How to enable Java 21 preview features

Enable preview features in Java 21

To enable Java 21 preview features, you must follow a two-step process:

1. Compile your Java 21 code with the --release 21 --enable-preview flag.
2. Run your Java 21 code with the --enable-preview flag.

If you miss either step, you’ll encounter both compile-time errors and runtime errors.

Java 21 preview feature commands

To test out Java 21’s new instance main methods and unnamed classes, I created a simple Java file named BlackJackHelloWorld.java. The command to compile the code containing the Java 21 preview features is as follows:

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

To run the code, use the following command:

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

Java 21 preview flags must exist at both compile time and runtime.

New Java 21 features

The September Java 21 release is scheduled to include a variety of preview, GA and incubating features, including the following:

• Vector API (Sixth Incubator).
• String templates (Preview).
• Unnamed patterns and variables (Preview).
• Unnamed classes and instance main methods (Preview).
• Scoped values (Preview).
• Foreign function & memory API (Third Preview).
• Structured concurrency (Preview).

Along with finalized features including sequenced collections and virtual threads, Java 21 has become a highly anticipated LTS release.