JDK 24 Preview: JNI Warnings, G1 Barrier Extensions, and Class File API Explained

Release Overview

JDK 24 is slated for release on March 18 2025 as a non‑LTS version, following JDK 23 released on 17 September. Oracle will provide six months of premium support, similar to the previous release.

Feature 1: JNI Usage Warning

Oracle proposes a warning mechanism that alerts developers when using the Java Native Interface (JNI) and restricts unsafe usage. The warning aligns the external function and memory (FFM) API introduced in JDK 22, aiming to standardise JNI usage and prepare future versions where native interop may be disabled by default.

Feature 2: G1 Garbage‑Collector Barrier Extension

The G1 GC receives a post‑barrier extension that moves barrier recording from the early C2 compilation pipeline to a later stage, reducing the impact on C2‑compiled code and helping developers understand G1 barriers without deep C2 knowledge. The goal is to preserve ordering invariants of memory accesses, safepoints and barriers.

Feature 3: Class File API

The previously previewed Class File API, introduced in JDK 22/23, becomes final in JDK 24 with minor changes. It offers a standard way to parse, generate and transform Java class files, tracking the class‑file format defined by the JVM specification and allowing migration away from the internal ASM copy.

Other Potential Features

Future additions may include stream collectors for custom intermediate operations, module‑import declarations, structured concurrency, value types for immutable data sharing, flexible constructor bodies, and enhancements to pattern matching (primitive type patterns, instanceof, switch) as well as a possible Vector API and accelerated class loading.

Support Timeline

JDK 21, the latest LTS release, was launched in September 2023 and will receive at least five years of Premier support. JDK 25 is planned as the next LTS in September 2025, meaning adoption of JDK 23 and JDK 24 may be limited until the next LTS arrives.