Java 8：ArrayDeque<>.poll 在并行环境中返回 null

Question

我试图维护多个线程中的项目列表，每个线程一个(例如，为每个线程为一个套接字连接(。我在ArrayDeque<>中维护此列表。我面临的问题是ArrayDeque<>超过项目的NO胜过。线程池中的线程。

这是我的代码：

package com.practice;
import java.util.ArrayDeque;
import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
public class CompletableFutureApp implements AutoCloseable {
    private static void sleep(long timeMS) {
        try {
            Thread.sleep(timeMS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
    public static void main(String[] args) {
        try (CompletableFutureApp completableFutureApp = new CompletableFutureApp()) {
            Runnable[] tasksList1 = new Runnable[100];
            for (int i = 0; i < tasksList1.length; i++) {
                String msg1 = "TaskList 1 no.: " + i;
                tasksList1[i] = () -> {
                    //System.out.println(msg1);
                    sleep(300);
                };
            }
            completableFutureApp
                    .executeTasks(tasksList1)
                    .thenRun(() -> System.out.println("All tasks completed successfully"));
        }
    }
    private ExecutorService threadPool = Executors.newWorkStealingPool(10);
    private ArrayDeque<Integer> itemsAvailable = new ArrayDeque<>();
    private CompletableFuture executeTasks(Runnable... tasks) {
        CompletableFuture[] futures = new CompletableFuture[tasks.length];
        for (int i = 0; i < tasks.length; i++) {
            Runnable task = tasks[i];
            futures[i] = CompletableFuture.runAsync(() -> {
                Integer item = itemsAvailable.poll();
                if (item == null)
                    item = new Random().nextInt();
                task.run();
                itemsAvailable.add(item);
                System.out.println("Items available: " + itemsAvailable.size());
            }, threadPool);
        }
        return CompletableFuture.allOf(futures).thenRun(() -> System.out.println("Items available at last: " + itemsAvailable.size()));
    }
    @Override
    public void close() {
        threadPool.shutdown();
        try {
            threadPool.awaitTermination(100, TimeUnit.SECONDS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

就像我的代码中一样，我正在创建一个偷窃尺寸10 的工作，任何人都可以让我知道为什么不。在这种情况下，ArrayDeque<>中的元素超过10？

Answer 1

来自Javadoc：

[阵列deques]不是线程安全；在没有外部同步的情况下，它们不支持多个线程的并发访问。

使用外部同步，或选择专门为并发访问设计的不同类。