ThreadPoolExecutor的PriorityBlockingQueue型別轉化問題
最近在使用ThreadPoolExecutor的時候碰到點問題,因為專案原因在使用ThreadPoolExecutor準備把BlockingQueue替換為PriorityBlockingQueue,從而實現對優先順序任務處理的執行緒池,貼下程式碼先
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的型別是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介面,但是我提交的Event
是實現了Comparable
介面的,究竟是因為什麼原因導致其成為了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
被轉化為了FutureTask
了,而FutureTask
沒有實現Comparable
,所以才會報錯
現在解決的方法有:
- 用一個ComparableFutureTask
繼承FutureTask
並實現Comparable
介面,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.