【Effective Java】同步访问共享的可变数据

package cn.xf.cp.ch02.item66;

import java.util.concurrent.TimeUnit;

import org.junit.Test;

public class StopThread
{
    /**
     * 停止线程变量
     */
    private static boolean stopRequested;
    
    //吧对变量的读和写方法都进行同步
    private static synchronized void requestStop()
    {
        stopRequested = true;
    }
    
    private static synchronized boolean stopRequested()
    {
        return stopRequested;
    }
    
    /**
     * 停止线程变量,这个使用关键字volatile使每个线程都是获取到最新的值
     */
    private static volatile boolean stopRequested2;
    
    @Test
    public void test()
    {
        Thread backgroundThread = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                int i = 0;
                while(!stopRequested())
                {
                    ++i;
                }
            }
        });
        //启动线程
        backgroundThread.start();
        
        //休眠1秒
        try
        {
            TimeUnit.SECONDS.sleep(1);
        }
        catch (InterruptedException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        requestStop();
    }
    
    @Test
    public void test2()
    {
        Thread backgroundThread = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                int i = 0;
                System.out.println("这里使用最新的stopRequested2值");
                while(!stopRequested2)
                {
                    ++i;
                }
            }
        });
        
        //启动线程
        backgroundThread.start();
        
        //停下1s之后执行变量修改程序
        try
        {
            TimeUnit.SECONDS.sleep(1);
        }
        catch (InterruptedException e)
        {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        //修改变量
        stopRequested2 = true;
    }
    
    
    public static void main(String[] args) throws InterruptedException
    {
        Thread backgroundThread = new Thread(new Runnable()
        {
            @Override
            public void run()
            {
                int i = 0;
                System.out.println(stopRequested); //false
                //这里停不下来是因为主线程对stopRequest进行修改的时候，这个线程并不可见
                while(!stopRequested)
                {
                    ++i;
                }
            }
        });
        //启动线程
        backgroundThread.start();
        
        //休眠1秒
        TimeUnit.SECONDS.sleep(1);
        stopRequested = true;
    }
}