I've compiled Mozilla 1.2.1 with Java support. Installed the Blackdown JRE and the SUN JRE. Using either Java profile, the Java plugin fails to initialize. The console output is as follows (using Blackdown): LoadPlugin: failed to initialize shared library /opt/blackdown-jre-1.3.1/plugin/i386/mozilla/javaplugin_oji.so [/opt/blackdown-jre-1.3.1/plugin/i386/mozilla/javaplugin_oji.so: undefined symbol: __vt_17nsGetServiceByCID] I have the C++ compat. libs installed. The Macromedia binary flash plugin and the Acrobat Reader plugin both work.
With a gcc3 compiled mozilla, you need a gcc3 java for this to work. There are numerous bugs about this already. It will be fixed when sun/whoever ships a gcc3 compiled java, else you could compiled it yourself.