Java 21 is scheduled for release on September 19, 2023, serving as the forthcoming long-term support (LTS) edition in Oracle's standard Java implementation. Java 21 will encompass a total of 15 noteworthy features:

String Templates (Preview) [JEP-430]
Sequenced Collections [JEP-431]
Generational ZGC [JEP-439]
Record Patterns [JEP-440]
Pattern Matching for switch [JEP-441]
Foreign Function & Memory API (Third Preview) [JEP-442]
Unnamed Patterns and Variables (Preview) [JEP-443]
Virtual Threads [JEP-444]
Unnamed Classes and Instance Main Methods (Preview) [JEP-445]
Scoped Values (Preview) [JEP-446]
Vector API (Sixth Incubator) [JEP-448]
Deprecate the Windows 32-bit x86 Port for Removal [JEP-449]
Prepare to Disallow the Dynamic Loading of Agents [JEP-451]
Key Encapsulation Mechanism API [JEP-452]
Structured Concurrency (Preview) [JEP-453]

Key Features of Java 21

Among other changes, the following are the main changes that we should be aware of as a Java developer.

Virtual Threads have been finalized.
The Record Patterns (Project Amber) have been finalized.
Pattern Matching for switch statements has been finalized.
A new collection, SequencedCollection, has been added that provides direct access to an ordered collection’s first and last elements.
String templates, unnamed classes and instance main() methods are available as preview features.

Virtual Threads (Project Loom)

Java 21's Virtual Threads: Boosting High-Concurrency Applications

Virtual threads, as managed by the JVM, are a game-changer for developing high-throughput concurrent applications. (Throughput refers to the number of units of information a system can process within a given time frame.) These advancements are encapsulated in JEP-425, JEP-436, and JEP-444, and in Java 21, virtual threads are primed for production deployment.

The introduction of virtual threads opens the door to executing countless virtual threads with just a handful of operating system threads. What's particularly advantageous is that no alterations are needed in existing Java code. The only requirement is to instruct our application framework to leverage virtual threads instead of platform threads.

To create a virtual thread, Java APIs provide options such as using the Thread class or Executors.

Exploring Java 21 Features Through Practical Examples