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.
A third feature, the class-file API, previously previewed in JDK 22 and JDK 23, would be finalized in JDK 24, with minor changes. This API provides a standard API for parsing, generating, and transforming Java class files. It aims to provide an API for processing class files that tracks the class file format defined by the Java Virtual Machine specification. A second goal is to enable JDK components to migrate to the standard API, and eventually remove the JDK’s internal copy of the third-party ASM library. Changes since the second preview include a renaming of enum values, removal of some fields, the addition of methods and method overloads, methods renamed, and removal of interfaces and methods deemed unnecessary.
