Compile Maven Module with Different Java Version

Question

My maven project has a few modules: server, web, etc.

I would like to build all but my server module on Java 6. For the server module, I'd like to compile it with Java 7.

Here's my pom.xml below, but I think that if I modify it to 1.7, then all of my modules will be compiled with Java 7. Also, does maven use the JAVA_HOME environment variable to determine which Java version to use?

    <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
                        <version>2.3.2</version>
        <configuration>
            <source>1.6</source>
            <target>1.6</target>
            <memmax>2048m</memmax>
        </configuration>
    </plugin>

EDIT Also, does the below output of

maven --version

indicate that maven is compiling my java code with 1.7?

vagrant@dev:~/bin/apache-tomcat-7.0.29/bin$ mvn --version
Apache Maven 3.0.4 (r1232337; 2012-01-17 08:44:56+0000)
Maven home: /home/vagrant/bin/apache-maven-3.0.4
Java version: 1.7.0_07, vendor: Oracle Corporation
Java home: /home/vagrant/bin/jdk1.7.0_07/jre
Default locale: en_US, platform encoding: UTF-8
OS name: "linux", version: "3.2.0-23-generic", arch: "amd64", family: "unix"

Thanks, Kevin

score 38 · Accepted Answer · edited Jun 07 '16 at 18:50

There are a number of hacks out there for compiling source code with a different version of the JDK than you are using to run Maven, for example you can use something like

<project>
  [...]
  <build>
    [...]
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>2.5.1</version>
        <configuration>
          <executable><!-- path-to-javac --></executable>
        </configuration>
      </plugin>
    </plugins>
    [...]
  </build>
  [...] 
</project>

The issue with this approach is that you now have hard-coded the path to the JDK into your POM. Everything will work just fine on your machine but when you have to rebuild your machine because the HDD failed, or when you want to build on a different machine, you will be stuck as the path will most likely not match up.

The correct best practice way to handle this is via Toolchains. This will see you creating a ~/.m2/toolchains.xml file that describes where each of the different toolchains in your system are. Then the version of the JDK can be applied by the Maven Toolchains Plugin, e.g.

<plugins>
 ...
  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-toolchains-plugin</artifactId>
    <version>1.0</version>
    <executions> 
      <execution>
        <phase>validate</phase>
        <goals>
          <goal>toolchain</goal>
        </goals>
      </execution>
    </executions>
    <configuration>
      <toolchains>
        <jdk>
          <version>1.6</version>
        </jdk>
      </toolchains>
    </configuration>
  </plugin>
  ...
</plugins>

The next thing is that you don't need this as often as you would think. For example by using the source and target values you can generate the correct bytecode for the JRE that you are targeting... the only issue that you will then hit is the use of methods that are new in JRE 1.7... which is where Mojo's Animal Sniffer Plugin comes in. Animal Sniffer can be used to ensure that you only use the methods of the JRE that you are targeting. The general community consensus is that the use of source and target configuration options in the Maven Compiler Plugin configuration coupled with the use of Mojo's Animal Sniffer virtually eliminates the need for toolchains on the Compiler end of things.... on the Surefire end of things there is still need for toolchains... and I have a few edge cases that I need to update the compiler plugin and the toolchains plugins for to handle but, realistically you will not hit those edge cases ;-)

Just to be sure that your original question is completely answered (since the above answers the question you wanted to ask - as opposed to the one you asked)

At present you are compiling with JDK 1.7 however depending on the version of the Maven Compiler Plugin you are using, you may be compiling with either <source>1.4</source><target>1.4</target> or <source>1.5</source><target>1.5</target> unless you have changed the configuration of the Maven Compiler Plugin in your pom.xml. That will dictate which language features are available to you, but not which classes... so you would be generating code that will work on JRE 1.7 and provided you have not used any new classes/methods introduced since 1.4/1.5 (Such as String.isEmpty()) should also work on JRE 1.4/1.5... the only way to be sure if it works on such an old JVM is to either: run it on the old JVM OR use Animal Sniffer.

Just like to add a little short cut to adding maven-compiler-plugin to your pom and adding the and configuration options, is to set some properties that the compiler plugin checks ' 1.7${java.version} ${java.version}' — dan carter, Jul 16 '13 at 21:11
Would it make sense to edit this top answer to clarify that the new best practice for correct cross-compilation is using javac `--release` flag (see https://stackoverflow.com/a/43103038/)? It is way easier in terms of configuration, faster (no need to fork the process for each module), works for Java 7+. Toolchains may still be useful, e.g., if one needs to also _run tests_ on the target JVM or compile for ancient Java (6 or earlier). — Dmitry Timofeev, Feb 24 '20 at 10:38

Vlad Mihalcea · Answer 2 · 2021-01-08T16:10:29.213

Maven Toolchains

To use multiple Java versions, you need to use Maven Toolchains, which require you to create a toolchains.xml file in your ~/.m2 Maven folder, containing all Java versions installed on your machine:

<toolchains>
  <toolchain>
    <type>jdk</type>
    <provides>
      <id>Java13</id>
      <version>13</version>
    </provides>
    <configuration>
      <jdkHome>${env.JAVA_HOME_13}</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <id>Java9</id>
      <version>9</version>
    </provides>
    <configuration>
      <jdkHome>${env.JAVA_HOME_9}</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <id>Java8</id>
      <version>8</version>
    </provides>
    <configuration>
      <jdkHome>${env.JAVA_HOME_8}</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <id>Java7</id>
      <version>7</version>
    </provides>
    <configuration>
      <jdkHome>${env.JAVA_HOME_7}</jdkHome>
    </configuration>
  </toolchain>
  <toolchain>
    <type>jdk</type>
    <provides>
      <id>Java6</id>
      <version>6</version>
    </provides>
    <configuration>
      <jdkHome>${env.JAVA_HOME_6}</jdkHome>
    </configuration>
  </toolchain>
</toolchains>

The JAVA_HOME_13, JAVA_HOME_9, JAVA_HOME_8, JAVA_HOME_7, JAVA_HOME_6 environment variables are configured so that they reference the path where the associated Java version is installed.

The FlexyPool parent pom.xml configuration file

The parent pom.xml Maven configuration file of the FlexyPool project defines the global Java version settings

<properties>
    <jdk.version>8</jdk.version>
    ...
</properties>

Now, we need to instruct both the compiler and the test plugins to use the configured java version.

<build>
    <plugins>
        <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>${jdk.version}</version>
                    </jdk>
                </toolchains>
            </configuration>
        </plugin>
        
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>${maven-compiler-plugin.version}</version>
            <configuration>
                <source>${jdk.version}</source>
                <target>${jdk.version}</target>
                <showDeprecation>true</showDeprecation>
                <showWarnings>true</showWarnings>
            </configuration>
        </plugin>
        
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-surefire-plugin</artifactId>
            <version>${maven-surefire-plugin.version}</version>
        </plugin>
    </plugins>
</build>

The FlexyPool child Maven module pom.xml using a different Java version

The flexy-pool-core-java9 child Maven module that requires a different Java version only needs to override the default jdk.version Maven property:

<properties>
    <jdk.version>9</jdk.version>
</properties>

And that's it, we can now build each module using its own minimum viable Java version.

env variables are [going away soon](http://grokbase.com/t/maven/users/077ymp66n4/passing-system-variables-to-maven), but are still supported in `$M2_HOME/conf/settings.xml` ([source](https://maven.apache.org/settings.html)) — TheLonelyGhost, Sep 30 '16 at 15:42

score 3 · Answer 3 · answered Sep 13 '12 at 21:14

3

use the setup for the JDK6 on your top pom, it will be inherited by all the module, and overwrite it for your server pom with the different configuration required.

As for the path of the JDK, you can specify it, see here: http://maven.apache.org/plugins/maven-compiler-plugin/examples/compile-using-different-jdk.html

answered Sep 13 '12 at 21:14

Farid

2,265
18
14

Based on the maven --version output that I included in my original post, does that mean that mvn is compiling with 1.7? – Kevin Meredith Sep 13 '12 at 21:21
based on your output, you will be compiling your application with a jdk 1.7, but your code will be compiled for a jdk 1.6 (or above), which is fine with your need. – Farid Sep 14 '12 at 08:09
Please explain what the significance of the "mvn --version" output versus the and values in my pom.xml. If I want to compile exclusively with 1.7, does that mean I need to change the and to 1.7? – Kevin Meredith Sep 14 '12 at 13:52
mvn --version just let you see the current environment being used. Regarding the source and target argument, those will specify how the same parameters will be set with the javac tool: see the description of those parameters in the following link: [http://docs.oracle.com/javase/6/docs/technotes/tools/windows/javac.html](http://docs.oracle.com/javase/6/docs/technotes/tools/windows/javac.html) – Farid Sep 14 '12 at 14:29
so in your context you are telling the JDK 1.7 to first make sure the source code contains only features available with a 1.6 VM and/or lower, and then you are telling it to generate binaries compatible with a JVM 1.6 or above. – Farid Sep 14 '12 at 14:34

Compile Maven Module with Different Java Version

3 Answers3

Maven Toolchains

The FlexyPool parent pom.xml configuration file

The FlexyPool child Maven module pom.xml using a different Java version

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