ClassCopyDemo

ClassCopyDemo.java

import java.lang.reflect.Field;

import sun.misc.Unsafe;
import javassist.ClassPool;
import javassist.CtClass;

public class ClassCopyDemo {

    /*
         <!-- Javassist dependency -->
         <dependency>
            <groupId>org.javassist</groupId>
            <artifactId>javassist</artifactId>
            <version>3.17.1-GA</version>
         </dependency>   
     */
    
    private static final Unsafe UNSAFE;
    
    static {
        try {
            Field unsafeField = Unsafe.class.getDeclaredField("theUnsafe");
            unsafeField.setAccessible(true);
            UNSAFE = (Unsafe) unsafeField.get(null);
        } catch (NoSuchFieldException e) {
            throw new RuntimeException("Unable to get unsafe", e);
        } catch (IllegalAccessException e) {
            throw new RuntimeException("Unable to get unsafe", e);
        }
    }
    
    public static void main(String[] args) throws InstantiationException, IllegalAccessException {
        Class<?> copyFooClass = copyClass(Foo.class);
        
        Incrementer fooInstance = Foo.class.newInstance();
        Incrementer copyFooInstance = (Incrementer) copyFooClass.newInstance();
    
        System.out.println(fooInstance.getClass() + ".increment(int x): " + fooInstance.increment(1));
        System.out.println(copyFooInstance.getClass() + ".increment(int x): " + copyFooInstance.increment(1));
        System.out.println(fooInstance.getClass() + " == " + copyFooInstance.getClass() + ": " + 
                           fooInstance.getClass().equals(copyFooInstance.getClass()));
    }
    
    private static Class<?> copyClass(Class<?> classToBeCopied) {
        try {
            String copyClassName = classToBeCopied.getName() + "$Copy";
            ClassPool cp = ClassPool.getDefault();
            CtClass ctClass = cp.getAndRename(classToBeCopied.getName(), copyClassName);
            byte[] bytecode = ctClass.toBytecode();
            return UNSAFE.defineClass(
                    copyClassName, 
                    bytecode, 
                    0, 
                    bytecode.length, 
                    classToBeCopied.getClassLoader(), 
                    classToBeCopied.getProtectionDomain());
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
    
    public interface Incrementer {
        int increment(int x);
    }
    
    public static class Foo implements Incrementer {
        
        @Override
        public int increment(int x) {
            return x + 1;
        }
        
    }

}