The new version of jarc now supports cross-compilation, i.e. the possibility to build jar files that works on a version of the JRE lower than the the version of the JRE used to run the jarc program. The jarc program itself will always run under the most recent JRE and by default will build a jar file targeted to the same version. But adding an X-Jarc-Target option will request jarc to build for a lower version. For example this manifest file means that the resulting jar file will work with a 1.5 JRE:
Manifest-Version: 1.0
X-Jarc-Target: 1.5
Without this line, the jar file would have been created for the 1.6 JRE so executing it on a 1.5 JRE would fail.
It is also possible to request a specific JRE vendor:
Manifest-Version: 1.0
X-Jarc-Target: 1.6_openjdk
Note that there is no need to install the JDK for all versions of the JRE. Only the JDK for the latest version (i.e. the one that is used to run jarc) is needed. But at least one installed JRE is needed for each potential X-Jarc-Target value, because the rt.jar file is needed for the cross-compilation.
It is even possible to configure jarc to use the JDK 1.7 by modifying the configuration in /etc/jarc.conf (see comments at the end of the file). The jarc package itself is built by jarc running on 1.7 but targeted to 1.6.
The current version of jarc (0.2.2) can build for the JDKs 1.6 and 1.5 and supports sun and openjdk JVMs. A future version will also support the JVMs generated by java-package and perhaps the gcj JVM.