在 ThreadPoolExecutor 中实现优先级队列

已经为此苦苦挣扎了 2 天多了。

实施了我在这里看到的答案Java中指定任务顺序执行 https://stackoverflow.com/questions/12722396/specify-task-order-execution-in-java/12722648#12722648

public class PriorityExecutor extends ThreadPoolExecutor {

public PriorityExecutor(int corePoolSize, int maximumPoolSize,
                        long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
    super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
}
//Utitlity method to create thread pool easily
public static ExecutorService newFixedThreadPool(int nThreads) {
    return new PriorityExecutor(nThreads, nThreads, 0L,
            TimeUnit.MILLISECONDS, new PriorityBlockingQueue<Runnable>());
}
//Submit with New comparable task
public Future<?> submit(Runnable task, int priority) {
    return super.submit(new ComparableFutureTask(task, null, priority));
}
//execute with New comparable task
public void execute(Runnable command, int priority) {
    super.execute(new ComparableFutureTask(command, null, priority));
}
}

public class ComparableFutureTask<T> extends FutureTask<T>
    implements
    Comparable<ComparableFutureTask<T>> {

volatile int priority = 0;

public ComparableFutureTask(Runnable runnable, T result, int priority) {
    super(runnable, result);
    this.priority = priority;
}
public ComparableFutureTask(Callable<T> callable, int priority) {
    super(callable);
    this.priority = priority;
}

@Override
public int compareTo(ComparableFutureTask<T> o) {
    return Integer.valueOf(priority).compareTo(o.priority);
}
}

我使用的Runnable：MyTask

public class MyTask implements Runnable{

 public MyTask(File file, Context context, int requestId) {
    this._file = file;
    this.context = context;
    this.requestId = requestId;
}

@Override
public void run() {
      // some work
    } catch (IOException e) {
        Log.e("Callable try", post.toString());

    }
}

我的服务：MediaDownloadService

public class MediaDownloadService extends Service {

private DBHelper helper;
Notification notification;
HashMap<Integer,Future> futureTasks = new HashMap<Integer, Future>();
final int _notificationId=1;
File file;

@Override
public IBinder onBind(Intent intent) {
    return sharonsBinder;
}


@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    helper = new DBHelper(getApplicationContext());
    PriorityExecutor executor = (PriorityExecutor) PriorityExecutor.newFixedThreadPool(3);
    Log.e("requestsExists", helper.requestsExists() + "");
   if(helper.requestsExists()){
        // map of the index of the request and the string of the absolute path of the request
        Map<Integer,String> requestMap = helper.getRequestsToExcute(0);
        Set<Integer> keySet = requestMap.keySet();
        Iterator<Integer> iterator = keySet.iterator();
        Log.e("MAP",requestMap.toString());
        //checks if the DB requests exists
        if(!requestMap.isEmpty()){
            //execute them and delete the DB entry
            while(iterator.hasNext()){
                int iteratorNext = iterator.next();
                Log.e("ITREATOR", iteratorNext + "");
                file = new File(requestMap.get(iteratorNext));
                Log.e("file", file.toString());
                Log.e("thread Opened", "Thread" + iteratorNext);
                Future future = executor.submit(new MyTask(file, this, iteratorNext),10);
                futureTasks.put(iteratorNext, future);
                helper.requestTaken(iteratorNext);
            }
            Log.e("The priority queue",executor.getQueue().toString());
        }else{

            Log.e("stopself", "stop self after this");
            this.stopSelf();
        }
    }
    return START_STICKY;
}

这条线上不断出现错误： Future future = executor.submit(new MyTask(file, this, iteratorNext),10);

即使是 executor.submit();假设返回一个我不断得到的未来对象

Caused by: java.lang.ClassCastException: java.util.concurrent.FutureTask cannot be cast to java.lang.Comparable
        at java.util.concurrent.PriorityBlockingQueue.siftUpComparable(PriorityBlockingQueue.java:318)
        at java.util.concurrent.PriorityBlockingQueue.offer(PriorityBlockingQueue.java:450)
        at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1331)
        at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:81)
        at com.vit.infibond.test.PriorityExecutor.submit(PriorityExecutor.java:26)
        at com.vit.infibond.test.MediaDownloadService.onStartCommand(MediaDownloadService.java:65)

谁能把我从这场噩梦中拯救出来？

我也尝试按照这个答案的建议去做在 ThreadPoolExecutor 中测试 PriorityBlockingQueue https://stackoverflow.com/questions/16833951/testing-priorityblockingqueue-in-threadpoolexecutor/16834473#16834473

通过添加 for Task 覆盖只是为了再次获得强制转换异常，但这次是针对 RunnableFuture。

我知道我的理解中缺少一些基本的东西，并且希望得到深入的解释......

通过查看源代码java.util.concurrent.ThreadPoolExecutor在提交 future 时让这个工作似乎真的很痛苦。您必须重写感觉是内部的受保护方法并执行一些令人讨厌的转换。

我建议你简单地使用execute方法代替。没有包装Runnable在那里进行，所以它应该可以工作。

如果您需要等待工作结果，我建议您自己实施，以避免浪费时间ThreadPoolExecutor内部结构。