线程共享所带来的问题

我就是我 2022-12-02 04:15 103阅读 0赞

假设两个线程共享一个计数变量，线程1对计数变量自增5000，线程2对计数变量自减5000，最终的结果是0吗？

不一定，会发生`线程不安全`问题。

import lombok.extern.slf4j.Slf4j;
    
    @Slf4j
    public class Test共享问题 {
        
    
        private static int count = 0;
    
        public static void main(String[] args) throws InterruptedException {
        
            Thread t1 = new Thread(new Runnable() {
        
                public void run() {
        
                    for (int i = 0; i < 5000; i++) {
        
                        count ++;
                    }
                }
            },"t1");
    
            Thread t2 = new Thread(new Runnable() {
        
                public void run() {
        
                    for (int i = 0; i < 5000; i++) {
        
                        count --;
                    }
                }
            },"t2");
    
            t1.start();
            t2.start();
            //等待两个线程结束
            t1.join();
            t2.join();
    
            log.debug("count = {}",count);
        }
    }

结果展示：

18:01:26.977 [main] DEBUG Test共享问题 - count = -53

### 实现互斥访问 ###

可以使用synchronized加锁

import lombok.extern.slf4j.Slf4j;
    
    @Slf4j
    public class Test共享问题 {
        
    
        private static int count = 0;
        private static Object obj = new Object();
    
        public static void main(String[] args) throws InterruptedException {
        
            Thread t1 = new Thread(new Runnable() {
        
                public void run() {
        
    
                    for (int i = 0; i < 5000; i++) {
        
                        synchronized (obj) {
        
                            count++;
                        }
                    }
                }
            },"t1");
    
            Thread t2 = new Thread(new Runnable() {
        
                public void run() {
        
                    for (int i = 0; i < 5000; i++) {
        
                        synchronized (obj){
        
                            count --;
                        }
                    }
                }
            },"t2");
    
            t1.start();
            t2.start();
            //等待两个线程结束
            t1.join();
            t2.join();
    
            log.debug("count = {}",count);
        }
    }

结果展示：

18:33:46.541 [main] DEBUG Test共享问题 - count = 0