What are Toolchains?

Maven Toolchains provide a way for a project to specify the JDK (or other tools) used to build the project, without needing to configure this in each plugin or in every pom.xml.

When Maven Toolchains are used to specify the JDK, a project can be built by a specific version of the JDK independent of the one Maven is running with. This is similar to how JDK versions can be set in IDEs like IDEA, NetBeans and Eclipse.

Using Toolchains in Your Project

There are two essential components that you need to configure in order to use toolchains:

1. the maven-toolchains-plugin in your project POM,
2. the toolchains.xml file on the building machine.

The maven-toolchains-plugin is the one that sets the toolchain to be used by the toolchain-aware plugins in your project.

For example, you want to use a different JDK version to build your project than the version used to run Maven, you can configure the version you want to use via this plugin as shown in the pom.xml below:

<plugins>
 ...
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.1</version>
  </plugin>
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-toolchains-plugin</artifactId>
    <version>1.1</version>
    <executions>
      <execution>
        <goals>
          <goal>toolchain</goal>
        </goals>
      </execution>
    </executions>
    <configuration>
      <toolchains>
        <jdk>
          <version>1.8</version>
          <vendor>openjdk</vendor>
        </jdk>
      </toolchains>
    </configuration>
  </plugin>
  ...
</plugins>

As you can see in the example above, a JDK toolchain with <version> "1.8" and <vendor> "openjdk" is to be used. Now how does the plugin know where this JDK is installed? This is where the toolchains.xml file comes in.

The toolchains.xml file (see below) is the configuration file where you set the installation paths of your toolchains. This file should be put in the ${user.home}/.m2 directory. When the maven-toolchains-plugin executes, it looks for the toolchains.xml file, reads it and looks for a toolchain matching the toolchains requirements configured in the plugin. In this example, that is a JDK toolchain with <version> "1.8" and <vendor> "openjdk". Once a match is found, the plugin then stores the toolchain to be used in the MavenSession. As you can see in the toolchains.xml below, there is indeed a JDK toolchain with <version> "1.8" and <vendor> "openjdk" configured. So when the maven-compiler-plugin configured in the pom.xml above executes, it sees that a JDK toolchain is set in the MavenSession and will use that toolchain (that would be the JDK installed at /path/to/jdk/1.8 in this example) to compile the sources.

Starting with Maven 3.3.1 you can put the toolchains.xml file wherever you like by using the --global-toolchains file option, but it is recommended to locate it into ${user.home}/.m2/.

<?xml version="1.0" encoding="UTF-8"?>
<toolchains>
  <!-- JDK toolchains -->
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>1.8</version>
      <vendor>openjdk</vendor>
    </provides>
    <configuration>
      <jdkHome>/path/to/jdk/1.8</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <version>17</version>
      <vendor>azul</vendor>
    </provides>
    <configuration>
      <jdkHome>/path/to/jdk/17</jdkHome>
    </configuration>
  </toolchain>

  <!-- other toolchains -->
  <toolchain>
    <type>netbeans</type>
    <provides>
      <version>5.5</version>
    </provides>
    <configuration>
      <installDir>/path/to/netbeans/5.5</installDir>
    </configuration>
  </toolchain>
</toolchains>

You can configure as many toolchains as you want in your toolchains.xml file.