This API previously was incubated in JDK 16 through JDK 23. It would be re-incubated in JDK 24 with no API changes and no substantial implementations relative to JDK 23. Goals of the proposal include clearly and concisely expressing a wide range of vector computations in an API that is platform-agnostic, that offers reliable runtime compilation and performance on x64 and AArch54 architectures, that degrades gracefully and still functions when a vector computation cannot be expressed at runtime, and that aligns with Project Valhalla, leveraging enhancements to the Java object model.
The first JDK 24-targeted feature, officially called “Prepare to Restrict the Use of JNI,” calls for issuing warnings about uses of JNI and adjusting the foreign function and memory (FFM) API, featured in JDK 22, to issue warnings in a consistent manner. These warnings are intended to prepare for a future release that ensures integrity by default by uniformly restricting JNI and the FFM API. Goals of the plan include preserving JNI as a standard way to interoperate with native code, preparing the Java ecosystem for future releases that disallow interoperation with native code by default, and aligning the use of JNI and the FFM API so library maintainers can migrate from one to the other without requiring developers to change command-line options.
The second feature, late barrier expansion for the G1 garbage collector, is intended to simplify the implementation of G1’s barriers. The G1 garbage collector’s barriers record information about application memory accesses, by shifting their expansion from early in the C2 compilation pipeline to later. Goals include reducing the execution time of C2 compilation when using the G1 collector, making G1 barriers comprehensible to HotSpot developers who lack a deep understanding of C2, and guaranteeing that C2 preserves invariants about the relative ordering of memory accesses, safepoints, and barriers. A fourth feature is preserving the quality of C2-generated JIT (just-in-time)-compiled code, in terms of speed and size.
