import javassist.*; public class HelloWorldModifier { public static void main(String[] args) throws Exception { ClassPool cp = ClassPool.getDefault(); CtClass cc = cp.get("com.example.HelloWorld"); CtMethod newMethod = CtNewMethod.make("public static void newMethod() { System.out.println(\"Hello, Javassist!\"); }", cc); cc.addMethod(newMethod); CtMethod oldMethod = cc.getDeclaredMethod("sayHello"); oldMethod.insertBefore("{ System.out.println(\"Hello, Javassist!\"); }"); cc.writeFile(); } }