Javassist: Create copy of a class

Using 'getAndRename' method of ClassPool, we can reads a class file and constructs a CtClass object with a new name.

 

Example

CtClass pointClass = classPool.getAndRename("com.sample.app.model.Point", "com.sample.app.model.MyPoint");

Class<?> MyPointClass = pointClass.toClass();

Object obj = MyPointClass.newInstance();

 

Find the below working application.

 

Point.java

package com.sample.app.model;

public class Point {
    public int x;
    public int y;

}

App.java

package com.sample.app;

import java.lang.reflect.Field;

import com.sample.app.model.Point;

import javassist.ClassPool;
import javassist.CtClass;

public class App {

    public static boolean set(Object object, String fieldName, Object fieldValue) {
        Class<?> clazz = object.getClass();
        while (clazz != null) {
            try {
                Field field = clazz.getDeclaredField(fieldName);
                field.setAccessible(true);
                field.set(object, fieldValue);
                return true;
            } catch (Exception e) {
                throw new IllegalStateException(e);
            }
        }
        return false;
    }

    @SuppressWarnings("unchecked")
    public static <V> V get(Object object, String fieldName) {
        Class<?> clazz = object.getClass();
        while (clazz != null) {
            try {
                Field field = clazz.getDeclaredField(fieldName);
                field.setAccessible(true);
                return (V) field.get(object);
            } catch (Exception e) {
                throw new IllegalStateException(e);
            }
        }
        return null;
    }

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

        CtClass pointClass = classPool.getAndRename("com.sample.app.model.Point", "com.sample.app.model.MyPoint");

        Class<?> MyPointClass = pointClass.toClass();

        System.out.println(MyPointClass.getName());

        Object obj = MyPointClass.newInstance();
        set(obj, "x", 10);
        set(obj, "y", 20);

        Object valueOfX = get(obj, "x");
        Object valueOfY = get(obj, "y");

        System.out.println("Value of x is : " + valueOfX);
        System.out.println("Value of y is : " + valueOfY);

        Point point = new Point();
        point.x = 123;
        point.y = 124;

        System.out.println("Value of x is : " + point.x);
        System.out.println("Value of y is : " + point.y);

    }

}

Run App.java, you will see below messages in console.

 

com.sample.app.model.MyPoint

Value of x is : 10

Value of y is : 20

Value of x is : 123

Value of y is : 124

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