Java – Maven: javac: source release 1.6 requires target release 1.6

javajavacmaven

NOTE: This appears to be a limit in the "javac" program.

I have Java 6 code that needs to be built for a Java 5 JVM. My previous work with the javac ant target (both with the JDK compiler and with ecj) led me to believe that it would simply be a matter of setting source and target for javac. Hence this pom.xml fragment:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>2.3.2</version>
    <configuration>
        <source>1.6</source>
        <target>1.5</target>
    </configuration>
</plugin>

which works as expected from within Eclipse 3.7 with Maven support. Unfortunately, running Maven directly from the command line give me

javac: source release 1.6 requires target release 1.6

which is the same as generated by javac -source 1.6 -target 1.5. To clarify, this is the official OpenJDK 6 for Ubuntu

x@JENKINS:~$ javac -version
javac 1.6.0_20
x@JENKINS:~$ javac -source 1.6 -target 1.5
javac: source release 1.6 requires target release 1.6
x@JENKINS:~$

The official Oracle Java 7 JDK for Windows show the same behavior.

Note: I do not want to build against Java 5 libraries or anything. Just that the active javac generates Java 5 compatible bytecode.

How do I get what I want while still being compatible with the Eclipse Maven plugin?

(EDIT: In addition to the @Override I also want to compile against the JAX-WS libraries in Java 6 when used, but still generated Java 5 byte code – I can then add the JAX-WS libraries deliberately in the web container when deploying to a Java 5 installation)


EDIT: It turns out that maven-compiler-plugin can be told to use another compiler, and the Eclipse compiler can do this:

        <plugin>
            <!-- Using the eclipse compiler allows for different source and target, 
                which is a good thing (outweighing that this is a rarely used combination, 
                and most people use javac) This should also allow us to run maven builds 
                on a JRE and not a JDK. -->

            <!-- Note that initial experiments with an earlier version of maven-compiler-plugin 
                showed that the eclipse compiler bundled with that gave incorrect lines in 
                the debug information. By using a newer version of the plexus-compiler-eclipse 
                plugin this is hopefully less of an issue. If not we must also bundle a newer 
                version of the eclipse compiler itself. -->

            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.0</version>
            <configuration>
                <source>1.6</source>
                <target>1.5</target>
                <debug>true</debug>
                <optimize>false</optimize>
                <fork>true</fork>
                <compilerId>eclipse</compilerId>
            </configuration>
            <dependencies>
                <dependency>
                    <groupId>org.codehaus.plexus</groupId>
                    <artifactId>plexus-compiler-eclipse</artifactId>
                    <version>2.1</version>
                </dependency>
            </dependencies>
        </plugin>

which compiles the class to Java 1.5 bytecode without complaints. This is also supported "out of the box" for m2e for Eclipse Java EE 4.2.2.

EDIT: I found that of all things the javadoc tool dislikes the output from the Eclipse compiler.

EDIT 2015-06-28: I did a quick test recently and the latest ecj (corresponding to Eclipse 4.4) worked fine with javadoc.

Best Solution

The limitation is in javac. The solution is to tell maven to use another compiler. See question for details.