正确运用synchronized和二次判断 实现多线程安全

正确运用synchronized和二次判断 实现多线程安全，做出高效二符合预期的程序，特别是多个线程跑一个对象的时候，如下图所示： 

测试代码如下： 
特别注意if(shutdownRequested){ *部分不同的写法。

不然就会输出与逻辑不符的现象： 
如：

runner—-false—-我没有关闭。。。

runner—-true—-我没有关闭。。。

runner—-true—-我关闭了=====»>

---

package com.xue.gang.volatiler;

import java.util.concurrent.CountDownLatch;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

public class VolatileRunner{

public static void main(String args[]) throws InterruptedException {    int size=1000;    CountDownLatch countDownLatch = new CountDownLatch(size);    TomRunner tomRunner = new TomRunner(false,countDownLatch,"runner");    ExecutorService executorService = Executors.newCachedThreadPool();    for(int i=1;i<=size;i++){        executorService.execute(new Thread2RunTomRunner(countDownLatch,tomRunner,i+"_号"));    }    countDownLatch.await();    executorService.shutdown();    //new Thread(volatileRunner).start();}static class Thread2RunTomRunner implements Runnable{    private CountDownLatch  countDownLatch;    private TomRunner tomRunner;    private String name;    public Thread2RunTomRunner(CountDownLatch countDownLatch,            TomRunner tomRunner, String name) {        super();        this.countDownLatch = countDownLatch;        this.tomRunner = tomRunner;        this.name = name;    }    public void run() {        System.out.println(this.name+":running...");        this.tomRunner.doWork();        System.out.println(this.name+":结束...");        this.countDownLatch.countDown();    }}static class TomRunner{    volatile boolean shutdownRequested = false;    //boolean shutdownRequested = false;    String name;    public TomRunner(boolean shutdownRequested,            CountDownLatch countDownLatch, String name) {        super();        this.shutdownRequested = shutdownRequested;        this.name = name;    }    public void shutdown() {        this.shutdownRequested = true;    }    public void doWork() {        while (true) {            /**             *多个线程的代码去执行：  System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我没有关闭。。。");             * */               /*if(shutdownRequested){                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我关闭了=====>>>");                    break;                }else{                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我没有关闭。。。");                    shutdown();                }*/            /**             * 如果没有二次判断，也会出现比较脏数据.             * */            /*                if(shutdownRequested){                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我关闭了=====>>>");                    break;                }                synchronized (this) {                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我没有关闭。。。");                    shutdown();                }*/            /**             * 加上二次判断,能够正确             * */            if(shutdownRequested){                System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我关闭了=====>>>");                break;            }                synchronized (this) {                if(!shutdownRequested){                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我没有关闭。。。");                    shutdown();                }            }                   }    }}

}

原：http://my.oschina.net/u/177808/blog/165442