<p>Which CMS are you using in Java and what is your experience with it (in terms of extensibility, usage comfort, framework API, memory usage, performance etc.). I am looking for suggestions.</p>

<p>Specifically any one that supports a search engine(probably lucene or similar).</p>

We are using
opencms
.
I haven't tried to extend it beyond changing page templates (writing JSPs) but where usage is concerned it gets the job done, albeit the feeling you get is that you fight the system all the way through.
Memory consumption on the JVM running opencms is right now 161 Mb, the JVM running since January 2008. This is on a low traffic site serving around 6000 hits per month with an average traffic of 1800 Mb per month.