Maven: javac: source release 1.6 requires target r

问题:

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 By using a newer version of the plexus-compiler-eclipse 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 If not we must also bundle a newer 
                version of the eclipse compiler itself.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

回答1:

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

回答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>

回答3:

I believe you need to set -bootclasspath as well so that javac compiles against JDK 1.5 bootstrap classes.

Try:

javac -source 1.6 -target 1.5 -bootclasspath /path/to/jdk1.5/lib/rt.jar -extdirs "" Foo.java

UPDATE:

Try removing the -source option, but keep the -target option.

I just tested it out:

# no source, only target => COMPILES to 1.5
$ javac -target 1.5 Foo.java
$ javap -v  Foo | grep version
  minor version: 0
  major version: 49

# no source, no target => COMPILES to 1.6
$ javac Foo.java
$ javap -v  Foo | grep version
  minor version: 0
  major version: 50

# both source and target => ERROR
$ javac -source 1.6 -target 1.5 Foo.java
javac: source release 1.6 requires target release 1.6

$ javac -version
javac 1.6.0_21

回答4:

What Java 6 language features are you using that are not present in Java 5? As far as I can tell, the only "feature" that's been added to the language is the use of the @Override annotation in interfaces. Otherwise, Java 6 and Java 5 are source-compatible. What happens when you use:

<source>1.5</source>
<target>1.5</target>

in your Maven build file?

回答5:

I had the same error when I upgraded my IntelliJ IDE, it was fixed with the replacement of 1.5 with 1.6 as below.

       <plugin>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>1.6</source>
                <target>1.6</target>
            </configuration>
        </plugin>

回答6:

The real question is thus to use @Override in source files that need to be compiled in Java 1.5 class files.

javac -source 1.5 -target 1.5 aFileWithOverride.java

will do just that. In jdk 7, this will lead to a warning

[options] bootstrap class path not set in conjunction with -source 1.5

which is absent in jdk 6.

To get rid of this warning a special boot class jar can be created by adding the java 6 (or higher) java.lang.annotation package to a java 5 rt.jar.