Eric Lafortune has released ProGuard 4.2, an open source Java "class file shrinker, optimizer, and obfuscator. It can detect and remove unused classes, fields, methods, and attributes. It can then optimize bytecode and remove unused instructions. Finally, it can rename the remaining classes, fields, and methods using short meaningless names. The resulting jars are smaller and harder to reverse-engineer." Version 4.2 fixes bugs and improves optimization. Proguard is published under the GPL.