JDK 21 · JSR 396 · Long-term support

Java 21

JDK 21 is Java SE 21, JSR 396, generally available on 19 September 2023. Vendors treat it as a long-term support release. Virtual threads, pattern matching for switch, record patterns, and sequenced collections are final. String templates, scoped values, structured concurrency, unnamed patterns, and unnamed classes are previews. The existing Java 21 example and the concurrency chapter stay the detailed sources.

How to read the status

Final. The change shipped in this release as a permanent part of that release.
Preview. It can be tried with --enable-preview. A preview is not a permanent language or API guarantee.
Incubator. An incubator API is not a supported standard API. It can change or be removed.

Status

Language

Virtual Threads

JEP 444 · Final · Language

A virtual thread is a lightweight thread for blocking I/O. The JVM schedules many of them onto a smaller set of platform threads.

Preview in Java 19 (JEP 425) and Java 20 (JEP 436).

OpenJDK JEP 444 (leaves JavaMastery)

Pattern Matching for switch

JEP 441 · Final · Language

A switch can match type patterns, record patterns, and null, and a guarded pattern can use when.

Preview in Java 17 through Java 20.

OpenJDK JEP 441 (leaves JavaMastery)

Record Patterns

JEP 440 · Final · Language

A pattern can deconstruct a record in switch or instanceof.

Preview in Java 19 (JEP 405) and Java 20 (JEP 432).

OpenJDK JEP 440 (leaves JavaMastery)

Related reading: Records and sealed classes

Curriculum file: versions6Java21Features.java

String Templates (Preview)

JEP 430 · Preview · Language

String templates are a preview for interpolating values into a string or another result. They are not a permanent feature.

Second preview in Java 22 (JEP 459). They are not in the JDK 23, JDK 24, or JDK 25 feature lists.

OpenJDK JEP 430 (leaves JavaMastery)

Unnamed Patterns and Variables (Preview)

JEP 443 · Preview · Language

An underscore can stand for a variable or a pattern component that the program does not use. This is a preview.

Final in Java 22 (JEP 456).

OpenJDK JEP 443 (leaves JavaMastery)

Unnamed Classes and Instance Main Methods (Preview)

JEP 445 · Preview · Language

A small program can omit the class wrapper and use an instance main method. This is a preview.

The idea continues through later previews and is finalized in Java 25 as compact source files (JEP 512).

OpenJDK JEP 445 (leaves JavaMastery)

APIs / Libraries

Sequenced Collections

JEP 431 · Final · APIs / Libraries

SequencedCollection, SequencedSet, and SequencedMap add getFirst, getLast, and reversed to ordered collections.

OpenJDK JEP 431 (leaves JavaMastery)

Key Encapsulation Mechanism API

JEP 452 · Final · APIs / Libraries

javax.crypto adds an API for key encapsulation mechanisms.

OpenJDK JEP 452 (leaves JavaMastery)

Scoped Values (Preview)

JEP 446 · Preview · APIs / Libraries

Scoped values are a preview API for sharing immutable data down a call, including across virtual threads.

Incubated in Java 20 (JEP 429). Final in Java 25 (JEP 506).

OpenJDK JEP 446 (leaves JavaMastery)

Structured Concurrency (Preview)

JEP 453 · Preview · APIs / Libraries

Structured concurrency is a preview API. Related tasks form one unit of work that can cancel and report errors together.

Incubated in Java 19 and Java 20. Still a preview in Java 25 (JEP 505).

OpenJDK JEP 453 (leaves JavaMastery)

Foreign Function & Memory API (Third Preview)

JEP 442 · Preview · APIs / Libraries

The foreign function and memory API is previewed a third time. It is not final in Java 21.

Final in Java 22 (JEP 454).

OpenJDK JEP 442 (leaves JavaMastery)

Vector API (Sixth Incubator)

JEP 448 · Incubator · APIs / Libraries

The Vector API incubates a sixth time.

Still incubating in Java 25 (JEP 508).

OpenJDK JEP 448 (leaves JavaMastery)

JVM / Runtime

Generational ZGC

JEP 439 · Final · JVM / Runtime

ZGC can keep young and old objects in separate generations. This is a collector change, not a source change.

OpenJDK JEP 439 (leaves JavaMastery)

Examples

versions6Java21Features.javaJava 21 example · Requires Java 21Virtual threads, record patterns, pattern matching for switch, and sequenced collections, in their Java 21 final form. String templates, structured concurrency, and scoped values were previews and are not shown.
concurrency6VirtualThreadsDemo.javaJava 21 example · Requires Java 21Virtual threads in the concurrency chapter. Thread.ofVirtual is the final Java 21 API. In Java 21 a virtual thread can stay pinned inside synchronized; Java 24 changed that.
advconcurrency7StructuredConcurrency.javaStable demo; Java 25 preview API is only in the comment · Requires Java 8The main method uses CompletableFuture and runs without --enable-preview. The comment shows the Java 25 preview API: StructuredTaskScope.open with a Joiner. JEP 505 is still a preview.

Historical notes

Since Java 17, this release finalizes virtual threads, switch patterns, and record patterns, and it adds sequenced collections. Structured concurrency, scoped values, string templates, unnamed variables, and unnamed classes remain previews. The Java 21 source file is unchanged and does not demonstrate the preview APIs.

Sources. General availability, JSR 396, and the JEP list: OpenJDK JDK 21. The example file versions6Java21Features.java is unchanged. OpenJDK JDK 21 (leaves JavaMastery)

All versions