JASM is an assembler/disassembler for JVM bytecode. It provides a nice syntax for writing JVM classes in a bytecode-focused assembly language, and can also disassemble any Java .class file to JASM ...
If you want to build a music mod in Arma 3 for mission triggers or to use whilst zeusing, you may be discouraged by the painfully long time it takes to create the file hierarchy and various code files ...