Does anyone know why Gradle does not support the most recent JDK's, like 20/21? It's a build tool th...

# gradle

[Gavin Ray]

Does anyone know why Gradle does not support the most recent JDK's, like 20/21?
It's a build tool that wraps javac right, at a 10,000ft view, so if there is a java compiler on the system capable of compiling those versions why can't Gradle use it?

For instance, why can't I run the Gradle daemon with JDK17, and ask it to use JDK21 to compile my .java files 🤔

[mbonnin]

Have you tried the toolchains?

[mbonnin]

https://docs.gradle.org/current/userguide/toolchains.html

[Chris Lee]

some Gradle versions had fixed constants for the JavaLanguageVersion used in toolchains. Looking at Gradle 8 that limitation appears to be removed (unsure exactly when that was done).

[Chris Lee]

This is the issue: https://github.com/gradle/gradle/issues/22396

[Gavin Ray]

Ah you can do it with Toolchains?

Does anyone know how to do it? I'm using 8.0-rc

[Gavin Ray]

I'm familiar with toolchains in general and love them for normal use, I didn't know you could use them for this sort of thing though

[Gavin Ray]

If I set my Gradle JVM to 19, and use the following, I get the below error:

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(21))
    }
}

2023-01-10T14:43:46.776-0500 [ERROR] [system.err] 	java.lang.UnsupportedClassVersionError: postgres/wire/protocol/App has been compiled by a more recent version of the Java Runtime (class file version 65.0), this version of the Java Runtime only recognizes class file versions up to 63.0

[mbonnin]

So 63 is Java19

[mbonnin]

I'm guessing 65 is Java21. Can you post the full error with --stacktrace ?

[mbonnin]

And including the failing task?

[mbonnin]

Made a small test and it's working fine for me

[mbonnin]

Of course you need to run your jar with a 21 JVM

[mbonnin]

That's my build file:

plugins {
    id("java")
    id("application")
}

repositories {
    mavenCentral()
}

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(21))
    }
}

application {
    mainClass.set("com.example.Main")
}

[mbonnin]

(main is trivial)

[mbonnin]

(I installed 21 through sdk uninstall java 21.ea.4-open )

[tapchicoma]

FYI: if, for example, you want to use some JDK preview with "toolchain" in Kotlin - you could use second approach via Tasks DSL:

tasks.withType<UsesKotlinJavaToolchain>().configureEach {
    kotlinJavaToolchain.jdk.use(
        "/path/to/local/jdk", // Put a path to your JDK
        JavaVersion.<LOCAL_JDK_VERSION> // For example, JavaVersion.17
    )
}

Note though that this will not configure any Java/Groovy etc tasks with this toolchain.

[Gavin Ray]

Huh very strange, thank you for investigating @mbonnin and @tapchicoma

This is what ended up finally working, maybe this gives some hints as to what is wrong with my current setup?

java {
    toolchain {
        // Tell Gradle itself to use JDK19
        languageVersion.set(JavaLanguageVersion.of(19))
    }
}

java {
    sourceCompatibility = JavaVersion.VERSION_21
    targetCompatibility = JavaVersion.VERSION_21
}

val JDK_VERSION = JavaLanguageVersion.of(21)
val JDK_FLAGS = listOf("--enable-preview")

val JDK_21_COMPILER: Provider<JavaCompiler> = javaToolchains.compilerFor {
    languageVersion.set(JDK_VERSION)
}

val JDK_21_LAUNCHER: Provider<JavaLauncher> = javaToolchains.launcherFor {
    languageVersion.set(JDK_VERSION)
}

tasks.withType<JavaCompile>().configureEach {
    javaCompiler.set(JDK_21_COMPILER)
    options.isIncremental = true
    options.isFork = true
    options.compilerArgs = JDK_FLAGS
}

tasks.withType<Test>().configureEach {
    useJUnitPlatform()
    javaLauncher.set(JDK_21_LAUNCHER)
    jvmArgs = JDK_FLAGS
}

tasks.withType<JavaExec>().configureEach {
    javaLauncher.set(JDK_21_LAUNCHER)
    jvmArgs = JDK_FLAGS
}