Question

我有以下代码的变体：

package com.test.package;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.PriorityBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class TestClass {

    public static class MyRunnable implements Runnable {

        @Override
        public void run() {
            System.out.println("Called");
        }

    }

    public void method() {
        PriorityBlockingQueue<MyRunnable> queue = new PriorityBlockingQueue<MyRunnable>();
        method2(queue);
    }

    public void method2(BlockingQueue<? extends Runnable> queue) {
        System.out.println(queue);

        // Getting error here because BlockingQueue<? extends Runnable> is not a
        // subtype of BlockingQueue<Runnable>.
        ThreadPoolExecutor threadPool = new ThreadPoolExecutor(200, 200, 0L,
            TimeUnit.MILLISECONDS, queue);
    }
}

正如您所看到的，我的队列与ThreadPoolExecutor构造函数不兼容。有没有办法解决这个问题，而不是将我的队列强制转换为(BlockingQueue<Runnable>)？我显然无法修补Java标准库。

Answer 1

不，你不应该。

您的BlockingQueue<MyRunnable>当然应该只包含MyRunnable。但是ThreadPoolExecutor可以将任意Runnable个任务提交给您提供的队列：请参阅execute(Runnable command)。

如果发生这种情况，您的队列中可能会有一个非MyRunnable实例。然后，您尝试从该队列的引用中进行轮询（键入为BlockingQueue<MyRunnable>），并获得ClassCastException。

简单示例：

PriorityBlockingQueue<MyRunnable> queue = new PriorityBlockingQueue<>();
ThreadPoolExecutor threadPool = new ThreadPoolExecutor(200, 200, 0L,
        TimeUnit.MILLISECONDS, queue);
threadPool.execute(new WhateverRunnable());
MyRunnable myRunnable = queue.poll(); // this could throw ClassCastException

如果queue.poll()在线程池有机会出列WhateverRunnable实例之前发生，则上述代码将抛出异常。

如何使我的泛型代码与此方法签名兼容？

1 个答案: