Debugging, Profiling, Tracing, and Visualizing Programs 2: Java Bytecode — BCEL

BCEL (formerly JavaClass) allows you to load a class, iterate through the methods and fields, change methods, add new methods and fields, etc. BCEL also makes it easy to create new classes from scratch. [2] 620 —Fall 2005 — 2 Getting started To open a class file for editing you do the following: org.apache.bcel.classfile.ClassParser p = new org.apache.bcel.classfile.ClassParser(classFile); org.apache.bcel.classfile.JavaClass jc = p.parse() org.apache.bcel.generic.ClassGen cg = new org.apache.bcel.generic.ClassGen(jc); org.apache.bcel.generic.ConstantPoolGen cp = new org.apache.bcel.generic.ConstantPoolGen( jc.getConstantPool()); [3] 620 —Fall 2005 — 2 Getting started... A org.apache.bcel.classfile.JavaClass-object represents the parsed class file. You can query this object to get any information you need about the class, for example its methods, fields, code, super-class, etc. A org.apache.bcel.generic.ClassGen-object represents a class that can be edited. You can add methods and fields, you can modify existing methods, etc. A org.apache.bcel.generic.ConstantPoolGen-object represents a constant pool to which new constants can be added. [4] 620 —Fall 2005 — 2 Finishing up When you are finished editing the class you must close and save it to the new class file: org.apache.bcel.classfile.JavaClass jc1 = cg.getJavaClass(); jc1.setConstantPool(cp.getFinalConstantPool()); jc1.dump(classFile1); [5] 620 —Fall 2005 — 2 Editing a Method org.apache.bcel.classfile.Method represents a method that has been read from a class file. org.apache.bcel.generic.MethodGen is the BCEL class that represents a method being edited: org.apache.bcel.generic.ConstantPoolGen cp =...; org.apache.bcel.generic.ClassGen cg =...; org.apache.bcel.classfile.Method method = cg.containsMethod(methodName, methodSignature); org.apache.bcel.generic.MethodGen mg = new org.apache.bcel.generic.MethodGen(method,cname,cp)