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 By using a newer version of the plexus-compiler-eclipse                  plugin this is hopefully less of an issue. If not we must also bundle a newer                  version of the eclipse compiler itself.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.

Answer 2

It seems if you want to do cross compilation you need to supply a couple of extra arguments -bootclasspath and -extdirs, although I believe you only need the first. For using Javac and example can be found here with an explanation of the additional options here (Cross-Compilation Options section).

You would then need to configure these options for your maven-compiler-plugin. From what I understand you need to set to plugin to fork so that it will use the compiler arguments rather than the built in compiler. You can find a listing of all the options here

 <build>         <plugins>             <plugin>                 <artifactId>maven-compiler-plugin</artifactId>                 <configuration>                     <source>1.6</source>                     <target>1.5</target>                     <fork>true</fork>                     <compilerArguments>                         <bootclasspath>${1.5.0jdk}\lib\rt.jar</bootclasspath>                    </compilerArguments>                </configuration>            </plugin>         ....        </plugins>    </build>