为什么要在多线程中进行数据同步

package com.mhm.test;public class Test7 extends Thread {public static int n = 0;public void run() {int m = n;yield();m++;n = m;}public static void main(String[] args) throws Exception {Test7 myThread = new Test7();Thread threads[] = new Thread[100];for (int i = 0; i < threads.length; i++)threads[i] = new Thread(myThread);for (int i = 0; i < threads.length; i++)threads[i].start();for (int i = 0; i < threads.length; i++)threads[i].join();System.out.println("n = " + Test7.n);}}

?当这段代码执行完后，结果可能不是100，因为出现了脏数据

解决办法是在run() 前加synchronized

转载于http://java.chinaitlab.com/line/779590.html

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在23种设计模式中的单件（Singleton）模式如果按传统的方法设计，也是线程不安全的，下面的代码是一个线程不安全的单件模式。

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class Singleton{    private static Singleton sample;    private Singleton()    {    }    public static Singleton getInstance()    {        if (sample == null)        {            Thread.yield(); // 为了放大Singleton模式的线程不安全性            sample = new Singleton();        }        return sample;    }}public class MyThread extends Thread{    public void run()    {        Singleton singleton = Singleton.getInstance();        System.out.println(singleton.hashCode());    }    public static void main(String[] args)    {        Thread threads[] = new Thread[5];        for (int i = 0; i < threads.length; i++)            threads[i] = new MyThread();        for (int i = 0; i < threads.length; i++)            threads[i].start();    }}要想使上面的单例模式线程安全，只要在静态方法getInstance()前加上synchronized就可以了