Why Generational Shenandoah GC in JDK 25 Is a Game‑Changer for Java Performance

Birth and Evolution of Shenandoah

Understanding Shenandoah starts with JDK 12, where it was introduced as an experimental feature focused on ultra‑low pause times. It became a formal product feature in JDK 15, and in JDK 24 the generational mode was added via JEP 404, still requiring the -XX:+UnlockExperimentalVMOptions flag.

JDK 12: Shenandoah introduced as an experimental feature for ultra‑low pause times.

JDK 15: Graduated to a standard product feature.

JDK 24: Added generational mode (experimental) requiring -XX:+UnlockExperimentalVMOptions.

Traditional Shenandoah is non‑generational (single‑generation), managing all objects in one heap region. Although its pause times are extremely short, its throughput can lag behind modern collectors like G1 and ZGC in certain high‑throughput scenarios.

The breakthrough is that Shenandoah moves objects concurrently with application threads, removing the most time‑consuming memory‑copy operation from stop‑the‑world pauses.

Generational Hypothesis

Most objects in typical applications are short‑lived ("young"), so generational GC divides the heap into a young generation for frequent reclamation and an old generation for occasional collection, dramatically improving efficiency.

JEP 521

From JDK 24’s JEP 404 to JDK 25’s JEP 521, the generational Shenandoah GC graduates to a production‑ready feature. The experimental flag -XX:+UnlockExperimentalVMOptions is no longer required. However, the default GC remains G1, and Shenandoah still runs in non‑generational mode by default for compatibility.

Core Benefits

Performance leap:

Younger‑generation collection is faster, reducing pause times.

Throughput improves by avoiding unnecessary full‑heap scans.

Memory efficiency rises, lowering overall heap usage.

Production readiness:

Extensive validation with DaCapo, SPECjbb2015, SPECjvm2008 benchmarks.

Real‑world usage in high‑load enterprise applications.

Stable on Linux, Windows, and macOS platforms.

Zero‑cost migration:

Command‑line compatibility: old experimental flags no longer cause errors.

Safe default: generational mode is not auto‑enabled, avoiding unexpected behavior changes.

Explicit activation via -XX:ShenandoahGCMode=generational when desired.

Comprehensive Breakthrough

In the era of ultra‑large heaps, the difference between generational and non‑generational Shenandoah becomes stark, especially in pause‑time versus throughput trade‑offs.

Generational Shenandoah transforms from a specialist into an all‑rounder, retaining its ultra‑low latency while closing the throughput gap, making it a compelling GC choice for new JDK 25+ projects.

Four Major Changes for Java

GC family becomes stronger: Shenandoah fills the generational gap, offering a more balanced option.

Low‑latency applications benefit: e‑commerce, finance, gaming gain a new performance weapon.

Experimental feature matures: showcases OpenJDK’s rigorous feature incubation process.

JDK 25 highlight: further solidifies Java’s position in high‑performance domains.

How to Enable in JDK 25

java -XX:+UseShenandoahGC \
    -XX:ShenandoahGCMode=generational \
    -jar your-app.jar

Conclusion

JEP 521 and the release of JDK 25 mark a new milestone for Java garbage‑collection technology, representing both an evolution of Shenandoah and the ecosystem’s relentless pursuit of extreme performance.

Future work includes AI‑driven adaptive generational sizing and tighter integration with Project Loom’s virtual‑thread‑aware GC.