CriticalNative

@Target([AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.PROPERTY_SETTER]) class CriticalNative

An ART runtime built-in optimization for native methods to speed up JNI transitions: Methods that are annotated with @CriticalNative use the fastest available JNI transitions from managed code to the native code and back. This annotation can be applied only to native methods that do not use managed objects (in parameters or return values, or as an implicit this).

The @CriticalNative annotation changes the JNI transition ABI. The native implementation must exclude the JNIEnv and jclass parameters from its function signature.

While executing a @CriticalNative 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. (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 @CriticalNative performance gain can become insignificant and overshadowed by potential GC delays.)

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 @CriticalNative 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 @CriticalNative 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-, so the ABI mismatch would lead to wrong argument marshalling and likely crashes.

A similar annotation, @FastNative, exists for methods that need fast transitions but absolutely need to use managed objects, whether as the implicit this for non-static methods, or method arguments, return values or to otherwise call back to managed code (say, static methods), or access managed heap objects (say, static fields).

Performance of JNI transitions:

• Regular JNI cost in nanoseconds: 115
• Fast (!) JNI cost in nanoseconds: 60
• @FastNative cost in nanoseconds: 35
• @CriticalNative cost in nanoseconds: 25

Deadlock Warning: As a rule of thumb, any native locks acquired in a @CriticalNative call (despite the above warning that this is an unbounded operation that can block GC for a long time) must be released before returning to managed code.

Say some code does: critical_native_call_to_grab_a_lock(); does_some_java_work(); critical_native_call_to_release_a_lock();

This code can lead to deadlocks. Say thread 1 just finishes critical_native_call_to_grab_a_lock() and is in does_some_java_work(). GC kicks in and suspends thread 1. Thread 2 now is in critical_native_call_to_grab_a_lock() but is blocked on grabbing the native lock since it's held by thread 1. Now thread suspension can't finish since thread 2 can't be suspended since it's doing CriticalNative JNI.

Normal natives don't have the issue since once it's executing in native code, it is considered suspended from the runtime's point of view. CriticalNative natives however don't do the state transition done by the normal natives.

This annotation has no effect when used with non-native methods.

The runtime shall throw a java.lang.VerifyError during verification if this annotation is present on a native method that is non-static, or contains object parameters, or returns an object.

Summary

Public constructors

CriticalNative()