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66 lines
3.1 KiB
66 lines
3.1 KiB
# SdkExtensions module
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SdkExtensions module is responsible for:
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- deciding the extension SDK level of the device;
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- providing APIs for applications to query the extension SDK level;
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- determining the values for the BOOTCLASSPATH, DEX2OATBOOTCLASSPATH, and
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SYSTEMSERVERCLASSPATH environment variables.
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## General information
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### Structure
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The module is packaged in an apex, `com.android.sdkext`, and has several
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components:
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- `bin/derive_classpath`: a native binary that runs early in the device boot
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process. It reads individual classpath configs files from the system and
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other modules, merges them, and defines the definition of *CLASSPATH environ
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variables.
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- `bin/derive_sdk`: native binary that runs early in the device boot process and
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reads metadata of other modules, to set system properties relating to the
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extension SDK (for instance `build.version.extensions.r`).
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- `javalib/framework-sdkextension.jar`: this is a jar on the bootclasspath that
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exposes APIs to applications to query the extension SDK level.
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### Deriving extension SDK level
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`derive_sdk` is a program that reads metadata stored in other apex modules, in
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the form of binary protobuf files in subpath `etc/sdkinfo.pb` inside each
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apex. The structure of this protobuf can be seen [here][sdkinfo-proto]. The
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exact steps for converting a set of metadata files to actual extension versions
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is likely to change over time, and should not be depended upon.
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### Reading extension SDK level
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The module exposes a java class [`SdkExtensions`][sdkextensions-java] in the
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package `android.os.ext`. The method `getExtensionVersion(int)` can be used to
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read the version of a particular sdk extension, e.g.
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`getExtensionVersion(Build.VERSION_CODES.R)`.
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### Deriving classpaths
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`derive_classpath` service reads and merges individual config files in the
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`/system/etc/classpaths/` and `/apex/*/etc/classpaths`. Each config stores
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protobuf message from [`classpaths.proto`] in a proto binary format. Exact
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merging algorithm that determines the order of the classpath entries is
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described in [`derive_classpath.cpp`] and may change over time.
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[`classpaths.proto`]: packages/modules/SdkExtensions/proto/classpaths.proto
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[`derive_classpath.cpp`]: packages/modules/SdkExtensions/derive_classpath/derive_classpath.cpp
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[sdkinfo-proto]: packages/modules/SdkExtensions/proto/sdk.proto
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[sdkextensions-java]: framework/java/android/os/ext/SdkExtensions.java
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## Developer information
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### Adding a new extension version
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For every new Android SDK level a new extension version should be defined. These
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are the steps necessary to do that:
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- Add the new modules in this extension version to the SdkModule enum in
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sdk.proto.
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- Update `derive_sdk.cpp` by:
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* mapping the modules' package names to the new enum values
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* creating a new set with the new enum values
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* set a new sysprop to the value of `GetSdkLevel` with the new enum set
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* add a unit test to `derive_sdk_test.cpp` verifying the new extensions works
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- Make the `SdkExtensions.getExtensionVersion` API support the new extensions.
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- Extend the CTS test to verify the above two behaviors.
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- Update `RollbackManagerServiceImpl#getExtensionVersions` to account for the
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new extension version.
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