单例模式是否线程安全

今天领导问了个问题说单例模式中，对象的变量是否是线程安全的。起初都不太确定，后来本着装的目的，试了一下，结果是否定的。在多线程环境下，对象的成员变量是不安全的。

package com.zhuyang.test;

public class Singleton {
	private static final Singleton singleton;
	//variable  will be updated in muti-thread
	private int count;
	static {
		singleton = new Singleton();
	}

	public static synchronized Singleton getInstance() {
		return singleton;
	}

	public int getCount() {
		return count;
	}

	public void setCount(int count) {
		this.count = count;
	}

}

上面的代码是一段标准的单例模式。 其中有个成员变量count.他的值，会再多线程环境下不断修改。

package com.zhuyang.test;

public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ThreadTest t1=new ThreadTest(1111);
		ThreadTest t2=new ThreadTest(2222);
		ThreadTest t3=new ThreadTest(3333);
		ThreadTest t4=new ThreadTest(4444);
		ThreadTest t5=new ThreadTest(5555);
		ThreadTest t6=new ThreadTest(6666);
		ThreadTest t7=new ThreadTest(7777);
		t1.start();
		t2.start();
		t3.start();
		t4.start();
		t5.start();
		t6.start();
		t7.start();
	}

}

class ThreadTest extends Thread {
	private int threadID;

	public ThreadTest(int threadID) {
		this.threadID = threadID;
	}
	@Override
	public void run() {
		Singleton singleton = Singleton.getInstance();
		
			singleton.setCount(threadID);
			System.out.println(this.getName()+"is updating count value is="+this.getThreadID()+", count value of singleton is="+singleton.getCount());
		
	}
	public int getThreadID() {
		return threadID;
	}

	public void setThreadID(int threadID) {
		this.threadID = threadID;
	}

}

运行上面的代码结果：
Thread-0is updating count value is=1111, count value of singleton is=4444
Thread-4is updating count value is=5555, count value of singleton is=4444
Thread-5is updating count value is=6666, count value of singleton is=4444
Thread-2is updating count value is=3333, count value of singleton is=4444
Thread-6is updating count value is=7777, count value of singleton is=4444
Thread-3is updating count value is=4444, count value of singleton is=4444
Thread-1is updating count value is=2222, count value of singleton is=4444

说明，多线程中，单例模式并非线程安全，即便是加上了synchronized关键字。

那么如何做到线程安全呢？做法就是把对象给synchronized掉，看代码。

package com.zhuyang.test;

public class Test {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub
		ThreadTest t1=new ThreadTest(1111);
		ThreadTest t2=new ThreadTest(2222);
		ThreadTest t3=new ThreadTest(3333);
		ThreadTest t4=new ThreadTest(4444);
		ThreadTest t5=new ThreadTest(5555);
		ThreadTest t6=new ThreadTest(6666);
		ThreadTest t7=new ThreadTest(7777);
		t1.start();
		t2.start();
		t3.start();
		t4.start();
		t5.start();
		t6.start();
		t7.start();
	}

}

class ThreadTest extends Thread {
	private int threadID;

	public ThreadTest(int threadID) {
		this.threadID = threadID;
	}
	@Override
	public void run() {
		Singleton singleton = Singleton.getInstance();
		synchronized (singleton) {
			singleton.setCount(threadID);
			System.out.println(this.getName()+"is updating count value is="+this.getThreadID()+", count value of singleton is="+singleton.getCount());
		}
	}

	public int getThreadID() {
		return threadID;
	}

	public void setThreadID(int threadID) {
		this.threadID = threadID;
	}

}

结果：

Thread-0is updating count value is=1111, count value of singleton is=1111
Thread-1is updating count value is=2222, count value of singleton is=2222
Thread-2is updating count value is=3333, count value of singleton is=3333
Thread-3is updating count value is=4444, count value of singleton is=4444
Thread-4is updating count value is=5555, count value of singleton is=5555
Thread-6is updating count value is=7777, count value of singleton is=7777
Thread-5is updating count value is=6666, count value of singleton is=6666