单例模式的方法不是线程安全的

最新推荐文章于 2023-10-16 09:16:07 发布

mgodot

最新推荐文章于 2023-10-16 09:16:07 发布

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文章标签：单例线程安全 java 函数 class

本文链接：https://blog.youkuaiyun.com/mgodot/article/details/53155542

版权

本文探讨了Java中的单例模式在多线程环境下可能存在的线程安全问题。通过测试用例展示了在并发情况下，非线程安全的单例实现可能导致实例创建的不一致性。

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题目是结论：

代码是测试用例，可自行通过修改注释切换单例与非单例

测试结果最后几行在最下面.

package Thread;

import java.util.concurrent.CountDownLatch;

public class SingletonThreadSaveTest
{

	 private static SingletonThreadSaveTest ASTPMgr;

	public static volatile int c = 0;

//	public SingletonThreadSaveTest()
//	{
//
//	}
	private SingletonThreadSaveTest(){}
	
	 public static SingletonThreadSaveTest getInstance()
	 {
	 if(ASTPMgr == null)
	 {
	 synchronized(SingletonThreadSaveTest.class)
	 {
	 if(ASTPMgr == null){
	 ASTPMgr = new SingletonThreadSaveTest();
	 }
	 }
	 }
	 return ASTPMgr;
	 }

	public void add()
	{
		for (int i = 0; i < 10000; i++)
		{
			c++;
			Thread current = Thread.currentThread();
			System.out.println("Id:" + current.getId() +"Add:" + String.valueOf(c)+ " Name:" + current.getName() + " activeCount:" + Thread.activeCount() + "\r\n");
			
		}
	}

	public void des()
	{
		for (int i = 0; i < 10000; i++)
		{
			c--;
			Thread current = Thread.currentThread();
			System.out.println("Id:" + current.getId() +"Des:" + String.valueOf(c)+ " Name:" + current.getName() + " activeCount:" + Thread.activeCount() + "\r\n");
		}
	}

	/**
	 * @param args
	 */
	public static void main(String[] args)
	{
		final CountDownLatch controller = new CountDownLatch(2);
		Thread current = Thread.currentThread();
		System.out.println("Id:" + current.getId() + " Name:" + current.getName() + " activeCount:" + Thread.activeCount() + "\r\n");

		 new Thread(
		new Runnable()
		{
			@Override
			public void run()
			{
				// TODO Auto-generated method stub
				 SingletonThreadSaveTest.getInstance().add();
				//new SingletonThreadSaveTest().add();
				controller.countDown();
			}
		}).start();

		 new Thread(
		new Runnable()
		{

			@Override
			public void run()
			{
				// TODO Auto-generated method stub
				 SingletonThreadSaveTest.getInstance().des();
				//new SingletonThreadSaveTest().des();
				controller.countDown();
			}
		}).start();

		 try
		 {
		 controller.await();
		 } catch (InterruptedException e)
		 {
		 // TODO Auto-generated catch block
		 e.printStackTrace();
		 }
		 System.out.print("\r\n");
		System.out.print(c);

	}
}

结果：

Id:11Add:-6 Name:Thread-0 activeCount:2

Id:11Add:-5 Name:Thread-0 activeCount:2

Id:11Add:-4 Name:Thread-0 activeCount:2

Id:11Add:-3 Name:Thread-0 activeCount:2

Id:11Add:-2 Name:Thread-0 activeCount:2

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