A correct which java is not evidence enough, since /usr/bin/ will likely be in your PATH anyway. Check
$ echo $JAVA_HOME
for evidence. Or run
$ JAVA_HOME=/path/to/your/java/home mvn clean javadoc:jar package
On OS X you can set your JAVA_HOME via:
$ export JAVA_HOME=$(/usr/libexec/java_home)
which on my machine points to
/System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home