package singleton;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.lang.reflect.Constructor;

/**
 * 用反射和反序列化的方法破解單例模式
 * @author weiyx15
 *
 */
public class SingletonCrack {
	public static void main(String[] args) throws Exception
	{
		// 正常創(chuàng)建單例對(duì)象
		SingletonLazy s1 = SingletonLazy.getInstance();
		SingletonLazy s2 = SingletonLazy.getInstance();
		System.out.println(s1);
		System.out.println(s2);
		
		// 用反射破解單例
		Class<SingletonLazy> cls = (Class<SingletonLazy>) Class.forName("singleton.SingletonLazy");		// 獲取SingletonLazy類
		Constructor<SingletonLazy> cons = cls.getDeclaredConstructor(null);		// 獲取SingletonLazy的構(gòu)造方法
		cons.setAccessible(true);			// 跳過方法的可見性檢查
		SingletonLazy s3 = cons.newInstance();	// 調(diào)用構(gòu)造方法生成新對(duì)象
		SingletonLazy s4 = cons.newInstance();	// 調(diào)用構(gòu)造方法生成新對(duì)象
		System.out.println(s3);
		System.out.println(s4);
		
		// 用反序列化破解單例
		FileOutputStream fos = new FileOutputStream("object.out");	// 文件輸出流
		ObjectOutputStream oos = new ObjectOutputStream(fos);		// 對(duì)象輸出流
		oos.writeObject(s1);										// 向文件序列化對(duì)象
		oos.close();												// 關(guān)閉對(duì)象輸出流
		fos.close();												// 關(guān)閉文件輸出流
		
		FileInputStream fis = new FileInputStream("object.out");	// 文件輸入流
		ObjectInputStream ois = new ObjectInputStream(fis);			// 對(duì)象輸入流
		SingletonLazy s5 = (SingletonLazy) ois.readObject();		// 從文件反序列化對(duì)象
		ois.close();												// 關(guān)閉對(duì)象輸入流
		fis.close();												// 關(guān)閉文件輸入流
		System.out.println(s5);
	}
}

package singleton;

import java.io.ObjectStreamException;
import java.io.Serializable;

/**
 * 排除了反射漏洞和反序列化漏洞的懶漢式單例模式
 * @author weiyx15
 *
 */
public class SingletonLazySafe implements Serializable{
	private static SingletonLazySafe instance;
	
	private SingletonLazySafe() {
		// 防止反射漏洞通過再次調(diào)用私有構(gòu)造方法實(shí)例化新的instance
		if (instance != null)
		{
			throw new RuntimeException();	// 拋出運(yùn)行時(shí)異常
		}
	}
	
	public static synchronized SingletonLazySafe getInstance() {
		if (instance == null)	// 如果未實(shí)例化，則先實(shí)例化
		{
			instance = new SingletonLazySafe();	// 調(diào)用getInstance方法后再實(shí)例化對(duì)象
		}
		return instance;
	}
	
	/**
	 * 從I/O流讀取對(duì)象時(shí)會(huì)調(diào)用readResolve接口
	 * 在readResolve接口中直接返回instance對(duì)象
	 * 避免反序列化時(shí)重新實(shí)例化對(duì)象
	 * @return 單例對(duì)象
	 * @throws ObjectStreamException
	 */
	private Object readResolve() throws ObjectStreamException {
		return instance;
	}
}