java单例模式在多线程环境下的测试

java单例模式在多线程环境下的测试,进攻自己学习和复习用.

1. 不加双重判断, 会生成2个不同的对象. 代码如下:

 

package com.thred;

import java.util.Date;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class TestSingleton {

	private static TestSingleton s = null;
	
	private TestSingleton() {}

	public static TestSingleton getInstance() {
		if (s == null) {
			System.out.println( new Date() + " " + Thread.currentThread().getName() + "-1->" +  s);
			try {
				Thread.sleep(1);
				System.out.println( new Date() + " " + Thread.currentThread().getName() + "-2->" +  s);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			synchronized (TestSingleton.class){
				//if(s == null){
					System.out.println( new Date() + " " + Thread.currentThread().getName() + "-3->" +  s);
					s = new TestSingleton();
					System.out.println( new Date() + " " + Thread.currentThread().getName() + "-4->" +  s);
				//}else{
				//	System.out.println("s isn't null");
				//}
			}
			
		}
		return s;
	}
	
	/**
	 * @param args
	 */
	public static void main(String[] args) throws Exception{
		ExecutorService service = Executors.newFixedThreadPool(10);
		for(int i = 0 ;i < 2;i++){
			service.submit(new Runnable(){
				public void run(){
					String str =   new Date() +" " +  Thread.currentThread().getName() + "-run invoke->" +  TestSingleton.getInstance().toString();
					System.out.println(str);
				}
			});
		}
	}
}

 

测试结果,会生成2个不同的对象,不是单例,在第四步的没加判断会new 对象.

Mon Jul 23 22:22:54 CST 2012 pool-1-thread-1-1->null
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-2-1->null
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-1-2->null
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-1-3->null
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-1-4->com.thred.TestSingleton@fa7e74
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-1-run invoke->com.thred.TestSingleton@fa7e74
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-2-2->com.thred.TestSingleton@fa7e74
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-2-3->com.thred.TestSingleton@fa7e74
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-2-4->com.thred.TestSingleton@133796
Mon Jul 23 22:22:54 CST 2012 pool-1-thread-2-run invoke->com.thred.TestSingleton@133796

 

2.加双重判断: 在同步代码块中加入判断,只贴出不同的部分.

 

synchronized (TestSingleton.class){
	if(s == null){
		System.out.println( new Date() + " " 
			+ Thread.currentThread().getName() + "-3->" +  s);
		s = new TestSingleton();
		System.out.println( new Date() + " " 
			+ Thread.currentThread().getName() + "-4->" +  s);
	}else{
		System.out.println("s isn't null");
	}
}

 

 

测试结果: 返回同一个对象,是单例

Mon Jul 23 22:27:11 CST 2012 pool-1-thread-2-1->null
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-1-1->null
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-2-2->null
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-2-3->null
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-2-4->com.thred.TestSingleton@183f74d
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-2-run invoke->com.thred.TestSingleton@183f74d
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-1-2->com.thred.TestSingleton@183f74d
s isn't null
Mon Jul 23 22:27:11 CST 2012 pool-1-thread-1-run invoke->com.thred.TestSingleton@183f74d

 

 结论:在写单例的时候一定要加上双重判断呀~~切记