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The Android platform contains a large number of shared Java libraries that can optionally be included in the classpath of apps with the <uses-library> tag in the app manifest. Apps link against these libraries, so treat them like the rest of the Android API in terms of compatibility, API review, and tooling support. Note, however, that most libraries don't have these features.

The java_sdk_library module type helps manage libraries of this kind. Device manufacturers can use this mechanism for their own shared Java libraries, to maintain backward compatibility for their APIs. If device manufacturers use their own shared Java libraries through the <uses-library> tag instead of the bootclass path, java_sdk_library can verify that those Java libraries are API-stable.

The java_sdk_library implements optional SDK APIs for use by apps. Libraries implemented through java_sdk_library in your build file (Android.bp) perform the following operations:

• The stubs libraries are generated to include stubs, stubs.system, and stubs.test. These stubs libraries are created by recognizing @hide, @SystemApi, and @TestApi annotations.
• The java_sdk_library manages API specification files (such as current.txt) in an API subdirectory. These files are checked against the latest code to ensure that theyre the most current versions. If they arent, you receive an error message that explains how to update them. Manually review all update changes to ensure that they match your expectations.
  
  To update all APIs, use m update-api. To verify that an API is up-to-date, use m checkapi.
• The API specification files are checked against the most recently published Android versions to ensure that the API is backward-compatible with earlier releases. The java_sdk_library modules provided as part of AOSP place their previously released versions in prebuilts/sdk/<latest number>.
• With respect to the API specification files checks, you can do one of the following three things:
• Allow the checks to proceed. (Dont do anything.)
• Disable checks by adding the following to java_sdk_library:
  unsafe_ignore_missing_latest_api: true,
• Provide empty APIs for new java_sdk_library modules by creating empty text files named module_name.txt in the version/scope/api directory.
• If the implementation library for the runtime is installed, an XML file gets generated and installed.

How java_sdk_library works

A java_sdk_library called X creates the following:

1. Two copies of the implementation library: one library called X and another called X.impl. Library X is installed on-device. Library X.impl is there only if explicit access to the implementation library is needed by other modules, such as for use in testing. Note that explicit access is rarely needed.
2. Scopes can be enabled and disabled to customize access. (Similar to Java keyword-access modifiers, a public scope provides a wide range of access; a test scope contains APIs only used in testing.) For each enabled scope the library creates the following:
• A stubs source module (of droidstubs module type) - consumes the implementation source and outputs a set of stub sources along with the API specification file.
• A stubs library (of java_library module type) - is the compiled version of the stubs. The libs used to compile this arent the same as those supplied to the java_sdk_library, which ensures the implementation details dont leak into the API stubs.
• If you need additional libraries to compile the stubs, use the stub_only_libs and stub_only_static_libs properties to supply them.

If a java_sdk_library is called X, and is being compiled against as X, always refer to it that way and dont modify it. The build will select an appropriate library. To ensure that you have the most appropriate library, inspect your stubs to see if the build introduced errors. Make any necessary corrections using this guidance:

• Verify you have an appropriate library by looking on the command line and inspecting which stubs are listed there to determine your scope:
• Scope is too wide: The depending library needs a certain scope of APIs. But you see APIs included in the library that fall outside of that scope, such as system APIs included with the public APIs.
• Scope is too narrow: The depending library doesnt have access to all the requisite libraries. For example, the depending library needs to use the system API but gets the public API instead. This usually results in a compilation error because needed APIs are missing.
• To fix the library, do only one of the following:
• Change the sdk_version to select the version you need. OR
• Explicitly specify the appropriate library, such as <X>.stubs or <X>.stubs.system.

java_sdk_library X usage

The implementation library X gets used when its referenced from apex.java_libs. However, due to a Soong limitation, when library X is referenced from another java_sdk_library module within the same APEX library, X.impl explicitly must be used, not library X.

When the java_sdk_library is referenced from elsewhere, a stubs library is used. The stubs library is selected according to the depending modules sdk_version property setting. For example, a module that specifies sdk_version: "current" uses the public stubs, whereas a module that specifies sdk_version: "system_current" uses the system stubs. If an exact match cant be found, the closest stub library gets used. A java_sdk_library that only provides a public API will supply the public stubs for everyone.

Examples and sources

The srcs and api_packages properties must be present in the java_sdk_library.

java_sdk_library {
        name: "com.android.future.usb.accessory",
        srcs: ["src/**/*.java"],
        api_packages: ["com.android.future.usb"],
    }

AOSP recommends (but doesnt require) that new java_sdk_library instances explicitly enable the API scopes they want to use. You can also (optionally) migrate existing java_sdk_library instances to explicitly enable the API scopes theyll use:

java_sdk_library {
         name: "lib",
         public: {
           enabled: true,
         },
         system: {
           enabled: true,
         },
         
    }

To configure the impl library used for runtime, use all the normal java_library properties, such as hostdex, compile_dex, and errorprone.

java_sdk_library {
        name: "android.test.base",

        srcs: ["src/**/*.java"],

        errorprone: {
          javacflags: ["-Xep:DepAnn:ERROR"],
        },

        hostdex: true,

        api_packages: [
            "android.test",
            "android.test.suitebuilder.annotation",
            "com.android.internal.util",
            "junit.framework",
        ],

        compile_dex: true,
    }

To configure stubs libraries, use the following properties:

• merge_annotations_dirs and merge_inclusion_annotations_dirs.
• api_srcs: The list of optional source files that are part of the API but not part of the runtime library.
• stubs_only_libs: The list of Java libraries that are in the classpath when building stubs.
• hidden_api_packages: The list of package names that must be hidden from the API.
• droiddoc_options: Additional argument for metalava.
• droiddoc_option_files: Lists the files that can be referenced from within droiddoc_options using $(location <label>), where <file> is an entry in the list.
• annotations_enabled.

The java_sdk_library is a java_library, but it isnt a droidstubs module and so doesn't support all of the droidstubs properties. The following example was taken from the android.test.mock library build file.

java_sdk_library {
        name: "android.test.mock",

        srcs: [":android-test-mock-sources"],
        api_srcs: [
            // Note: The following arent APIs of this library. Only APIs under the
            // android.test.mock package are taken. These do provide private APIs
            // to which android.test.mock APIs reference. These classes are present
            // in source code form to access necessary comments that disappear when
            // the classes are compiled into a Jar library.
            ":framework-core-sources-for-test-mock",
            ":framework_native_aidl",
        ],

        libs: [
            "framework",
            "framework-annotations-lib",
            "app-compat-annotations",
            "Unsupportedappusage",
        ],

        api_packages: [
            "android.test.mock",
        ],
        permitted_packages: [
            "android.test.mock",
        ],
        compile_dex: true,
        default_to_stubs: true,
    }

Maintain backward compatibility

The build system checks whether the APIs have maintained backward compatibility by comparing the latest API files with the generated API files at build time. The java_sdk_library performs the compatibility check using the information provided by prebuilt_apis. All libraries built with java_sdk_library must have API files in the latest version of api_dirs in prebuilt_apis. When you release the version, the API lists files and stubs libraries can be obtained with dist build with PRODUCT=sdk_phone_armv7-sdk.

The api_dirs property is list of API version directories in prebuilt_apis. The API-version directories must be located at the Android.bp directory level.

prebuilt_apis {
       name: "foo",
       api_dirs: [
           "1",
           "2",
             ....
           "30",
           "current",
       ],
    }

Configure the directories with the version/scope/api/ structure under the prebuilts directory. version corresponds to the API level and scope defines whether the directory is public, system, or test.

• version/scope contains Java libraries.
• version/scope/api contains API .txt files. Create empty text files named module_name.txt and module_name-removed.txt here.

    30
       public
          api
             android.test.mock-removed.txt
             android.test.mock.txt
          android.test.mock.jar
       system
          api
             android.test.mock-removed.txt
             android.test.mock.txt
          android.test.mock.jar
       test
           api
              android.test.mock-removed.txt
              android.test.mock.txt
           android.test.mock.jar
    Android.bp