Projects act as containers for storing things such as code and resource files. The SDK tools
expect your projects to follow a specific structure so it can compile and package your
application correctly, so it is highly recommended that you create them with Eclipse and ADT or
android tool on the command line. There are three types of projects, and
they all share the same general structure but differ in function:
- Android Projects
- An Android project is the container for your application's source code, resource files, and
files such as the Ant build and Android Manifest file. An application project is the main type
of project and the contents are eventually built into an
.apkfile that you install on a device.
- Test Projects
- These projects contain code to test your application projects and are built into applications that run on a device.
- Library Projects
- These projects contain shareable Android source code and resources that you can reference
in Android projects. This is useful when you have common code that you want to reuse.
Library projects cannot be installed onto a device, however, they are
pulled into the
.apkfile at build time.
When you use the Android development tools to create a new project, the essential files and
folders will be created for you. There are only a handful of files and folders generated for you,
and some of them depend on whether you use the Eclipse plugin or the
android tool to
generate your project. As your application grows in complexity, you might require new kinds of
resources, directories, and files.
Android projects are the projects that eventually get built into an
.apk file that you install
onto a device. They contain things such as application source code and resource files.
Some are generated for you by default, while others should be created if
required. The following directories and files comprise an Android project:
- Contains your stub Activity file, which is stored at
src/your/package/namespace/ActivityName.java. All other source code files (such as
.aidlfiles) go here as well.
- Output directory of the build. This is where you can find the final
.apkfile and other compiled resources.
- Contains native code sources developed using the Android NDK. For more information, see the Android NDK documentation.
- Contains the Java files generated by ADT, such as your
R.javafile and interfaces created from AIDL files.
- This is empty. You can use it to store raw asset files. Files that you save here are
compiled into an
.apkfile as-is, and the original filename is preserved. You can navigate this directory in the same way as a typical file system using URIs and read files as a stream of bytes using the the
AssetManager. For example, this is a good location for textures and game data.
Contains application resources, such as drawable files, layout files, and string values. See
Application Resources for more
- For XML files that are compiled into animation objects. See the Animation resource type.
- For XML files that describe colors. See the Color Values resource type.
- For bitmap files (PNG, JPEG, or GIF), 9-Patch image files, and XML files that describe Drawable shapes or a Drawable objects that contain multiple states (normal, pressed, or focused). See the Drawable resource type.
- XML files that are compiled into screen layouts (or part of a screen). See the Layout resource type.
- For XML files that define application menus. See the Menus resource type.
- For arbitrary raw asset files. Saving asset files here instead of in the
assets/directory only differs in the way that you access them. These files are processed by aapt and must be referenced from the application using a resource identifier in the
Rclass. For example, this is a good place for media, such as MP3 or Ogg files.
- For XML files that are compiled into many kinds of resource. Unlike other resources in
res/directory, resources written to XML files in this folder are not referenced by the file name. Instead, the XML element type controls how the resources is defined within them are placed into the
- For miscellaneous XML files that configure application components. For example, an XML
file that defines a
AppWidgetProviderInfo, or Searchability Metadata. See Application Resources for more information about configuring these application components.
- Contains private libraries.
- The control file that describes the nature of the application and each of its components. For instance, it describes: certain qualities about the activities, services, intent receivers, and content providers; what permissions are requested; what external libraries are needed; what device features are required, what API Levels are supported or required; and others. See the AndroidManifest.xml documentation for more information
- This file contains project settings, such as the build target. This file is integral to the project, so maintain it in a source revision control system. To edit project properties in Eclipse, right-click the project folder and select Properties.
- Customizable computer-specific properties for the build system. If you use Ant to build
the project, this contains the path to the SDK installation. Because the content of the file
is specific to the local installation of the SDK, the
local.propertiesshould not be maintained in a source revision control system. If you use Eclipse, this file is not used.
- Customizable properties for the build system. You can edit this file to override default build settings used by Ant and also provide the location of your keystore and key alias so that the build tools can sign your application when building in release mode. This file is integral to the project, so maintain it in a source revision control system. If you use Eclipse, this file is not used.
- The Ant build file for your project. This is only applicable for projects that you build with Ant.
Library project example code
The SDK includes an example application called
TicTacToeMain that shows how a dependent
application can use code and resources from an Android Library project. The TicTacToeMain
application uses code and resources from an example library project called TicTacToeLib.
To download the sample applications and run them as projects in your environment, use the Android SDK Manager to download the "Samples for SDK API 8" (or later) package into your SDK.
For more information and to browse the code of the samples, see the TicTacToeMain application.
An Android library project is a development project that holds shared Android
source code and resources. Other Android application projects can reference the library project
and, at build time, include its compiled sources in their
.apk files. Multiple
application projects can reference the same library project and any single application project
can reference multiple library projects.
Note: You need SDK Tools r14 or newer to use the new library project feature that generates each library project into its own JAR file. You can download the tools and platforms using the Android SDK Manager, as described in Exploring the SDK.
If you have source code and resources that are common to multiple Android projects, you can move them to a library project so that it is easier to maintain across applications and versions. Here are some common scenarios in which you could make use of library projects:
- If you are developing multiple related applications that use some of the same components, you move the redundant components out of their respective application projects and create a single, reuseable set of the same components in a library project.
- If you are creating an application that exists in both free and paid versions. You move the part of the application that is common to both versions into a library project. The two dependent projects, with their different package names, will reference the library project and provide only the difference between the two application versions.
Structurally, a library project is similar to a standard Android application project. For
example, it includes a manifest file at the project root, as well as
res/ and similar directories. The project can contain the same types of source
code and resources as a standard Android project, stored in the same way. For example, source
code in the library project can access its own resources through its
However, a library project differs from an standard Android application project in that you
cannot compile it directly to its own
.apk and run it on an Android device.
Similarly, you cannot export the library project to a self-contained JAR file, as you would do
for a true library. Instead, you must compile the library indirectly, by referencing the
library in the dependent application and building that application.
When you build an application that depends on a library project, the SDK tools compile the
library into a temporary JAR file and uses it in the main project, then uses the
result to generate the
.apk. In cases where a resource ID is defined in both the
application and the library, the tools ensure that the resource declared in the application gets
priority and that the resource in the library project is not compiled into the application
.apk. This gives your application the flexibility to either use or redefine any
resource behaviors or values that are defined in any library.
To organize your code further, your application can add references to multiple library projects, then specify the relative priority of the resources in each library. This lets you build up the resources actually used in your application in a cumulative manner. When two libraries referenced from an application define the same resource ID, the tools select the resource from the library with higher priority and discard the other.
Once you have added references to library projects to your Android project, you can set their relative priority. At build time, the libraries are merged with the application one at a time, starting from the lowest priority to the highest.
Library projects can reference other library projects and can import an external library (JAR) in the normal way.
As you develop your library project and dependent applications, keep the points listed below in mind:
Since the tools merge the resources of a library project with those of a dependent application project, a given resource ID might be defined in both projects. In this case, the tools select the resource from the application, or the library with highest priority, and discard the other resource. As you develop your applications, be aware that common resource IDs are likely to be defined in more than one project and will be merged, with the resource from the application or highest-priority library taking precedence.
Use prefixes to avoid resource conflicts
To avoid resource conflicts for common resource IDs, consider using a prefix or other consistent naming scheme that is unique to the project (or is unique across all projects).
You cannot export a library project to a JAR file
A library cannot be distributed as a binary file (such as a JAR file). This will be added in a future version of the SDK Tools.
A library project can include a JAR library
You can develop a library project that itself includes a JAR library, however you need to manually edit the dependent application project's build path and add a path to the JAR file.
A library project can depend on an external JAR library
You can develop a library project that depends on an external library (for example, the Maps external library). In this case, the dependent application must build against a target that includes the external library (for example, the Google APIs Add-On). Note also that both the library project and the dependent application must declare the external library in their manifest files, in a
Library projects cannot include raw assets
The tools do not support the use of raw asset files (saved in the
assets/directory) in a library project. Any asset resources used by an application must be stored in the
assets/directory of the application project itself. However, resource files saved in the
res/directory are supported.
Platform version must be lower than or equal to the Android project
A library is compiled as part of the dependent application project, so the API used in the library project must be compatible with the version of the Android library used to compile the application project. In general, the library project should use an API level that is the same as — or lower than — that used by the application. If the library project uses an API level that is higher than that of the application, the application project will not compile. It is perfectly acceptable to have a library that uses the Android 1.5 API (API level 3) and that is used in an Android 1.6 (API level 4) or Android 2.1 (API level 7) project, for instance.
No restriction on library package names
There is no requirement for the package name of a library to be the same as that of applications that use it.
Each library project creates its own R class
When you build the dependent application project, library projects are compiled and merged with the application project. Each library has its own
Rclass, named according to the library's package name. The
Rclass generated from main project and the library project is created in all the packages that are needed including the main project's package and the libraries' packages.
Library project storage location
There are no specific requirements on where you should store a library project, relative to a dependent application project, as long as the application project can reference the library project by a relative link. What is important is that the main project can reference the library project through a relative link.
Test projects contain Android applications that you write using the Testing and Instrumentation framework. The framework is an extension of the JUnit test framework and adds access to Android system objects. The file structure of a test project is the same as an Android project.
- Includes your test source files. Test projects do not require an Activity
.javafile, but can include one.
- This contains the Java files generated by ADT, such as your
R.javafile and interfaces created from AIDL files.
- This is empty. You can use it to store raw asset files.
- A folder for your application resources, such as drawable files, layout files, string values, etc. See Application Resources.
- The Android Manifest for your project. See The AndroidManifest.xml File. Test
Projects have a special
<instrumentation>element that connects the test project with the application project.
- This file contains project settings, such as the build target and links to the project being tested. This file is integral to the project, so maintain it in a source revision control system. To edit project properties in Eclipse, right-click the project folder and select Properties.
- Customizable computer-specific properties for the build system. If you use Ant to build the project, this contains the path to the SDK installation. Because the content of the file is specific to the local installation of the SDK, it should not be maintained in a Source Revision Control system. If you use Eclipse, this file is not used.
- Customizable properties for the build system. You can edit this file to override default build settings used by Ant and provide the location to your keystore and key alias, so that the build tools can sign your application when building in release mode. This file is integral to the project, so maintain it in a source revision control system. If you use Eclipse, this file is not used.
- The Ant build file for your project. This is only applicable for projects that you build with Ant.
For more information, see the Testing section.
Testing a Library Project
There are two recommended ways of setting up testing on code and resources in a library project:
- You can set up a test project that instruments an application project that depends on the library project. You can then add tests to the project for library-specific features.
- You can set up a standard application project that depends on the library and put the instrumentation in that project. This lets you create a self-contained project that contains both the tests/instrumentations and the code to test.