Oracle announced the general availability of JDK 21 on September 19, 2023. Oracle will offer long term support for Java 21 for at least eight years.

What is New?

Java 21 delivers tens of new features and enhancements along with thousands of performance, stability and security updates.

The 15 JEPs (JDK Enhancement Proposals) delivered with Java 21 are grouped into 6 categories mapping to key long-term Java technology projects and hardware support.

1. Project Amber: The goal of Project Amber is to explore and incubate smaller, productivity-oriented Java language features.
   • JEP 430: String Templates (Preview)
   • JEP 440: Record Patterns
   • JEP 441: Pattern Matching for switch
   • JEP 443: Unnamed Patterns and Variables (Preview)
   • JEP 445: Unnamed Classes and Instance Main Methods (Preview)
2. Project Loom: Project Loom strives to simplify maintaining concurrent applications in Java by introducing virtual threads and an API for structured concurrency, among other things.
   • JEP 444: Virtual Threads
   • JEP 446: Scoped Values (Preview)
   • JEP 453: Structured Concurrency (Preview)
3. Project Panama: Project Panama aims to improve the connection between the JVM and foreign (non-Java) libraries.
   • JEP 442: Foreign Function & Memory API (3rd Preview)
   • JEP 448: Vector API (6th Incubator)
4. Core Libraries
   • JEP 431: Sequenced Collections
5. Performance Updates
   • JEP 439: Generational ZGC
   • JEP 452: Key Encapsulation Mechanism API
6. Maintenance and Deprecation
   • JEP 449: Deprecate the Windows 32-bit x86 Port for Removal
   • JEP 451: Prepare to Disallow the Dynamic Loading of Agents

Details for each JEP can be read from the links above. I marked the most important JEPs as bold for readability.

References:

1. https://www.oracle.com/en/news/announcement/ocw-oracle-releases-java-21-2023-09-19/
2. https://inside.java/2023/09/19/the-arrival-of-java-21/
3. https://foojay.io/today/java-21-is-available-today-and-its-quite-the-update/