I recently released the first version of a mostly-complete Java implementation of the XAPI server. The source code is available on my GitHub with a readme describing generally how to get it set up.
Running this database with a full-planet requires some large, fast disks. The database is roughly 350GB and is slowly increasing:
CPU and memory usage are useful for the initial import (indexing especially), but any subsequent queries will be limited by the disk speed.
Feel free to use it! Serge W. created an XAPI UI framework that I set up at http://openstreetmap.us/uixapi/xapi.html to build the XAPI query URLs.
At the moment, the Java XAPI seems to have a problem returning relations and doesn't implement child element predicates or special tags. If you find other problems, please file a bug.