Java多线程中读写不一致问题

问题：
假设有个全局变量var初始化为0
MyThread线程循环+1
MyThread2线程检测到var大于10时退出循环
问题来了，我们发现MyThread一直没有退出循环
也就是说线程没有及时刷新内存
解决方法：给全局变量添加 volatile关键字

Java提供了volatile来保证可见性。当一个变量被volatile修饰后，表示着线程本地内存无效，当一个线程修改共享变量后他会立即被更新到主内存中，当其他线程读取共享变量时，它会直接从主内存中读取。

点击查看代码

abstract class ThreadMemo implements Runnable {
    private Thread t;
    protected String threadName;

    // public static int var = 0;
    volatile public static int var = 0; // 只需要添加volatile关键字就可以了

    public ThreadMemo(String name) {
        threadName = name;
        System.out.println("Creating " + threadName);
    }

    abstract public void run();

    public void start() {
        System.out.println("Starting " + threadName);
        if (t == null) {
            t = new Thread(this, threadName);
            t.start();
        }
    }

}

class MyThread extends ThreadMemo {

    MyThread(String name) {
        super(name);
    }

    public void run() {
        var = super.var;
        while(true) {
            var = var + 1;
            // sleep(50);
            // time.sleep(50);
            try {
                Thread.sleep(50);
            } catch (InterruptedException e) {
                System.out.println("Thread " + threadName + " interrupted.");
            }
            // System.out.println("mythread 1 var: " + var);
        }
        
    }
}

class MyThread2 extends ThreadMemo {

    MyThread2(String name) {
        super(name);
    }

    public void run() {
        var = super.var;
        while(true) {
            if(var < 10) {
                // try {
                //     Thread.sleep(1);
                // } catch (InterruptedException e) {
                //     System.out.println("Thread " + threadName + " interrupted.");
                // }
                // Object o = new Object();

                continue;
            } 
            System.out.println("mythread2 var = " + var);
            break;
        }
    }
}



public class Test2 {
    // public static int var = 0;
    public static void main(String[] args) {
        ThreadMemo t1 = new MyThread("Thread-1");
        ThreadMemo t2 = new MyThread2("Thread-2");

        Thread a = new Thread(t1);
        Thread b = new Thread(t2);
        a.start();
        b.start();
    }
}

然后我们发现，给读线程的循环中添加一些语句，循环就能退出了
V2EX上发现相同的问题 Java 多线程并发，线程什么时候会刷新 "工作内存"

据说是遇到IO阻塞才会刷新内存