SdkExtensions module

SdkExtensions module is responsible for:

deciding the extension SDK level of the device;
providing APIs for applications to query the extension SDK level;
determining the values for the BOOTCLASSPATH, DEX2OATBOOTCLASSPATH, and SYSTEMSERVERCLASSPATH environment variables.

General information

Structure

The module is packaged in an apex, com.android.sdkext, and has several components:

bin/derive_classpath: a native binary that runs early in the device boot process. It reads individual classpath configs files from the system and other modules, merges them, and defines the definition of *CLASSPATH environ variables.
bin/derive_sdk: native binary that runs early in the device boot process and reads metadata of other modules, to set system properties relating to the extension SDK (for instance build.version.extensions.r).
javalib/framework-sdkextension.jar: this is a jar on the bootclasspath that exposes APIs to applications to query the extension SDK level.

Deriving extension SDK level

derive_sdk is a program that reads metadata stored in other apex modules, in the form of binary protobuf files in subpath etc/sdkinfo.pb inside each apex. The structure of this protobuf can be seen here. The exact steps for converting a set of metadata files to actual extension versions is likely to change over time, and should not be depended upon.

Reading extension SDK level

The module exposes a java class SdkExtensions in the package android.os.ext. The method getExtensionVersion(int) can be used to read the version of a particular sdk extension, e.g. getExtensionVersion(Build.VERSION_CODES.R).

Deriving classpaths

derive_classpath service reads and merges individual config files in the /system/etc/classpaths/ and /apex/*/etc/classpaths. Each config stores protobuf message from classpaths.proto in a proto binary format. Exact merging algorithm that determines the order of the classpath entries is described in derive_classpath.cpp and may change over time.

Developer information

Adding a new extension version

For every new Android SDK level a new extension version should be defined. These are the steps necessary to do that:

Add the new modules in this extension version to the SdkModule enum in sdk.proto.
Update derive_sdk.cpp by:

mapping the modules' package names to the new enum values
creating a new set with the new enum values
set a new sysprop to the value of GetSdkLevel with the new enum set
add a unit test to derive_sdk_test.cpp verifying the new extensions works

Make the SdkExtensions.getExtensionVersion API support the new extensions.
Extend the CTS test to verify the above two behaviors.