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