Working on multiple projects can require interacting with multiple versions of the Java language. Even within a single project different parts of the codebase may be fixed to a particular language level due to backward compatibility requirements. This means different versions of the same tools (a toolchain) must be installed and managed on each machine that builds the project.
A Java toolchain is a set of tools to build and run Java projects, which is usually provided by the environment via local JRE or JDK installations.
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.
By default, Gradle uses the same Java toolchain for running Gradle itself and building JVM projects. However, this may only sometimes be 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.
In order to improve reproducibility of the builds and make build requirements clearer, Gradle allows configuring toolchains on both project and task levels.
You can define what toolchain to use for a project by stating the Java language version in the java
extension block:
Executing the build (e.g. using gradle check
) will now handle several things for you and others running your build:
-
Gradle configures all compile, test and javadoc tasks to use the defined toolchain.
-
Gradle detects locally installed toolchains.
-
Gradle chooses a toolchain matching the requirements (any Java 17 toolchain for the example above).
-
If no matching toolchain is found, Gradle can automatically download a matching one based on the configured toolchain download repositories.
Note
|
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. |
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.
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.
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.
Note
|
The Java major version, the vendor (if specified) and implementation (if specified) will be tracked as an input for compilation and test execution. |
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.
Usage of invalid instances of JavaToolchainSpec
results in a build error since Gradle 8.0.
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 Java 8.
Additionally, we introduce a new Test
task that will run our unit tests using a JDK 17.
In addition, in the application
subproject, we add another Java execution task to run our application with JDK 17.
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
Any task 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 the 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:
As another example, you can configure the Java Home for a task as follows:
If you require a path to a specific tool such as Java compiler, you can obtain it as follows:
Warning
|
The examples above use tasks with Doing respectively |
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
-
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.
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 yourgradle.properties
file.
If Gradle can’t find a locally available toolchain that matches the requirements of the build, it can automatically download one (as long as a toolchain download repository has been configured; for detail, see relevant section). Gradle installs the downloaded JDKs in the Gradle User Home directory.
Note
|
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.
Toolchain download repository definitions are added to a build by applying specific settings plugins. For details on writing such plugins, consult the Toolchain Resolver Plugins page.
One example of a toolchain resolver plugin is the Disco Toolchains Plugin, based on the foojay Disco API. It even has a convention variant, which automatically takes care of all the needed configuration, just by being applied:
In general, when applying toolchain resolver plugins, the toolchain download resolvers provided by them also need to be configured. Let’s illustrate with an example. Consider two toolchain resolver plugins applied by the build:
-
One is the Foojay plugin mentioned above, which downloads toolchains via the
FoojayToolchainResolver
it provides. -
The other contains a FICTITIOUS resolver named
MadeUpResolver
.
The following example uses these toolchain resolvers in a build via the toolchainManagement
block in the settings file:
-
In the
toolchainManagement
block, thejvm
block contains configuration for Java toolchains. -
The
javaRepositories
block defines named Java toolchain repository configurations. Use theresolverClass
property to link these configurations to plugins. -
Toolchain declaration order matters. Gradle downloads from the first repository that provides a match, starting with the first repository in the list.
-
You can configure toolchain repositories with the same set of authentication and authorization options used for dependency management.
Warning
|
The |
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
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.
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 agradle.properties
file.
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 JRE17
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,JRE17
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 toolchains.
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
Note
|
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 precedence rules. |
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):
-
ADOPTIUM
-
ADOPTOPENJDK
-
AMAZON
-
APPLE
-
AZUL
-
BELLSOFT
-
GRAAL_VM
-
HEWLETT_PACKARD
-
IBM
-
MICROSOFT
-
ORACLE
-
SAP
-
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.
When creating a plugin or a task that uses toolchains, it is essential to provide sensible defaults and allow users to override them.
For JVM projects, it is usually safe to assume that the java
plugin has been applied to the project.
The java
plugin is automatically applied for the core Groovy and Scala plugins, as well as for the Kotlin plugin.
In such a case, using the toolchain defined via the java
extension as a default value for the tool property is appropriate.
This way, the users will need to configure the toolchain only once on the project level.
The example below showcases how to use the default toolchain as convention while allowing users to individually configure the toolchain per task.
-
We declare a
JavaLauncher
property on the task. The property must be marked as a@Nested
input to make sure the task is responsive to toolchain changes. -
We obtain the toolchain spec from the
java
extension to use it as a default. -
Using the
JavaToolchainService
we get a provider of theJavaLauncher
that matches the toolchain. -
Finally, we wire the launcher provider as a convention for our property.
In a project where the java
plugin was applied, we can use the task as follows:
-
The toolchain defined on the
java
extension is used by default to resolve the launcher. -
The custom task without additional configuration will use the default Java 8 toolchain.
-
The other task overrides the value of the launcher by selecting a different toolchain using
javaToolchains
service.
When a task needs access to toolchains without the java
plugin being applied the toolchain service can be used directly.
If an unconfigured toolchain spec is provided to the service, it will always return a tool provider for the toolchain that is running Gradle.
This can be achieved by passing an empty lambda when requesting a tool: javaToolchainService.launcherFor({})
.
You can find more details on defining custom tasks in the Authoring tasks documentation.