Setting up a Library Project

Setting up a Library Project

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A library project is a standard Android project, so you can create a new one in the same way as you would a new application project.

When you are creating the library project, you can select any application name, package, and set other fields as needed, as shown in figure 1.

Next, set the project's properties to indicate that it is a library project:

1. In the Package Explorer, right-click the library project and select Properties.
2. In the Properties window, select the "Android" properties group at left and locate the Library properties at right.
3. Select the "is Library" checkbox and click Apply.
4. Click OK to close the Properties window.

The new project is now marked as a library project. You can begin moving source code and resources into it, as described in the sections below.

You can also convert an existing application project into a library. To do so, simply open the Properties for the project and select the "is Library" checkbox. Other application projects can now reference the existing project as a library project.

Figure 1. Marking a project as an Android library project.

Creating the manifest file

A library project's manifest file must declare all of the shared components that it includes, just as would a standard Android application. For more information, see the documentation for AndroidManifest.xml.

For example, the TicTacToeLib example library project declares the Activity GameActivity:

<manifest>  ...  <application>    ...    <activity android:name="GameActivity" />    ...  </application></manifest>

Referencing a library project

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If you are developing an application and want to include the shared code or resources from a library project, you can do so easily by adding a reference to the library project in the application project's Properties.

To add a reference to a library project, follow these steps:

1. In the Package Explorer, right-click the dependent project and select Properties.
2. In the Properties window, select the "Android" properties group at left and locate the Library properties at right.
3. Click Add to open the Project Selection dialog.
4. From the list of available library projects, select a project and click OK.
5. When the dialog closes, click Apply in the Properties window.
6. Click OK to close the Properties window.

As soon as the Properties dialog closes, Eclipse rebuilds the project, including the contents of the library project.

Figure 2 shows the Properties dialog that lets you add library references and move them up and down in priority.

Figure 2. Adding a reference to a library project in the properties of an application project.

If you are adding references to multiple libraries, note that you can set their relative priority (and merge order) by selecting a library and using the Up and Down controls. The tools merge the referenced libraries with your application starting from lowest priority (bottom of the list) to highest (top of the list). If more than one library defines the same resource ID, the tools select the resource from the library with higher priority. The application itself has highest priority and its resources are always used in preference to identical resource IDs defined in libraries.

Declaring library components in the manifest file

In the manifest file of the application project, you must add declarations of all components that the application will use that are imported from a library project. For example, you must declare any <activity>, <service>, <receiver>,<provider>, and so on, as well as <permission>, <uses-library>, and similar elements.

Declarations should reference the library components by their fully-qualified package names, where appropriate.

For example, the TicTacToeMain example application declares the library Activity GameActivity like this:

<manifest>  ...  <application>    ...    <activity android:name="com.example.android.tictactoe.library.GameActivity" />    ...  </application></manifest>

For more information about the manifest file, see the documentation for AndroidManifest.xml.