《学习多线程03》关键字volatile

超、凢脫俗 2023-06-30 01:37 20阅读 0赞

## volatile的基础知识 ##

1.可见性，保证不同线程操作该变量的可见性，即一个线程修改该变量，这个新的值对其他线程来说是立刻知道的

2.禁止进行指令重排序（虚拟机层面，有自己的规则，对代码进行排序），有序性

3.volatile只能保证单次读/写的原子性。i++这种不能保证

## 简单实战 ##

**1.volatile定义的值改变，其他线程能否查看（可见性）**

public class VolatileTestOne {
         private volatile int n = 0;
    
        public static void main(String[] args) {
            volatileTest();
        }
    
        //单次读写保证原子性
        public void addNumber(){
            n++;
        }
    
        public static void volatileTest(){
            VolatileTestOne volatileTestOne = new VolatileTestOne();
            ExecutorService executorService = Executors.newFixedThreadPool(3);
            executorService.execute(()->{
                System.out.println("one");
                for (int i=0;i<10;i++){
                    volatileTestOne.addNumber();
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("oneOver");
            });
            executorService.execute(()->{
                System.out.println("two");
                for (int i=0;i<10;i++){
                    System.out.println(volatileTestOne.n);
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
                System.out.println("twoOver");
            });
            executorService.shutdown();
            while (!executorService.isTerminated()){
            }
            System.out.println(" "+volatileTestOne.n);
        }
    }

执行结果  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70]  
**2.进行自增操作，不能确保原子性。**

public class VolatileTestTwo {
        private volatile int n = 0;
    
        public static void main(String[] args) {
            volatileTest();
        }
    
        public static void volatileTest() {
            VolatileTestTwo volatileTestTwo = new VolatileTestTwo();
            ExecutorService executorService = Executors.newFixedThreadPool(5);
            for (int j=0;j<5;j++){
                executorService.execute(() -> {
                    for (int i=0; i < 1000; i++) {
                        volatileTestTwo.n++;
                    }
                });
            }
            executorService.shutdown();
            while (!executorService.isTerminated()){
            }
            System.out.println(" "+volatileTestTwo.n);
        }
    }

执行结果  
![在这里插入图片描述][20200113161637367.png]  
多次执行我们会发现还会有其他的结果  
![在这里插入图片描述][20200113161734645.png]  
不能确保原子性

**3.如何解决自增并保证原子性？**

public class VolatileTestThree {
    
        private volatile int n = 0;
    
        Lock lock = new ReentrantLock();
    
        public static void main(String[] args) {
            volatileTest();
        }
        //法（1）进行synchronized加锁
    //    public synchronized void addNumber(){
    //        n++;
    //    }
        //法（2）进行lock加锁
        public void addNumber(){
            lock.lock();
            try {
                n++;
            }finally {
                lock.unlock();
            }
        }
    
        public static void volatileTest() {
            VolatileTestThree volatileTestThree = new VolatileTestThree();
            ExecutorService executorService = Executors.newFixedThreadPool(10);
            for (int j=0;j<10;j++){
                executorService.execute(() -> {
                    for (int i=0; i < 1000; i++) {
                        volatileTestThree.addNumber();
                    }
                });
            }
            executorService.shutdown();
            while (!executorService.isTerminated()){
            }
            System.out.println(" "+volatileTestThree.n);
        }
    }

执行结果  
![在这里插入图片描述][20200113161837833.png]  
加锁后不管怎么执行，都会正确。

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![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 1]

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70]: https://img-blog.csdnimg.cn/20200113161531658.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw,size_16,color_FFFFFF,t_70
[20200113161637367.png]: https://img-blog.csdnimg.cn/20200113161637367.png
[20200113161734645.png]: https://img-blog.csdnimg.cn/20200113161734645.png
[20200113161837833.png]: https://img-blog.csdnimg.cn/20200113161837833.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/2020011317323178.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2JheHhkd3lw,size_16,color_FFFFFF,t_70