FastNative

public abstract @interface FastNative
implements Annotation

An ART runtime built-in optimization for native methods to speed up JNI transitions: Compared to normal native methods, native methods that are annotated with @FastNative use faster JNI transitions from managed code to the native code and back. Calls from a @FastNative method implementation to JNI functions that access the managed heap or call managed code also have faster internal transitions.

While executing a @FastNative method, the garbage collection cannot suspend the thread for essential work and may become blocked. Use with caution. Do not use this annotation for long-running methods, including usually-fast, but generally unbounded, methods. In particular, the code should not perform significant I/O operations or acquire native locks that can be held for a long time. Never acquire a native lock that can also be held while a thread invokes Java code. That virtually guarantees deadlocks. (Otherwise some logging or native allocations, which internally acquire native locks for a short time, are generally OK. However, as the cost of several such operations adds up, the @FastNative performance gain can become insignificant and overshadowed by potential GC delays.) Acquiring managed locks is OK as it internally allows thread suspension.

For performance critical methods that need this annotation, it is strongly recommended to explicitly register the method(s) with JNI RegisterNatives instead of relying on the built-in dynamic JNI linking.

The @FastNative optimization was implemented for system use since Android 8 and became CTS-tested public API in Android 14. Developers aiming for maximum compatibility should avoid calling @FastNative methods on Android 13-. The optimization is likely to work also on Android 8-13 devices (after all, it was used in the system, albeit without the strong CTS guarantees), especially those that use unmodified versions of ART, such as Android 12+ devices with the official ART Module. The built-in dynamic JNI linking is working only in Android 12+, the explicit registration with JNI RegisterNatives is strictly required for running on Android versions 8-11. The annotation is ignored on Android 7-.

Deadlock Warning: As a rule of thumb, any native locks acquired in a @FastNative call (despite the above