import org.apache.bcel.*; import org.apache.bcel.classfile.*; import org.apache.bcel.generic.*; public class HelloWorldBCEL { public static void main(String[] args) throws Exception { JavaClass helloWorldClass = Repository.lookupClass("HelloWorld"); ClassGen classGen = new ClassGen(helloWorldClass); ConstantPoolGen constantPoolGen = classGen.getConstantPool(); Method[] methods = helloWorldClass.getMethods(); for (Method method : methods) { if (method.getName().equals("sayHello")) { MethodGen methodGen = new MethodGen(method, helloWorldClass.getClassName(), constantPoolGen); InstructionList instructionList = methodGen.getInstructionList(); InstructionFactory instructionFactory = new InstructionFactory(classGen, constantPoolGen); instructionList.insert(instructionFactory.createFieldAccess("java.lang.System", "out", new ObjectType("java.io.PrintStream"), Constants.GETSTATIC)); instructionList.insert(new PUSH(constantPoolGen, "Hello, BCEL!")); instructionList.insert(instructionFactory.createInvoke("java.io.PrintStream", "println", Type.VOID, new Type[] { Type.STRING }, Constants.INVOKEVIRTUAL)); methodGen.setMaxStack(); methodGen.setMaxLocals(); methodGen.update(); } } classGen.getJavaClass().dump("HelloWorld.class"); } }