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 By using a newer version of the plexus-compiler-eclipse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 If not we must also bundle a newer
version of the eclipse compiler itself.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.
The limitation is in javac. The solution is to tell maven to use another compiler. See question for details.
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>
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
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?
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>
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.
来源：https://stackoverflow.com/questions/8970920/maven-javac-source-release-1-6-requires-target-release-1-6