java - 如何使我的泛型代码与此方法签名兼容？

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);
    }
}

如您所见，我的队列与 ThreadPoolExecutor 不兼容构造函数。有什么办法可以解决这个问题，而不是将我的队列转换为 (BlockingQueue<Runnable>) ？我显然无法修补 Java 标准库。

Answer 1

不，你不应该。

你的 BlockingQueue<MyRunnable>当然，应该只包含 MyRunnable秒。但是 ThreadPoolExecutor可以提交任意Runnable任务到你给它的队列:见 execute(Runnable command) .

如果发生这种情况，您可能会得到一个非 MyRunnable队列中的实例。然后您尝试从您对该队列的引用(输入为 BlockingQueue<MyRunnable> )进行轮询，并获得 ClassCastException .

简单的例子:

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() 上面的代码将抛出异常在线程池有机会出列 WhateverRunnable 之前发生实例。