Java 21 to drop generational Shenandoah GC

Java Development Kit (JDK) 21 is in an initial rampdown phase as of June 8. With the feature set now frozen, one feature is set to be dropped: generational Shenandoah, an experimental capability to enhance the Shenandoah garbage collector (GC).

The JDK Enhancement Proposal (JEP) page for JDK 21, which is due as a production release September 19, now proposes cutting back the official feature set from 16 features down to 15, with generational Shenandoah getting the ax. Removal centers on a lack of readiness. The author of the JEP for generational Shenandoah, Roman Kennke of Amazon, has decided to remove it from JDK, or simply, Java 21, with the intent to evaluate it for a future JDK release when ready, according to Oracle, which oversees development of standard Java.

Generational Shenandoah would enhance Shenandoah with experimental generational collection capabilities to improve sustainable throughput, load-spike resilience, and memory utilization. Goals of the plan include providing an experimental generational mode without breaking non-generational Shenandoah, with the intent of making it the default in a future release. Other goals include reducing the sustained memory footprint without sacrificing low GC pauses, cutting CPU and power usage, sustaining high throughput, and decreasing the risk of incurring degenerated and full collections during allocation spikes. Non-generational Shenandoah would not be replaced.

The proposed removal, set for a review period until June 14, only is relevant for users of the Shenandoah GC in OpenJDK builds that support it; this does not impact the G1 GC, Z Garbage Collector (ZGC), or other modern garbage collectors. It also does not impact Java distributions that use G1 GC and ZGC, such as Oracle OpenJDK and the Oracle JDK. JDK 21, with other features such as a preview of structured concurrency, is set to be a long-term support (LTS) release of Java and receive multiple years of support. The last LTS release was JDK 17, which arrived in September 2021. JDK releases occur every six months, with LTS releases happening every two years. Feature releases arriving otherwise only get six months of support. The current feature release is JDK 20, which was published in March.