多线程访问内部类

Question

我创建了3个线程，它们正在访问ThreadSaroundinnerClasss外部类的内部类MyInnerClass。

package com.test;

public class ThreadsAroundInnerClasses {
    public static void main(String[] args) {
        Thread t1 = new Thread(new MyThread(), "THREAD-1");
        Thread t2 = new Thread(new MyThread(), "THREAD-2");
        Thread t3 = new Thread(new MyThread(), "THREAD-3");
        t1.start();
        t2.start();
        t3.start();
    }

    static class MyInnerClass {
        static int counter = 0;
        public void printIt(String threadName) {
            System.out.println("I am inside inner class, counter value is " + ++counter + " and thread name is " + threadName);
        }
    }
}

class MyThread implements Runnable {
    @Override
    public void run() {
        ThreadsAroundInnerClasses.MyInnerClass innerObj = new ThreadsAroundInnerClasses.MyInnerClass();
        innerObj.printIt(Thread.currentThread().getName());
    }
}

在输出中，我可以看到MyInnerClass类中的counter静态变量没有按顺序更新。

I am inside inner class, counter value is 1 and thread name is THREAD-1
I am inside inner class, counter value is 3 and thread name is THREAD-2
I am inside inner class, counter value is 2 and thread name is THREAD-3

如果有人能解释在多线程情况下如何处理内部类，将会有很大帮助？我们能同步整个内部类吗？

事先谢谢你的帮助。

Answer 1

试试看：

    static class MyInnerClass {
    static int counter = 0;
    public void printIt(String threadName) {
        synchronized(MyInnerClass.class) {
            System.out.println("I am inside inner class, counter value is " + ++counter + " and thread name is " + threadName);
        }
    }
}

您的计数器是静态的，因此需要在整个类对象上进行同步。

Answer 2

类是否是内部类是不相关的。您在这里看到的是预期的输出：您的程序中没有任何同步，因此线程调度程序可以在需要时自由切换到其他线程。下面是一个可能的操作序列，它将导致您看到的输出。

1. T1:增量计数器
2. T1:共轭酸
3. T1:打印
4. T3:增量计数器
5. T3:串联
6. T2:增量计数器
7. T2:串联
8. T2:打印
9. T3:打印

如果您希望计数器增量、级联和打印是原子操作,那么您应该确保它在唯一锁上同步：

synchronized MyInnerClass.class {
    System.out.println("I am inside inner class, counter value is " + ++counter + " and thread name is " + threadName);
}