By default, Gradle uses the same Java version for running Gradle itself and building JVM projects.

This is not always desirable. Building projects with different Java versions on different developer machines and CI servers may lead to unexpected issues. Additionally, you may want to build a project using a Java version that is not supported for running Gradle.

A Java Toolchain (from now on referred to simply as toolchain) is a set of tools, usually taken from a local JRE/JDK installation that are used to configure different aspects of a build. Compile tasks may use javac as their compiler, test and exec tasks may use the java command while javadoc will be used to generate documentation.

Consuming Toolchains

A build can globally define what toolchain it targets by stating the Java Language version it needs and optionally the vendor:

buildSrc/src/main/groovy/myproject.java-conventions.gradle
java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(11)
    }
}
buildSrc/src/main/kotlin/myproject.java-conventions.gradle.kts
java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
    }
}

Executing the build (e.g. using gradle check) will now handle several things for you and others running your build

  1. Setup all compile, test and javadoc tasks to use the defined toolchain which may be different than the one Gradle itself uses

  2. Gradle detects locally installed JVMs

  3. Gradle chooses a JRE/JDK matching the requirements of the build (in this case a JVM supporting Java 11)

  4. If no matching JVM is found, it will automatically download a matching JDK from AdoptOpenJDK

Toolchain support is available in the Java plugins and for the tasks they define. For the Groovy plugin, compilation is supported but not yet Groovydoc generation. For the Scala plugin, compilation and Scaladoc generation are supported.

Using toolchains by specific vendors

In case your build has specific requirements from the used JRE/JDK, you may want to define the vendor for the toolchain as well. JvmVendorSpec has a list of well-known JVM vendors recognized by Gradle. The advantage is that Gradle can handle any inconsistencies across JDK versions in how exactly the JVM encodes the vendor information.

build.gradle
java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(11)
        vendor = JvmVendorSpec.ADOPTIUM
    }
}
build.gradle.kts
java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
        vendor.set(JvmVendorSpec.ADOPTIUM)
    }
}

If the vendor you want to target is not a known vendor, you can still restrict the toolchain to those matching the java.vendor system property of the available toolchains.

Given the snippet below, only toolchain are taken into accounts whose java.vendor property contains the given match string. Matching is done case-insensitive.

build.gradle
java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(11)
        vendor = JvmVendorSpec.matching("customString")
    }
}
build.gradle.kts
java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
        vendor.set(JvmVendorSpec.matching("customString"))
    }
}

Selecting toolchains by their virtual machine implementation

If your project requires a specific implementation, you can filter based on the implementation as well. Currently available implementations to choose from are:

VENDOR_SPECIFIC

Acts as a placeholder and matches any implementation from any vendor (e.g. hotspot, zulu, …​)

J9

Matches only virtual machine implementations using the OpenJ9/IBM J9 runtime engine.

For example, to use an IBM Semeru JVM, distributed via AdoptOpenJDK, you can specify the filter as shown in the example below.

build.gradle
java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(11)
        vendor = JvmVendorSpec.IBM_SEMERU
        implementation = JvmImplementation.J9
    }
}
build.gradle.kts
java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(11))
        vendor.set(JvmVendorSpec.IBM_SEMERU)
        implementation.set(JvmImplementation.J9)
    }
}

The Java major version, the vendor (if specified) and implementation (if specified) will be tracked as an input for compilation and test execution.

Specify custom toolchains for individual tasks

In case you want to tweak which toolchain is used for a specific task, you can specify the exact tool a task is using. For example, the Test task exposes a JavaLauncher property that defines which java executable to use for launching the tests.

In the example below, we configure all java compilation tasks to use JDK8. Additionally, we introduce a new Test task that is going to run our unit tests but using a JDK 14.

list/build.gradle
tasks.withType(JavaCompile).configureEach {
    javaCompiler = javaToolchains.compilerFor {
        languageVersion = JavaLanguageVersion.of(8)
    }
}
task('testsOn14', type: Test) {
    javaLauncher = javaToolchains.launcherFor {
        languageVersion = JavaLanguageVersion.of(14)
    }
}
list/build.gradle.kts
tasks.withType<JavaCompile>().configureEach {
    javaCompiler.set(javaToolchains.compilerFor {
        languageVersion.set(JavaLanguageVersion.of(8))
    })
}

tasks.register<Test>("testsOn14") {
    javaLauncher.set(javaToolchains.launcherFor {
        languageVersion.set(JavaLanguageVersion.of(14))
    })
}

In addition, in the application subproject, we add another Java execution task to run our application with JDK 14.

application/build.gradle
task('runOn14', type: JavaExec) {
    javaLauncher = javaToolchains.launcherFor {
        languageVersion = JavaLanguageVersion.of(14)
    }

    classpath = sourceSets.main.runtimeClasspath
    mainClass = application.mainClass
}
application/build.gradle.kts
tasks.register<JavaExec>("runOn14") {
    javaLauncher.set(javaToolchains.launcherFor {
        languageVersion.set(JavaLanguageVersion.of(14))
    })

    classpath = sourceSets["main"].runtimeClasspath
    mainClass.set(application.mainClass)
}

Depending on the task, a JRE might be enough while for other tasks (e.g. compilation), a JDK is required. By default, Gradle prefers installed JDKs over JREs if they can satisfy the requirements.

Toolchains tool providers can be obtained from the javaToolchains extension.

Three tools are available:

  • A JavaCompiler which is the tool used by the JavaCompile task

  • A JavaLauncher which is the tool used by the JavaExec or Test tasks

  • A JavadocTool which is the tool used by the Javadoc task

Integration with tasks relying on a Java executable or Java home

Any tasks that can be configured with a path to a Java executable, or a Java home location, can benefit from toolchains.

While you will not be able to wire a toolchain tool directly, they all have metadata that gives access to their full path or to the path of the Java installation they belong to.

For example, you can configure the executable for a Kotlin compile tasks as follows:

build.gradle
def compiler = javaToolchains.compilerFor {
    languageVersion = JavaLanguageVersion.of(11)
}

tasks.withType(KotlinJvmCompile).configureEach {
    kotlinOptions.jdkHome = compiler.get().metadata.installationPath.asFile.absolutePath
}
build.gradle.kts
val compiler = javaToolchains.compilerFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.withType<KotlinJvmCompile>().configureEach {
    kotlinOptions.jdkHome = compiler.get().metadata.installationPath.asFile.absolutePath
}

Similarly, doing compiler.get().executablePath would give you the full path to javac for the given toolchain. Please note however that this may realize (and provision) a toolchain eagerly.

Auto detection of installed toolchains

By default, Gradle automatically detects local JRE/JDK installations so no further configuration is required by the user. The following is a list of common package managers, tools, and locations that are supported by the JVM auto-detection.

JVM auto-detection knows how to work with:

Among the set of all detected JRE/JDK installations, one will be picked according to the Toolchain Precedence Rules.

How to disable auto-detection

In order to disable auto-detection, you can use the org.gradle.java.installations.auto-detect Gradle property:

  • Either start gradle using -Porg.gradle.java.installations.auto-detect=false

  • Or put org.gradle.java.installations.auto-detect=false into your gradle.properties file.

Auto Provisioning

If Gradle can’t find a locally available toolchain which matches the requirements of the build, it will automatically try to download it from Adoptium or AdoptOpenJDK. By default, it will request a HotSpot JDK matching the current operating system and architecture. Provisioned JDKs are installed in the Gradle User Home directory.

Gradle will only download JDK versions for GA releases. There is no support for downloading early access versions.

Once a provisioned JDK has been installed in the Gradle User Home directory it becomes one of the JDKs visible to auto detection and can be used by any subsequent builds, just like any other JDK installed on the system. Since auto-provisioning only kicks in when auto-detection fails to find a matching JDK, auto-provisioning can only download new JDKs and is in no way involved in updating any of the already installed ones. None of the auto-provisioned JDKs will ever be revisited and automatically updated by auto-provisioning, even if there is a newer minor version available for them.

By default, the public Adoptium and AdoptOpenJDK APIs are used to determine and download a matching JDK. Gradle will try Adoptium first, and if it fails to find a matching JDK, it will try AdoptOpenJDK.

Due to changes in AdoptOpenJDK and the migration to Eclipse Adoptium, the endpoint now serves JDKs from Eclipse Adoptium or IBM Semeru and no longer builds from AdoptOpenJDK. Using JvmVendorSpec.ADOPTOPENJDK and having it resolved through auto provisioning will result in a deprecation warning.

In case you want to use another server that is compatible with v3 of the AdoptOpenJDK API, you can point Gradle to use a different host. For that you use the Gradle property as in the example below:

org.gradle.jvm.toolchain.install.adoptopenjdk.baseUri=https://api.company.net/

You can also override the Adoptium API host with org.gradle.jvm.toolchain.install.adoptium.baseUri.

Only secure protocols like https are accepted. This is required to make sure no one can tamper with the download in flight.

Viewing and debugging toolchains

Gradle can display the list of all detected toolchains including their metadata.

For example, to show all toolchains of a project, run:

gradle -q javaToolchains
Output of gradle -q javaToolchains
> gradle -q javaToolchains

 + Options
     | Auto-detection:     Enabled
     | Auto-download:      Enabled

 + AdoptOpenJDK 1.8.0_242
     | Location:           /Users/username/myJavaInstalls/8.0.242.hs-adpt/jre
     | Language Version:   8
     | Vendor:             AdoptOpenJDK
     | Architecture:       x86_64
     | Is JDK:             false
     | Detected by:        system property 'org.gradle.java.installations.paths'

 + Microsoft JDK 16.0.2+7
     | Location:           /Users/username/.sdkman/candidates/java/16.0.2.7.1-ms
     | Language Version:   16
     | Vendor:             Microsoft
     | Architecture:       aarch64
     | Is JDK:             true
     | Detected by:        SDKMAN!

 + OpenJDK 15-ea
     | Location:           /Users/user/customJdks/15.ea.21-open
     | Language Version:   15
     | Vendor:             AdoptOpenJDK
     | Architecture:       x86_64
     | Is JDK:             true
     | Detected by:        environment variable 'JDK16'

 + Oracle JDK 1.7.0_80
     | Location:           /Library/Java/JavaVirtualMachines/jdk1.7.0_80.jdk/Contents/Home/jre
     | Language Version:   7
     | Vendor:             Oracle
     | Architecture:       x86_64
     | Is JDK:             false
     | Detected by:        macOS java_home

This can help to debug which toolchains are available to the build, how they are detected and what kind of metadata Gradle knows about those toolchains.

How to disable auto provisioning

In order to disable auto-provisioning, you can use the org.gradle.java.installations.auto-download Gradle property:

  • Either start gradle using -Porg.gradle.java.installations.auto-download=false

  • Or put org.gradle.java.installations.auto-download=false into a gradle.properties file.

Custom Toolchain locations

If auto-detecting local toolchains is not sufficient or disabled, there are additional ways you can let Gradle know about installed toolchains.

If your setup already provides environment variables pointing to installed JVMs, you can also let Gradle know about which environment variables to take into account. Assuming the environment variables JDK8 and JRE14 point to valid java installations, the following instructs Gradle to resolve those environment variables and consider those installations when looking for a matching toolchain.

org.gradle.java.installations.fromEnv=JDK8,JRE14

Additionally, you can provide a comma-separated list of paths to specific installations using the org.gradle.java.installations.paths property. For example, using the following in your gradle.properties will let Gradle know which directories to look at when detecting JVMs. Gradle will treat these directories as possible installations but will not descend into any nested directories.

org.gradle.java.installations.paths=/custom/path/jdk1.8,/shared/jre11

Custom toolchains are not prioritized over auto-detected ones. If auto-detection is turned on, custom toolchains can still be specified but all they will do is to extend the set of detected JRE/JDK locations. Among these locations, one will be picked according to the Toolchain Precedence Rules.

Precedence

Gradle will sort all the JDK/JRE installations matching the toolchain specification of the build and will pick the first one. Sorting is done based on the following rules:

  1. the installation currently running Gradle is preferred over any other

  2. JDK installations are preferred over JRE ones

  3. certain vendors take precedence over others; their ordering (from the highest priority to lowest):

    1. ADOPTIUM

    2. ADOPTOPENJDK

    3. AMAZON

    4. APPLE

    5. AZUL

    6. BELLSOFT

    7. GRAAL_VM

    8. HEWLETT_PACKARD

    9. IBM

    10. IBM_SEMERU

    11. MICROSOFT

    12. ORACLE

    13. SAP

    14. everything else

  4. higher major versions take precedence over lower ones

  5. higher minor versions take precedence over lower ones

  6. installation paths take precedence according to their lexicographic ordering (last resort criteria for deterministically deciding between installations of the same type, from the same vendor and with the same version)

All these rules are applied as multilevel sorting criteria, in the order they are shown. Let’s illustrate with an example. Let’s say that the only criteria we have added to our toolchain specification is that we want Java version 17 and that on our local machine following matching installations have been detected: Oracle JRE v17.0.0, Oracle JDK v17.0.1, Microsoft JDK 17.0.0, Microsoft JRE 17.0.1, Microsoft JDK 17.0.1. Let’s also assume that Gradle is being run by an installation with a different major version, so it’s not a match of the toolchain specification (the point being that otherwise that installation would have priority over every other one and our example would be less informative).

When we apply the above rules to sort this set we will end up with following ordering:

  1. Microsoft JDK 17.0.1

  2. Microsoft JDK 17.0.0

  3. Oracle JDK v17.0.0

  4. Microsoft JRE v17.0.1

  5. Oracle JRE v17.0.1

The JREs end up last after applying the rule of preferring JDKs over JREs. Then in both categories the Microsoft installations end up first after applying the rule of preferring that vendor over Oracle. Then the two Microsoft JDKs get ordered further by the rule saying that we prefer higher version numbers over lower ones. Applying any more rules won’t change the final order.

The installation that will be picked is the first one, the Microsoft JDK 17.0.1.

Toolchains for plugin authors

Custom tasks that require a tool from the JDK should expose a Property<T> with the desired tool as generic type. The property should be declared as a @Nested input. By injecting the JavaToolchainService in the plugin or task, it is also possible to wire a convention in those properties by obtaining the JavaToolchainSpec from the java extension on the project. The example below showcases how to use the default toolchain as convention while allowing users to individually configure the toolchain per task.

build.gradle
import javax.inject.Inject;

abstract class CustomTaskUsingToolchains extends DefaultTask {

    @Nested
    abstract Property<JavaLauncher> getLauncher()

    @Inject
    CustomTaskUsingToolchains() {
        // Access the default toolchain
        def toolchain = project.getExtensions().getByType(JavaPluginExtension.class).toolchain

        // acquire a provider that returns the launcher for the toolchain
        JavaToolchainService service = project.getExtensions().getByType(JavaToolchainService.class)
        Provider<JavaLauncher> defaultLauncher = service.launcherFor(toolchain);

        // use it as our default for the property
        launcher.convention(defaultLauncher);
    }

    @TaskAction
    def showConfiguredToolchain() {
        println launcher.get().executablePath
        println launcher.get().metadata.installationPath
    }
}

plugins {
    id 'java'
}

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(8)
    }
}

tasks.register('showDefaultToolchain', CustomTaskUsingToolchains)

tasks.register('showCustomToolchain', CustomTaskUsingToolchains) {
    launcher = javaToolchains.launcherFor {
        languageVersion = JavaLanguageVersion.of(16)
    }
}
build.gradle.kts
import javax.inject.Inject;

abstract class CustomTaskUsingToolchains : DefaultTask {

    @get:Nested
    abstract val launcher: Property<JavaLauncher>

    @Inject
    constructor() {
        // Access the default toolchain
        val toolchain = project.extensions.getByType<JavaPluginExtension>().toolchain

        // acquire a provider that returns the launcher for the toolchain
        val service = project.extensions.getByType<JavaToolchainService>()
        val defaultLauncher = service.launcherFor(toolchain)

        // use it as our default for the property
        launcher.convention(defaultLauncher);
    }

    @TaskAction
    fun showConfiguredToolchain() {
        println(launcher.get().executablePath)
        println(launcher.get().metadata.installationPath)
    }
}

plugins {
    java
}

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(8))
    }
}

tasks.register<CustomTaskUsingToolchains>("showDefaultToolchain")

tasks.register<CustomTaskUsingToolchains>("showCustomToolchain") {
    launcher.set(javaToolchains.launcherFor {
        languageVersion.set(JavaLanguageVersion.of(16))
    })
}

With the property correctly configured as @Nested, it will automatically track the Java major version, the vendor (if specified) and implementation (if specified) as an input.