java - ArrayBlockingQueue NoSuchElementException

Question

只是为了学习，我编写了以下用于自定义线程池的代码，引用并编辑显示的代码 here.

如代码所示，我使用 ArrayBlockingQueue 作为任务队列。

代码:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.TimeUnit;

public class ThreadPoolService {
    private final BlockingQueue<Runnable> taskQueue;
    private final int corePoolSize;

    private ThreadPoolService(int corePoolSize) {
        this.corePoolSize = corePoolSize;
        this.taskQueue = new ArrayBlockingQueue<>(corePoolSize);
        ThreadPool[] threadPool = new ThreadPool[corePoolSize];
        for (int i = 0; i < corePoolSize; i++) {
            threadPool[i] = new ThreadPool();
            threadPool[i].start();
        }
    }

    public static ThreadPoolService newFixedThreadPool(int size) {
        return new ThreadPoolService(size);
    }

    public void execute(Runnable task) {
        try {
            taskQueue.offer(task, 10, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    private class ThreadPool extends Thread {
        Runnable task;

        @Override
        public void run() {
            while (true) {
                try {
                    while (!taskQueue.isEmpty()) {
                        task = taskQueue.remove();
                        task.run();
                    }
                } catch (RuntimeException ex) {
                    ex.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        ThreadPoolService pool = ThreadPoolService.newFixedThreadPool(10);
        Runnable task1 = () -> {
            System.out.println(" Wait for sometime: -> " + Thread.currentThread().getName());
            try {
                TimeUnit.SECONDS.sleep(2);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        };

        Runnable task2 = () -> System.out.println(" Do  Task 2 -> " + Thread.currentThread().getName());
        Runnable task3 = () -> System.out.println(" Do  Task 3 -> " + Thread.currentThread().getName());
        Runnable task4 = () -> System.out.println(" Do  Task 4 -> " + Thread.currentThread().getName());
        List<Runnable> taskList = new ArrayList<>();
        taskList.add(task1);
        taskList.add(task2);
        taskList.add(task3);
        taskList.add(task4);
        for (Runnable task : taskList) {
            pool.execute(task);
        }
    }
}

此代码有时运行良好，有时会出错。

成功输出:

Do  Task 2 -> Thread-2
Wait for sometime: -> Thread-8
Do  Task 3 -> Thread-6
Do  Task 4 -> Thread-7

失败输出:

Do  Task 4 -> Thread-3
Do  Task 3 -> Thread-6
Wait for sometime: -> Thread-4
Do  Task 2 -> Thread-7
java.util.NoSuchElementException
    at java.util.AbstractQueue.remove(AbstractQueue.java:117)
    at com.interview.java.ThreadPoolService$ThreadPool.run(ThreadPoolService.java:43)
java.util.NoSuchElementException
    at java.util.AbstractQueue.remove(AbstractQueue.java:117)
    at com.interview.java.ThreadPoolService$ThreadPool.run(ThreadPoolService.java:43)
java.util.NoSuchElementException
    at java.util.AbstractQueue.remove(AbstractQueue.java:117)
    at com.interview.java.ThreadPoolService$ThreadPool.run(ThreadPoolService.java:43)

我发现错误的原因是在队列为空时尝试删除元素。但它不应该，因为我正在第 42 行进行队列空检查(while (!taskQueue.isEmpty()))。代码有什么问题以及为什么有时运行时没有错误？

Answer 1

在“while”检查和实际删除之间，队列可能会被另一个线程修改，可能会导致您提到的错误。这就是所谓的“竞争条件”。

因此，为了解决这个问题，您需要一种方法来阻止其他线程对队列的访问，可以通过“锁定”，或者使用带有共享锁对象的“同步” block 。或者简单地通过“轮询”而不是删除。