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Java中多线程安全问题实例分析

2020-12-16 17:31 218 查看

案例

1 package com.duyang.thread.basic.basethread;
2
3 /**
4  * @author ：jiaolian
5  * @date ：Created in 2020-12-16 14:02
6  * @description：线程不安全分析
7  * @modified By：
8  * 公众号:叫练
9  */
10 public class ThreadUnsafe {
11
12     public static void main(String[] args) {
13         Thread task = new Task();
14         Thread threadA = new Thread(task,"A");
15         Thread threadB = new Thread(task,"B");
16         Thread threadC = new Thread(task,"C");
17         Thread threadD = new Thread(task,"D");
18         Thread threadE = new Thread(task,"E");
19         threadA.start();
20         threadB.start();
21         threadC.start();
22         threadD.start();
23         threadE.start();
24     }
25
26     private static class Task extends Thread {
27
28         int count = 5;
29         @Override
30         public void run() {
31             /**
32              * jvm分3步骤;
33              * 1.获取count(从主内存获取值)
34              * 2.count减1(在各自寄存器完成)
35              * 3.保存count(刷新到主内存)
36              *
37              * 说下可能执行的过程...
38              * A线程获取cpu的count值为5，A线程先减去1，保存count值为4刷新到主内存,此时还没有执行System.out.println count
39              * 切换到B线程，此时B线程的count值为4，因为B线程是从主内存取的，B线程count值减去1为3，此时刷新到主内存，主内存值变为3
40              * 切换到A线程，执行System.out.println count=3
41              * 切换到B线程，执行System.out.println count=3
42              * 情况就是这样的
43              *
44              */
45             count--;
46
56c
System.out.println(Thread.currentThread().getName() + " "+count);
47         }
48     }
49 }