连续执行线程以打印1A 2B 3C 4D

Question

我实现了两个线程，一个打印整数，另一个打印英文字母。这些线程应该串联工作。

我使用同步块，一次只有一个线程将保持锁。但不知何故，AtomicInteger 没有按预期工作。一个线程对计数变量所做的更改对其他不同线程不可见。这怎么可能。我错过了什么吗？

我的实施

package com.concurrency;

导入java.util.concurrent.atomic.原子整数；

公共类ThreadWaitEg{

// Single monitor object for both threads
private final Object monitor = new Object();
// Thread-safe counter for both threads
private final AtomicInteger count = new AtomicInteger(0);

private class PrintNumber implements Runnable 
{

    @Override
    public void run() 
    {
        for (int i = 0; i < 26; i++) 
        {
            synchronized (monitor) 
            {
                System.out.println("From PrintNumber "+count.get());
                try 
                {
                    while ((count.get()) % 2 != 0) 
                    {
                        monitor.wait();
                    }

                } catch (InterruptedException e) 
                {
                    e.printStackTrace();
                }
                System.out.println(i);
                count.getAndIncrement();
                monitor.notifyAll();

            }
        }

    }

}

private class PrintChar implements Runnable 
{

    @Override
    public void run() 
    {

        for (int charr = 'A'; charr <= 'Z'; charr++) 
        {
            synchronized (monitor) 
            {
                System.out.println("From PrintChar "+count.get());
                try 
                {
                    while ((count.get()) % 2 == 0) 
                    {
                        monitor.wait();
                    }

                } catch (InterruptedException e) 
                {
                    e.printStackTrace();
                }
                System.out.println((char) charr);
                count.getAndIncrement();
                monitor.notifyAll();
            }
        }
    }

}

public static void main(String[] args) 
{
    System.out.println("from main thread");
    Runnable runInt = new ThreadWaitEg().new PrintNumber();
    new Thread(runInt).start();
    Runnable runChar = new ThreadWaitEg().new PrintChar();
    new Thread(runChar).start();
    System.out.println(" main thread completes ");
}

}

输出

从主线程 从打印编号 0 0 从打印


编号 1 主
线程完成
从打印字符 0

Answer 1

您的主要问题是您创建了主ThreadWaitEg类的两个实例——每个实例都有自己的“monitor”和“count”对象的副本，因此这些对象不能在两个线程之间共享。最快的解决方法是只创建一个实例，也就是说，将main()方法替换为:

public static void main(String[] args) {
    System.out.println("from main thread");
    ThreadWaitEg threadWaitEg = new ThreadWaitEg();
    Runnable runInt = threadWaitEg.new PrintNumber();
    new Thread(runInt).start();
    Runnable runChar = threadWaitEg.new PrintChar();
    new Thread(runChar).start();
    System.out.println(" main thread completes ");
}