As of this writing, the open source Apache Harmony JDK (http://harmony.apache.org) runs Tomcat 6.0. During light testing, we encountered no problems with Tomcat itself, however, some webapps had some trouble. It is possible that your webapp may run successfully. Unless you try your webapp running on Apache Harmony, you won't know if it works, nor will you know if it performs better or worse.

This may become mostly a nonissue for most people once Sun's JDK is available as pure open source. But, if you need to redistribute the JDK with your Tomcat and webapp(s), and if your company needs or wants to make any modifications to the JDK or to the Java core class libraries that you must keep closed, Harmony's more permissive license terms may be an important differentiating factor.

To try it out, download an archive of the Harmony JDK from http://harmony.apache.org/downloads.html. Here's how it looked when we installed Harmony on Linux:

# cd /opt
# tar zxvf ˜/harmony-jdk-linux-x86_64.tar.gz

Then, set JAVA_HOME and put Harmony's bin directory on the front of your shell's PATH:

# JAVA_HOME="/opt/harmony-jdk-r533200"
# export JAVA_HOME
# PATH=$JAVA_HOME/bin:$PATH
# export PATH
# which java
/opt/harmony-jdk-r533200/bin/java
# java -version
Apache Harmony Launcher : (c) Copyright 1991, 2006 The Apache Software Foundation or i
ts licensors, as applicable.
java version "1.5.0"
pre-alpha : not complete or compatible
svn = r533200, (Apr 28 2007), Linux/em64t/gcc 3.3.3, release build
http://incubator.apache.org/harmony

Try running Tomcat and your webapp on Harmony, and tell the Harmony community what worked and what didn't on the Harmony developer mailing list. You can find the mailing list details at http://harmony.apache.org/mailing.html.