Javassist: Add new method and invoke it for an existing class

Follow below steps to add and invoke new method.

Step 1: Get an object of ClassPool.

ClassPool classPool = ClassPool.getDefault();

Step 2: Get CtClass object for given class.

CtClass ctClass = classPool.get("com.sample.app.Test");

Step 3: Define and add new method to CtClass instance.

CtMethod newmethod = CtNewMethod.make("public void print() { System.out.println(\"Hey are you there\"); }",ctClass);

ctClass.addMethod(newmethod);

   

Step 4: Publish byte code changes.

ctClass.toClass();

Step 5: Call the added method using reflection.   

Test test = new Test();

Method method = test.getClass().getMethod("print", new Class[] {});

method.invoke(test, new Object[] {});

Find the below working application.

Test.java

package com.sample.app;

public class Test {

}

App.java

package com.sample.app;

import java.lang.reflect.Method;

import javassist.ClassPool;
import javassist.CtClass;
import javassist.CtMethod;
import javassist.CtNewMethod;

public class App {

  public static void main(String args[]) throws Exception {
    ClassPool classPool = ClassPool.getDefault();

    CtClass ctClass = classPool.get("com.sample.app.Test");

    CtMethod newmethod = CtNewMethod.make("public void print() { System.out.println(\"Hey are you there\"); }",
        ctClass);
    ctClass.addMethod(newmethod);
    
    ctClass.toClass();
    
    Test test = new Test();
    Method method = test.getClass().getMethod("print", new Class[] {});
    method.invoke(test, new Object[] {});
  }
}

Run App.java, you will get below message in console.

Hey are you there

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