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构造函数不兼容。除了将队列转换到(BlockingQueue<Runnable>)之外,还有什么方法可以解决此问题?我显然无法修补Java标准库。

最佳答案

不,你不应该。

当然,您的BlockingQueue<MyRunnable>应该只包含MyRunnable。但是ThreadPoolExecutor可以将任意Runnable任务提交到您给它的队列中:请参阅 execute(Runnable command)

如果发生这种情况,您的队列中可能有一个非MyRunnable实例。然后,您尝试从该队列的引用中轮询(键入为BlockingQueue<MyRunnable>),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()发生在线程池有机会使WhateverRunnable实例出队之前,则上面的代码将引发异常。

关于java - 如何使我的泛型代码与此方法签名兼容?,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/24636199/

10-10 16:19