JDK 21 GA: All New Features, Virtual Threads & Generational ZGC Explained

JDK 21 has reached General Availability (GA), meaning it is the officially released, fully tested version.

GA stands for General Availability, indicating a production‑ready release.

Version naming conventions:

Alpha – internal testing, many bugs, not for external use.

Beta – public testing, major bugs fixed, still adding features.

Gamma – near‑final, minor changes before release candidate.

RC (Release Candidate) – feature‑complete, focused on bug fixing.

GA – final production version.

Release – final version, often shown with © symbol.

Stable – fully vetted open‑source release.

LTS (Long‑Term Support) versions receive extended maintenance and security updates; in Java, LTS releases include Java 8, 11, and 17.

JDK 21 introduces fifteen new features grouped into four JEP categories: core Java libraries, language specifications, HotSpot, and security libraries.

Key feature: JEP 444 – Virtual Threads, a lightweight concurrency model that can be adopted with minimal code changes.

To enable virtual threads, replace the thread‑pool creation with the following code:

Other notable features include:

Core library: JEP 431 (Sequence Collections), JEP 442 (Foreign Function & Memory API, third preview), JEP 444 (Virtual Threads), JEP 446 (Scoped Values, preview), JEP 448 (Vector API, sixth incubator), JEP 453 (Structured Concurrency, preview).

Language: JEP 430 (String Templates, preview), JEP 440 (Record Patterns), JEP 441 (Switch Pattern Matching), JEP 443 (Unnamed Patterns & Variables, preview), JEP 445 (Unnamed Classes & Main Method, preview).

HotSpot: JEP 439 (Generational ZGC), JEP 449 (Deprecate Windows x86 port), JEP 451 (Prohibit dynamic loading agents).

Security: JEP 452 (Key Encapsulation Mechanism API).

JEP 445 simplifies the classic “Hello, World!” program; the preview version can be compiled and run with:

javac --release 21 --enable-preview Main.java
java --enable-preview Main

Generational ZGC, introduced in JEP 439, adds a generational mode to the Z Garbage Collector, which is disabled by default and can be enabled via configuration.

Shenandoah was planned for JDK 21 but was delayed due to review risks; it is targeted for JDK 22.

Overall, JDK 21 brings significant improvements to concurrency, memory handling, and language ergonomics, offering developers a modern platform for production workloads.