线程安全

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1.什么是线程安全？*

线程安全就是当多个线程访问某一个类（对象或方法）时，这个对象始终都能表现出正确的行为，那么这个类（对象或方法）就是线程安全的。

2.举例说明

public class MyThread extends Thread{    private int count = 5 ;    //没加锁    public void run(){        count--;        System.out.println(this.currentThread().getName() + " count = "+ count);    }    public static void main(String[] args) {        MyThread myThread = new MyThread();        Thread t1 = new Thread(myThread,"t1");        Thread t2 = new Thread(myThread,"t2");        Thread t3 = new Thread(myThread,"t3");        Thread t4 = new Thread(myThread,"t4");        Thread t5 = new Thread(myThread,"t5");        t1.start();        t2.start();        t3.start();        t4.start();        t5.start();    }}

输出结果

t2 count = 3t3 count = 2t1 count = 3t4 count = 1t5 count = 0

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而在run方法上加了synchronized关键字后，输出结果为

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t1 count = 4t2 count = 3t3 count = 2t4 count = 1t5 count = 0