServer Pages by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 365 Web Design and Development (CS506) Lecture 38: JSP Custom Tags
To begin with, let’s review our last code example of lecture 36 i.e. Displaying course
outline. We incorporated JavaBeans to minimize the database logic from the JSP. But
still, we have to write some lines of java code inside java.jsp & web.jsp. As
discussed earlier, JSPs are built for presentation purpose only, so all the other code that
involves business and database logic must be shifted elsewhere like we used JavaBeans
for such purpose.
There is also another problem attached to it. Generally web page designers which have enough
knowledge to work with HTML and some scripting language, faced lot of difficulties in
writing some simple lines of java code. To overcome these issues, java provides us the
mechanism of custom tags.
38.1 Motivation
To give you an inspiration, first have a glance over the code snippet we used in JSP of the course
outline example of last lecture. Of course, not all code is given here; it’s just for your reference to
give you a hint.
<%
CourseDAO courseDAO = new CourseDAO(); ………………
// iterating over ArrayList
for (…………………… ) {
……………………
…………………… //displaying courseoutline
}
………………
%>
Can we replace all the above code with one single line? Yes, by using custom tag we can write like
this:
<mytag:coursetag
pageName=“java” />
By only specifying the course/page name, this tag will display the course outline in
tabular format. Now, you must have realized how significant changes custom tags can
bring on.
38.2 What is a Custom Tag?
•
•
In simplistic terms, “a user defined component that is used to perform certain action”.
This action could be as simple as displaying “hello world” or it can be as complex as
displaying course outline of selected course after reading it from database.
It provides mechanism for encapsulating complex functionality for use in JSPs. Thus
© Copyright Virtual University of Pakistan 366 Web Design and Development (CS506) •
facilitates the non-java coders.
We already seen & used many built in tags like:
o < jsp:useBean …… />
o < jsp:include …… />
o < jsp:forward …… /> etc.
38.3 Why Build Custom Tag?
•
•
•
We introduced action <jsp:useBean> and JavaBeans to incorporate complex,
encapsulated functionality in a JSP.
However, JavaBeans cannot manipulate JSP content and Web page designers
must have some knowledge to use JavaBeans in a page
With Custom tags, it is possible for web page designers to use complex
functionality without knowing any java
38.4 Advantages of using Custom Tags
•
•
•
Provides cleaner separation of processing logic and presentation, than JavaBeans.
Have access to all JSP implicit objects like out, request etc.
Can be customized by specifying attributes.
38.5 Types of Tags
Three types of can be constructed. These are:
1. Simple Tag
2. Tag with Attribute
3. Tag with Body
38.5.1 Simple Tag
A simple tag has the following characteristics:
•
•
•
•
Start and End of tag
No body is specified within tag
No attributes
For example
© Copyright Virtual University of Pakistan 367 Web Design and Development (CS506) 38.5.2 Tag with Attributes
A tag with attributes has the following characteristics:
•
•
•
•
Start and End of tag
Attributes within tag
No body enclosed
For example
< mytag:hello
attribute = “value” />
38.5.3 Tag with Body
A tag with body has the following characteristics:
•
•
•
•
Start and End of tag
May be attributes
Body enclosed within tag
For example
< mytag:hello
optional_attributes ………… >
some body
</ mytag:hello >
38.6 Building Custom Tags
So far, we have used many built-in tags. Now the time has come to build your own one. Custom
tags can be built either by using JSP 1.2 specification or JSP 2.0 (latest) specification.
To develop custom tags using JSP 1.2 involves lot of cumbersome (too difficult for James
Gossling also ). However, JSP 2.0 brings lots of goodies like
•
•
•
•
Simple tag extensions to build custom tags
Integrated Expression Language (will be discussed in coming lecture)
Also provides an alternate mechanism for building custom tags using tag files
(.tag)
Improved XML syntax etc.
38.6.1 Steps for Building Custom Tags
The following steps are used in order to develop your own custom tag. These are:
1. Develop the Tag Handler class
2. Write Tag library Descriptor (.tld) file
3. Deployment
© Copyright Virtual University of Pakistan 368 Web Design and Development (CS506) 38.6.2 Develop the Tag Handler class
•
•
•
•
Tag Handler is also a java class that is implicitly called when the associated tag is
encountered in the JSP.
Must implement SimpleTag interface
Usually extend from SimpleTagSupport class that has already implemented
SimpleTag interface.
For example,
public class MyTagHandler extends SimpelTagSupport {
•
•
•
•
………………………
………………………
}
doTag() method
o By default does nothing
o Need to implement / override to code/write functionality of tag
o Invoked when the end element of the tag encountered.
JSP implicit objects (e.g. out etc) are available to tag handler class through
pageContext object.
pageContext object can be obtained using getJspContext() method.
For example to get the reference of implicit out object, we write.
o PageContext pc = (PageContext) getJspContext();
o JspWriter out = pc.getOut();
38.6.3 Write Tag Library Discriptor (.tld) file
•
•
It is a XML based document.
Specifies information required by the JSP container such as:
o Tag library version
o JSP version
o Tag name
o Tag Handler class name
o Attribute names etc.
Note: If you are using any IDE (like netBeans® 4.1, in order to build custom tags,
the IDE will write .tld file for you.
38.6.4 Deployment
•
•
Place Tag Handler class in myapp/WEB-INF/classes folder
application.
Place .tld file in myapp/WEB-INF/tlds folder of web application.
of
web
Note: Any good IDE will also perform this step on your behalf
© Copyright Virtual University of Pakistan 369 Web Design and Development (CS506) 38.7 Using Custom Tags
Use taglib directive in JSP to refer to the tag library. For example
<%@ taglib uri=”TLD file name” prefix=“mytag” %>
The next step is to call the tag by its name as defined in TLD. For example, if tag
name is hello then we write:
< mytag:hello />
where mytag is the name of prefix specified in taglib directive.
What actually happened behind the scenes? Container calls the doTag()
method of appropriate tag handler class. After that, Tag Handler will write the
appropriate response back to the page.
Example Code: Building simple tag that displays “Hello World”
Enough we have talked about what are custom tags, their types. Now, it is a time to
build a custom tag that displays “Hello World”.
Approach
•
•
•
Extend Tag Handler class from SimpleTagSupport class and override
doTag() method
Build TLD file
Deploy
Note: As mentioned earlier, if you are using any IDE (like netBeans® 4.1), the last two steps will
be performed by the IDE.
WelcomeTagHandler.java
package vu;
// importing required packages
import javax.servlet.jsp.*;
import javax.servlet.jsp.tagext.*;
// inheriting from SimpleTagSupport class
public class WelcomeTagHandler extends SimpleTagSupport {
// overriding doTag() method
public void doTag() throws JspException {
© Copyright Virtual University of Pakistan 370 Web Design and Development (CS506) // obtaining the reference of out implicit object
PageContext pageContext = (PageContext)getJspContext();
JspWriter out = pageContext.getOut();
try {
out.println(" Hello World ");
} catch (java.io.IOException ex) {
throw new JspException(ex.getMessage());
}
} // end doTag() method
} // end WelcomeTagHandler class
customtags.tld
If using IDE, this file will be written automatically. In this file you specify the tag name
along with Tag Handler class.
<?xml version="1.0" encoding="UTF-8"?>
<taglib version="2.0" xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee webjsptaglibrary_2_0.xsd">
<tlib-version>1.0</tlib-version>
<short-name>mytag</short-name>
<!—the value of uri will be used in JSP to refer to this tld -->
<uri>/WEB-INF/tlds/customtags</uri>
<!—
Specifying the tag name and tag class. Also mentioning that
this tag has no body
-->
<tag>
<name>welcome</name>
<tag-class>vu.WelcomeTagHandler</tag-class>
<body-content>empty</body-content>
</tag>
</taglib>
© Copyright Virtual University of Pakistan 371 Web Design and Development (CS506) index.jsp
<%-using taglib directive, specifying the tld file name as well as
prefix. Note that you you use any value for the prefix attribtute
--%>
<%@taglib uri="/WEB-INF/tlds/customtags.tld" prefix="mytag" %>
<html>
<body>
<h2>A Simple Tag Example</h2>
<h3>
<%-- calling welcome tag with the help of prefix --%>
<mytag:welcome />
</h3>
</body>
</html>
38.8 Building tags with attributes
If you want to build a tag that can also take attributes, for example
<mytag:hello attribute=”value” />
To handle attributes, you need to add
Instance variables and Corresponding setter methods
Behind the scenes, container will call these setter methods implicitly and pass the value
of the custom tag attribute as an argument.
Example Code: Building tag with attribute
In this example, we will modify our course outline example to incorporate tags. Based on
attribute value, the tag will display the respective course outline in tabular format.
Approach
•
Extend Tag Handler class from SimpleTagSupport class
o Add instance variable of type String
o Write setter method for this attribute
© Copyright Virtual University of Pakistan 372 Web Design and Development (CS506) •
•
o Override doTag() method
Build TLD file
Deploy
CourseOutlineBean.java
This is the same file used in the last example
package vubean;
import java.io.*;
public class CourseOutlineBean implements Serializable{
private int sessionNo;
private String topic;
private String assignment;
// no argument constructor
public CourseOutlineBean() {
sessionNo = 0;
topic = "";
assignment = "";
}
// setters
public void setSessionNo(int s){
sessionNo = s;
}
public void setTopic(String t){
topic = t;
}
public void setAssignment(String a){
assignment = a;
}
// getters
public int getSessionNo( ){
return sessionNo;
}
public String getTopic( ){
return topic;
}
public String getAssignment( ){
return assignment;
}
} // end class
© Copyright Virtual University of Pakistan 373 Web Design and Development (CS506) CourseDAO.java
No changes are made to this file too.
package vu;
import
import
import
import
java.io.*;
java.sql.*;
java.util.*;
vubean.*;
public class CourseDAO implements Serializable{
private Connection con;
public CourseDAO() {
establishConnection();
}
//********** establishConnection method ********************
// method used to make connection with database
private void establishConnection(){
try{
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:CourseDSN";
con = DriverManager.getConnection(conUrl);
}catch(Exception ex){
System.out.println(ex);
}
}
//*********** retrieveCourseList method ********************
public ArrayList retrieveCourseList(String cName){
ArrayList courseList = new ArrayList();
try{
String sql = " SELECT sessionNo, topic, assignment " +
" FROM Course, SessionDetail" +
" WHERE courseName = ? " +
" AND Course.courseId = SessionDetail.courseID ";
PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString(1, cName);
© Copyright Virtual University of Pakistan 374 Web Design and Development (CS506) ResultSet rs = pStmt.executeQuery();
int sNo;
String topic;
String assignment;
while ( rs.next() ) {
sNo
= rs.getInt("sessionNo");
topic = rs.getString("topic");
assignment = rs.getString("assignment");
if (assignment == null){
assignment = "";
}
// creating a CourseOutlineBean object
CourseOutlineBean cBean = new CourseOutlineBean();
cBean.setSessionNo(sNo);
cBean.setTopic(topic);
cBean.setAssignment(assignment);
// adding a bean to arraylist
courseList.add(cBean);
}
}catch(Exception ex){
System.out.println(ex);
} finally {
// to close connection
releaseResources();
}
// returning ArrayList object
return courseList;
} // end retrieveCourseOutline
//********** releaseResources method ********************
private void releaseResources(){
try{
if(con != null){
con.close();
}
}catch(Exception ex){
System.out.println();
© Copyright Virtual University of Pakistan 375 Web Design and Development (CS506) }
} // end releaseResources
}// end CourseDAO
MyTagHandler.java
The tag handler class uses JavaBeans (CourseOutlineBean.java &
CourseDAO.java), and includes the logic of displaying course outline in tabular
format.
package vutag;
// importing package that contains the JavaBeans
import vubean.*;
import vu.*;
import javax.servlet.jsp.tagext.*;
import javax.servlet.jsp.*;
import java.util.*;
public class MyTagHandler extends SimpleTagSupport {
/*
Declaration of pageName property.
*/
private String pageName;
public void doTag() throws JspException {
CourseDAO courseDAO = new CourseDAO();
ArrayList courseList = courseDAO.retrieveCourseList(pageName);
// to display course outline in tabular form, this method is
// used – define below
display(courseList);
}
/*
Setter for the pageName attribute.
*/
public void setPageName(java.lang.String value) {
this.pageName = value;
}
/*
display method used to print courseoutline in tabular form
*/
private void display(ArrayList courseList)throws JspException{
© Copyright Virtual University of Pakistan 376 Web Design and Development (CS506) PageContext pc = (PageContext)getJspContext();
JspWriter out = pc.getOut();
try{
// displaying table headers
out.print("<TABLE BORDER=1 >");
out.print("<TR>");
out.print("<TH> Session No </TH>");
out.print("<TH> Topic </TH>");
out.print("<TH> Assignment </TH>");
out.print("</TR>");
// loop to iterate over courseList
for (int i=0; i<courseList.size(); i++){
CourseOutlineBean courseBean =
(CourseOutlineBean)courseList.get(i);
// displaying one row
out.print("<TR>");
out.print("<TD>" + courseBean.getSessionNo() + "</TD>");
out.print("<TD>" + courseBean.getTopic() + "</TD>");
out.print("<TD>" + courseBean.getAssignment() + "</TD>");
out.print("</TR>");
}catch(java.io.IOException ex){
throw new JspException(ex.getMessage());
}
}
} // end clas MyTagHandler.java
mytaglibrary.tld
<?xml version="1.0" encoding="UTF-8"?>
<taglib version="2.0" xmlns="http://java.sun.com/xml/ns/j2ee"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee webjsptaglibrary_2_0.xsd">
<tlib-version>1.0</tlib-version>
<short-name>mytaglibrary</short-name>
<!—the value of uri will be used in JSP to refer to this tld -->
<uri>/WEB-INF/tlds/mytaglibrary</uri>
<!—
© Copyright Virtual University of Pakistan 377 Web Design and Development (CS506) Specifying the tag name and tag class. Also mentioning that
this tag has no body
-->
<tag>
<name>coursetag</name>
<tag-class>vutag.MyTagHandler</tag-class>
<body-content>empty</body-content>
<!—
Specifying the attribute name and its type
-->
<attribute>
<name>pageName</name>
<type>java.lang.String</type>
</attribute>
</tag>
</taglib>
out.print("</TABLE>");
index.jsp
This page is used to display the course options to the user in the radio button form.
<html>
<body>
<h2>Select the page you want to visit</h2>
<form name="myForm" action="controller.jsp" >
<h3>
<input type="radio" name = "page" value="web"/>
Web Design & Development
</h3>
<br>
<h3>
<input type="radio" name = "page" value="java"/>
Java
</h3>
<br>
<input type="submit" value="Submit" />
</form>
</body>
</html>
© Copyright Virtual University of Pakistan 378 Web Design and Development (CS506) controller.jsp
Based upon the selection made by the user, this page will redirect the user to respective
pages. Those are web.jsp and java.jsp
<html>
<body>
<!-- scriptlet -->
<%
String pageName = request.getParameter("page");
if (pageName.equals("web")) {
response.sendRedirect("web.jsp");
} else if (pageName.equals("java") )
response.sendRedirect("java.jsp");
}
%>
{
</body>
</html>
java.jsp
<%-- using taglib directive, specifying the tld file and prefix -%>
<%@taglib uri="/WEB-INF/tlds/mytaglibrary.tld" prefix="mytag"%>
<html>
<body>
<center>
<h2> Welcome to Java Learning Center </h2>
<h3> Course Outline</h3>
<%-calling coursetag and specifying java as attribute
value
--%>
<mytag:coursetag pageName="java" />
</center>
</body>
</html>
© Copyright Virtual University of Pakistan 379 Web Design and Development (CS506) web.jsp
<%-- using taglib directive, specifying the tld file and prefix -%>
<%@taglib uri="/WEB-INF/tlds/mytaglibrary.tld" prefix="mytag"%>
<html>
<body>
<center>
<h2> Welcome to Java Learning Center </h2>
<h3> Course Outline</h3>
<%-calling coursetag and specifying java as attribute
value
--%>
<mytag:coursetag pageName="java" />
</center>
</body>
</html>
38.9 References:
•
•
Java A Lab Course by Umair Javed.
Core Servlets and JavaServer Pages by Marty Hall
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 380 Web Design and Development (CS506) Lecture 39: MVC + Case Study
We have covered an adequate amount of Servlets and JSPs in detail. Now, the time has come to learn
different architectures that are most commonly used for the sake of web development. These architectures
also help us to understand where these components best fit in. In this handout, we’ll cover the most
widely used/popular architecture i.e. Model
View Controller (MVC).
A small case study “Address Book” is also part of this handout that is based on MVCModel 1. Before
moving on to MVC, let’s see what error pages are and how they are
used?
39.1 Error Page
Error Pages enables you to customize error messages. You can even hide them from the user's view
entirely, if you want. This also makes possible to maintain a consistent look and feel throughout an
application, even when those dreaded error messages are thrown.
By means of page directive, a JSP can be given the responsibility of an Error page. An Error JSP is called
by the web server when an uncaught exception gets occurred. This exception is passed as an instance of
java.lang.Throwable to Error JSP (also accessible via implicit exception object).
39.1.1 Defining and Using Error Pages
isErrorPage attribute of a page directive is used to declare a JSP as an error page.
JSP pages are informed about the error page by setting errorPage attribute of page directive
In the figure below, error.jsp is defined as JSP Error page and index.jsp is informed to call
error.jsp if any uncaught exception rose. This is done by setting attributes errorPage and
isErrorPage of the page directive on these JSPs.
© Copyright Virtual University of Pakistan 381 Web Design and Development (CS506) 39.2 Case Study – Address Book
What we have learned is going to be implemented in this Address Book example. Here MS-Access is being
used as DBMS. This database will have only one table, Person with following attributes
39.2.1 Ingredients of Address Book
Java Beans, Java Server Pages and Error Page that are being used in this Address Book Example are: -
Java Beans
• PersonInfo – Has following attributes:
o name
o address
o phoneNum
• PersonDAO
o Encapsulates database logic.
o Therefore, it will be used to save and retrieve PersonInfo data.
Java Server Pages
• addperson.jsp
o Used to collect new person info that will be saved in database.
• saveperson.jsp
o Receives person info from addperson.jsp
o Saves it to database
• searchperson.jsp
o Used to provide search criteria to search Person’s info by providing name
• showperson.jsp
o This page receive person’s name from searchperson.jsp to search in
o database
o Retrieves and displays person record found against person name
Error Page
• addbookerror.jsp
o This page is declared as an error page and used to identify the type of exception.
o In addition to that, it also displays the message associated with the received
exception to the user.
© Copyright Virtual University of Pakistan 382 Web Design and Development (CS506) addperson.jsp takes person’s information from the user and sends it to
saveperson.jsp. After receiving request, saveperson.jsp makes an object of
PersonInfo using received information and saves it into the database using PersonDAO
Java bean.
Similarly, searchperson.jsp takes search criteria (name) from the user and passes it to
showperson.jsp that searches the record in database using PersonDAO and shows the
results to the user.
If any uncaught exception is generated on these JSP, addbookerror.jsp is called
implicitly, which displays an appropriate message to the user after identifying the exception type.
Code for the Case Study
Let’s have a look on the code of each component used in the case study; first start from
JavaBeans.
PersonInfo
PersonInfo represents the record of one person and its objects are used to interrupt the
information about persons.
package vu;
import java.io.*;
public class PersonInfo implements Serializable{
private String name;
private String address;
© Copyright Virtual University of Pakistan 383 Web Design and Development (CS506) private int phoneNum;
// no argument constructor
public PersonInfo() {
name = "";
address = "";
phoneNum = 0;
}
// setters
public void setName(String n){
name = n;
}
public void setAddress(String a){
address = a;
}
public void setPhoneNum(int pNo){
phoneNum = pNo;
}
// getters
public String getName( ){
return name;
}
public String getAddress( ){
return address;
}
public int getPhoneNum( ){
return phoneNum;
}
} // end class PersonInfo
PersonDAO
This class will help in retrieving and storing person’s records in database. The code is
given below:
package vu;
import java.util.*;
import java.sql.*;
public class PersonDAO{
private Connection con;
© Copyright Virtual University of Pakistan 384 Web Design and Development (CS506) // default constructor
public PersonDAO() throws ClassNotFoundException , SQLException
{
establishConnection();
}
// method used to establish connection with db
private void establishConnection() throws ClassNotFoundException
, SQLException
{
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:PersonDSN";
con = DriverManager.getConnection(conUrl);
}
// used to search the person records against name and returns
// the ArrayList that contains only those PersonInfo objects
// which matches the search criteria i.e. name
public ArrayList retrievePersonList(String pName) throws
SQLException
{
ArrayList personList = new ArrayList();
// preparing query
String sql = " SELECT * FROM Person WHERE name = ?";
PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString( 1, pName);
// executing query
ResultSet rs = pStmt.executeQuery();
String name;
String add;
int pNo;
while ( rs.next() ) {
name = rs.getString("name");
add = rs.getString("address");
pNo = rs.getInt("phoneNumber");
// creating a CourseOutlineBean object
PersonInfo personBean = new PersonInfo();
© Copyright Virtual University of Pakistan 385 Web Design and Development (CS506) personBean.setName(name);
personBean.setAddress(add);
personBean.setPhoneNum(pNo);
// adding a bean to arraylist
personList.add(personBean);
} // end while
return personList;
} // end retrievePersonList
// this method accepts an object of PersonInfo, and stores it
// into the database
public void addPerson(PersonInfo person) throws SQLException{
String sql = " INSERT INTO Person(name, address, phoneNumber)
VALUES (?, ?, ?)";
PreparedStatement pStmt = con.prepareStatement(sql);
String name = person.getName();
String add = person.getAddress();
int pNo = person.getPhoneNum();
pStmt.setString( 1 , name );
pStmt.setString( 2 , add );
pStmt.setInt( 3 , pNo );
pStmt.executeUpdate();
} // end addPerson
// overriding finalize method to release acquired resources
public void finalize( ) {
try{
if(con != null){
con.close();
}
}catch(SQLException sqlex){
System.out.println(sqlex);
}
} // end finalize
} // end PersonDAO class
© Copyright Virtual University of Pakistan 386 Web Design and Development (CS506) Now let’s take a look at the code for JSP pages
addperson.jsp
This JSP page gets person record’s information from the user. It contains three Input Fields for
name, address and phone number as shown in the diagram. This page sends this information to
saveperson.jsp for further processing.
The code that is used to generate the above page is given below:
<%-Although there are no chances of exception to arise on this page,
for consistency, error page is defined on top of all JSPs
--%>
<%@page errorPage="addbookerror.jsp" %>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Add New Person</h3>
<%-- Form that contains Text input fields and sending it to
saveperson.jsp
--%>
<form name ="register" action="saveperson.jsp" />
<TABLE BORDER="1" >
© Copyright Virtual University of Pakistan 387 Web Design and Development (CS506) <TR>
<TD> <h4 > Name </h4> </TD>
<TD> <input type="text" name="name" />
</TD>
</TR>
<TR>
<TD> <h4> Address </h4> </TD>
<TD> <input type="text" name="address" />
</TD>
</TR>
<TR>
<TD> <h4>Phone Number</h4> </TD>
<TD> <input type="text" name="phoneNum" />
</TD>
</TR>
<TR>
<TD COLSPAN="2" ALIGN="CENTER" >
<input type="submit" value="save" />
<input type="reset" value="clear" />
</TD>
</TR>
</TABLE>
</form>
<h4>
<%-- A link to searchperson.jsp --%>
<a href="searchperson.jsp" > Search Person </a>
</h4>
</center>
</body>
</html>
saveperson.jsp
This JSP page gets data from the addperson.jsp, makes an object of PersonInfo and
saves it to the database using PersonDAO class. Apart from these, it also displays an
informative message to the user if new person record is saved successfully into thedatabase and
two hyperlinks to navigate on to the desired pages as shown in the following diagram:
© Copyright Virtual University of Pakistan 388 Web Design and Development (CS506) The code of this page is given below:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<%@ page import="java.sql.*" %>
<html>
<body>
<%-- creating PersonDAO object and storing in page scope --%>
<jsp:useBean id="pDAO" class="vu.PersonDAO" scope="page" />
<%-- creating PersonBean object and storing in page scope --%>
<jsp:useBean id="personBean" class="vu.PersonInfo" scope = "page"
/>
<%-setting all properties of personBean object with input
parameters using *
--%>
<jsp:setProperty name="personBean" property="*" />
<%-to save Person record into the database, calling addperson
method of PersonDAO
--%>
<%
pDAO.addPerson(personBean);
%>
<center>
<h3> New Person Record is saved successfully!</h3>
<h4>
<a href="addperson.jsp" > Add Person </a>
</h4>
<h4>
<a href="searchperson.jsp" > Search Person </a>
</h4>
</center>
</body>
</html>
© Copyright Virtual University of Pakistan 389 Web Design and Development (CS506) searchperson.jsp
It gets search criteria from the user (i.e. name) and sends it to showperson.jsp to display the
search results. The outlook of the page is given below:
The code used to generate the above page given page is:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Search Person</h3>
<%-Form that contains Text input field and sending it to
showperson.jsp
--%>
<form name ="search" action="showperson.jsp" />
<TABLE BORDER="1" >
<TR>
<TD> <h4 >Name</h4>
</TD>
<TD> <input type="text" name="name" />
</TR>
<TR>
<TD COLSPAN="2" ALIGN="CENTER"">
© Copyright Virtual University of Pakistan </TD>
390 Web Design and Development (CS506) <input type="submit" value="search" />
<input type="reset" value="clear" />
</TD>
</TR>
</TABLE>
</form>
<h4>
<a href="addperson.jsp" > Add Person </a>
</h4>
</center>
</body>
</html>
showperson.jsp
showperson.jsp receives search criteria (i.e. name) from the searchperson.jsp,
that is entered by the user to find the matching record. This page retrieves the complete list of
matching records from the database using PersonDAO, and shows them to the user.
This following figure gives you the sight, when person named “saad” is searched.
Below, the code of showperson.jsp is given:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<%-- importing required packages --%>
<%@page import="java.util.*" %>
<%@page import="vu.*" %>
<html>
© Copyright Virtual University of Pakistan 391 Web Design and Development (CS506) <body>
<center>
<h2> Address Book </h2>
<h3> Following results meet your search criteria</h3>
<TABLE BORDER="1" >
<TR>
<TH> Name </TH>
<TH> Address </TH>
<TH> PhoneNum </TH>
</TR>
<jsp:useBean id="pDAO" class="vu.PersonDAO" scope="page" />
<%
// getting search criteria sent by searchperson.jsp
String pName = request.getParameter("name");
// retrieving matching records from the Database using
// retrievePersonList() method of PersonDAO
ArrayList personList = personDAO.retrievePersonList(pName);
PersonInfo person = null;
// Showing all matching records by iterating over ArrayList
for(int i=0; i<personList.size(); i++) {
person = (PersonInfo)personList.get(i);
%>
<TR>
<TD> <%= person.getName()%> </TD>
<TD> <%= person.getAddress()%> </TD>
<TD> <%= person.getPhoneNum()%> </TD>
</TR>
<%
} // end for
%>
</TABLE >
<a href="addperson.jsp" > Add Person </a>
<a href="searchperson.jsp" > Search Person </a>
</center>
</body>
</html>
addbookerror.jsp
This JSP error page is called implicitly by all other JSP pages whenever any uncaught
© Copyright Virtual University of Pakistan 392 Web Design and Development (CS506) /unhandled exception occurs. It also finds out the type of the exception that is generated,and
shows an appropriate message to the user:
<%-- indicating that this is an error page --%>
<%@page isErrorPage="true" %>
<%-- importing class --%>
<%@page import = "java.sql.SQLException" %>
<html>
<head>
<title>Error</title>
</head>
<body>
<h2>
Error Page
</h2>
<h3>
<%-- scriptlet to determine exception type --%>
<%
if (exception instanceof SQLException) {
%>
An SQL Exception
<%
} else if (exception instanceof ClassNotFoundException){
%>
A Class Not Found Exception
<%
} else {
%>
A Exception
<%
} // end if-else
%>
<%-- end scriptlet to determine exception type --%>
occured while interacting with the database
</h3>
© Copyright Virtual University of Pakistan 393 Web Design and Development (CS506) <h3>
The Error Message was
<%= exception.getMessage() %>
</h3>
<h3 > Please Try Again Later! </h3>
<%-hyperlinks to return back to addperson.jsp or
searchperson.sjp
--%>
<h3>
<a href="controller.jsp?action=addperson" >
Add Person
</a>
<a href="controller.jsp?action=searchperson" >
Search Person
</a>
</h3>
</body>
</html>
39.3 Model View Controller (MVC)
Now, more than ever, enterprise applications need to support multiple types of users with multiple
types of interfaces. For example, an online store may require an HTML front for Web customers,
a WML front for wireless customers, a JavaTM (JFC) / Swing interface for administrators, and an
XML-based Web service for suppliers
Also, several problems can arise when applications contain a mixture of data access code,business
logic code, and presentation code. Such applications are difficult to maintain,because
interdependencies between all of the components cause strong ripple effects whenever a change is
© Copyright Virtual University of Pakistan 394 Web Design and Development (CS506) made anywhere. High coupling makes classes difficult or impossible to reuse because they
depend on so many other classes. Adding new data views often requires re-implementing or
cutting and pasting business logic code, which then requires maintenance in multiple places. Data
access code suffers from the same problem, being cut and pasted among business logic methods.
The Model-View-Controller architecture solves these problems by decoupling data access,
business logic, and data presentation and user interaction. Such separation allows multiple views
to share the same enterprise data model, which makes supporting multiple clients easier to
implement, test, and maintain.
39.3.1 Participants and Responsibilities
The individual’s responsibility of three participants (model, view & controller) is given below:
• Model
The model represents the state of the component (i.e. its data and the methods required to
manipulate it) independent of how the component is viewed or rendered.
• View
The view renders the contents of a model and specifies how that data should be presented.
There can be multiple views for the same model within single applications or model may
have different views in different applications or operating systems.
• Controller
The controller translates interactions with the view into actions to be performed by the
model. In a web application, they appear as GET and POST HTTP requests. The actions
performed by the model include activating business processes or changing the state of the
model. Based on the user interactions and the outcome of the model actions, the controller
responds by selecting an appropriate view.
39.3.2 Evolution of MVC Architecture
In the beginning, we used no MVC. Then we had MVC Model 1 and MVC Model 2 architectures.
And people came up with so called web application frameworks such as Apache Struts based on
Model 2 architecture. And finally we have a standard web based application framework i.e.
JavaServer Faces (JSF).
In this handout, we’ll only talk about MVC Model 1.
39.3.2.1 MVC Model 1
A Model 1 architecture consists of a Web browser directly accessing Web-tier JSP pages.The JSP
pages access JavaBeans that represent the application model. And the next view to display
(JSP page, servlet, HTML page, and so on) is determined either by hyperlinks selected in the
source document or by request parameters.
© Copyright Virtual University of Pakistan 395 Web Design and Development (CS506) In Model 1 architecture, view selection is decentralized, because the current page being displayed
determines the next page to display. In addition, each JSP page or servlet processes its own inputs
(parameters from GET or POST). And this is hard to maintain, for example, if you have to change
the view selection, then several JSP pages need to be changed. In some Model 1 architectures,
choosing the next page to display occurs in scriptlet code, but this usage is considered poor form.
In MVC Model 1 architecture, the JSP page alone is responsible for processing the incoming
request and replying back to the client. There is still separation of presentation from content,
because all data access is performed using JavaBeans.
Although the Model 1 architecture should be perfectly suitable for simple applications, it may not
be desirable for complex implementations. Random usage of this architecture usually leads to a
significant amount of scriptlets or Java code embedded within the JSP page, especially if there is a
significant amount of request processing to be performed. While this may not seem to be much of
a problem for Java developers, it is certainly an issue if your JSP pages are created and maintained
by designers which are only aware of HTML and some scripting language.
Note: Probably some of you must be thinking about the case study discussed earlier inthis
handout. Indeed, it is based on MVC Model 1 architecture.
39.4 References:
•
•
•
Java A Lab Course by Umair Javed
Java BluePrints - J2EE Patterns
http://java.sun.com/blueprints/patterns/MVC-detailed.html
Exploring the MVC Design Pattern
http://www.javaworld.com/javaworld/jw-12-1999/jw-12-ssj-jspmvc.html
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 396 Web Design and Development (CS506) Lecture 40: MVC Model 2 Architecture
We have studied page-centric approach and page-with-bean approach until now. You must be
wondering when we had covered these. Probably these buzz words are new one for you but we
already covered these topics. Let’s review these once again.
40.1 Page-Centric Approach
A web application that is collection of JSPs. Generally this approach is followed to get started with
developing web applications. This approach is represented in the following diagram:
The page-centric approach has lot of draw backs such as the code becomes a mixture of
presentation, business and data access logic. The maintenance and up-gradation of the application
becomes a nightmare. Scaling of such kind of application is also difficult and lots of code is also
get duplicated.
40.1.1 Page-with-Bean Approach (MVC Model1)
This approach is different from page-centric approach in a way that all the business logic goes
into JavaBeans. Therefore, the web application is a collection of JSPs and JavaBeans. But still
this approach is insufficient to separate different kind of logics. We have made an address book
example in the last handout using this approach.
© Copyright Virtual University of Pakistan 397 Web Design and Development (CS506) 40.2 MVC Model 2 Architecture
This architecture introduces a controller. This controller can be implemented using JSP or
servlet. Introducing a controller gives the following advantages:
It centralizes the logic for dispatching requests to the next view based on:
•
•
•
The Request URL
Input Parameters
Application state
It gives the single point of control to perform security checks and to record logging information
It also encapsulates the incoming data into a form that is usable by the back-end MVC model.
We’ll discuss it with the help of an example.
The following figure will help you to understand the architecture and functioning of the
application that is built using MVC Model 2 architecture.
The client (browser) sends all the requests to the controller. Servlet/JSP acts as the Controller and
is in charge of the request processing and creation of any beans or objects(Models) used by the
JSP.
JSP is working as View and there is not much processing logic within the JSP page itself,it is
simply responsible for retrieving objects and/or beans, created by the Servlet,extracting dynamic
content from them and put them into the static templates.
© Copyright Virtual University of Pakistan 398 Web Design and Development (CS506) 40.3 Case Study: Address Book using MVC Model 2
The address book example that is built using page-with-bean approach will be modified to
incorporate controller. We’ll show you how to implement controller using JSP as well as with
servlet. Let’s first incorporate controller using JSP.
40.3.1 Introducing a JSP as Controller
Add another JSP (controller.jsp) that
•
•
•
•
Acts as a controller Recieves requests from addperson.jsp & searchperson.jsp
Identifies the page which initiates the request
Uses JavaBeans to save or search persons to/from database
Forwards or redirects the request to appropriate (saveperson.jsp or
showperson.jsp) page.
The program flow of this example is shown in the following diagram:
As you can see in the diagram that all the requests are submitted to controller which uses the
JavaBeans and forwards/redirects the user to another view (JSP)? If any exception arises on
controller or JSPs, the control would automatically be transferred to addbookerror.jsp to
display an appropriate message.
40.3.2 How controller differentiates between requests?
Most likely, you must be thinking about it. The simplest solution lies in using the consistent
name (e.g. action) of the submit button across all the pages but with different and unique values.
© Copyright Virtual University of Pakistan 399 Web Design and Development (CS506) The same rule applies to hyperlinks that send the action parameter along with value by using
query string technique.
This eases the controller’s job to identify which page is actually generated the request and what
to do next. The controller simply retrieves the value of action parameter using
request.getParameter() method. Now, if-else structure can be used to compare the
possible values of action to act upon the requested task.
Now, let’s first see the code of JavaBean that is used in this example.
PersonInfo
This JavaBean is used to represent one person record. The code is given below:
package vu;
import java.io.*;
public class PersonInfo implements Serializable{
private String name;
private String address;
private int phoneNum;
// no argument constructor
public PersonInfo() {
name = "";
address = "";
phoneNum = 0;
}
// setters
public void setName(String n){
name = n;
}
public void setAddress(String a){
address = a;
}
public void setPhoneNum(int pNo){
phoneNum = pNo;
}
// getters
public String getName( ){
return name;
}
public String getAddress( ){
return address;
© Copyright Virtual University of Pakistan 400 Web Design and Development (CS506) }
public int getPhoneNum( ){
return phoneNum;
}
} // end class PersonInfo
PersonDAO
This class will help in retrieving and storing person’s records in database. The code is given
below:
package vu;
import java.util.*;
import java.sql.*;
public class PersonDAO{
private Connection con;
// default constructor
public PersonDAO() throws ClassNotFoundException , SQLException
{
establishConnection();
}
// method used to establish connection with db
private void establishConnection() throws ClassNotFoundException
,SQLException
{
// establishing conection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:PersonDSN";
con = DriverManager.getConnection(conUrl);
}
// used to search the person records against name and returns
// the ArrayList that contains only those PersonInfo objects
// which matches the search criteria i.e. name
public ArrayList retrievePersonList(String pName) throws
SQLException
(
ArrayList personList = new ArrayList();
// preparing query
String sql = " SELECT * FROM Person WHERE name = ?";
© Copyright Virtual University of Pakistan 401 Web Design and Development (CS506) PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString( 1, pName);
// executing query
ResultSet rs = pStmt.executeQuery();
String name;
String add;
int pNo;
while ( rs.next() ) {
name = rs.getString("name");
add = rs.getString("address");
pNo = rs.getInt("phoneNumber");
// creating a CourseOutlineBean object
PersonInfo personBean = new PersonInfo();
personBean.setName(name);
personBean.setAddress(add);
personBean.setPhoneNum(pNo);
// adding a bean to arraylist
personList.add(personBean);
} // end while
return personList;
} // end retrievePersonList
// this method accepts an object of PersonInfo, and stores it
// into the database
public void addPerson(PersonInfo person) throws SQLException{
String sql = " INSERT INTO Person(name, address, phoneNumber)
VALUES (?, ?, ?)";
PreparedStatement pStmt = con.prepareStatement(sql);
String name = person.getName();
String add = person.getAddress();
int pNo = person.getPhoneNum();
pStmt.setString( 1 , name );
© Copyright Virtual University of Pakistan 402 Web Design and Development (CS506) pStmt.setString( 2 , add );
pStmt.setInt( 3 , pNo );
pStmt.executeUpdate();
} // end addPerson
// overriding finalize method to release acquired resources
public void finalize( ) {
try{
if(con != null){
con.close();
}
}catch(SQLException sqlex){
System.out.println(sqlex);
}
} // end finalize
} // end PersonDAO class
addperson.jsp
This page is used for entering a new person record into the database. Note that a hyperlink is also
given at the bottom of the page that takes the user to searchperson.jsp.
Note: Since we are following MVC model 2 architecture, so all the hyperlinks will also sends the
request to controller first which redirects the user to requested page.
© Copyright Virtual University of Pakistan 403 Web Design and Development (CS506) The code of above page is given below:
<%-Although there are no chances of exception to arise on this page,
for consistency, error page is defined on top of all JSPs
--%>
<%@page errorPage="addbookerror.jsp" %>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Add New Person</h3>
<%-As mentioned in MVC2, all the requests are submitted to
controller, that’s why action’s contains the value of
“controller.jsp”
--%>
<form name ="register" action="controller.jsp" />
<TABLE BORDER="1" >
<TR>
<TD> <h4> Name </h4>
</TD>
<TD> <input type="text" name="name" /> </TD>
</TR>
<TR>
<TD> <h4> Address </h4>
</TD>
<TD> <input type="text" name="address" /> </TD>
</TR>
<TR>
<TD> <h4>Phone Number</h4>
</TD>
<TD> <input type="text" name="phoneNum" /> </TD>
</TR>
<TR>
<TD COLSPAN="2" ALIGN="CENTER">
<%-As described above the technique to differentiate
between the requests, the name of the button is
“action” with value “save”.
--%>
<input type="submit" name ="action" value="save" />
<input type="reset" value="clear" />
</TD>
</TR>
© Copyright Virtual University of Pakistan 404 Web Design and Development (CS506) </TABLE>
</form>
<h4>
<%-The hyperlink will also sends the request to controller
Note the action parameter with its value are also part of
hyperlink using the query string technique.
--%>
<a href="controller.jsp?action=searchperson" >
Search Person
</a>
</h4>
</center>
</body>
</html>
searchperson.jsp
This JSP is used to search the person record against name given in the text field. A hyperlink is
also given at the bottom of addperson.jsp.
The code that is used to generate that above page is given below:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Search Person</h3>
<form name ="search" action="controller.jsp" />
<TABLE BORDER="1" >
<TR>
© Copyright Virtual University of Pakistan 405 Web Design and Development (CS506) <TD> <h4> Name </h4></TD>
<TD> <input type="text" name="name" />
</TR>
</TD>
<TR>
<TD COLSPAN="2" ALIGN="CENTER">
<%-The name of the button is still “action” but with
different value “search”.
--%>
<input type="submit" name ="action" value="search" />
<input type="reset" value="clear" />
</TD>
</TR>
</TABLE>
</form>
<h4>
<%-The action parameter with different value “addperson” are
part of hyperlink here as well.
--%>
<a href="controller.jsp?action=addperson" >
Add Person
</a>
</h4>
</center>
</body>
</html>
controller.jsp
As mentioned earlier that controller.jsp identifies the page which initiates the request and
use JavaBeans to save/search persons to/from database. Also its job list includes redirecting the
user to appropriate page.
Since this JSP is doing only processing therefore no view available. Let’s check it out its code:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<%-- importing required packages. package vu contains JavaBeans -%>
<%@page import ="java.util.*" %>
© Copyright Virtual University of Pakistan 406 Web Design and Development (CS506) <%@page import = "vu.*" %>
<html>
<body>
<%-- declaring PersonDAO object--%>
<jsp:useBean id="pDAO" class="vu.PersonDAO" scope="page" />
<%-scriptlet to identify JSP for redirection purpose if request
comes from hyperlinks
--%>
<%
// retrieving action parameter value
// Remember that “action” is the name of buttons as well
// it is used in hyperlinks in making of query string
String action = request.getParameter("action");
// if "Add Person" hyperlink is clicked
if (action.equals("addperson") ){
response.sendRedirect("addperson.jsp");
// if "Search Person" hyperlink is clicked
} else if (action.equals("searchperson")){
response.sendRedirect("searchperson.jsp");
// if "save" button is clicked of addperson.jsp
}else if (action.equals("save")) {
%>
// declaring PersonInfo obeject
<jsp:useBean id="personBean" class="vu.PersonInfo" scope="page"/>
<%-setting all properties of personBean object with input
parameters using *
--%>
<jsp:setProperty name="personBean" property="*" />
<%-- to insert record into database--%>
<%
pDAO.addPerson(personBean);
// redirecting user to saveperson.jsp
response.sendRedirect("saveperson.jsp");
%>
© Copyright Virtual University of Pakistan 407 Web Design and Development (CS506) <%-- if "search" button is clicked on searchperson.jsp --%>
<%
}else if (action.equals("search") ) {
String pName = request.getParameter("name");
ArrayList personList = pDAO.retrievePersonList(pName);
// storing personList(contains PersonInfo objects) into
// request hashmap
request.setAttribute("list", personList);
%>
<%-forwarding request to showperson.jsp to retrieve stored arraylist
(“list”)
--%>
<jsp:forward page="showperson.jsp" />
<%
} // end if page == search
%>
</body>
</html>
saveperson.jsp
This page displays a successful message indicating that person record is saved. Its also give the
options to the user to move on to addperson.jsp or searchperson.jsp through
hyperlinks. Note that these hyperlinks also first take the user to controller.jsp then on to
requested page.
The code of saveperson.jsp is given below:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<html>
<body>
<center>
© Copyright Virtual University of Pakistan 408 Web Design and Development (CS506) <h3> New Person Record is saved successfully!</h3>
<h4>
<a href="controller.jsp?action=addperson" >
Add Person
</a>
</h4>
<h4>
<a href="controller.jsp?action=searchperson" >
Search Person
</a>
</h4>
</center>
</body>
</html>
showperson.jsp
This following figure gives you the view when name “saad” is searched.
Below, the code of showperson.jsp is given:
<%-- defining error page --%>
<%@page errorPage="addbookerror.jsp" %>
<%-- importing required packages --%>
<%@page import="java.util.*" %>
<%@page import="vu.*" %>
<html>
© Copyright Virtual University of Pakistan 409 Web Design and Development (CS506) <body>
<center>
<h2> Address Book </h2>
<h3> Following results meet your search criteria</h3>
<TABLE BORDER="1" >
<TR>
<TH> Name </TH>
<TH> Address </TH>
<TH> PhoneNum </TH>
</TR>
<%
// retrieving arraylist stored on controller.jsp to display
// PersonInfo objects
ArrayList personList =
(ArrayList)request.getAttribute("list");
PersonInfo person = null;
for(int i=0; i<personList.size(); i++) {
person = (PersonInfo)personList.get(i);
%>
<%-- displaying PersonInfo details--%>
<TR>
<TD> <%= person.getName()%> </TD>
<TD> <%= person.getAddress()%> </TD>
<TD> <%= person.getPhoneNum()%> </TD>
</TR>
<%
} // end for
%>
</TABLE >
<h4>
<a href="controller.jsp?action=addperson"> Add Person </a>
<a href="controller.jsp?action=searchperson">Search Person</a>
</h4>
</center>
</body>
</html>
addbookerror.jsp
User will view this page only when any sort of exception is generated. The code of this page is
given below:
© Copyright Virtual University of Pakistan 410 Web Design and Development (CS506) <%-- indicating that this is an error page --%>
<%@page isErrorPage="true" %>
<%-- importing class --%>
<%@page import = "java.sql.SQLException" %>
<html>
<head>
<title>Error</title>
</head>
<body>
<h2>
Error Page
</h2>
<h3>
<%-- scriptlet to determine exception type --%>
<%
if (exception instanceof SQLException) {
%>
An SQL Exception
<%
} else if (exception instanceof ClassNotFoundException){
%>
A Class Not Found Exception
<%
} else {
%>
A Exception
<%
} // end if-else
%>
<%-- end scriptlet to determine exception type --%>
occured while interacting with the database
</h3>
<h3>
The Error Message was
© Copyright Virtual University of Pakistan 411 Web Design and Development (CS506) <%= exception.getMessage() %>
</h3>
<h3 > Please Try Again Later! </h3>
<%-hyperlinks to return back to adperson.jsp or
searchperson.sjp
--%>
<h3>
<a href="controller.jsp?action=addperson" >
Add Person
</a>
<a href="controller.jsp?action=searchperson" >
Search Person
</a>
</h3>
</body>
</html>
JSP is the Right Choice as a Controller?
Since JSP that is performing the job of controller is doing only processing and there is no view
available of it. It includes the logic of selecting JSP and to retrieve/store records from/to dataset
using JavaBeans.
But remember the reason for introducing JSPs? JavaServer Pages are built for presentation (view)
only so JSP is really not a good place for such kind of logic. Concluding, what’s the option we
have? The answer is, use Servlets as controller.
Introducing a Servlet as Controller
Remove the controller.jsp from the previous example code and add
ControllerServlet.java (a
servlet)
into
this
example.This
ControllerServlet.java performs the same job that was previously performed by
controller.jsp.
Besides adding ControllerServlet.java, you have to modify all the addresses which
are previously pointing to controller.jsp. For example the value of action attribute of
form tag & the address of hyperlink in all concerned pages.
If controller is defined in web.xml as an alias of ControllerServlet.java, consider the
following fragment of code which shows the value of action attribute of form tag before and after
introducing change.
When controller.jsp is acting as a controller
<form name ="register" action="controller.jsp" />
When ControllerServlet.java is acting as a controller then value of action attribute becomes:
© Copyright Virtual University of Pakistan 412 Web Design and Development (CS506) <form name ="register" action="controller" />
Similarly, the following comparison shows the code of hyperlinks used in the previous example
before and after making changes
When controller.jsp is acting as a controller
<a href="controller.jsp?action=searchperson" >
Search Person
</a>
When ControllerServlet.java is acting as a controller
<a href="controller?action=searchperson" >
Search Person
</a>
Passing Exceptions to an Error JSP from a Servlet
Servlet can use existing error pages (like addbookerror.jsp) to pass on the exceptions. Set the
request attribute to javax.servlet.jsp.JspExcpetion with the exception object
you want to pass. After that forwards the request to error page.
For example, the following code snippet is taken from ControllerServlet.java to
demonstrate how to pass SQLException to addbookerror.jsp
………………
………………
}catch (SQLException sqlex){
// setting SQLException instance
request.setAttribute("javax.servlet.jsp.JspException" , sqlex);
RequestDispatcher rd =
request.getRequestDispatcher("addbookerror.jsp");
rd.forward(request, response);
} // end catch
ControllerServlet.java
The following code is of servlet that is acting as a controller
package controller;
import vu.*;
import java.io.*;
import java.net.*;
import java.sql.*;
© Copyright Virtual University of Pakistan 413 Web Design and Development (CS506) import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ControllerServlet extends HttpServlet {
// This method only calls processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// This method only calls processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
protected void processRequest(HttpServletRequest request,
HttpServletResponse response) throws ServletException,
IOException
{
// retrieving value of action parameter
String userAction = request.getParameter("action");
// if request comes to move to addperson.jsp from hyperlink
if (userAction.equals("addperson") ) {
response.sendRedirect("addperson.jsp");
// if request comes to move to searchperson.jsp from hyperlink
} else if (userAction.equals("searchperson")) {
response.sendRedirect("searchperson.jsp");
// if “save” button clicked on addperson.jsp to add new record
}
if (userAction.equals("save")) {
// this method defined below
addPerson(request,response);
// if “search” button clicked on searchperson.jsp for search
} else if (userAction.equals("search"))
{
// this method defined below
© Copyright Virtual University of Pakistan 414 Web Design and Development (CS506) searchPerson(request,response);
}
} // end processRequest()
// if request comes to add/save person
private void addPerson(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
try
{
// creating PersonDAO object
PersonDAO pDAO = new PersonDAO();
// creating PersonInfo object
PersonInfo person = new PersonInfo();
// setting properties of Person object
// setting name property
String pName = request.getParameter("name");
person.setName(pName);
// setting address propertyt
String add = request.getParameter("address");
person.setAddress(add);
// setting phoneNumb property
String pNo = request.getParameter("phoneNum");
int phoneNum = Integer.parseInt(pNo);
person.setPhoneNum(phoneNum);
// calling PersonDAO method to save data into database
pDAO.addPerson(person);
// redirecting page to saveperson.jsp
response.sendRedirect("saveperson.jsp");
}catch (SQLException sqlex){
// setting SQLException instance
request.setAttribute("javax.servlet.jsp.JspException" , sqlex);
RequestDispatcher rd =
request.getRequestDispatcher("addbookerror.jsp");
© Copyright Virtual University of Pakistan 415 Web Design and Development (CS506) rd.forward(request, response);
}catch (ClassNotFoundException cnfe){
// setting ClassNotFoundException instance
request.setAttribute("javax.servlet.jsp.JspException" , cnfe);
RequestDispatcher rd =
request.getRequestDispatcher("addbookerror.jsp");
rd.forward(request, response);
}
}// end addperson()
// if request comes to search person record from database
private void searchPerson(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException{
try {
// creating PersonDAO object
PersonDAO pDAO = new PersonDAO();
String pName = request.getParameter("name");
// calling DAO method to retrieve personlist from database
// against name
ArrayList personList = pDAO.retrievePersonList(pName);
request.setAttribute("list", personList);
// forwarding request to showpeson, so it can render personlist
RequestDispatcher rd =
request.getRequestDispatcher("showperson.jsp");
rd.forward(request, response);
}catch (SQLException sqlex){
// setting SQLException instance
request.setAttribute("javax.servlet.jsp.JspException" , sqlex);
RequestDispatcher rd =
request.getRequestDispatcher("addbookerror.jsp");
rd.forward(request, response);
}catch (ClassNotFoundException cnfe){
// setting ClassNotFoundException instance
request.setAttribute("javax.servlet.jsp.JspException" , cnfe);
RequestDispatcher rd =
request.getRequestDispatcher("addbookerror.jsp");
© Copyright Virtual University of Pakistan 416 Web Design and Development (CS506) rd.forward(request, response);
}
}// end searchPerson()
} // end ControllerServlet
web.xml
As you already familiar, for accessing a servlet, you need to define a URL pattern in
web.xml. This is shown below:
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name> ControllerServlet </servlet-name>
<servlet-class> controller.ControllerServlet </servlet-class>
</servlet>
<servlet-mapping>
<servlet-name> ControllerServlet </servlet-name>
<url-pattern> /controller </url-pattern>
</servlet-mapping>
</web-app>
40.4 References:
•
•
Java A Lab Course by Umair Javed.
Java E-commerce course at Stanford
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 417 Web Design and Development (CS506) Lecture 41: Layers and Tiers
How do you structure an application to support such operational requirements as maintainability,
reusability, scalability and robustness? The answer lies in using Layers and Tiers? What different
technologies Java provides to support layered or tiered architectures. The answer to these questions
will remain our focus in this handout. A small case study will also be used to comprehend the
concept of layers.
41.1 Layers vs. Tiers
Layers are merely logical grouping of the software components that make up the application or
service, whereas Tiers refer to the physical residence of those layers.
In general,
Layers – represents the logical view of application
Tiers – represents physical view of application
However, both terms are used intractably very often. You must be confused what does logical &
physical view mean? Let’s elaborate layers and tiers further in detail to differentiate between them.
41.1.1 Layers
The partitioning of a system into layers such that each layer performs a specific type of
functionality and communicates with the layer that adjoins it.
The separation of concerns minimizes the impact of adding services/features to an application.
The application developed in layers also enables tiered distribution(discussed later). Furthermore
easier maintenance, reuse of code, high cohesion & loose coupling sort of additional benefits are
also enjoyed by the use of tiered architecture.
To begin with, layered architecture based on three layers. These are
• Presentation Layer
• Business Layer
• Data Layer
Note: However, there is no upper limit of number of layers an application can have. Each layer
can also be further break down into several layers depending upon the requirements and size of
the application.
© Copyright Virtual University of Pakistan 418 Web Design and Development (CS506) The figure given below shows a simplified view of an application and its layers.
As you can see in the figure, users can only interact with the presentation layer. The presentation
layer passes the user request to the business layer, which further passes the request to the data
layer. The data layer communicates with the data sources (like Database etc.) or other external
services in order to accomplish the user request.
Let’s discuss each layer’s responsibility in detail:
41.1.1.1 Presentation Layer
It provides a user interface to the client/user to interact with the application. This is the only part
of the application visible to client.
Its job list includes collecting user’s input, validating user’s input (on client side using JavaScript
like technologies OR on server side), presenting the results of the request made by the user and
controlling the screen flow (which page/view will be visible to the user).
41.1.1.2 Business Layer
Also called application layer, it is only concerned with the application specific functionality. It is
used to implement business rules and to perform business tasks.
For example, in a banking system, this layer will provide the functionality of banking functions
such as opening an account, transferring of balance from one account to another, calculation of
taxes etc.
41.1.1.3 Data Layer
It is concerned with the management of the data & data sources of the system. Data sources can
be database, XML, web services, flat file etc. Encapsulates data retrieval & storage logic For
© Copyright Virtual University of Pakistan 419 Web Design and Development (CS506) example, the address book application needs to retrieve all person records from a database to
display them to the user.
41.1.2 Tiers
As mentioned, layers help in building a tiered architecture. Like layers, there is no restriction on
using number of tiers. An application can be based on Single-tier, Two-tier,Three-tier or N-Tier
(application which have more than three tiers). The choice of using a tiered architecture is
contingent to the business requirements and the size of the application etc.
Tiers are physically separated from each other. Layers are spread across tiers to build up an
application. Two or more layers can reside on one tier. The following figure presents a three-tier
architectural view of an application.
The client tier represents the client machine where actually web browser is running and usually
displays HTML. You can think of a Presentation as of two parts; one is on client side, for
example, HTML. There is also a presentation layer that is used to generate the client presentation
often called server presentation. We’ll discuss about it later.
The server machine can consist on a single server machine or more. Therefore, it is possible web
server is running on one server machine while application server on another. Web server is used
to execute web pages like JSPs whereas application server is used to run special business objects
like Enterprise JavaBeans (discussed later). The web layer and applications server can be on two
separate machines or they can be on same tier as shown in the diagram.
The database server is often running on a separate tier, i.e. DB machine often called Enterprise
information tier.
© Copyright Virtual University of Pakistan 420 Web Design and Development (CS506) 41.2 Layers Support in Java
The secret of wide spread use of Java lies in providing specific technology for each layer.This not
only eases the development by freeing the programmer for caring operational features but only
reduces the production time of the software.
In the following figure, Presentation is bifurcated into two layers. These are Client Presentation
layer and Server Presentation Layer. What client sees in a browser forms client presentation layer
while server presentation layer includes the Java technology components (JSP and Servlets etc.)
that are used to generate the client presentation.
On business layer, JavaBeans (also referred as Plain Old Java Objects (POJO) ) can be used.
While moving towards a bigger architecture, the J2EE provides the special class that fits in
business layer i.e. Enterprise JavaBean (EJB).
EJBs are special java classes that are used to encapsulate business logic. They provide additional
benefits in building up an application such as scalability, robustness,scalability etc.
On data layer, Data Access Objects (DAO) can be used. Similarly you can use connectors. There
are other different specialized components provided in java that ease the development of data
layer.
41.3 J2EE Multi-Tiered Applications
In a typical J2EE Multi-Tiered application, a client can either be a swing based application or a
web based. As you can see in the following figure, clients can access the web server from behind
the firewall as well.
© Copyright Virtual University of Pakistan 421 Web Design and Development (CS506) Suppose, our client is HTML based. Client does some processing on HTML and transports it to
web server. JSP and Servlets are possible technologies that can be used in a web server. However,
there are some Frameworks such as JSF etc that can be used in a web server. The classes which
form the presentation layer reside on web server and of course controllers are also used over here.
If web server, wants to perform some business process, it usually gets help from some business
layer components. The business layer component can be a simple JavaBean (POJO) but in a
typical J2EE architecture, EJBs are used. Enterprise JavaBeans interacts with the database or
information system to store and retrieve data.
EJBs and JSP/Servlets works in two different servers. As you already know, JSP and Servlets
runs in a web server where as EJBs requires an application server. But, generally application
server contains the web server as well.
Application server including web server generally resides on a single tier (machine),which is
often called middle tier. This tier stores and retrieves data from the Enterprise Information Tier
(EIS) which is a separate tier. The response sends back to the client by the middle tier can be
HTML, XML etc. This response can be seen on the separate tier know as client tier.
41.4 Case Study: Matrix Multiplication using Layers
Problem Statement
Calculate product of two matrices of order 2 * 2
Result of multiplication should be stored in DB as well as shown to the user.
Format
• Input format
© Copyright Virtual University of Pakistan 422 Web Design and Development (CS506) •
•
o input will be in 4,2,6,5 format separated by commas where 4,2 represents entries of
the first row
Display format
o Displays the matrix as a square
Storage format for DB
o Matrix will be stored as a string in the database along with the order of the matrix
o The following figure shows the table design that will be used to store the results.
Layer by Layer View
A picture’s worth than thousand words. Therefore, before jumping on to code, let’s put a glance
over layers that will be used in this small case study. The classes that will be used on each layer
and what functionality each class will perform will also be discussed.
First, look on the following picture that will describe the whole story.
© Copyright Virtual University of Pakistan 423 Web Design and Development (CS506) The data layer has a class MatrixDAO that is used to save the matrix result into database. As
mentioned in the problem statement, that resultant matrix should be saved in the database. So,
MatrixDAO is used to accomplish that.
MatrixDAO called by the MatrixMultiplier, a business layer class. The functionality list
of MatrixMultiplier includes:
- Converting the user input string (e.g. 2,3,4,1) into a proper object i.e. a matrix data structure.
[
- Helps in calculating product of two matrices.
Controller layer’s class ControllerServlet calls the MatrixMultiplier. This layer
calls the various business methods (like multiplication of two matrices) of business layer class
and got the resultant matrix. Furthermore, ControllerServlet sends the output to the
matrixresult.jsp and receives the input from
matrixinput.jsp.
The MatrixBean representing matrix data structure, as you can see in the figure is used across
several layers. In fact, the object formed by MatrixMultiplier from a user
input string is of MatrixBean type. It is used to transfer data from one layer to another.
First, look on the MatrixBean code given below:
MatrixBean
package bo;
import java.io.*;
public class MatrixBean implements Serializable{
// a 2D array representing matrix
public int matrix[ ][ ] ;
// constructor
public MatrixBean()
{
matrix = new int[2][2];
matrix[0][0] = 0;
matrix[0][1] = 0;
matrix[1][0] = 0;
matrix[1][1] = 0;
}
// setter that takes 4 int values and assigns these to array
public void setMatrix(int w, int x, int y, int z)
{
matrix[0][0] = w;
matrix[0][1] = x;
© Copyright Virtual University of Pakistan 424 Web Design and Development (CS506) matrix[1][0] = y;
matrix[1][1] = z;
}
// getter returning a 2D array
public int[ ][ ] getMatrix()
{
return matrix;
}
// used to convert 2D array into string
public String toString()
{
return matrix[0][0] + "," + matrix[0][1] + "," +
matrix[1][0] + "," +matrix[1][1] ;
}
} // end MatrixBean
matrixinput.jsp
This JSP is used to collect the input for two matrices in the form of string such as 2,3,5,8.The data
will be submitted to ControllerServlet from this page.
<html>
<body>
<h2>
Enter Two Matrices of order 2 * 2 to compute Product
</h2>
<h3>
<%-“controller” is an alias/URL pattern of ControllerServlet
--%>
<form name="matrixInput" action="controller" >
First Matrix:
<input type="text" name = "firstMatrix" /> E.g. 2,3,4,1
<br/>
Second Matrix:
<input type="text" name = "secondMatrix" />
<br/>
<input type = "submit" value = "Calculate Product" />
</form>
</h3>
</body>
</html>
© Copyright Virtual University of Pakistan 425 Web Design and Development (CS506) ControllerServlet
This servlet acting as a controller receives the input from matrixinput.jsp. Furthermore,it will
interact with the business layer class MatrixMultiplier to convert the string into a MatrixBean
object, and to multiply two matrices.
package controller;
import bl.*;
import bo.* ;
import java.io.*;
import java.net.*;
import javax.servlet.*;
import javax.servlet.http.*;
public class ControllerServlet extends HttpServlet {
// This method only calls processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
// This method only calls processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
// retrieving values from input fields of matrixinput.jsp
String sMatrix1 = request.getParameter("firstMatrix");
String sMatrix2 = request.getParameter("secondMatrix");
// Creating MatrixMultipler object
MatrixMultiplier mm = new MatrixMultiplier();
// Passing Strings to convertToObject() method of
MatrixMultiplier
© Copyright Virtual University of Pakistan 426 Web Design and Development (CS506) // convertToObject() is used to convert strings into MatrixBean
MatrixBean fMatrix = mm.convertToObject(sMatrix1);
MatrixBean sMatrix = mm.convertToObject(sMatrix2);
// passing MatrixBean’s objects to multiply() method of
// MatrixMultiplier and receiving the product matrix in the form
// of MatrixBean
MatrixBean rMatrix = mm.multiply(fMatrix, sMatrix);
// saving results in database
mm.saveResult(rMatrix);
// storing the product of matrices into request, so that it can
be
// retrieved on matrixresult.jsp
request.setAttribute("product", rMatrix);
// forwarding request to matrixresult.jsp
RequestDispatcher rd =
request.getRequestDispatcher("matrixresult.jsp");
rd.forward(request, response);
} // end processRequest()
} // end ControllerServlet
MatrixMultiplier
The business layer class that’s primary job is to calculate product of tow matrices given in the
form of MatrixBean. This class also has a method convertToObject that takes a String and
returns back a MatrixBean object. MatrixMultiplier will also interact with the data layer class
MatrixDAO to store results in the database.
package bl;
import bo.*;
import dal.*;
public class MatrixMultiplier {
//constructor
public MatrixMultiplier( ) {
}
// used to convert a String (like 2,3,4,5) into a MatrixBean
object
public MatrixBean convertToObject(String sMatrix){
//splitting received string into tokens by passing “,” as
//delimeter
© Copyright Virtual University of Pakistan 427 Web Design and Development (CS506) String tokens[] = sMatrix.split(",");
//creating MatrixBean object
MatrixBean matrixBO = new MatrixBean();
// converting tokens into integers
int w = Integer.parseInt(tokens[0]);
int x = Integer.parseInt(tokens[1]);
int y = Integer.parseInt(tokens[2]);
int z = Integer.parseInt(tokens[3]);
// setting values into MatrixBean object by calling setter
matrixBO.setMatrix(w , x , y, z);
return matrixBO;
} // end convertToObject()
// used to multiply two matrices , receives two MatrixBean
objects
// and returns the product in the form of MatrixBean as well
public MatrixBean multiply(MatrixBean fMatrix , MatrixBean
sMatrix)
{
// creating MatrixBean object where product of the matrices will
// be
// stored
MatrixBean resultMatrix = new MatrixBean();
// retrieving two dimensional arrays from MatrixBeans object to
// perform multipication
int matrixA[ ][ ] = fMatrix.getMatrix();
int matrixB[ ][ ] = sMatrix.getMatrix();
int matrixC[ ][ ] = resultMatrix.getMatrix();
// code to multiply two matrices
for (int i=0; i<2; i++) {
for (int j=0; j<2; j++) {
for (int k=0; k<2; k++) {
matrixC[i][j] += (matrixA[i][k] * matrixB[k][j]);
}
}
// storing the product from 2d array to MatrixBean object by
// calling setter
resultMatrix.setMatrix( matrixC[0][0], matrixC[0][1],
matrixC[1][0], matrixC[1][1] );
return resultMatrix;
© Copyright Virtual University of Pakistan 428 Web Design and Development (CS506) } // end multiply()
// save results (MatrixBean containg product of two matrices)
//into
// database using DAO
public void saveResult( MatrixBean resultMatrix )
{
MatrixDA0 dao = null;
try{
dao = newMatrixDAO();}
catch(ClassNotFoundException e){}
catch(SQLException e){}
dao.saveMatrix(resultMatrix);
}
} // end MatrixMulitplier
MatrixDAO
As class name depicts, it is used to store product results into database. Let’s look on the
code to see how it is accomplished.
package dal;
import java.util.*;
import java.sql.*;
import bo.*;
public class MatrixDAO{
private Connection con;
// constructor
public MatrixDAO() throws ClassNotFoundException , SQLException
{
establishConnection();
}
// method used to establish connection with db
private void establishConnection() throws ClassNotFoundException
,SQLException
{
// establishing conection
class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:MatrixDSN";
con = DriverManager.getConnection(conUrl);
}
// used to store MatrixBean into database after converting it to
// a String
public void saveMatrix(MatrixBean matrix){
try
{
© Copyright Virtual University of Pakistan 429 Web Design and Development (CS506) String sql = "INSERT INTO Matrix(mOrder, mValues) VALUES (?,?)";
PreparedStatement pStmt = con.prepareStatement(sql);
// converting MatrixBean into String by calling toString()
String sMatrix = matrix.toString();
// setting order of matrix
pStmt.setString( 1 , "2*2" );
// setting matrix values in the form of string
pStmt.setString( 2 , sMatrix );
pStmt.executeUpdate();
}catch(SQLException sqlex){
System.out.println(sqlex);
}
} // end saveMatrix
// overriding finalize method to release acquired resources
public void finalize( ) {
try{
if(con != null){
con.close();
}
}catch(SQLException sex){
System.out.println(sex);
}
} // end finalize
} // end MatrixDAO class
matrixresult.jsp
Used to display resultant product of two matrices. The code is given below:
<%-- importing “bo” package that contains MatrixBean --%>
<%@ page import="bo.*"%>
<html>
<body>
<h1>The resultant Matrix is </h1>
© Copyright Virtual University of Pakistan 430 Web Design and Development (CS506) <%-retrieving MatrixBean object from request, that was set on
ControllerServlet
--%>
<%
MatrixBean productMatrix =
(MatrixBean)request.getAttribute("product");
// retrieving values in 2d array so that it can be displayed
int matrix[][] = productMatrix.getMatrix() ;
%>
<%-- displaying MatrixBean’s object values --%>
<TABLE>
<TR>
<TD> <%= matrix[0][0] %> </TD>
<TD> <%= matrix[0][1] %> </TD>
</TR>
<TR>
<TD> <%= matrix[1][0] %> </TD>
<TD> <%= matrix[1][1] %> </TD>
</TR>
</TABLE>
</body>
</html>
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name> ControllerServlet </servlet-name>
<servlet-class> controller.ControllerServlet </servlet-class>
</servlet>
© Copyright Virtual University of Pakistan 431 Web Design and Development (CS506) <servlet-mapping>
<servlet-name> ControllerServlet </servlet-name>
<url-pattern> /controller </url-pattern>
</servlet-mapping>
</web-app>
41.5 References:
•
•
Java A Lab Course by Umair Javed.
Java Passion by Sang Shin
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 432 Web Design and Development (CS506) Lecture 42: Expression Language
Sun Microsystems introduced the Servlet API, in the later half of 1997, positioning it as a
powerful alternative for CGI developers who were looking around for an elegant solution that was
more efficient and portable than CGI (Common Gateway Interface)programming. However, it
soon became clear that the Servlet API had its own drawbacks, with developers finding the
solution difficult to implement, from the perspective of code maintainability and extensibility. It
is in some ways, this drawback that prompted the community to explore a solution that would
allow embedding Java Code in HTML - JavaServer Pages (JSP) emerged as a result of this
exploration.
Java as the scripting language in JSP scares many people particularly web page designers which
have enough knowledge to work with HTML and some scripting language, faced lot of
difficulties in writing some simple lines of java code. Can we simplify this problem to ease the
life of web designer? Yes, by using Expression Language (EL).
JavaServer Pages Standard Tag Library (JSTL) 1.0 introduced the concept of the EL but it was
constrained to only the JSTL tags. With JSP 2.0 you can use the EL with template text.
Note: - JSTL will be discussed in the following Handout.
42.1 Overview
The Expression Language, not a programming or scripting language, provides a way to simplify
expressions in JSP. It is a simple language that is geared towards looking up objects, their
properties and performing simple operations on them. It is inspired form both the ECMAScript
and the XPath expression language.
42.2 JSP Before and After EL
To add in motivational factor so that you start learning EL with renewed zeal and zest, a
comparison is given below that illustrates how EL affects the JSPs.
The following figure depicts the situation of a JSP before EL. We have to declare a variable
before using it, data type must be known in advance and most importantly have to use awkward
syntax and many more. All these problems are highlighted in the following figure:
© Copyright Virtual University of Pakistan 433 Web Design and Development (CS506) Contrary to the above figure, have a look on the subsequent figure that gives you a hint how
useful EL can be?
© Copyright Virtual University of Pakistan 434 Web Design and Development (CS506) 42.3 Expression Language Nuggets
We’ll discuss the following important pieces of EL. These are:
•
•
•
•
•
•
Syntax of EL
Expressions & identifiers
Arithmetic, logical & relational operators
Automatic type conversion
Access to beans, arrays, lists & maps
Access to set of implicit objects
42.3.1 EL Syntax
The format of writing any EL expression is:
$ { validExpression }
The valid expressions can consist on these individuals or combination of these given below:
•
•
•
•
Literals
Operators
Variables (object references)
Implicit call to function using property name
•
EL Literals
The list of literals that can be used as an EL expression and their possible values are given in the
tabular format below:
Examples of using EL literals are:
${ false } <%-- evaluates to false --%>
${ 8*3 } <%-- evaluates to 24 --%>
© Copyright Virtual University of Pakistan 435 Web Design and Development (CS506) •
EL Operators
The lists of operators that can be used in EL expression are given below:
Let us look at some examples that use operators as valid expression:
•
•
•
•
${ (6*5) + 5 } <%-- evaluate to 35 --%>
${ (x >= min) && (x <= max) }
${ empty name }
o Returns true if name is
Empty string (“”),
Null etc.
EL Identifiers
Identifiers in the expression language represent the names of objects stored in one of the JSP
scopes: page, request, session, or application. These types of objects are referred to scoped
variables throughout this handout.
EL has 11 reserved identifiers, corresponding to 11 implicit objects. All other identifiers
assumed to refer to scoped variables.
• EL implicit Objects
The Expression Language defines a set of implicit objects given below in tabular format:
© Copyright Virtual University of Pakistan 436 Web Design and Development (CS506) Category
Implicit Object
Operator
The context for the JSP page, used to access the JSP
JSP
pageContext
implicit objects such as request, response, session,
Scopes
pageScope
A Map associating names & values of page scoped
requestScope
attributes
A Map associating names & values of request scoped
attributes
sessionScope
A Map associating names & values of session scoped
attributes
applicationScope
A Map associating names & values of application
scoped attributes
Request
param
Maps a request parameter name to a single String
parameter value.
Parameters
paramValues
Maps a request parameter name to an array of values
Request
header
Maps a request header name to a single header value.
Headers
headerValues
Maps a request header name to an array of value.
Cookies
cookie
A Map storing the cookies accompanying the request by
name
Initialization
Parameters
initParam
A Map storing the context initialization parameters of
the web application by name
Examples
of using implicit objects are:
•
•
•
${ pageContext.response }
o Evaluates to response implicit object of JSP
${ param.name }
o This expression is equivalent to calling request.getParameter(“name”);
${ cookie.name.value }
o Returns the value of the first cookie with the given name
o Equivalent to
if (cookie.getName().equals(“name”){
String val = cookie.getValue();
}
© Copyright Virtual University of Pakistan 437 Web Design and Development (CS506) Example Code: Summation of Two Numbers using EL
This simple example demonstrates you the capabilities of EL. index.jsp is used to collect
input for two numbers and their sum is displayed on result.jsp using EL.
Let’s first see the code of index.jsp
index.jsp
<html>
<body>
Enter two numbers to see their sum
<form action="result.jsp" >
First Number :
<input type="text" name="num1" />
<br>
Second Number:
<input type="text" name="num2" />
<input type="submit" value="Calculate Sum" />
</form>
</body>
</html>
result.jsp
<html>
<body>
<%-- The code to sum two numbers if we used scriptlet
<%
String no1 = request .getParameter("num1");
String no2 = request .getParameter("num2");
int num1 = Integer.parseInt(no1);
int num2 = Integer.parseInt(no2);
%>
Result is: <%= num1 + num2 %>
--%>
<%-- implicit Object param is used to access request parameters
By Using EL summing two numbers
--%>
Result is: ${param.num1 + param.num2}
</body>
</html>
© Copyright Virtual University of Pakistan 438 Web Design and Development (CS506) 42.3.2 EL Identifiers (cont.)
We had started our discussion on EL identifiers. Let’s find out how these identifiers (variables)
can be stored/retrieved in/from different scopes.
•
Storing Scoped Variables
By using java code, either in pure servlet or in a scriptlet of JSP, we can store variables in a
particular scope. For example,
o Storing a variable in session scope using Java code
Assume that we have PersonInfo class and we want to store its object p in
session scope then we can write the following lines of code to accomplish that:
HttpSession ses = request.getSession(true);
PersonInfo p = new PersonInfo();
p.setName(“ali”);
ses.setAttribute(“person” , p);
o Storing a variable in request scope using Java code
For the following lines of code, assume that request is of HttpServletRequest type. To
store PersonInfo object p in request scope, we’ll write:
PersonInfo p = new PersonInfo();
p.setName(“ali”);
request.setAttribute(“person” , p);
You must be thinking of some another method (with which you are already familiar) to store
a variable in a scope, certainly by using JSP action tags, we learned how to store a variable in
any particular scope.
o Storing a variable in request scope using JSP action tag
If we want to store p of type PersonInfo in request scope by using JSP action tags,
then we’ll write:
<jsp:useBean id=”p” class=”PersonInfo”
scope=”request”/>
Later, you can change the properties of object p by using action tag as well.For example
<jsp:setProperty name=“p” property=“name” value=“ali”
/>
•
Retrieving Scoped Variables
You are already very much familiar of retrieving any stored scoped variable by using java
code and JSP action tags. Here, we’ll discuss how EL retrieves scoped variables. As already
mentioned, identifiers in the valid expression represent the names of objects stored in one of
the JSP scopes: page, request, session and application.
© Copyright Virtual University of Pakistan 439 Web Design and Development (CS506) When the expression language encounters an identifier, it searches for a scoped variable with
that name first in page scope,then in request scope,then in session scope,and finally in
application scope
Note: - If no such object is located in four scopes, null is returned.
For example, if we’ve stored PersonInfo object p in session scope by mean of any
mechanism discussed previously and have written the following EL expression to access the name
property of p
${p.name}
Then EL searches for p first in page scope, then in request scope, then in session scope where it
found p. After that it calls p.getName() method. This is also shown in pictorial form below:
42.3.3 EL Accessors
The dot (.) and bracket ([ ]) operator let you access identifies and their properties. The dot
operator typically used for accessing the properties of an object and the bracket operator is
generally used to retrieve elements of arrays and collections.
•
Dot (.) operator
Assume that JavaBean PersonInfo has name property and its object person is stored in
some scope. Then to access the name property of person object, we’ll write the following
expression using EL:
© Copyright Virtual University of Pakistan 440 Web Design and Development (CS506) ${person.name}
The EL accesses the object’s properties using the JavaBeans conventions therefore getName()
must be defined in PersonInfo. Moreover, if property being accessed itself an object, the dot
operator can be applied recursively. For example
•
Bracket ([ ]) operator
This operator can be applied to arrays & collections implementing List interface e.g.
ArrayList etc.
o Index of the element appears inside brackets
o For example, ${ personList[2] } returns the 3rd element stored in it
Moreover, this operator can also be applied to collections implementing Map interface e.g.
HashMap etc.
o Key is specified inside brackets
o For example, ${ myMap[“id”] } returns the value associated with the
42.3.4 EL – Robust Features
Some powerful characteristics of Expression Language are:
•
Multiple expressions can be combined and intermixed with static text. For example
$ { “Hello” ${user.firstName} ${user.lastName} }
•
EL also supports automatic type conversion; as a result primitive can implicitly wrap and
unwrap into/from their corresponding java classes. For example
•
Most importantly, if object/identifier is null, no NullPointerException would be
thrown . For example. If the expression written is:
${person.name}
© Copyright Virtual University of Pakistan 441 Web Design and Development (CS506) Assume that person is null, then no exception would be thrown and the result would also be
null.
42.3.5 Using Expression Language
Expression Language can be used in following situations
•
As attribute values in standard & custom actions. E.g.
<jsp:setProperty id = “person” value = ${….} />
•
In template text – the value of the expression is inserted into the current output. E.g.
<h3> $ { …… } </h3>
•
With JSTL (discussed in the next handout)
Example Code: AddressBook using EL
So far, we have shown you implementation of AddressBook example in number of different ways.
This time EL will be incorporated in this example. AddressBook code example consists on
searchperson.jsp,
showperson.jsp,ControllerServlet, PersonInfo and
PersonDAO classes. Let’s look on the code of each of these components:
PersonInfo.java
The JavaBean used to represent one person record.
package vu;
import java.io.*;
public class PersonInfo implements Serializable{
private String name;
private String address;
private int phoneNum;
// no argument constructor
public PersonInfo() {
name = "";
address = "";
phoneNum = 0;
}
// setters
public void setName(String n){
name = n;
}
public void setAddress(String a){
address = a;
© Copyright Virtual University of Pakistan 442 Web Design and Development (CS506) }
public void setPhoneNum(int pNo){
phoneNum = pNo;
}
// getters
public String getName( ){
return name;
}
public String getAddress( ){
return address;
}
public int getPhoneNum( ){
return phoneNum;
}
}
PersonDAO.java
It is used to retrieve/search person records from database.
package vu;
import java.util.*;
import java.sql.*;
public class PersonDAO{
private Connection con;
// constructor
public PersonDAO() throws ClassNotFoundException , SQLException {
establishConnection();
}//used to establish connection with database
private void establishConnection()
throws ClassNotFoundException , SQLException{
// establishing connection
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
String conUrl = "jdbc:odbc:PersonDSN";
con = DriverManager.getConnection(conUrl);
}
// used to search person records against name
public ArrayList retrievePersonList(String pName)
throws SQLException
{
ArrayList personList = new ArrayList();
© Copyright Virtual University of Pakistan 443 Web Design and Development (CS506) String sql = " SELECT * FROM Person WHERE name = ?";
PreparedStatement pStmt = con.prepareStatement(sql);
pStmt.setString( 1, pName);
System.out.println("retrieve person list");
ResultSet rs = pStmt.executeQuery();
String name;
String add;
int pNo;
while ( rs.next() ) {
name = rs.getString("name");
add = rs.getString("address");
pNo = rs.getInt("phoneNumber");
// creating a PersonInfo object
PersonInfo personBean = new PersonInfo();
personBean.setName(name);
personBean.setAddress(add);
personBean.setPhoneNum(pNo);
// adding a bean to arraylist
personList.add(personBean);
} // end while
return personList;
} // end retrievePersonList
//overriding finalize method to release resources
public void finalize( ) {
try{
if(con != null){
con.close();
}
}catch(SQLException sex){
System.out.println(sex);
}
} // end finalize
} // end class
searchperson.jsp
This JSP is used to gather person’s name from the user and submits this data to the
ControllerServlet.
© Copyright Virtual University of Pakistan 444 Web Design and Development (CS506) <html>
<body>
<center>
<h2> Address Book </h2>
<h3> Search Person</h3>
<FORM name ="search" action="controllerservlet" />
<TABLE BORDER="1" >
<TR>
<TD> <h4 >Name</h4> </TD>
<TD> <input type="text" name="name" />
</TR>
</TD>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Search Person</h3>
<FORM name ="search" action="controllerservlet" />
<TABLE BORDER="1" >
<TR>
<TD> <h4 >Name</h4> </TD>
<TD> <input type="text" name="name" />
</TR>
</TD>
ControllerServlet.java
The Controller Servlet receives request from searchperson.jsp and after fetching search results
from database, forwards the request to showperson.jsp.
package controller;
import
import
import
import
import
import
vu.*;
java.util.*;
java.io.*;
java.net.*;
javax.servlet.*;
javax.servlet.http.*;
public class ControllerServlet extends HttpServlet {
© Copyright Virtual University of Pakistan 445 Web Design and Development (CS506) // This method only calls processRequest()
protected void doGet(HttpServletRequest request,
HttpServletResponse response throws ServletException, IOException
e)
{
processRequest(request, response);
}
// This method only calls processRequest()
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
processRequest(request, response);
}
protected void processRequest(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
// defined below
searchPerson(request, response);
} // end processRequest()
protected void searchPerson(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
try {
// creating PersonDAO object
PersonDAO pDAO = new PersonDAO();
// retrieving request parameter “name” entered on showperson.jsp
String pName = request.getParameter("name");
// calling DAO method to retrieve personlist from database
// against the name entered by the user
ArrayList personList = pDAO.retrievePersonList(pName);
// storing personlist in request scope, later it is retrieved
// back on showperson.jsp
request.setAttribute("plist", personList);
// forwarding request to showperson, so it renders personlist
RequestDispatcher rd =
request.getRequestDispatcher("showperson.jsp");
rd.forward(request, response);
© Copyright Virtual University of Pakistan 446 Web Design and Development (CS506) }catch (Exception ex) {
System.out.println("Exception is" + ex);
}
} // end searchPerson
} // end ControllerServlet
showperson.jsp
This page is used to display the search results. To do so, it reclaims the stored ArrayList
(personList) from the request scope. Furthermore, this page also uses the Expression Language
to display records.
<%-- importing required packages--%>
<%@page import="java.util.*" %>
<%@page import="vu.*" %>
<html>
<body>
<center>
<h2> Address Book </h2>
<h3> Following results meet your search criteria</h3>
<TABLE BORDER="1" >
<TR>
<TH>Name</TH>
<TH>Address</TH>
<TH>PhoneNum</TH>
</TR>
<%-- start of scriptlet --%>
<%
// retrieving ArrayList from request scope
ArrayList personList =(ArrayList)request.getAttribute("plist");
PersonInfo person = null;
for(int i=0; i<personList.size(); i++) {
person = (PersonInfo)personList.get(i);
// storing PersonInfo object in request scope
/* As mentioned, an object must be stored in
some scope to work with Expression Language*/
request.setAttribute("p", person);
%>
<%-- end of scriptlet --%>
<TR>
<%-- accessing properties of stored PersonInfo
object with name “p” using EL --%>
© Copyright Virtual University of Pakistan 447 Web Design and Development (CS506) <TD> ${ p.name } </TD>
<TD> ${ p.address} </TD>
<TD> ${ p.phoneNum} </TD>
<%-- The following expressions are now replaced
by EL statements written above--%>
<%-- <%= person.getName()%> --%>
<%-- <%= person.getAddress()%> --%>
<%-- <%= person.getPhoneNum()%> --%>
</TR>
<%
} // end for
%>
</TABLE >
</center>
</body>
</html>
web.xml
<?xml version="1.0" encoding="UTF-8"?>
<web-app>
<servlet>
<servlet-name> ControllerServlet </servlet-name>
<servlet-class> controller.ControllerServlet </servlet-class>
</servlet>
<servlet-mapping>
<servlet-name> ControllerServlet </servlet-name>
<url-pattern> /controllerservlet </url-pattern>
</servlet-mapping>
</web-app>
42.4 References:
•
•
•
Java A Lab Course by Umair Javed.
Expression Language Tutorial by Sun
http://java.sun.com/j2ee/1.4/docs/tutorial/doc/JSPIntro7.html
The JSTL Expression Language by David M. Geary
http://www.informit.com/articles/article.asp?p=30946&rl=1
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 448 Web Design and Development (CS506) Lecture 43: JavaServer Pages Standard Tag Library (JSTL)
43.1 Introduction
The JSP Standard Tag Library (JSTL) is a collection of custom tag libraries that implement
general-purpose functionality common to Web applications, including iteration and
conditionalization, data management formatting, manipulation of XML, and database access. Like
JSP, JSTL is also a specification not an implementation. The development theme of JSTL is
“scriptlet free JSP”.
These tag libraries provide a wide range of custom action functionality that most JSP authors have
found themselves in need of in the past. Having a defined specification for how the functionality
is implemented means that a page author can learn these custom actions once and then use and
reuse them on all future products on all application containers that support the specification.
Using the JSTL will not only make your JSPs more readable and maintainable, but will allow you
to concentrate on good design and implementation practices in your pages.
43.2 JSTL & EL
JSTL includes supports for Expression Language thus EL can be used to specify dynamic
attribute values for JSTL actions without using full-blown programming language. Prior to JSP
2.0, EL can only be used in attributes of JSTL tags but EL now becomes a standard part of JSP
2.0. This allows the use of EL anywhere in the document.
43.3 Functional Overview
As mentioned, JSTL encapsulates common functionality that a typical JSP author would
encounter. This set of common functionality has come about through the input of the various
members of the expert group. Since this expert group has a good cross-section of JSP authors and
users, the actions provided in the JSTL should suit a wide audience.While the JSTL is commonly
referred to as a single tag library, it is actually composed of four separate tag libraries:
Core−>contains tags for conditions, control flow and to access variables etc.
XML manipulation−>contains tags for XML parsing and processing
SQL−>contains tags for accessing and working with database.
Internationalization and formatting−>contains tags to support locale messages, text, numbers
and date formation
43.4 Twin Tag Libraries
JSTL comes in two flavors to support various skill set personal
•
Expression Language (EL) version
o Dynamic attribute values of JSTL tags are specified using JSTL expression
© Copyright Virtual University of Pakistan 449 Web Design and Development (CS506) language (i.e. ${ expression } )
o The EL based JSTL tag libraries along with URIs and preferred prefixes are given
below in tabular format
Library
URI
Prefix
Core
http://java.sun.com/jsp/jstl/core
c
SQL
http://java.sun.com/jsp/jstl/sql
sql
Internationalization/
http://java.sun.com/jsp/jstl/fmt
fmt
F
XML
http://java.sun.com/jsp/jstl/xml
x
•
Request Time (RT) version
o Dynamic attribute values of JSTL tags are specified using JSP expression
(i.e. <%= expression %> )
o The RT based JSTL tag libraries along with URIs and preferred prefixes are given
below in tabular format
Library
URI
Prefix
Core
http://java.sun.com/jsp/jstl/core_rt
c_rt
SQL
http://java.sun.com/jsp/jstl/sql_rt
sql_rt
Internationalization/
http://java.sun.com/jsp/jstl/fmt_rt
fmt_rt
F
XML
http://java.sun.com/jsp/jstl/xml_rt
x_rt
43.5 Using JSTL
As we discussed earlier, JSTL includes four standard tag libraries. As is true with any JSP custom
tag library, a taglib directive must be included in any page that you want to be able to use this
library's tags.
For example, to use EL based core tag library, the taglib directive appears as:
<%@taglib prefix=“c” uri=http://java.sun.com/jsp/jstl/core %>
And to use RT based core tag library, the taglib directive appears as:
<%@taglib prefix=“c_rt” uri=http://java.sun.com/jsp/jstl/core_rt
%>
© Copyright Virtual University of Pakistan 450 Web Design and Development (CS506) 43.6 Working with Core Actions (tags)
The set of tags that are available in the Core tag library come into play for probably most anything
you will be doing in your JSPs such as:
•
•
•
•
•
•
Manipulation of scoped variables
Output
Conditional logic
loops
URL manipulation
and Handling errors.
Let’s walk through some important core actions:
c:set
Provides a tag based mechanism for creating and setting scope based variables. Its syntax is as
follows:
<c:set var=“name”
scope = “scope” value = “expression” />
Where the var attribute specifies the name of the scoped variable, the scope attribute indicates
which scope (page | request | session | application) the variable resides in, and the value attribute
specifies the value to be bound to the variable. If the specified variable already exists, it will simply
be assigned the indicated value. If not, a new scoped variable is created and initialized to that value.
The scope attribute is optional and default to page.
Three examples of using c:set are given below. In the first example, a page scoped variable
“timezone” is set to a value“Asia / Karachi”.
<c:set var=“timezone” value=“Asia/Karachi” />
In the second example, a request scoped variable “email” email is set to a value
“[email protected]”
<c:set var=“email” scope=”request” value=“[email protected]” />
In the third example, a page scoped variable “email” is set to value of request parameter“email”
by using param implicit object. If email parameter is defined in JSP page as:
<input type=”text” value = “email” />
Then c:set tag would be used as:
<c:set var=“email” scope=”request” value=“param.email” />
Using c:set with JavaBeans & Map
c:set tag can also be used to change the property of a bean or the value against some key. For
this purpose, the syntax of the c:set tag would look like this:
© Copyright Virtual University of Pakistan 451 Web Design and Development (CS506) <c:set
target=“bean/map”
property=”property/key”
value=“value”
/>
If target is a bean, sets the value of the property specified. This process is equivalent to
<jsp:setProperty … /> JSP action tag.
If target is a Map, sets the value of the key specified
And of course, these beans and maps must be stored in some scope prior to any attempt is made
to change their properties.
For example, consider the following snippet of code that stores PersonInfo’s object person into
request scope using <jsp:useBean … /> tag. Then using c:set tag,
person’s name property is set to “ali”.
<jsp:useBean
/>
id=“person”
class=“vu.PersonInfo”
scope=“request”
<c:set target=“person” property =“name” value = “ali” />
c:out
A developer will often want to simply display the value of an expression, rather than store it.
This can be done by using c:out core tag, the syntax of which appears below:
<c:out value = “expression” default = “expression” />
This tag evaluates the expression specified by its value attribute, and then prints the result. If
the optional default attribute is specified, the c:out action will print its
(default) value if the value attribute's expression evaluates either to null or an empty String.
This tag is equivalent to JSP expression i.e. <%=expression %>.
Consider the following examples in which the usage of c:out tag has shown. In the first
example, string “Hello” would be displayed
<c:out value = “Hello” />
In the second example, if request parameter num evaluates to null or an empty string then default
value “0” would be displayed.
<c:out value = “${param.num}”
default = “0” />
The above fragment of code is equivalent to following scriptlet:
<%
String no = request.getParameter(“num”);
if (no == null || no.equals(“”)) {
© Copyright Virtual University of Pakistan 452 Web Design and Development (CS506) System.out.println(0);
}else{
Out.println(no);
}
%>
If we want to display the property of a bean like name, we’ll write
<c:out value= “${person.name}” default = “Not Set” />
c:remove
As its name suggests, the c:remove action is used to delete a scoped variable, and takes two
attributes. The var attribute names the variable to be removed, and the optional scope attribute
indicates the scope from which it should be removed and defaults to page.
For example, to remove a variable named square from page scope, we’ll write:
<c:remove var = “square” />
And if variable email is required to be removed from request scope, then c:removetag will
look like:
<c:remove var = “email” scope = “request” />
c:forEach
In the context of Web applications, iteration is primarily used to fetch and display collections of
data, typically in the form of a list or sequence of rows in a table. The primary JSTL action for
implementing iterative content is the c:forEach core tag. This tag supports two different
styles of iteration:
Iteration over an integer range (like Java language's for statement)
Iteration over a collection (like Java language's Iterator and Enumeration classes).
Iteration over an Integer range
To iterate over a range of integers, the syntax of the c:forEach tag will look like:
<c:forEach var=“name” begin=“expression” end=“expression”
step=“expression” >
Body Content
</c:forEach>
The begin and end attributes should be either constant integer values or expressions
evaluating to integer values. They specify the initial value of the index for the iteration and the
index value at which iteration should cease, respectively. When iterating over a range of integers
using c:forEach, these two attributes are required and all others are optional.
© Copyright Virtual University of Pakistan 453 Web Design and Development (CS506) The step attribute specifies the amount to be added to the index after each iteration. Thus the
index of the iteration starts at the value of the begin attribute, is incremented by the value of
the step attribute, and halts iteration when it exceeds the value of the end attribute. Note that
if the step attribute is omitted, the step size defaults to 1.
If the var attribute is specified, then a scoped variable with the indicated name will be created
and assigned the current value of the index for each pass through the iteration. This scoped
variable has nested visibility that is it can only be accessed within the body of the c:forEach
tag.
For example to generate squares corresponding to range of integer values, the c:forEach tag
will be used as:
<c:forEach var=“x” begin=“0” end=“10” step=“2” >
<c:out value=“${x * x}” />
</c:forEach>
By executing the above code, following output would appear:
4 16 36 64 100
Iteration over a Collection
When iterating over the members of a collection and arrays etc, one additional attribute of the
c:forEach tag is used: the items attribute. Now the c:forEach tag will look similar to
this:
<c:forEach var=“name” items=“expression” >
Body Content
</c:forEach>
When you use this form of the c:forEach tag, the items attribute is the only required
attribute. The value of the items attribute should be the collection/array over whose members
the iteration is to occur, and is typically specified using an EL expression. If a variable name is
also specified using var attribute, then the named variable will be bound to successive elements
of the collection for each iteration pass.
For example, to iterate over a String array (messages) using java code, we used to write in JSP:
<%
for(int i=0; i<messages.length; i++) {
String msg = messages[i];
%>
<%= msg %>
<%
} // end for
%>
© Copyright Virtual University of Pakistan 454 Web Design and Development (CS506) This can be done using c:forEach tag in much simpler way as shown below:
<c:forEach var=“msg” items=“${messages}” >
<c:out value= “${msg}” />
</c:forEach>
Similarly, to iterate over a persons ArrayList that contains PersonInfo objects, w used to
write in JSP:
<%
ArrayList persons = (ArrayList)request.getAttribute(“pList”);
for(int i=0; i<persons.size(); i++) {
PersonInfo p == (PersonInfo)persons.get(i);
String name = p.getName();
%>
<%= name %>
<%
} // end for
%>
Indeed, the above task can be achieved in much simpler way using c:forEach tag as shown
below:
<c:forEach var=“p” items=“${persons}” >
<c:out value= “${p.name}” />
</c:forEach>
The c:forEach tag processes each element of this list(persons) in turn, assigning it to a
scoped variable named p. Note that typecast is also not required.
Furthermore, you can use the begin, end, and step attributes to restrict which elements of
the collection are included in the iteration.
c:if
Like ordinary Java’s if, used to conditionally process the body content. It simply evaluates a
single test expression and then processes its body content only if that expression evaluates to
true. If not, the tag's body content is ignored. The syntax for writing c:if tag is:
<c:if test= “expression” >
Body Content
</c:if>
For example, to display a message “a equals b” if two strings a & b are equal, the c:if tag is
used as:
<c:if test= “${a == b}” >
<h2> A equals B </h2>
© Copyright Virtual University of Pakistan 455 Web Design and Development (CS506) </c:if>
c:choose
c:choose the second conditionalization tag, used in cases in which mutually exclusively test
are required to determine what content should be displayed. The syntax is shown below:
<c:choose>
<c:when test= “expression” >
Body content
</c:when>
……………
<c:otherwise >
Body content
</c:otherwise>
</c:choose>
Each condition to be tested is represented by a corresponding <c:when> tag, of which there
must be at least one. Only the body content of the first <c:when> tag whose test evaluates to
true will be processed. If none of the <c:when> tests return true, then the body content of the
<c:otherwise> tag will be processed.
Note, though, that the <c:otherwise> tag is optional; a <c:choose> tag can have at most
one nested <c:otherwise> tag. If all <c:when> tests are false and no <c:otherwise>
action is present, then no <c:choose> body content will be processed.
The example code given below illustrates the usage of c:choose tag in which two strings a &
b are compared and appropriates messages are displayed:
<c:choose>
<c:when test= “a == b” >
<h2> a equals b</h2>
</c:when>
<c:when test= “a <= b” >
<h2> a is less than b</h2>
</c:when>
<c:otherwise >
<h2> Don’t know what a equals to </h2>
</c:otherwise>
</c:choose>
43.7 netBeans 4.1 and JSTL
If you are using netBeans 4.1 IDE then you have to add JSTL library to your project manually.
To do so, right click on the libraries folder, you can find it under project’s name and select the
Add Library option. This is also shown in the following figure:
© Copyright Virtual University of Pakistan 456 Web Design and Development (CS506) The Add Library dialog box opens in front of you. Select JSTL 1.1 option and press Add Library
button. Now you can refer to any JSTL library in your JSPs.
Note: Remember that the JSTL 1.1 library is only added to current project. You have to repeat
this step for each project in which you want to incorporate JSTL. Example Code: AddressBook using JSTL core tags
This is the modified version of AddressBook that was built using Expression Language in the last
handout. Only showperson.jsp is modified to incorporate JSTL core tags along with
Expression Language in place of scriptlets. The remaining participants searchperson.jsp,
ControllerServlet, PersonInfo and PersonDAO left unchanged. Let’s look on the
© Copyright Virtual University of Pakistan 457 Web Design and Development (CS506) code of each of these components:
PersonInfo.java
The JavaBean used to represent one person record.
package vu;
import java.io.*;
public class PersonInfo implements Serializable{
private String name;
private String address;
private int phoneNum;
// no argument constructor
public PersonInfo() {
name = "";
address = "";
phoneNum = 0;
}
// setters
public void setName(String n){
name = n;
}
public void setAddress(String a){
address = a;
}
public void setPhoneNum(int pNo){
phoneNum = pNo;
}
// getters
public String getName( ){
return name;
}
public String getAddress( ){
return address;
}
public int getPhoneNum( ){
return phoneNum;
}}
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 458 Web Design and Development (CS506) Lecture 44: Client Side Validation & JavaServer Faces (JSF)
In this handout, we’ll talk about client side validation and also learn about growing in demand
Java technology i.e. JSF. First start with client side validation
44.1 Client Side Validation
Forms validation on the client-side is essential -- it saves time and bandwidth, and gives you
more options to point out to the user where they've gone wrong in filling out the form.
Furthermore, the browser doesn't have to make a round-trip to the server to perform routine
client-side tasks. For example, you wouldn't want to send the browser to the server to validate
that all of the required fields on a form were filled out.
Any scripting language can be used to achieve the said objective. However, JavaScript and
VBScript are t wo popular options
44.1.1 Why is Client Side Validation Good?
There are two good reasons to use client-side validation:
•
•
It's a fast form of validation: if something's wrong, the alarm is triggered upon submission
of the form.
You can safely display only one error at a time and focus on the wrong field, to help ensure
that the user correctly fills in all the details you need.
Code Example: Form Validation using JavaScript
For example on the following form, we want to make sure that text filed for name should not be
left empty and age field does not contain any negative value. To accomplish this we’ll use
JavaScript. If user forgets to provide name and/or enters a negative value, a message would be
displayed to the user that indicates what was went wrong? However, if user conforms to
requirements, he/she would be taken to another page that displays a greeting message.
Note: In this example, JavaScript semantics isn’t discussed over here as I am assuming that you
might be familiar with some scripting language. Otherwise, www.w3schools.com is an excellent
resource to learn about scripting languages
© Copyright Virtual University of Pakistan 459 Web Design and Development (CS506) The code that is used to generate this page is given below:
<HTML>
<HEAD>
<!—- start of scripting code and mentioning type
<SCRIPT TYPE = "text/javascript">
-->
/* defining a function that receives form’s reference, defined
inside the body and returns false if any requirement violated
*/
function validateForm(thisform)
{
/* checking the value of the name field, if it is left empty
then displaying a message
*/
if (thisform.name.value == null || thisform.name.value == "")
{
alert("Username is required");
return false;
}
// if value of age is negative, displaying a message
if (thisform.age.value < 0 )
{
alert("Age can't be negative");
return false;
} // end of function
</SCRIPT> <!—-end of script-- >
</HEAD>
<BODY>
<!—- validateForm method is called and specified as a value of
onsubmit value, if this method returns false, the user remains
on the same page -->
<FORM method="post" onsubmit="return validateForm(this)"
action = “greeting.jsp” >
<h2> Client Side Validation Example </h2>
<BR/>
Name: <INPUT type="text" name="name" size="30" />
<BR/> <BR/>
Age: <INPUT type="text" name="age" size="30" />
© Copyright Virtual University of Pakistan 460 Web Design and Development (CS506) <BR/> <BR/>
<INPUT type="submit" value="Submit">
</FORM>
</BODY>
</HTML>
44.2 JavaServer Faces (JSF)
JSF technology simplifies building the user interface for web applications. It does this by
providing a higher-level framework for working with your web applications. Some distinct
features will be discussed provided by this technology. To begin with, have a look on some
popular existing frameworks
44.2.1 Different existing frameworks
• Struts
A popular open source JSP-based Web application framework helps in defining a structured
programming model (MVC), also validation framework and reduces tedious coding but…
o Adds complexity and doesn’t provide UI tags
o Very Java programmer centric
• Tapestry
Another popular framework that is extensively used in the industry is Tapestry. It has almost
similar sort of problems as with Struts.
44.2.2 JavaServer Faces
A framework which provides solutions for:
•
•
•
•
•
•
•
•
Representing UI components
Managing their state
Handling events
Input validation
Data binding
Automatic conversion
Defining page navigation
Supporting internationalization and accessibility.
If you are familiar with Struts and Swing (the standard Java user interface framework for desktop
applications), think of JavaServer Faces as a combination of those two frameworks. Like Swing,
JSF provides a rich component model that eases event handling and component rendering; and
like Struts, JSF provides Web application lifecycle management through a controller servlet
© Copyright Virtual University of Pakistan 461 Web Design and Development (CS506) 44.2.3 JSF UI Components
Some of the standard JavaServer Faces components are shown below:
Some custom JavaServer Faces components are
And some open course JavaServer Faces components are also available like:
© Copyright Virtual University of Pakistan 462 Web Design and Development (CS506) And some third-party Java Server Faces components are also available:
44.2.4 JSF Events Handling
A JSF application works by processing events triggered by the JSF components on the pages.
These events are caused by user actions. For example, when the user clicks button, the button
triggers an event. You, the JSF programmer, decide what the JSF application will do when a
particular event is fired. You do this by writing event listeners.In other words, a JSF application
is event-driven.
For example, if you write a JSF code to create a button, you will write:
<h:commandButton value="Login“
actionListener=“#{customer.loginActionListener}”
action=“#{customer.login}” />
The value attribute specifies the text that appeared on the face of a button, the
actionListener attributes specifies to call the loginActionListener method written
somewhere in a Customer class if an event is triggered and on which to go next,is decided by
the login method of Customer class and given as a value of action attribute.
The method specified in action attribute should return a String value as the returned
String value is used in page navigation.
Note: Many IDE provides visual support for JSF so you can drag and drop components instead
of writing tedious coding for defining JSF components as shown above. Sun Studio Creator® is
a free open source IDE that provides visual support for JSF and can be downloaded form Sun
site.
© Copyright Virtual University of Pakistan 463 Web Design and Development (CS506) The code examples are also built using this IDE.
class Customer {
public void loginActionListener(ActionEvent e)
{
…………………
}
public String login() {
return “OK”;
}}
Example Code: Hello User
The example code (“hello user 1”) is given along with the handout. It is strongly advised that
you must see the lecture video in order to learn how this example is built.
User will provide a name in the text field and his/her name after appending “hello” to it,would
be displayed on the same page.
44.2.5 JSF Validators
Validators make input validation simple and save developers hours of programming. JSF
provides a set of validator classes for validating input values entered into input components.
Alternatively, you can writeyour own validator if none of the standard validators suits your
needs.
Some built-in validators are:
•
DoubleRangeValidator
Any numeric type, between specified maximum and minimum values
•
LongRangeValidator
Any numeric type convertible to long, between specified maximum and minimum
values
•
LengthValidator
Ensures that the length of a component's local value falls into a certain range
(between minimum & maximum). The value must be of String type.
Example Code: Hello User
The example code (“hello user 2”) is given along with the handout. You can open it using Sun
Studio Creator IDE. It is strongly advised that you must see the lecture video in order to learn
how this example is built.
It is actually a modified version of the last example. This time, we’ll make sure that user couldn’t
left blank the name field and must enter a name between ranges of 2 to 10 characters. If any
condition fails, an appropriate message would be displayed.
© Copyright Virtual University of Pakistan 464 Web Design and Development (CS506) 44.2.6 JSF – Managed Bean-Intro
These are JavaBeans defined in the configuration file and are used to hold the data from JSF
components. Managed beans represent the data model, and are passed between business logic
and pages. Some other salient features are:
•
•
•
Use the declarative model
Entry point into the model and event handlers
Can have beans with various states
Here is an example of a managed-bean element whose scope is session, meaning that an instance
of this bean is created at the beginning of a user session.
<managed-bean>
<managed-bean-name>myBean</managed-bean-name>
<managed-bean-class>myPackage.MyBean</managed-bean-class>
<managed-bean-scope>session</managed-bean-scope>
</managed-bean>
44.2.7 JSF – Value Binding
Value binding expressions can be used inside of JSF components to:
•
•
•
•
Automatically instantiate a JavaBean and place it in the request or session scope.
Override the JavaBean's default values through its accessor methods.
Quickly retrieve Map, List, and array contents from a JavaBean.
Synchronize form contents with value objects across a number of requests.
The syntax of binding expressions is based on the JavaServer Pages (JSP) 2.0 Expression
Language. In JSP, expressions are delimited with "${}", but in JSF they are delimited
with "#{}".
44.2.8 JSF – Method Binding
Unlike a value binding, a method binding does not represent an accessor method. Instead,
a method binding represents an activation method.
For example, binding an event handler to a method
<h:commandButton ……
actionListener=“#{customer.loginActionListener}”
……… />
44.2.9 JSF Navigation
Page navigation determines the control flow of a Web application. JSF provides a default
navigational handler and this behavior can be configured in configuration. However, you
can do it visually in most tools like Sun Studio Creator
© Copyright Virtual University of Pakistan 465 Web Design and Development (CS506) Note: We have quickly breezed through the JSF technology essentials due to shortage of
time. You must explore it by yourself to excel on it. You can find the resources in
the last handout to acquire further skills.
44.3 References:
•
•
•
Java A Lab Course by Umair Javed
Intrduction to JavaServer Faces by Sun
http://java.sun.com
JavaServer Faces Programming by Kumiawan
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 466 Web Design and Development (CS506) Lecture 45: JavaServer Faces
In the last lecture, we have covered the basic nutshells of JSF. Having a belief on“learning by
doing”, in this lecture another example is also given to show you the capabilities of JSF.
Example Code: Addition of Two Numbers
The example code (“AddNumbers”) is given along with the handout. It is strongly advised that
you must see the lecture video in order to learn the making plus working of this example.
This example demonstrates the usage of value and method binding expressions, managed beans,
and how to use page navigation technique using IDE etc.
45.1 Web Services
In the remaining handout, we’ll take an overview of web services’ potential, their types and
working model. Resources are given at the end for those who are interested in learning new
technologies.
45.1.1 Introduction
Web services are Web-based enterprise applications that use open, XML-based standards and
transport protocols to exchange data with calling clients.
Web Service is becoming one of those overly overloaded buzzwords these days. Due to their
increasing popularity, Java platform Enterprise Edition (J2EE) provides the APIs and tools you
need to create and deploy interoperable web services and clients.
45.1.2 Web service, Definition by W3C
W3C recently has come up with a decent definition of web services. According to W3C,“A Web
service is a software application identified by a URI, whose interfaces and binding are capable of
being defined, described and discovered by XML artifacts and supports direct interactions with
other software applications using XML based messages via internet-based protocols”.
45.1.3 Distributed Computing Evolution
Let's think a little bit on how distributed computing technology has evolved.
© Copyright Virtual University of Pakistan 467 Web Design and Development (CS506) In the beginning, things were built and deployed typically in the form of client and server model
in which clients talk to a single server, for example, remote procedure calls (RPC).
The second phase can be called web-based computing in which many clients talk to many
servers through the net. In this phase, communicating partners still have to go through some prearrangement in terms of what common object model they have to use or what common
communication protocol they have to agree upon.
Finally, the web services model in which service users and service providers can be dynamically
connected. And the pretty much every computing device and application participates as both
service user and service provider.
45.1.4 Characteristics of Web services
Web services are XML-based throughout. Pretty much everything in the domain of Web services
is defined in XML. For example, the format of the data being exchanged between service user
and service provider is defined in XML or the description of web service is defined in XML.
Because the only contract that has to be agreed upon between service user and service provider is
syntax and semantics of XML messages, as long as valid messages can be generated and
understood, it does not matter what programming language is used. So a web service is said to be
programming language independent.
Web services can be dynamically located and invoked. And typically they will be accessed and
invoked over both internet and intranet.
45.1.5 Why Web services?
Interoperable
Connect across heterogeneous networks using ubiquitous web-based standards
Economical
Recycle components, no installation and tight integration of software
Automatic
No human intervention required even for highly complex transactions
© Copyright Virtual University of Pakistan 468 Web Design and Development (CS506) Accessible
Legacy assets & internal apps are exposed and accessible on the web
Available
Services on any device, anywhere, anytime
Scalable
No limits on scope of applications and amount of heterogeneous applications
45.1.6 Types of Web service
Data providers
For example, a service providing stock quotes
Business-to-business process integration
For example, purchase orders
Enterprise application integration
Different applications work together simply by adding a webservice wrapper
Comparison between Web page & Web service
Just to give you a sense on the difference between a web page and a web service,consider
the following table:
Web page
Web Service
Has a UI
No GUI
Interacts with user
Interacts with application
Works with web browser client
Works with any type of client
45.2 Web service Architectural Components
Following are the core building blocks of web service architecture:
•
Service Description-How do clients know how it works (which functions, parameters
etc.)?
At the minimum, you need a standard way of describing a web service that is universally
understood by all potential service users and service providers. This is important because
without commonly agreed upon description of service, a service provider might have to
produce individually tailored way of describing its service to all its potential service users.
Web Service Description Language (WSDL pronounced as viz-dal) is industry agreed upon
XML language that can be used to describe web service. It provides XML format for
describing web services in terms of methods, properties, data types and protocols.
© Copyright Virtual University of Pakistan 469 Web Design and Development (CS506) •
Service Registration (Publication) and Discovery
There has to be registry by which a service can be published and discovered.
Universal Description, Discovery & Integration (UDDI), a way to publish and find web
services. A repository of web services on the internet where a machine or a human can find
different web services. www.uddi.org
•
Service Invocation
Then there has to be standard way of invoking a service. Finally, for business transactions in
which secure and reliable message delivery is important, there has to be a standard electronic
business framework.
The following figure represents simplified web service architecture and summarizes the working
of web services:
45.3 References:
•
Java A Lab Course by Umair Javed
•
Web services overview by sang shin
Note: Coding exercises in working condition for this lecture are also available on “Downloads”
section of LMS.
© Copyright Virtual University of Pakistan 470 Web Design and Development (CS506) 45.4 Resources:
•
An excellent resource for learning Java related technologies is:
http://www.apl.jhu.edu/~hall/java/
•
http://java.sun.com
•
http://www.javaworld.com
•
http://www.theserverside.com
•
http://www.jsfcentral.com
•
http://www.jspolympus.com
• http://www.onjava.com
© Copyright Virtual University of Pakistan 471 
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