java - Maven: javac: source release 1.6 requires target release 1.6

Question

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 By using a newer version of the plexus-compiler-eclipse 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 If not we must also bundle a newer 
                version of the eclipse compiler itself.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.

Answer 1

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