From Fedora Project Wiki

It is often useful to test Fedora Core JPackages against GCJ HEAD. java-gcjHEAD-compat is a package that provides a GCJ HEAD JVM alternative alongside the normal java-gcj-compat and SDK alternatives.

These instructions assume a ~/.rpmmacros file like this:

$ cat ~/.rpmmacros
%_topdir /home/overholt/rpmbuild

and a GCC installation in /home/overholt/install.

1. Check java-1.4.2-gcj-compat out of Fedora CVS

$ cvs -d co java-1.4.2-gcj-compat

2. Change to the devel directory

$ cd java-1.4.2-gcj-compat/devel

3. Build an SRPM

$ make srpm

4. Ensure that your built gcc is ahead of the system compiler in your path

$ export PATH=/home/overholt/install/bin:$PATH

5. Build java-1.4.2-gcjHEAD-compat RPMS

$ rpmbuild --rebuild \
--define "custom 1" \
--define "gcj_prefix /home/overholt/install" \
--define "origin gcjHEAD" \

6. Install the RPMs produced by rpmbuild

$ sudo rpm -Uvh ~/rpmbuild/RPMS/<arch>/*gcjHEAD*

7. Set the java and javac alternatives to gcjHEAD

$ sudo /usr/sbin/update-alternatives --config javac
$ sudo /usr/sbin/update-alternatives --config java

8. Verify the alternatives

$ readlink -f /usr/bin/java
$ readlink -f /usr/bin/javac

This approach is general, so it can be applied to any number of GCC installations by modifying the arguments to rpmbuild.