顺序线程执行打印 1A 2B 3C 4D

Question

我实现了两个线程，一个用于打印整数，另一个用于打印英文字母。这些线程应该协同工作。

我使用同步块，一次只有一个线程持有锁。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的线程不可见。这怎么可能。我错过了什么吗？

我的实现

package com.concurrency;

进口java.util.concurrent.atomic.AtomicInteger;

public class 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

来自主线程
来自 PrintNumber 0
0
来自 PrintNumber 1
主线程完成
来自 PrintChar 0

Answer 1

您的主要问题是您创建了主 ThreadWaitEg class 的两个实例 - 每个实例都有自己的 "monitor" 和 "count" 对象副本，因此对象不共享在两个线程之间。最快的解决方法是只创建一个实例，即将 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 ");
}

顺序线程执行打印 1A 2B 3C 4D

Sequential thread execution to print 1A 2B 3C 4D

java

multithreading

atomic

thread-safety