ThreadPoolExecutor的PriorityBlockingQueue類型轉化問題

最近在使用ThreadPoolExecutor的時候碰到點問題,因爲項目原因在使用ThreadPoolExecutor準備把BlockingQueue替換爲PriorityBlockingQueue,從而實現對優先級任務處理的線程池,貼下代碼先

public abstract class Event<T> implements Callable<T>,Comparable<Event>

ExecutorService executorService=new ThreadPoolExecutor(1,1,1,TimeUnit.SECONDS,new PriorityBlockingQueue());

然後當我進行以下操作的時候

executorService.submit(new Event<Object>() {
            @Override
            public Object call() throws Exception {
                ...
                return null;
            }
        });

出現瞭如下錯誤

java.lang.ClassCastException: java.util.concurrent.FutureTask cannot be cast to java.lang.Comparable
    at java.util.concurrent.PriorityBlockingQueue.siftUpComparable(PriorityBlockingQueue.java:357)
    at java.util.concurrent.PriorityBlockingQueue.offer(PriorityBlockingQueue.java:489)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1361)
    at java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:134)

找到java.util.concurrent.PriorityBlockingQueue.siftUpComparable方法：

private static <T> void siftUpComparable(int k, T x, Object[] array) {
        Comparable<? super T> key = (Comparable<? super T>) x;
        while (k > 0) {
            int parent = (k - 1) >>> 1;
            Object e = array[parent];
            if (key.compareTo((T) e) >= 0)
                break;
            array[k] = e;
            k = parent;
        }
        array[k] = key;
    }

是在Comparable<? super T> key = (Comparable<? super T>) x;上出現問題，根據java.util.concurrent.FutureTask cannot be cast to java.lang.Comparable知道x的類型是java.util.concurrent.FutureTask。現在看看FutureTask：

public class FutureTask<V> implements RunnableFuture<V>
public interface RunnableFuture<V> extends Runnable, Future<V>

可見FutureTask的確沒有實現Comparable接口,但是我提交的Event是實現了Comparable接口的,究竟是因爲什麼原因導致其成爲了FutureTask呢，結果在ThreadPoolExecutor的submit(Callable<T> task)找到原因，它是在AbstractExecutorService中實現的。

public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

重點在newTaskFor方法

protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

所以我提交的Event被轉化爲了FutureTask了，而FutureTask沒有實現Comparable，所以纔會報錯
現在解決的方法有:
- 用一個ComparableFutureTask繼承FutureTask並實現Comparable接口，但也必須要override ThreadPoolExecutor的newTaskFor方法

另外需要注意的是PriorityBlockingQueue的實現是一個最小堆.