Managing Projects

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 with the 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 .apk file 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 .apk file 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

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:

src/
Contains your stub Activity file, which is stored at src/your/package/namespace/ActivityName.java. All other source code files (such as .java or .aidl files) go here as well.
bin
Output directory of the build. This is where you can find the final .apk file and other compiled resources.
jni
Contains native code sources developed using the Android NDK. For more information, see the Android NDK documentation.
gen/
Contains the Java files generated by ADT, such as your R.java file and interfaces created from AIDL files.
assets/
This is empty. You can use it to store raw asset files. Files that you save here are compiled into an .apk file 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.
res/
Contains application resources, such as drawable files, layout files, and string values. See Application Resources for more information.
anim/
For XML files that are compiled into animation objects. See the Animation resource type.
color/
For XML files that describe colors. See the Color Values resource type.
drawable/
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.
layout/
XML files that are compiled into screen layouts (or part of a screen). See the Layout resource type.
menu/
For XML files that define application menus. See the Menus resource type.
raw/
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 R class. For example, this is a good place for media, such as MP3 or Ogg files.
values/
For XML files that are compiled into many kinds of resource. Unlike other resources in the 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 R class.
xml/
For miscellaneous XML files that configure application components. For example, an XML file that defines a PreferenceScreen, AppWidgetProviderInfo, or Searchability Metadata. See Application Resources for more information about configuring these application components.
libs/
Contains private libraries.
AndroidManifest.xml
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
build.properties
Customizable properties for the build system. You can edit this file to override default build settings used by Ant and provide a pointer to your keystore and key alias so that the build tools can sign your application when built in release mode. If you use Eclipse, this file is not used.
build.xml
The Ant build file for your project. This is only applicable for projects that you create on the command line.
default.properties
This file contains project settings, such as the build target. This files is integral to the project, as such, it should be maintained in a Source Revision Control system. Do not edit the file manually.

Library Projects

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 r8 or newer to fully support library projects for all Android platform versions. You can download the tools and platforms using the Android SDK and AVD Manager, as described in Adding SDK Components.

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 src/, 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 R class.

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 and merge its sources with those in the main project, then use 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.

Note that a library project cannot itself reference another library project and that, at build time, library projects are not merged with each other before being merged with the application. However, note that a library can import an external library (JAR) in the normal way.

Development considerations

As you develop your library project and dependent applications, keep the points listed below in mind:

  • Resource conflicts

    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 is because the library project is compiled by the main project to use the correct resource IDs.

  • 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 <uses-library> element.

  • 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 R class, named according to the library's package name. The R class 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

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.

src/
Includes your test source files. Test projects do not require an Activity .java file, but can include one.
gen/
This contains the Java files generated by ADT, such as your R.java file and interfaces created from AIDL files.
assets/
This is empty. You can use it to store raw asset files.
res/
A folder for your application resources, such as drawable files, layout files, string values, etc. See Application Resources.
AndroidManifest.xml
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.
build.properties
Customizable properties for the build system. You can edit this file to override default build settings used by Ant and provide a pointer to your keystore and key alias so that the build tools can sign your application when built in release mode.
build.xml
The Ant build file for your project.
default.properties
This file contains project settings, such as the build target. This files is integral to the project, as such, it should be maintained in a Source Revision Control system. It should never be edited manually — to edit project properties, right-click the project folder and select "Properties".
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 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.
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