AsmTools (Assembler Tools) contains two Java .class file assembler/disassembler languages:

• JAsm/JDis –  Provides a Java-like declaration of member signatures while providing Java VM Specification-compliant mnemonics for bytecode instructions.  JAsm also provides a high-level syntax for constructs often found within classfile attributes.
• JCod/JDec – Provides bytecode containers of classfile constructs.

Resources

• Respository: http://hg.openjdk.java.net/code-tools/asmtools/
• Mailing list: asmtools-dev
• AsmTools is part of the  Code Tools Project