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 The command to compile the code containing the Java 21 preview features is as follows:

javac --release 21 --enable-preview

To run the code, use the following command:

javac --release 21 --enable-preview
enable Java 21 preview commands

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.

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