Jakarta BCEL
The Byte Code Engineering Library (BCEL) is a Java library enabling creation, analysis, and modification of Java .class files. According to its website, it “is already being used successfully in several projects such as compilers, optimizers, obsfuscators, code generators and analysis tools.” BCEL also includes a bytecode verifier that is designed to give more useful feedback about unverifiable bytecode than would be output by the JVM verifier.
Thanks to Mulhern for the suggestion.