Google Android - A Comprehensive Introduction - DAI

Google Android - A Comprehensive
Introduction
Technical Report: TUB-DAI 03/09-01
Hans-Gunther Schmidt, Karsten Raddatz,
Aubrey-Derrick Schmidt, Ahmet Camtepe, and Sahin
Albayrak
March 16, 2009
DAI-Labor der Technischen Universität Berlin
Prof. Dr.-Ing. habil. Sahin Albayrak
Technische Universität Berlin DAI-Labor
Institut für Wirtschaftsinformatik und Quantitative Methoden
Fachgebiet Agententechnologien in betrieblichen Anwendungen und der Telekommunikation
Sekretariat TEL 14
Ernst-Reuter-Platz 7
10587 Berlin
Telefon: (030) 314 74000
Telefax: (030) 314 74003
E-mail: Sekretariat@dai-labor.de
WWW: http://www.dai-labor.de
Disclaimer
This Technical Report has been kindly supported by DAI-Labor Technische Universität Berlin, Prof. Dr.-Ing. habil. Sahin Albayrak and was produced as a result of the
university project called course "Autonomous Security II".
2
Abstract
Google Android, Google’s new product and its first attempt to enter the mobile market,
might have an equal impact on mobile users like Apple’s hyped product, the iPhone.
In this Technical report we are going to present the Google Android platform, what
Android is, describe why it might be considered as a worthy rival to Apple’s iPhone.
We will describe parts of its internals, take a look "under the hood" while explaining
components of the underlying operating system. We will show how to develop applications for this platform, which difficulties a developer might have to face, and how
developers can possibly use other programming languages to develop for Android than
the propagated language Java.
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Contents
1 Introduction
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2 About Android
2.1 What Is Google Android? . . . . . . .
2.2 What Is the Google Android SDK? . .
2.3 The Google Android Architecture . .
2.4 Google’s Motivation Behind Android
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3 The Android SDK
3.1 General Structure Of The SDK . . . . . . . . .
3.2 Tools Within The SDK . . . . . . . . . . . . .
3.3 The Android Emulator . . . . . . . . . . . . .
3.4 Android Documentation . . . . . . . . . . . .
3.5 ADT - Android Development Tools For Eclipse
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4 The Android OS
4.1 The Android Emulator On OS-level
4.2 The Kernel . . . . . . . . . . . . .
4.3 Fileystem Layout . . . . . . . . . .
4.3.1 System Image . . . . . . . .
4.3.2 User Data Image . . . . . .
4.3.3 Cache Image . . . . . . . .
4.3.4 SD Card Image . . . . . . .
4.4 Android-Specific Daemons . . . . .
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5 Source Code Life Cycle On Android
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5.1 From Java Source Code To A Compiled Dex File . . . . . . . . . . . . 21
5.2 Packaging All Project Files Into An APK . . . . . . . . . . . . . . . . 21
5.3 How To Create And Deploy APKs Without Eclipse . . . . . . . . . . . 21
6 Facing Difficulties In Developing On Android
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6.1 Difficulties In Developing Dndroid Applications . . . . . . . . . . . . . 23
6.2 Difficulties Developing In C/C++ . . . . . . . . . . . . . . . . . . . . . 23
6.3 Difficulties Developing In Java . . . . . . . . . . . . . . . . . . . . . . 23
7 Interconnect Daemon and GetProcessList
7.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2 Handling Interprocess Communication . . . . . . . . . . .
7.3 Architecture Design And UML-Diagramm . . . . . . . . .
7.4 Interconnect Daemon . . . . . . . . . . . . . . . . . . . .
7.5 GetProcessList . . . . . . . . . . . . . . . . . . . . . . .
7.6 Out Of Scope . . . . . . . . . . . . . . . . . . . . . . . .
7.6.1 What Did We Not Implement? Why? . . . . . . .
7.6.2 What Else Could Be Done Based On This Project?
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References
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List of Figures
1
2
3
4
5
The Google Android Architecture overview
Greeting you with the welcome screen . . .
UML-diagram of GetProcessList . . . . . .
UML-diagram of GetProcessList . . . . . .
UML-diagram of GetProcessList . . . . . .
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15
26
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SDK Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
10
List of Tables
1
5
1 Introduction
For some time now, two operating systems for mobile phones have been dominating the
mobile market: Symbian OS and Windows Mobile.
In January 2007 [1], Apple, as third big company to enter the mobile market, introduced the iPhone running the in-house developed operating system Mac OS X. Leaving
the generation of keypads behing, the iPhone was one of the first phones to offer a
ground-breaking new touchpad interface which works very fast and sensitive. This,
combined with its appearance of both physical device and operating system, resulted in
a big hype for Apple’s new cash cow iPhone. In combination with iPhone market, offering thousands of applications which can easily be downloaded and installed, iPhone
has become a major success in Apple’s product history.
As last contestant, Google, famous for its dominating web search portal, entered the
market with a Linux-kernel-based product: Google Android. Google Android, a Linuxkernel-based operating system, comes along with a quite new credo within this market:
it is supposed to be open! This, in particular, means all elements used shall be provided
in source code form so that developers will have the possiblility to take a closer look into
the system to maybe alter it to their needs. In order to guarantee a great impact of Google
Android on the market, Google organized several developer contests in order to create
a rich set of ready-to-go applications for the Android plattform before the actual phone
is even available on the market, hoping to enter the competition with Apple’s iPhone
market. As strategic partners Google picked HTC as provider for the first physical
device running Android and T-Mobile as telephone provider who will offer the T-Mobile
G1, the name of the first upcoming device, coupled with a contract, for a consumerfriendly price.
Motivation of this Technical Report was to get a thorough understanding of the new
arising platform Google Android, its framework, its underlying operating systems and
possible differences to commonly known Linux operating systems. Furthermore, we
wanted to get a feel for how open the proposed system really is (as stated by Google),
meaning to identify difficulties in developing applications for Android, not only with the
proposed core language Java but also with other well-known programming languages,
e.g. C or C++. Once we identified and, finally, mastered those difficulties, we wanted
to show and prove that other languages than Java might as well be used to successfully
develop applications for the Android plattform. And, as an important point, we wanted
to show that such applications can coexist with given Android Java applications and
even communicate with those.
The layout of this Technical Report is as follows: While section 1 gives a brief introduction into the topic, section 2 will describe the Abouts of Google Android. Section
3 will cover the Android SDK, its structure, included tools, the emulator and so forth.
Section 4 will take a deeper look, "under the hood", being specific, a description of the
underlying operating system, the used kernel, filesystem layout and more. Interesting
for developers, we will then give a basic overview of the source code life cycle in section 5. Section 6 will shortly discuss difficulties in developing applications for Android,
6
using any kind of programming language for this task. To prove that Java is not the
only programming language that can be used on Android, we will, in section 6, give a
brief introduction into our basic server-/client-architecture Interconnect/GetProcessList,
a software set consisting of both Android-based Java-application and a unix service that
has been implemented in the programming language C. In the same section, we will
also show and discuss out-of-scope thoughts and what else could be done ontop of our
server-/client-stack.
After having given a short introduction into the topic, we will continue with the
Abouts of Google Android to get a common, basic understanding of what we are dealing
with.
7
2 About Android
Google Android, as the new contestant on the market, gained great popularity after being
first announced in November 2007. With its announcement, it also was accompanied by
the founding of the Open Handset Alliance for which Google Android is said to be the
flagship software. In following chapters, we will take a closer look at Google Android
and describe its parts, functionalities, eventual problems and more. But first, we will
start with a basic understanding of Google Android.
2.1 What Is Google Android?
According to the Google Android website [2], Google describes its platform as
a software stack for mobile devices including an operating system, middleware and key applications.
Basically said, Android consists of a UNIX-like operating system based on a 2.6series Linux kernel. The operating system has been enriched with all necessary elements that are required to provide basic functions like a networking stack, GSM/GPRS
abilities, and more. On top of all this, Google offers a framework containing a rich set
of Java methods enabling developers to create a wide range of software for mobile use
on Google Android.
Taking a closer look at the operating system, we are dealing with an ARMarchitecture based system (armv5tejl) which is quite common for small or handheld
devices. The provided 2.6 kernel (2.6.25-00350-g40fff9a in SDK version 1.0_r1) is
slightly modified - specialized for handheld devices - and offers support for the most
commonly used hardware devices in the embedded field of use (SD-Card, USB, and
more).
2.2 What Is the Google Android SDK?
As no officially obtainable physical device is available at time of writing, developers
will have to rely on the provided development kit: Google Android SDK.
A core element of the SDK is the actual Google Android Emulator which provides
a graphical emulation of a possible handheld device running Google Android. Furthermore, the SDK not only provides the core classes of the Android framework packed into
a Java Jar-file: it also includes the documentation in HTML-form and several tools that
improve the usability and interaction with the emulator.
Once the SDK has been downloaded, developers can directly start creating Androidapplications, compiling them and deploying them to the emulator. Nothing more than
an editor and a working JDK (Sun JDK) is required.
Though, to make things easier, Google suggests Eclipse as the IDE to be used. For
this, Google optionally provides an Android Plugin made for Eclipse that will take over
a few basic tasks like creating the Android Project layout on disk, the integration of your
SDK, eventually starting your emulator if not running yet and the deployment of you
application on the emulator ... all this with just a few clicks.
8
Figure 1: The Google Android Architecture overview
2.3 The Google Android Architecture
Figure 1 shows the Architecture overview that can be found on the Google Android
project website ( [2]). The architecture basically consists of four sections: the Linux
kernel (system) as underlying operating system interface, the libraries (e.g libc) as important part of the operating system; the Android framework providing all necessary
classes and methods in order to write Android-compatible applications; and, as top section, the actual Android applications.
2.4 Google’s Motivation Behind Android
It is hard to say what Google’s plans are for Android. The physical device T-Mobile G1,
which will be the first phone running with Android OS, will be Google’s first step into
the mobile market. If Android is going to advance to a sophisticated operating system
for smart mobile phones or if it is going to be stripped down to a very basic system for
every mobile phone hardware, we can only guess.
But, as we all know Google, Google often sets foot into a certain area with a basic
version of application or service and then simply waits what the actual user is going to
make out of it. In Android’s case, this strategy might just be like that: Offer the G1,
offer well-documented SDKs and APIs for Java, offer and promise openness ... and
watch what is going to happen next. The users will define Android. And as soon as
Google knows in which direction we are going, they are most likely making something
great out of it.
Let us wait and see how Android is going to develop.
9
Table 1: SDK Elements
android.jar
docs
LICENSE.txt
samples
tools
The Android classes
Android documentation (HTML)
License terms
Sample projects based on the Android framework
binaries, libraries, emulator images
3 The Android SDK
The Google Android SDK can be freely downloaded on the project website:
http://code.google.com/android/download.html
Google provides the Android SDK for three different platforms: Windows, Apple’s Mac OS X and Linux. All SDKs are quite similar in its structure, therefore we
will, from now on, refer to the Linux-version of the SDK.
In following sections, we will take a glance at each part of the SDK.
3.1 General Structure Of The SDK
Once the SDK-archive is unpacked on the host system, a quick glance at the filesystem
structure reveals following:
The tools directory is, by far, the most interesting directory to inspect: in its root
directory it offers several executables consisting of shell scripts, binaries, python scripts,
basically everything that enables the developer to create Android projects and work with
them. Furthermore, the tools/lib directory contains several Java-libraries in form of Jarfiles. And, last but not least, the tools/lib/images directory accomodates the emulator
base images which are loaded upon emulator start.
3.2 Tools Within The SDK
All important tools for successfully developing applications for the Android platform
can be found in the tools directory. If the developer uses Eclipse as standard development IDE, he will most likely not be confronted with any of these: the optionally
available Eclipse plugin provided on the Google Android project website takes over the
responsibility for all necessary tasks. One can simply concentrate on writing the code,
a simple click on the build button will automatically do the rest.
In case, you are not using any IDE or simply not using the plugin, following will give
a short overview of each tool and its meaning:
10
aapt - Android Asset Packaging Tool This tool enables the developer to view,
create and update Zip-compatible archives. It can also compile resources into
binary assets.
activityCreator The shell script is used to create basic filesystem structure for a new
Android project. From then on, any IDE or Editor can be used to alter the code.
adb - Android Debug Bridge A very powerful service can basically enables all
communication to and from the emulator. Connecting with the emulator via port
5555, it enables the user to execute remote shell commands or simply provides a
login shell.
aidl - Android Interface Definition Language The Android Interface Definition
Language tool is used to generate code that enables two processes on an Androidpowered device to communicate with each other using IPC.
ddms - Dalvik Debug Monitor Service This tool provides port-forwarding services, screen capturing on the device, thread and heap information, logcat and
much more (see [3]).
dmtracedump Alternative to generate graphical call-stack diagrams from trace log
files.
dx Generates bytecode from .class files. It converts the files into the .dex file format
which can be run in the Android environment.
mksdcard Creates a FAT32 disk image which can be loaded upon emulator start. Simulate the presence of an SD card.
sqlite3 The well-know sqlite database which saves its databases in plain files. Can be
used for debugging issues (inspecting databases in Android).
traceview Traceview is used to view tracefiles which can be created on the device.
emulator Executable to start the Android Emulator.
3.3 The Android Emulator
Based on Qemu (see [4]), the Google Android emulator works with several images
which can be found as files in the SDK:
hgschmidt@toyo:~/Android/SDK/android-sdk/tools/lib/images$ file *
kernel-qemu: data
NOTICE.txt:
ISO-8859 English text
ramdisk.img: gzip compressed data, from Unix, last modified: Mon Sep 22 22:37:59 2008
skins:
directory
system.img:
VMS Alpha executable
userdata.img: VMS Alpha executable
hgschmidt@toyo:~/Android/SDK/android-sdk/tools/lib/images$
11
Both system and user data images can be found here which represent two important
parts within the operating system. We will take a more detailled look at all images in
the next chapters so please hold on by now.
In order to start the emulator, the tools directory offers the emulator binary. The help
page to the command reveals a quite powerful background:
hgschmidt@toyo:~/Android/SDK/android-sdk/tools ./emulator -help
Android Emulator usage: emulator [options] [-qemu args]
options:
-system <dir>
read system image from <dir>
-datadir <dir>
write user data into <dir>
-kernel <file>
use specific emulated kernel
-ramdisk <file>
ramdisk image (default <system>/ramdisk.img
-image <file>
system image (default <system>/system.img
-initdata <file>
initial data image (default <system>/userdata.img
-data <file>
data image (default <datadir>/userdata-qemu.img
-cache <file>
cache partition image (default is temporary file)
-nocache
disable the cache partition
-sdcard <file>
SD card image (default <system>/sdcard.img
[...]
As already stated, the emulator works ontop of Qemu, therefore the emulator offers
nearly all parameters in order to tweak the emulated qemu system. As we can see here,
the used images can be placed at different locations making it possible to start several
instances of the emulator with all different user data and system images.
3.4 Android Documentation
No development without appropriate documentation. Google is aware of this and offers
two valuable sets of information resources.
SDK documentation Each SDK includes an offline documentation set including
the Android Java API and code examples for most basic issues when developing for
Android. The documentation can be easily observed with any given browser.
Online Resources Google offers an easy to understand API, enriched with various
code examples, guidelines for naming conventions, extensive descriptions to all parts
of the Android platform on the project website:
http://code.google.com/android/
The website will most obviously be the most valuable resource for Android developers.
3.5 ADT - Android Development Tools For Eclipse
Google provides developers with a rich framework to start developing applications for
Android. The SDK is one part of this framework and offers almost anything a developer
needs to start, given an editor to write the code. But, additionally, Google also offers
another valuable piece in the Android application development life-cycle: ADT, the
Android Development Tools, a plugin for the Eclipse IDE. Once installed into Eclipse,
Eclipse will from then on
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• know how to create Android projects,
• start up the emulator on compilation and deployment automatically if not running
yet,
• offer interfaces to some mechanisms in Android, e.g. logcat, heap monitoring,
power management, etc. for extensive debugging.
All steps that would have to be accomplished in a given shell can be handled with this
plugin. Eclipse becomes the only interface developers will ever need to code, compile
and deploy Android applications.
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4 The Android OS
Google announced Google Android to be the first open operating systems for handheld
devices. In order to make that possible, Google decided to use the open and freely
available operating system Linux. Linux has become a more and more used operating
system, commonly used in the server and data center area. But, at the same time, is also
quite often used as the running system for small, embedded devices.
Running on a handheld device, the Linux system is highly optimized for its environment: only elementary parts of Linux have been used like the basic and necessary
kernel providing the most common drivers that will be required during the lifetime of
the handheld device, a filesystem with a very own interpretation of a fileystem layout,
basic libraries that seem to be stripped down to the bare minimum to run the whole
system, and more. In following chapters, we will take a closer look at all of these elementary parts in Google Android. But before that, we will just take a short look at the
maybe most important part that is provided with the Android SDK: the Emulator.
4.1 The Android Emulator On OS-level
Being shipped with the SDK, Google provides an basis for the development tasks: an
emulator which emulates a possible GPhone with graphical user interface. Furthermore,
several scripts and daemons form the whole package of the provided emulator, simply
to make certain things easier or being able to actually communicate with the emulated
system.
First of all, to start the emulated Android system, one either has to deploy an existing,
already created Android project in Eclipse which will automatically cause the emulator
to start, given an installed Android plugin. Or, simply start the emulator executable from
the tools directory of the SDK
hgschmidt@toyo:~/Android/SDK/android-sdk-linux_x86-1.0_r1/tools$ ./emulator &
[1] 15015
hgschmidt@toyo:~/Android/SDK/android-sdk-linux_x86-1.0_r1/tools$
After a few seconds it will greet you with a graphical interface (see figure 2) of a
small handheld which is (almost) fully functional: outgoing and incoming calls are only
simulated.
After having clicked through the interface for a while in order to see that everything
seems to be working and after having found out that Google Android just looks great,
we get bored and start trying to find out what is beneath the shiny interface. In order to
do that, we will need the Android Debug bridge [5], a special services Google provides
in order to communicate with the emulator. adb is quite powerful enabling the developer
to do several tasks like:
• "push" files into the emulator
• "pull" files from the emulator
• view device log
• forward socket connections
14
Figure 2: Greeting you with the welcome screen
• and more ...
To get into the system, adb provides the shell parameter. After executing adb shell
the developer will find himself in a login shell within the emulator.
hgschmidt@toyo:~/Android/SDK/android-sdk-linux_x86-1.0_r1/tools$ ./adb shell
* daemon not running. starting it now *
* daemon started successfully *
#
One can go ahead and drop off a few first linux shell commands:
# ls -l
drwxrwxrwt
drwxrwx--d--------lrwxrwxrwx
-rwxr-x---rwxr-x---rwxr-x--drwxrwx--x
drwxr-xr-x
drwxr-xr-x
dr-xr-xr-x
drwxr-x---rw-r--r-drwx-----drwxr-xr-x
#
root
system
system
root
root
root
root
system
root
root
root
root
root
root
root
root
cache
system
root
root
root
root
system
root
root
root
root
root
root
root
98260
1564
8630
93
2008-10-20
2008-10-20
2008-10-20
2008-10-20
1970-01-01
1970-01-01
1970-01-01
2008-09-22
2008-09-22
1970-01-01
1970-01-01
1970-01-01
1970-01-01
1970-01-01
2008-10-20
18:21
18:21
18:21
18:21
01:00
01:00
01:00
22:44
22:41
01:00
01:00
01:00
01:00
01:00
18:21
sqlite_stmt_journals
cache
sdcard
etc -> /system/etc
init
init.goldfish.rc
init.rc
data
system
sys
proc
sbin
default.prop
root
dev
Once this has been accomplished, we have a common basis to take a closer look at
the specific parts of the operating system.
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4.2 The Kernel
As has already been stated, Google Android provides a 2.6 kernel (output from SDK
version 1.0_r1):
Linux localhost 2.6.25-00350-g40fff9a #1 Wed Jul 23 18:10:44 PDT 2008 armv5tejl unknown
The kernel configuration can be found in the proc filesystem as zipped archive:
# zcat /proc/config.gz
#
# Automatically generated make config: don’t edit
# Linux kernel version: 2.6.25
# Wed Jul 23 18:09:51 2008
#
CONFIG_ARM=y
CONFIG_SYS_SUPPORTS_APM_EMULATION=y
# CONFIG_GENERIC_GPIO is not set
CONFIG_GENERIC_TIME=y
CONFIG_GENERIC_CLOCKEVENTS=y
CONFIG_MMU=y
# CONFIG_NO_IOPORT is not set
[...]
The kernel can be rebuilt, the sources are available on the project website ( [6]). Once
downloaded and extracted, the above mentioned configuration can be used as basis for
further configurations. After having configured all necessary parts, having successfully
compiled the new kernel, it can be loaded via an emulator-parameter:
$ emulator -kernel <file>
4.3 Fileystem Layout
Google Android has been designed to offer, both, a very stable basis every developer can
rely on and, at the same time, enable developers to experiment and develop freely without any significant limitations. This is why Google obviously seperated distinguishable
parts into different images that are loaded and mounted upon emulator start:
• system image
• user data image
• cache image
Eventually, more filesystems are mounted upon start but do not require further investigation as they are quite standard and common on other Linux-variants, e.g. the proc
filesystem.
Here is a complete overview of the mounted fileystems within Google Android:
# /data/busybox/df -h
Filesystem
tmpfs
tmpfs
/dev/block/mtdblock0
/dev/block/mtdblock1
/dev/block/mtdblock2
#
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Size
46.2M
4.0M
64.0M
64.0M
64.0M
Used Available Use% Mounted on
0
46.2M
0% /dev
0
4.0M
0% /sqlite_stmt_journals
41.3M
22.7M 65% /system
26.3M
37.7M 41% /data
1.1M
62.9M
2% /cache
4.3.1 System Image
The system image contains the core parts of the operating system, for example:
• system binaries
• system libraries
• configuration files
• and more ...
Furthermore, the system image includes the actual Android framework with all necessary JAR-files:
# ls /system/framework
framework-res.apk
core.jar
ext.jar
framework.jar
am.jar
android.awt.jar
android.policy.jar
com.google.android.gtalkservice.jar
android.test.runner.jar
com.google.android.maps.jar
input.jar
monkey.jar
pm.jar
ssltest.jar
svc.jar
framework-tests.jar
itr.jar
services.jar
#
It also includes a basic set of Android applications:
• Alarmclock
• Calculator
• Contacs
• Camera
• Music player
• Phone
• Google Maps
• and more ...
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The system image is locked, so read-only access is granted but not more. Obviously,
this will be the place for Google to provide updates to the system without any interference by the user.
As it is locked, importing new applications, either written ontop of the Android
Framework or in any other programming language, is not possible at the moment. If
it would be possible we would still have to face the problem of very limited filesystem
size: for smaller applications it might be sufficient. For bigger applications with eventually big databases will easily exceed the available 21MBytes left over in the image.
4.3.2 User Data Image
While the system image contains the fairly basic parts of Google Android, the user data
image provides the main location to basically put files into the system. New applications
that have been either downloaded or simply self-written will be put here, if correctly
installed by the provided handlers of the emulator.
In its basic state, it already provides a quite complex filesystem structure:
# ls -l /data
drwxrwx--x system
drwxrwx--x system
drwxrwx--t system
drwxrwx--x shell
drwxrwx--x system
drwxrwx--x system
drwx------ root
drwxrwxrwx system
drwxrwx--x system
drwxrwxr-x system
drwxrwx--- root
#
system
system
misc
shell
system
system
root
system
system
system
root
2008-09-22
2008-09-22
2008-09-25
2008-09-25
2008-09-25
2008-09-25
2008-09-25
2008-09-25
2008-09-25
2008-09-25
2008-09-25
22:37
22:44
21:45
21:45
21:45
21:45
21:45
21:47
21:45
21:47
21:45
location
app
misc
local
data
app-private
property
anr
dalvik-cache
system
lost+found
On installation of new applications, obviously several things happen in order to integrate the new application into the fileystem:
• The apk (Android package) is placed into /data/app
• application-centric libraries are installed into /data/data/<application name>
• application-relevant databases are setup and eventually updated
The user data image is, most obviously, a writeable filesystem. New applications
which might not have been created upon the Android framework but eventually in a
different higher-level programming language like C or C++ will most likely find its way
into user data image.
Problems of the user data image is that, quite similar to the system image, Google
only provides a very limited area to import new applications: approx. 40Mbytes have
to be shared for all Java-based applications, configuration files and databases, and applications written in other programming languages than Java.
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4.3.3 Cache Image
The cache image offers the exact same size to interact as both system and user data
image: 65 MBytes. Except for being used for the typical lost+found directory, we
could not observe any other usage than that ... rendering it a good place for temporarily
saving files that will easily exceed the limitations of the other two images.
4.3.4 SD Card Image
Having such great limitations on both system and user data image, the SD Card seems
to be the perfect alternative to provide large applications within Android!
In order to use a simulated SD Card, one first has to create one with the provided
SDK-tool mksdcard. Providing a label and a size, the image is created and can be
used almost instantly. Once the emulator is started with appropriate parameter -sdcard
<location of SD Card image> the emulator will start and offer the mounted image at
/sdcard.
Major drawback when using the SD Card image in the emulator is: although, the
image can be created with no obvious size limitations, it cannot be used to execute provided shell scripts or applications written in e.g. C/C++. The problem is that certain
permissions have to be set to applications that are supposed to be run within the operating system. All files in the SD Card image will receive a certain user id, group id and
file permissions which cannot be altered in any way: no execution bit can be set and
the files cannot be allocated to user shell and group shell (the user/group for files to be
executed).
If this is going to change in future (when the first devices are actually available on the
market), is not known yet. Hopefully, Google will change this offering developers more
space for their applications.
4.4 Android-Specific Daemons
Apart from the standard server daemons that are available on Linux-based systems running kernel version 2.6.xx, Google added own daemons, supposedly in order to be able
to interact with the underlying operating system. Taken from the output of the ps command (list all system-wide processes), following additional, non-standard daemons will
be up and running in a running instance of the Google Android Emulator:
USER
[...]
system
root
root
radio
root
media
root
root
system
[...]
PID
PPID
VSIZE RSS
WCHAN
PC
NAME
19
20
21
22
24
25
28
31
55
1
1
1
1
1
1
1
1
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796
248
c017fb5c afe0b74c S /system/bin/servicemanager
1820 328
ffffffff afe0c0bc S /system/bin/mountd
652
248
c018a19c afe0c0bc S /system/bin/debuggerd
5308 636
ffffffff afe0bdbc S /system/bin/rild
69508 18672 c008be9c afe0b874 S zygote
16032 3048 ffffffff afe0b74c S /system/bin/mediaserver
788
296
c01eb8b0 afe0b50c S /system/bin/installd
812
308
c00aabf0 afe0c61c S /system/bin/qemud
196572 23612 ffffffff afe0b74c S system_server
Following list will give a short description of each daemon and, supposedly, its purpose:
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servicemanager Manages and provides access to services (a special application
form) on the Android platform
mountd The mount daemon providing access to the YAFFS2 images (system, data,
cache).
debuggerd Offers functions to handle crashes, logs, etc.
rild Radio daemon
zygote Started by /system/bin/app_process. Father process of each new application
process.
mediaserver Handle media libraries on the phone
installd As the name states, this daemon controls the install mechanism for Android
packages on the phone (Emulator).
qemud Qemu daemon. The SDK runs within a virtual environment provided by
Qemu.
system_server Part in the dalvik VM application execution chain. Will require more
analysis to identify the exact functionalities
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5 Source Code Life Cycle On Android
In this section, we will cover the source code life cycle of an Android application,
starting with the initial Android project layout, the development of the project written
in Java, the compilation of the Android package and, finally, the deployment of such a
project on the emulator.
5.1 From Java Source Code To A Compiled Dex File
Android introduces a new virtual environment and concept that is based on the Java
programming lanugage and a new file type: dex. Most parts of Android are open, source
code is available excluding the virtual environment. So far, we can say that when Java
sources are compiled, the final result is dex file which can then be run on the given
device with appropriate tools. The user will not obviously have to cope with dex files as
those are simply parts of the application package which a user will work with.
5.2 Packaging All Project Files Into An APK
An Android project consists of a given directory structure with given code sources.
Once the project is compiled and built, all files are packaged into one single file, an
apk (Android package). Compiled and packaged APKs can be found within the bin
directory of the project. An APK basically is a zip file, including all resource files,
compiled sources as dex files (Google’s own file format for running Java bytecode on
Androi), and the AndroidManifest file which defines the application and its activities.
APKs are required to successfully deploy applications onto the destination device:
Android’s mechanism will take a given APK, deploy it by unpacking all relevant files
in defined locations in filesystem, update the application database of the Android-based
device and finally start up the new application.
5.3 How To Create And Deploy APKs Without Eclipse
ADT in Eclipse takes over quite a few tasks in the code life-cycle. Not using ADT will
require to manually run through all necessary steps:
1. The base project layout needs to be created with with activitycreator which is part
of the SDK and can be found in the tools directory of the SDK.
2. One important product of the activity creator is the build.xml file holding all important project definitions (where to find the SDK, name of the project, etc).
3. ant, a Java build tool from the famous Apache Ant project, will require the
build.xml to successfully compile a given Android project. The product of the
ant run is a ready-to-deploy APK.
4. If using the emulator, the developer has to manually start the emulator from the
tools directory of the SDK.
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5. With adb - Android Debug Bridge and appropriate parameters a given APK can
easily be deployed on any Google Android-based target device.
6. Once installed on the target device, the application can be selected and started
from the Android device menu.
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6 Facing Difficulties In Developing On Android
Google offers a comprehensive API to help out in situations where the developer needs
help. In this section we will shortly discuss where Google cannot help, where developers
will have to face problems and difficulties.
6.1 Difficulties In Developing Dndroid Applications
All SDKs for each supported operating system platform are quite similar. Using the officially supported development IDE Eclipse abstracts all underlying steps automatically
taken on compilation and deployment of a given Android project on the emulator.
As soon as a developer refuses to use IDE (due to own preference) and instead use a
different editor, a few things have to be beared in mind:
• Android projects have to be created manually with the activitycreator.
• ant has to be used to compile the project and create the APK, and adb has to be
used to install the APK on the target device.
• If the developer wishes to manually compile his Android project with ant, ant
will require the build.xml. This file is ONLY created when the project was initially created via activitycreator. Eclipse with ADT will not create this important
configuration file and therefore a manual ant compilation run will fail.
6.2 Difficulties Developing In C/C++
A major issue when developing applications for Android in C/C++ is that the developer
will have to face the problem that he or she will be developing for a different platform:
ARM. This might require advanced knowledge of this platform type and also knowledge
in how to appropriately compile code for ARM architecture.
The Android Sources include a complete cross-compile toolchain for ARM. But, the
developer will have to face learning the Android Makefile system in order to successfully compile code on basis of the Android Sources’ cross-compile toolchain. Especially
getting all required includes correct in order to compile against the Android libraries and
header files will appear to be a major problem in the code life cycle.
If this gets a major problem, there are other solutions: CodeSourcery offers a crosscompilation ARM toolchain which can also be used to compile the sources. But, most
likely the includes of the header files will be different, the libraries might differ in its
functionalities. A way out is to statically compile and link all applications in order to
statically include the necessary library functions. The developer receives one big, allin-one static binary which can then be pushed to the device and run. This might be
a sensible solution for smaller projects. But, most obviously, not desired in situations
where applications get quite big and full-blow.
6.3 Difficulties Developing In Java
The official API is very well documented. Any problem going beyond the capabilities
of the API can be discussed on several mailing lists that are available for the Android
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project. Ranging from the beginners mailing list up to porting other existing applications to Android: Google made sure that everybody can find an appropriate answer.
Using ADT with Eclipse makes development very easy and straigh-forward. Other
IDEs, e.g. Netbeans can also be used as the community starting developing a pendant
to ADT, simply for Netbeans. Furthermore, Google did not restrict developers to keep
information for themselves. This resulted in various new websites dealing with Android
development giving hundreds of new ideas, code examples and more.
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7 Interconnect Daemon and GetProcessList
As a first demonstration of what is possible on Google Android, we decided to write
a small application stack, which both includes a Java-based part GetProcessList, using
the official Google Android Java APIs, and a helper application Interconnect Daemon,
written in C, which operates on OS-level.
Before going into the architecture description in detail, we will first cover the motivation behind this application stack and how to implement the common communication
channel between both distinct parts.
7.1 Motivation
Main intentions for this application stack were to demonstrate ...
• the inability of the Java-level to cope with certain OS-level tasks.
• that programming languages other than Java can be used to develop applications
for Google Android.
• how such applications can communicate with each other and therefore legitimate
their existance.
Various tasks can be mastered via the Android Java API but still though, a few remain
impossible to be done on Java-level. While the Java API offers a fairly big number of
methods and classes, it still cannot overcome the problem that each application is run
with the application’s user permissions. Thus, certain files or ressources are simply
unavailable on a Linux system (e.g. some log files on standard Linux distributions). In
Android, this counts for a rare number of files within the /sys and the /proc directories.
Altering important data on OS-level is an almost impossible task: e.g. network information data can be read but not easily altered.
7.2 Handling Interprocess Communication
The application stack works, as already mentioned, in two tiers: the stack is split up into
a Java-application, using the Android Java APIs, and a small Linux server daemon, the
Interconnect Daemon, written in C, which operates on OS-level.
As both parts work fairly independent, a common communication channel had to be
established. In order to implement such a common communication channel, several
concepts have to be taken under consideration:
JNI - Java Native Interface JNI offers a possibility to connect Java-applications to
shared libraries which are implemented in C/C++. The shared libraries offer functions to call libc system and library calls. The Java-application has to be the instance in control as it triggers each function from higher level. Once mastered
the page alignment issue, JNI is a very promising concept to quickly implement
application stacks with almost full access to the underlying OS-level.
For our usage, JNI did not fit as we opted for an independent server daemon on
OS-level which can communicate to various independent Java-applications.
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Figure 3: UML-diagram of GetProcessList
Sockets Sockets are a very commonly used concept for communication, especially
IPC (Interprocess Communication). While not digging to deep into this concept,
the following concept Named Pipes has some minor advantages over Sockets due
to non-blocking-issues in certain cases.
Named Pipes Named Pipes is also a very commonly used concept for IPC and it
is very simple to implement. The libc offers a simple function to create such
FIFOs (First In, First Out). A huge number of these can already found in the /dev
filesystem, e.g. in Google Android’s case for the logging system logcat.
7.3 Architecture Design And UML-Diagramm
Taking a top-down-view on the architecture design, GetProcessList is the application
that interacts with the user. It offers a graphical user interface based on the Android
Java API, which presents all results from the application stack on the screen.
The Interconnect Daemon runs as a regular server daemon on Linux-level from where
it gathers relevant system (process) data and passes this on to GetProcessList.
7.4 Interconnect Daemon
The Interconnect Daemon is a small server daemon, compiled for ARM architecture.
For the sake of easier handling, the application has been statically compiled and linked,
rendering it a single ELF-compatible binary file which can be transferred to the Android
Emulator or the actual physical device G1 and executed (Please note: There is not rootaccess on G1!)
Its main tasks are to:
• identify all currently available processes in the system, taken from the /proc
filesystem. This filesystem offers a common information base to exchange data
with the Linux kernel.
• record all processes and collect detailled information on each process.
• provide GetProcessList with this information.
The data is read from /proc and directly written to the named pipe.
Figure 3 will display the basic dependencies to given libraries.
In figure 4 you will see the call graph for the main function which handles all necessary steps to establish the communication via named pipes and send the process status
information to the receiver on Java-level.
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Figure 4: UML-diagram of GetProcessList
7.5 GetProcessList
GetProcessList is an application written in Java. It is responsible for following tasks:
• Receive all information sent via named pipe (the interprocess communication
channel between Interconnect and GetProcessList)
• Process each line sent via named pipe
• Save the data to its database
• Present the list of all processes in a graphical user interface
In figure 5 you will find the UML class diagram of GetProcessList.
7.6 Out Of Scope
Both Interconnect and GetProcessList are to be seen as a demonstration of what is possible on Google Android. Especially developing applications not based on the officially
supported language Java is, disregarding a few minor hurdles to take, possible and, in
some cases, a sensible choide.
As the complete application stack has to be considered as a demonstration software,
a few things have not been implemented due to time constraints, out-of-scope-issues or
the prototype character of this demonstrator. We will briefly cover what has not been
implemented but may be in future in order to make this application stack a useful and
usable product for end-users.
Furthermore, we will also cover eventual problems during development, problems
that aroused due to programming language characteristics or simply anything that
formed a given obstacle.
7.6.1 What Did We Not Implement? Why?
Following points were disregarded during development of the demonstrator application
stack:
Using Standard protocols For IPC Interconnect reads all process status files and
simply pipes its output, which basically represent strings (char-arrays in C), into the
established named pipe. No formatting of the strings sent over the pipe is done, requiring
the receiver application GetProcessList to know which string is coming when and how
each string has to be formed.
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Figure 5: UML-diagram of GetProcessList
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In a more advanced version of Interconnect, which has been used in following project,
this communication is based on sending standardized JSON Javascript Object Notation
[7] objects. Using JSON, several major advantages arise:
• Each object is a complete set of data including all necessary information. In
our case this would mean that each object includes all data concerning a single
process.
• Each object is one single string. Due to its optimal structure, all data concerning
one process is sent "in one go".
• Receiver has more freedom on how to parse the received object and how to postprocess it.
7.6.2 What Else Could Be Done Based On This Project?
Both Interconnect and GetProcessList could be extended to a useful ps-clonse (ps displays the process status on common UNIX- and Linux-based systems) including a
graphical user interface for Android. For end-users who want to know what exactly
is happening on their device, this tool could be very good solution.
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Acknowledgement
We hereby want to thank those people who supported us, always had an open ear whenever we had problems; simply all who have made all this possible. So special thanks
go to Aubrey-Derrick Schmidt, the project manager of this project, and Seyhit Ahmet
Camtepe, leader of our competence center CC-Security. Furthermore we would like
to thank DAI Labor, especially Prof. Dr. Sahin Albayrak for giving us this possibility to work in this, in our opinion, very interesting field of mobile telecommunication,
UNIX-based operating systems and security.
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References
[1] MacWorld.com, “Apple unveils iphone.”
[2] G. Inc., “Google android project website.”
[3] G. Inc., “Android dalvik debug monitor service.”
[4] bellard.org, “Qemu project website.”
[5] G. Inc., “Android debug bridge.”
[6] G. Inc., “Android code downloads.”
[7] “Json - javascript object notation.”
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