1.2.3实例变量与线程安全（共享数据的情况）

package demo;/** * Created by sunyifeng on 17/10/9. */public class MyThread extends Thread {    private int count = 5;    @Override    public void run(){        super.run();        count --;        System.out.println("由" + this.currentThread().getName() + "计算，count=" + count);    }}

package demo;/** * Created by sunyifeng on 17/10/9. */public class Run {    public static void main(String[] args) {        MyThread myThread = new MyThread();        Thread a = new Thread(myThread, "A");        Thread b = new Thread(myThread, "B");        Thread c = new Thread(myThread, "C");        Thread d = new Thread(myThread, "D");        Thread e = new Thread(myThread, "E");        //        a.start();        b.start();        c.start();        d.start();        e.start();    }}

运行结果：

由A计算，count=4
由C计算，count=2
由B计算，count=3
由D计算，count=1
由E计算，count=0

程序说明：

如果有多线程访问，则必定会出现多线程的安全问题。

package demo;/** * Created by sunyifeng on 17/10/9. */public class MyThread extends Thread {    private int count = 5;    @Override    synchronized public void run(){        super.run();        count --;        System.out.println("由" + this.currentThread().getName() + "计算，count=" + count);    }}

通过在run方法前加入synchrinized多个线程在执行run方法时，以排队的方式处理。当一个线程调用此方法前，先判断run方法有没有被上锁，如有锁，必须等到其他线程对run方法执行完之后才能执行。