OpenJDK 28 Early-Access Build 21 Introduces Enhanced ZGC Features
The latest OpenJDK 28 build brings significant ZGC improvements, including adaptive heap sizing and faster startup times, as development continues toward a March 2027 release.
OpenJDK 28 early-access build 21 is available under GPL v2 with the Classpath Exception.
Following build 20 from October 15, build 21 includes two Z Garbage Collector changes targeted in mid-October. JEP 546 introduces adaptive heap sizing, defaulting to a maximum heap of 100% of physical memory minus a small reserve instead of the current 25%. JEP 545 adds faster startup and warmup by having the collector acquire and prepare physical memory more quickly and efficiently.
General availability is scheduled for March 2027, preceded by a feature freeze in December 2026. Other scheduled work includes the finalized Structured Concurrency API in JEP 543 after seven preview rounds, the simple JSON API in incubation under JEP 540, strict field initialization in the JVM as a preview via JEP 539, and generational mode as the default for Shenandoah in JEP 535. Additionally, value objects under JEP 401 will require --enable-preview, and the macOS/x64 port is slated for deprecation under JEP 541.
You can test these builds and report bugs through the Java Bug Database.