volatile修饰数组

青旅半醒 2024-02-28 02:04 112阅读 0赞

> 结论：volatile修饰对象或数组时，只能保证他们的引用地址的可见性。

**非volatile数组的可见性问题**

public class Test {
        
        static int[] a = new int[]{
        1};
    
        public static void main(String[] args) {
        
            new Thread(() -> {
          //线程A
                try {
        
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
        
                    e.printStackTrace();
                }
                a[0] = 0;
            }).start();
    
            new Thread(()-> {
           //线程B
                try {
        
                    while (true) {
        
                        if (a[0] == 0) {
        
                            System.out.println("结束");
                            break;
                        }
                        //Thread.sleep(10);
                    }
                } catch (Exception e) {
        
                    e.printStackTrace();
                }
            }).start();
        }
    
    }

由此可见打印不出来‘’结束‘’  
![在这里插入图片描述][3c3f452ae1154a7ba16867fd952a7d4e.png]  
**volatile数组的可见性问题**

public class Test {
        
        static volatile int[] a = new int[]{
        1};
    
        public static void main(String[] args) {
        
            new Thread(() -> {
          //线程A
                try {
        
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
        
                    e.printStackTrace();
                }
                a[0] = 0;
            }).start();
    
            new Thread(()-> {
           //线程B
                try {
        
                    while (true) {
        
                        if (a[0] == 0) {
        
                            System.out.println("结束");
                            break;
                        }
                        //Thread.sleep(10);
                    }
                } catch (Exception e) {
        
                    e.printStackTrace();
                }
            }).start();
        }
    
    }

![在这里插入图片描述][7926cd3651474441a686f3b0de0d7a33.png]  
**发现结果和我们预期的相反**  
当ThreadB读取array时，因为array的引用被volatile修饰，所以ThreadB对所有变量都会从主内存去获取，当然也就包括array\[0\]。  
![在这里插入图片描述][a869a910a2db4d4d95ba4ac39d103933.png]  
[https://stackoverflow.com/questions/53753792/java-volatile-array-my-test-results-do-not-match-the-expectations][https_stackoverflow.com_questions_53753792_java-volatile-array-my-test-results-do-not-match-the-expectations]

其实如果不给数组加volatile就永远不会打印“结束”，这种绝对的认为是错误的，volatile保证了其变量及时可见性而不会引起线程安全的问题，就算不加volatile，cpu在空闲的时候也会将array\[0\]的值从线程缓存刷新到主存，只是while(true)太快了，导致cpu一直没空。我们可以使用Thread.sleep(10)让出CPU让其有空将线程变量刷新到主内存中去。记得先去掉volatile修饰

public class Test {
        
        static  int[] a = new int[]{
        1};
    
        public static void main(String[] args) {
        
            new Thread(() -> {
          //线程A
                try {
        
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
        
                    e.printStackTrace();
                }
                a[0] = 0;
            }).start();
    
            new Thread(()-> {
           //线程B
                try {
        
                    while (true) {
        
                        if (a[0] == 0) {
        
                            System.out.println("结束");
                            break;
                        }
                        Thread.sleep(10);
                    }
                } catch (Exception e) {
        
                    e.printStackTrace();
                }
            }).start();
        }
    
    }

[3c3f452ae1154a7ba16867fd952a7d4e.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/02/28/675bcb2effbe4820bec7128220242219.png
[7926cd3651474441a686f3b0de0d7a33.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/02/28/e76629af35f3467ba5405acb2b0b5398.png
[a869a910a2db4d4d95ba4ac39d103933.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/02/28/45dc68b5a0e74a52af94a3a16109cfb2.png
[https_stackoverflow.com_questions_53753792_java-volatile-array-my-test-results-do-not-match-the-expectations]: https://stackoverflow.com/questions/53753792/java-volatile-array-my-test-results-do-not-match-the-expectations