What is the best open source forum java-based software thats free and reliable?

I am working for a public school district and we are looking for free, reliable open source forum software thats java-based. If you guys know any that would considered clean and effective but still maintain professionalism.What is the best open source forum java-based software thats free and reliable?Sun Solaris is now open source.What is the best open source forum java-based software thats free and reliable?there are several but mvnForum seems to get good reports