The TechTrader Bytecode Toolkit is a high-level set of APIs for manipulating java bytecode. It is primarily targeted at making runtime and/or compile-time enhancements to existing java classes, or creating new java classes on the fly.
The Universal COBOL Compiler (UCC) is a Java-based COBOL to Java bytecode cross-compiler. It's made up of a COBOL compiler, a Java-based COBOL runtime, and a package for manipulating Java class files.