Under the hood, instead of javac, vscode-java uses ECJ, a Java 25 compiler provided by Eclipse JDT. It is used to compile against all other versions of Java. But the ECJ compiler only supports the latest JDK release when it comes to preview features. It means that, with the latest vscode-java release, preview features can not be enabled for projects compiling against Java 24 (or older), even if you configured the proper runtime. Those projects need to be updated to use Java 25 in vscode-java.

It's recommended you configure the java.configuration.runtimes preference in your user's settings.json:

"java.configuration.runtimes": [ { "name": "JavaSE-25", "path": "/path/to/jdk-25", "default": true }, ],

Now, depending on the style of Java projects you use, there are different ways to enable preview features.

When you open standalone Java files (i.e. which have no Eclipse/Maven/Gradle settings), preview features are enabled by default, without warnings, if vscode-java was started with a JDK 25 or JavaSE-25 is set to be the default in java.configuration.runtimes.

Maven projects need to have the --enable-preview flag added to the maven-compiler-plugin configuration, in their pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>foo.bar</groupId> <artifactId>demo</artifactId> <version>0.0.1-SNAPSHOT</version> <properties>	<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> </properties> <build> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.13.0</version> <configuration> <release>25</release> <compilerArgs>--enable-preview</compilerArgs> </configuration> </plugin> </plugins> </build> </project>

Eclipse projects need to add the the following preferences to .settings/org.eclipse.jdt.core.prefs:

eclipse.preferences.version=1 org.eclipse.jdt.core.compiler.codegen.inlineJsrBytecode=enabled org.eclipse.jdt.core.compiler.codegen.targetPlatform=25 org.eclipse.jdt.core.compiler.codegen.unusedLocal=preserve org.eclipse.jdt.core.compiler.compliance=25 org.eclipse.jdt.core.compiler.debug.lineNumber=generate org.eclipse.jdt.core.compiler.debug.localVariable=generate org.eclipse.jdt.core.compiler.debug.sourceFile=generate org.eclipse.jdt.core.compiler.problem.assertIdentifier=error org.eclipse.jdt.core.compiler.problem.enumIdentifier=error org.eclipse.jdt.core.compiler.release=enabled org.eclipse.jdt.core.compiler.source=25 org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures=enabled org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures=ignore 

⚠️ Gradle 9.10 minimum is required to build Java 25 projects.

Gradle projects can also maintain the same .settings/org.eclipse.jdt.core.prefs file. Alternatively, adding an eclipse.jdt.file.withProperties hook in build.gradle is possible, but requires the gradle compileJdt task to be invoked manually, as Buildship, the underlying Gradle integration tool, doesn't invoke it :

plugins { // Apply the java-library plugin to add support for Java Library id 'java-library' id 'eclipse' } sourceCompatibility = JavaVersion.VERSION_25 targetCompatibility = JavaVersion.VERSION_25 compileJava { options.compilerArgs += ["--enable-preview"] } compileTestJava { options.compilerArgs += ["--enable-preview"] } //Buildship doesn't use that hook (https://discuss.gradle.org/t/when-does-buildship-eclipse-customization-run/20781/2) //you need to run `gradle eclipse` separately eclipse.jdt.file.withProperties { props -> props['org.eclipse.jdt.core.compiler.problem.enablePreviewFeatures']= 'enabled' props['org.eclipse.jdt.core.compiler.problem.reportPreviewFeatures']= 'ignore' } test { jvmArgs '--enable-preview' }