正確運用synchronized和二次判斷實現多線程安全 - langtianya－IT工程師數位筆記本

文章出處

正確運用synchronized和二次判斷實現多線程安全，做出高效二符合預期的程序，特別是多個線程跑一個對象的時候，如下圖所示：

測試代碼如下：
特別注意if(shutdownRequested){ *部分不同的寫法。

不然就會輸出與邏輯不符的現象：
如：

runner—-false—-我沒有關閉。。。

runner—-true—-我沒有關閉。。。

runner—-true—-我關閉了=====»>

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+"_號"));
    }
    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+":結束...");
        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) {

            /**
             *多個線程的代碼去執行：  System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我沒有關閉。。。");
             * */
               /*if(shutdownRequested){
                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我關閉了=====>>>");
                    break;
                }else{
                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我沒有關閉。。。");
                    shutdown();
                }*/
            /**
             * 如果沒有二次判斷，也會出現比較臟數據.
             * */
            /*
                if(shutdownRequested){
                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我關閉了=====>>>");
                    break;
                }
                synchronized (this) {
                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我沒有關閉。。。");
                    shutdown();
                }*/
            /**
             * 加上二次判斷,能夠正確
             * */

            if(shutdownRequested){
                System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我關閉了=====>>>");
                break;
            }
                synchronized (this) {
                if(!shutdownRequested){
                    System.out.println(this.name + "----" + this.shutdownRequested +"----" +"我沒有關閉。。。");
                    shutdown();
                }
            }           
        }
    }
}