The Android SDK is made of mostly off-the-shelf components, plus some purpose-built components. In many cases, configurations, plug-ins, and extensions adapt these components to Android. The Android SDK is a study in the efficient development of a modern and complete SDK. Google took this approach in order to bring Android to market quickly. You will see this for yourself as you explore the components of the Android SDK. Eclipse, the Java language, QEMU, and other preexisting platforms, tools, and technologies comprise some of the most important parts of the Android SDK.
In creating the simple program that confirms that your SDK installation is correct, you have already used many of the components of the SDK. Here we will identify and describe the components of the SDK involved in creating your program, and other parts you have yet to use.
adb is a program that enables you to control both emulators and devices, and to run a shell in order to execute commands in the environment of an emulator or device. adb is especially handy for installing and removing programs from an emulator or device. Documentation on adb can be found at http://developer.android.com/guide/developing/tools/adb.html.
The Dalvik Debug Monitor Server (DDMS) is a traffic director between the single port that Eclipse (and other Java debuggers) looks for to connect to a Java Virtual Machine (JVM) and the several ports that exist for each Android device or virtual device, and for each instance of the Dalvik virtual machine (VM) on each device. The DDMS also provides a collection of functionality that is accessible through a standalone user interface or through an interface embedded in Eclipse via the ADT plug-in.
When you invoke the DDMS from the command line, you will see something similar to the window shown in Figure 1-12.
In the upper-left pane of the DDMS window, you will see listed the Android devices you have connected to your PC, plus any AVDs you have running. Listed under each device or virtual device are the tasks running in Dalvik VMs.
You can explore the filesystem on a device or virtual device using the DDMS filesystem explorer, accessible through the “File explorer” menu item in the Devices menu. It displays the file hierarchy in a window similar to the one shown in Figure 1-13.
The bottom pane of the DDMS window displays log output from processes on the selected device or virtual device. You can filter the log output by selecting a filter from among the buttons on the toolbar above the logging pane.
Detailed documentation on the DDMS is available at http://developer.android.com/guide/developing/tools/ddms.html.
Eclipse enables you to create specific project types, including several kinds of Java projects. The ADT plug-in adds the ability to make and use Android projects. When you make a new Android project, the ADT plug-in creates the project file hierarchy and all the required files for the minimal Android project to be correctly built. For Android projects, the ADT plug-in enables Eclipse to apply components of the ADT plug-in to editing, building, running, and debugging that project.
In some cases, components of the SDK can be used with Eclipse or in a standalone mode. But, in most of the Android application development cases covered in this book, the way these components are used in or with Eclipse will be the most relevant.
The ADT plug-in has numerous separate components, and, despite the connotations of a “plug-in” as a modest enhancement, it’s a substantial amount of software. Here we will describe each significant part of the ADT plug-in that you will encounter in using Eclipse for developing Android software.
Layouts for user interfaces in Android applications can be specified in XML. The ADT plug-in adds a visual editor that helps you to compose and preview Android layouts. When you open a layout file, the ADT plug-in automatically starts this editor to view and edit the file. Tabs along the bottom of the editing pane enable you to switch between the visual editor and an XML editor.
In earlier versions of the Android SDK, the Android Layout Editor was too limited to be of much use. Now, though, you should consider using visual editing of Android layouts as a preferred way of creating layouts. Automating the specification of layouts makes it more likely that your layouts will work on the widest range of Android devices.
In Android projects, a manifest file is included with the project’s software and resources when the project is built. This file tells the Android system how to install and use the software in the archive that contains the built project. The manifest file is in XML, and the ADT plug-in provides a specialized XML editor to edit the manifest.
Other Android XML files that hold information such as specifications for menus, or resources such as strings, or that organize graphical assets of an application, have specialized editors that are opened when you open these files.
Eclipse projects are usually built automatically. That means you will normally not encounter a separate step for turning the source code and resources for a project into a deployable result. Android requires Android-specific steps to build a file you can deploy to an Android emulator or device, and the ADT plug-in provides the software that executes these steps. For Android projects, the result of building the project is an .apk file. You can find this file for the test project created earlier in this chapter in the bin subfolder of the project’s file hierarchy in your Eclipse workspace.
The Android-specific builders provided in the ADT plug-in enable you to use Java as the language for creating Android software while running that software on a Dalvik VM that processes its own bytecodes.
When you run or debug an Android project from within Eclipse, the .apk file for that project is deployed and started on an AVD or Android device, using the ADB and DDMS to communicate with the AVD or device and the Dalvik runtime environment that runs the project’s code. The ADT plug-in adds the components that enable Eclipse to do this.
In The Dalvik Debug Monitor Server (DDMS) we described the Dalvik Debug Monitor and how to invoke the DDMS user interface from the command line. The DDMS user interface is also available from within Eclipse. You can access it by using the Window→Open Perspective→DDMS command in the Eclipse menu. You can also access each view that makes up the DDMS perspective separately by using the Window→Show View menu and selecting, for example, the LogCat view.
AVDs are made up of QEMU-based emulators that emulate the hardware of an Android device, plus Android system images, which consist of Android software built to run on the emulated hardware. AVDs are configured by the SDK and AVD Manager, which sets parameters such as the size of emulated storage devices and screen dimensions, and which enables you to specify which Android system image will be used with which emulated device.
AVDs enable you to test your software on a broader range of system characteristics than you are likely to be able to acquire and test on physical devices. Because QEMU-based hardware emulators, system images, and the parameters of AVDs are all interchangeable parts, you can even test devices and system images before hardware is available to run them.
QEMU is the basis of AVDs. But QEMU is a very general tool that is used in a wide range of emulation systems outside the Android SDK. While you will configure QEMU indirectly, through the SDK and AVD Manager, you may someday need to tweak emulation in ways unsupported by the SDK tools, or you may be curious about the capabilities and limitations of QEMU. Luckily, QEMU has a large and vibrant developer and user community, which you can find at http://www.qemu.org.
QEMU is a general-purpose emulator system. The Android SDK provides controls over the configuration of QEMU that make sense for creating emulators that run Android system images. The SDK and AVD Manager provides a user interface for you to control QEMU-based Android virtual devices.
In addition to the major tools you are likely to use in the normal course of most development projects, there are several other tools in the SDK, and those that are used or invoked directly by developers are described here. Still more components of the SDK are listed in the Tools Overview article in the Android documentation found at http://developer.android.com/guide/developing/tools/index.html.
The Hierarchy Viewer displays and enables analysis of the view hierarchy of the current activity of a selected Android device. This enables you to see and diagnose problems with your view hierarchies as your application is running, or to examine the view hierarchies of other applications to see how they are designed. It also lets you examine a magnified view of the screen with alignment guides that help identify problems with layouts. Detailed information on the Hierarchy Viewer is available at http://developer.android.com/guide/developing/tools/hierarchy-viewer.html.
Layoutopt is a static analyzer that operates on XML layout files and can diagnose some problems with Android layouts. Detailed information on Layoutopt is available at http://developer.android.com/guide/developing/tools/layoutopt.html.
Monkey is a test automation tool that runs in your emulator or device. You invoke this tool using another tool in the SDK: adb. Adb enables you to start a shell on an emulator or device, and Monkey is invoked from a shell, like this:
adb shell monkey --wait-dbg -p your.package.name 500
This invocation of Monkey sends 500 random events to the specified application (specified by the package name) after waiting for a debugger to be attached. Detailed information on Monkey can be found at http://developer.android.com/guide/developing/tools/monkey.html.
Android uses SQLite as the database system for many system
databases and provides APIs for applications to make use of
SQLite, which is convenient for data storage and presentation.
SQLite also has a command-line interface, and the
sqlite3 command enables developers to
dump database schemas and perform other operations on Android
These databases are, of course, in an
Android device, or they are contained in an AVD, and therefore the
sqlite3 command is available in
the adb shell. Detailed directions for how to access the
sqlite3 command line from inside the adb
shell are available at http://developer.android.com/guide/developing/tools/adb.html#shellcommands.
sqlite3 in Example Database Manipulation Using sqlite3.
keytool generates encryption keys, and is used by the ADT
plug-in to create temporary debug keys with which it signs code
for the purpose of debugging. In most cases, you will use this
tool to create a signing certificate for releasing your
applications, as described in Creating a self-signed certificate.
Zipalign enables optimized access to data for production releases of Android applications. This optimization must be performed after an application is signed for release, because the signature affects byte alignment. Detailed information on Zipalign is available at http://developer.android.com/guide/developing/tools/zipalign.html.
A 9 patch is a special kind of Android resource, composed of nine images, and useful when you want, for example, buttons that can grow larger without changing the radius of their corners. Draw9patch is a specialized drawing program for creating and previewing these types of resources. Details on draw9patch are available at http://developer.android.com/guide/developing/tools/draw9patch.html.
The command named
be used to invoke the SDK and AVD Manager from the command line, as we described in the SDK
installation instructions in The Android SDK.
It can also be used to create an Android project from the command
line. Used in this way, it causes all the project folders, the
manifest, the build properties, and the ant script for building
the project to be generated. Details on this use of the
android command can be found at http://developer.android.com/guide/developing/other-ide.html#CreatingAProject.