ClassReader cr2 = new ClassReader(fis);
ClassNode cn = new ClassNode();
cr2.accept(cn, flags);
//Now content of ClassNode can be modified and then serialized back into bytecode:
new TryCatchBlockHandler().instrument(cn);
ClassWriter cw2 = new ClassWriter(0);
cn.accept(cw2);
bytes = cw2.toByteArray();