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 (

Answer 1

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?

Answer 2

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>

Answer 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

Answer 4

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.

Answer 5

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>

Answer 6

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