Google has announced the upcoming Android Gradle Plugin (AGP), version 8.0, reduces build times by removing the Transform APIs and replacing them through the Artifacts API and Instrumentation API. All third-party plugins and apps relying on them are required to migrate to the new APIs.
The Transform API is being removed to improve build performance. Projects that use the Transform API force AGP to use a less optimized flow for the build that can result in large regressions in build times.
The Transform APIs are a collection of APIs provided by the Android Gradle plugin to process intermediate artifacts of the build process, enabling to transform bytecode, synthesize new classes, or analyse the whole program.
A typical use of this feature is instrumenting compiled classes to add traces, custom logging, performance analysis and so on. For example, the Android Hilt plugin uses the Transform APIs to embed Dagger dependency injection into an Android app and Firebase uses it for performance monitoring. While the Transform APIs is typically used in Gradle plugins, a number of libraries also exist that use them, such as mobile database Realm. Additionally, Transform features may be helpful to create test classes.
For developers, this change means they may need to migrate their own codebases, as well as to update any dependencies using the Transforms APIs to newer versions using the replacement APIs. While a number of plugins and libraries, including the mentioned Hilt, Firebase, and Realm, have been working on adopting the new APIs, it is not unlikely that migration may cause some headache for developers. In particular, upgrading to the latest version of a plugin/library may not be just as simple as changing its version number in a configuration file, but could also require modifications to the codebase that is using it.
To make the migration as easy as possible, Google is making the new replacement APIs available in AGP 7.2 so developers can adapt their code in preparation for version 8. AGP 7.2 will output a prominent warning for each use of the Transform APIs, which will cause a failure in AGP 8. To make AGP warning provide more detail about which component is causing it, you can set android.debug.obsoleteApi=true in gradle.properties.
As mentioned, there are two distinct APIs that replace Transform: the Instrumentation API, which can be used for transforming bytecode, and the Artifacts API, which enables adding new classes to an app. Google has not provided specific information regarding the build time reduction brought by the new plugin. Anyway a few details as to how it is achieved can be inferred from Google docs.
Specifically, to make the Instrumentation API fast, AGP restricts the instrumentation code to run independently on each class with no visibility of other classes. The first benefit this provides is that libraries can be instrumented in parallel rather than at the end of compilation. It also contributes to speed up incremental builds, since a change in a class will only require that class and those that depend on it to be processed again.
Regarding the Artifacts API, the key factor bringing the performance benefit seems to be the isolation of whole-program analysis transformation into a specific API, called Artifacts.forScope. This API should be used with caution, says Google, since it has a high build-performance cost and making it a separate part means you can handle cases where this kind of whole-program analysis is not used more efficient.