Java introduced preview features in the language since Java 12. This features can be tried out by developers, but are still subject to change and can even be removed in a next release. By default the preview features are not enabled when we want to compile and run our Java code. We must explicitly specify that we want to use the preview feature to the Java compiler and Java runtime using the command-line argument --enable-preview. In Gradle we can customize our build file to enable preview features. We must customize tasks of type JavaCompile and pass --enable-preview to the compiler arguments. Also tasks of type Test and JavaExec must be customized where we need to add the JVM argument --enable-preview.

In the following Gradle build script written in Kotlin we have a Java project written with Java 15 where we reconfigure the tasks to enable preview features:

plugins {
    java
    application
}

repositories {
    mavenCentral()
}

dependencies {
    testImplementation("org.junit.jupiter:junit-jupiter-api:5.7.1")
    testRuntimeOnly("org.junit.jupiter:junit-jupiter-engine:5.7.1")
}

application {
    mainClass.set("mrhaki.Patterns")
}

tasks {
    val ENABLE_PREVIEW = "--enable-preview"

    // In our project we have the tasks compileJava and
    // compileTestJava that need to have the
    // --enable-preview compiler arguments.
    withType<JavaCompile>() {
        options.compilerArgs.add(ENABLE_PREVIEW)

        // Optionally we can show which preview feature we use.
        options.compilerArgs.add("-Xlint:preview")

        // Explicitly setting compiler option --release
        // is needed when we wouldn't set the
        // sourceCompatiblity and targetCompatibility
        // properties of the Java plugin extension.
        options.release.set(15)
    }

    // Test tasks need to have the JVM argument --enable-preview.
    withType<Test>() {
        useJUnitPlatform()
        jvmArgs.add(ENABLE_PREVIEW)
    }

    // JavaExec tasks need to have the JVM argument --enable-preview.
    withType<JavaExec>() {
        jvmArgs.add(ENABLE_PREVIEW)
    }
}

Written with Gradle 6.8.3

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