Javassist简单入门实例 - 修改Class并写入磁盘

package com.lj.javassist;

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

public class Example01
{
	public static void main(String[] args) throws Exception
	{
		ClassPool pool=ClassPool.getDefault();
		CtClass cc=pool.get("com.lj.javassist.User");
		
		
		//byte[] b=cc.toBytecode();
		
		
		
		
		CtMethod[] cms=cc.getDeclaredMethods();
		
		for(CtMethod cm:cms){
			cm.insertAfter("System.out.println(\"hello : \"+this.name);");
		}
		
		Class clz=cc.toClass();
		User u=(User) clz.getConstructor(new Class[]{}).newInstance(new Object[]{});
		
		u.setName("alleni");
		
		System.out.println(u.getName());
		
		
		
	//	cc.writeFile();
	}
}

这个代码的运行结果是：
hello : alleni
hello : alleni
alleni

这里不能执行cc.writeFile(),  否则会报错:
com.lj.javassist.User was pruned.

看了一下官方文档， 貌似要加入
cc.stopPruning(true);
这段文档在Frozen class那段，
The default value of ClassPool.doPruning is false.

To disallow pruning a particular CtClass, stopPruning() must be called on that object in advance:

因此只要把这段代码放在

ClassPool pool=ClassPool.getDefault();
		CtClass cc=pool.get("com.lj.javassist.User");
		
		
		//byte[] b=cc.toBytecode();
		
		
		cc.stopPruning(true);

再次运行， 一切正常。



用FrontEnd反编译class文件， 可以看到如下代码

// Decompiled Using: FrontEnd Plus v2.03 and the JAD Engine
// Available From: http://www.reflections.ath.cx
// Decompiler options: packimports(3) 
// Source File Name:   User.java

package com.lj.javassist;

import java.io.PrintStream;

public class User
{

    public User()
    {
    }

    public int getId()
    {
_L2:
        return i;
        int i = id;
        System.out.println("hello : " + name);
        if(true) goto _L2; else goto _L1
_L1:
    }

    public void setId(int id)
    {
_L2:
        return;
        this.id = id;
        Object obj = null;
        System.out.println("hello : " + name);
        if(true) goto _L2; else goto _L1
_L1:
    }

    public String getName()
    {
_L2:
        return s;
        String s = name;
        System.out.println("hello : " + name);
        if(true) goto _L2; else goto _L1
_L1:
    }

    public void setName(String name)
    {
_L2:
        return;
        this.name = name;
        Object obj = null;
        System.out.println("hello : " + this.name);
        if(true) goto _L2; else goto _L1
_L1:
    }

    private int id;
    private String name;
}

虽然不知道那些if else语句和obj=null是什么玩意儿， 不过目标功能是实现了。
所有的方法都被加入了这句System.out.println(xxx);

Hibernate里面用到了Javassist,并通过这个东西实现了load查询方法的代理，和延迟加载。
具体内容以后再慢慢研究了。