Well, to date, there are two widely used Java-based MU* engines projects that are currently underway: CoffeeMUD and JavaMUD.
CoffeeMUD :
JavaMUD :
Do note: Java-based game engines take ALOT of ram and CPU time (CoffeeMUD is heard to average over 300MB+ of VRam just for running). the JavaMUD project doesn't use as much ram, however the CPU load can be spiked well over 80% in some cases, thus it would be best to run either of these applications on a its own co-located server.
-- M
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