javassist例子

Class klass = com.sun.tools.attach.VirtualMachine.class;
        String className = "com.sun.tools.attach.VirtualMachine";
        String methodName = "loadAgent";

        ClassLoader loader = klass.getClassLoader();
        ClassPool classPool = ClassPool.getDefault();
        classPool.appendClassPath(new javassist.LoaderClassPath(loader));
        CtClass ctClass = classPool.get(className);

        CtMethod method = ctClass.getDeclaredMethod(methodName);
        String name = method.getName();
        String nName = name + "$impl";
        method.setName(nName);

        CtMethod newMethod = CtNewMethod.copy(method, methodName, ctClass, null);
        StringBuilder mthBody = new StringBuilder();
        mthBody.append("{");
        mthBody.append("System.out.println(\"==========file=\"+$1);");
        mthBody.append("return " + name + "$impl($$);\n");
        mthBody.append("}");
        newMethod.setBody(mthBody.toString());// 替换新方法

        ctClass.addMethod(newMethod);
        ctClass.toClass();