Java线程脏读

绝地灬酷狼 2022-05-24 03:52 195阅读 0赞

**亭亭画舸系春谭,直待行人酒半酣**  
脏读,也就是说你拿到的值不是你想要的.当然数据库中也有脏读的情况出现,那是因为事务的原因.这里不做解释.  
Java线程中也会出现脏读的现象,示例代码如下:

public class ThreadTest { 
        public static void main(String[] args) {
            try{
                A a = new A();
                MyThread thread = new MyThread(a);
                thread.start();
                Thread.sleep(1000);
                a.getValue();
            }catch (Exception e) {
                e.printStackTrace();
            }
        }
    }
    
    class MyThread extends Thread{
        private A a;
        public MyThread(A a) {
            super();
            this.a = a;
        }
        @Override
        public void run() {
            super.run();
            a.setValue("bb","bbb");
        }
    }
    
    class A {
        private String name = "aa";
        private String pass = "aaa";
        synchronized public void setValue(String username,String password) {
            try {
                this.name = username;
                Thread.sleep(5000);
                this.pass = password;
                System.out.println("setValue Thread = " + Thread.currentThread().getName() + " name = " + name + " pass = " + pass);
            }catch (Exception e) {
                e.printStackTrace();
            }
        }
        public void getValue() {
            System.out.println("getValue Thread = " + Thread.currentThread().getName() + " name = " + name + " pass = " + pass);
        }
    }

运行结果:  
![这里写图片描述][70]  
这里getValue()读出来的值为name = bb,pass = aaa,这是因为线程首先调用的setValue()方法是同步的(synchronized),而getValue()方法并非同步,在设置name之后线程被睡了5秒

this.name = username;
    Thread.sleep(5000);
    this.pass = password;

而pass的值还没来得及设置,所以在调用getValue()方法时就会出现脏读的情况.  
解决这个问题的方法当然是把getValue()方法也加上同步(synchronized):

synchronized public void getValue() {
            System.out.println("getValue Thread = " + Thread.currentThread().getName() + " name = " + name + " pass = " + pass);
        }

再来看运行结果:  
![这里写图片描述][70 1]  
结论:  
**当线程A调用了加入synchronized关键字的X方法时,其他任何线程如果调用X方法,就必须等A线程执行完才可以.因为A线程已经拿到了X方法的对象的锁.但是其他线程可以任意调用没有synchronized关键字的Y方法,但是此时可能就会出现脏读;**  
**如果Y方法也加入了synchronized关键字,线程A先调用了X方法时,A就拿到了对象的锁,线程B如果调Y方法,就必须等待A调用完,释放锁才可以正常调用;这样也就不会出现脏读;**

[70]: /images/20220524/afb0f24377934e448beb202948f3e674.png
[70 1]: /images/20220524/bf8c423fa7d940f3a550c864e7e1f7d8.png