Toolchains for JVM projects

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:

Setup all compile, test and javadoc tasks to use the defined toolchain which may be different than the one Gradle itself uses
Gradle detects locally installed JVMs
Gradle chooses a JRE/JDK matching the requirements of the build (in this case a JVM supporting Java 11)
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.

The following snippet uses filtering to include a subset of available toolchains. This example only includes toolchains whose java.vendor property contains the given match string. The matching is done in a case-insensitive manner.

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.

Configuring toolchain specifications

Gradle allows configuring multiple properties that affect the selection of a toolchain, such as language version or vendor. Even though these properties can be configured independently, the configuration must follow certain rules in order to form a valid specification.

A JavaToolchainSpec is considered valid in two cases:

when no properties have been set, i.e. the specification is empty;
when languageVersion has been set, optionally followed by setting any other property.

In other words, if a vendor or an implementation are specified, they must be accompanied by the language version. Gradle distinguishes between toolchain specifications that configure the language version and the ones that do not. A specification without a language version, in most cases, would be treated as a one that selects the toolchain of the current build. If the language version is not set then Gradle will ignore values provided for other properties of the specification.

Usage of invalid instances of JavaToolchainSpec is deprecated and will be removed in the future versions of Gradle.

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 java executable for a task as follows:

build.gradle

def launcher = javaToolchains.launcherFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.named('sampleTask') {
    javaExecutable = launcher.map { it.executablePath }
}

build.gradle.kts

val launcher = javaToolchains.launcherFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.sampleTask {
    javaExecutable.set(launcher.map { it.executablePath })
}

Another example, you can configure the Java Home for a task as follows:

build.gradle

def launcher = javaToolchains.launcherFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.named('anotherSampleTask') {
    javaHome = launcher.map { it.metadata.installationPath }
}

build.gradle.kts

val launcher = javaToolchains.launcherFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.anotherSampleTask {
    javaHome.set(launcher.map { it.metadata.installationPath })
}

Yet another example, you can configure the Java compiler executable for a task as follows:

build.gradle

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

tasks.named('yetAnotherSampleTask') {
    javaCompilerExecutable = compiler.map { it.executablePath }
}

build.gradle.kts

val compiler = javaToolchains.compilerFor {
    languageVersion.set(JavaLanguageVersion.of(11))
}

tasks.yetAnotherSampleTask {
    javaCompilerExecutable.set(compiler.map { it.executablePath })
}

The examples above use tasks with RegularFileProperty and DirectoryProperty properties which allow lazy configuration.

Doing respectively launcher.get().executablePath, launcher.get().metadata.installationPath or compiler.get().executablePath instead will give you the full path for the given toolchain but note 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:

Operation-system specific locations: Linux, macOS, Windows
Package Managers: Asdf-vm, Jabba, SDKMAN!
Maven Toolchain specifications
IntelliJ IDEA installations

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 that matches the requirements of the build, it can automatically download one. By default, Gradle downloads an Adoptium or AdoptOpenJDK that matches the current operating system and architecture. Gradle installs these JDKs in the Gradle User Home directory. For further details see the Toolchain Download Repositories section below.

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

Once installed in the Gradle User Home directory, a provisioned JDK 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.

Default toolchain resolvers

Gradle uses the public Adoptium and AdoptOpenJDK APIs by default to determine and download a matching JDK. Gradle first attempts to download a JDK from Adoptium. If Adoptium does not provide a matching JDK, Gradle attempts to download from 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 results 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.

Toolchain Download Repositories

Starting in Gradle 7.6, you can configure arbitrary toolchain repositories. For details on writing plugins for toolchain provisioning, consult the Toolchain Resolver Plugins page.

When you configure a toolchain repository, Gradle no longer uses the built-in default Adoptium and OpenJDK toolchain respositories.

Consider two toolchain resolver plugins:

One contains a resolver named AzulResolver, which downloads toolchains from Azul.
The other contains a resolver named AdoptiumResolver, which duplicates the built-in AdoptiumJDK toolchain repository by downloading toolchains from AdoptiumJDK.

The following example uses these toolchain resolvers in a build via the toolchainManagement block in the settings file:

settings.gradle

toolchainManagement {
    jvm { (1)
        javaRepositories {
            repository('azul') { (2)
                resolverClass = AzulResolver
                credentials {
                    username "user"
                    password "password"
                }
                authentication {
                    digest(BasicAuthentication)
                } (3)
            }
            repository('adoptium') { (4)
                resolverClass = AdoptiumResolver
            }
        }
    }
}

settings.gradle.kts

toolchainManagement {
    jvm { (1)
        javaRepositories {
            repository("azul") { (2)
                resolverClass.set(AzulResolver::class.java)
                credentials {
                    username = "user"
                    password = "password"
                }
                authentication {
                    create<DigestAuthentication>("digest")
                } (3)
            }
            repository("adoptium") { (4)
                resolverClass.set(AdoptiumResolver::class.java)
            }
        }
    }
}

1	In the `toolchainManagement` block, the `jvm` block contains configuration for Java toolchains.
2	The `javaRepositories` block defines named Java toolchain repository configurations. Use the `resolverClass` property to link these configurations to plugins.
3	You can configure toolchain repositories with the same set of authentication and authorization options used for dependency management.
4	Toolchain declaration order matters. Gradle downloads from the first repository that provides a match, starting with the first repository in the list.

The jvm block in toolchainManagement only resolves after applying a toolchain resolver plugin.

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

Gradle does not prioritize custom toolchains over auto-detected toolchains. If you enable auto-detection in your build, custom toolchains extend the set of toolchain locations. Gradle picks a toolchain 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:

the installation currently running Gradle is preferred over any other
JDK installations are preferred over JRE ones
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
higher major versions take precedence over lower ones
higher minor versions take precedence over lower ones
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 shown. Let’s illustrate with an example. A toolchain specification requests Java version 17. Gradle detects the following matching installations:

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

Assume that Gradle runs on a major Java version other than 17. Otherwise, that installation would have priority.

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

Microsoft JDK 17.0.1
Microsoft JDK 17.0.0
Oracle JDK v17.0.0
Microsoft JRE v17.0.1
Oracle JRE v17.0.1

Gradle prefers JDKs over JREs, so the JREs come last. Gradle prefers the Microsoft vendor over Oracle, so the Microsoft installations come first. Gradle prefers higher version numbers, so JDK 17.0.1 comes before JDK 17.0.0.

So Gradle picks the first match in this order: 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(17)
    }
}

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(17))
    })
}

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.