Java那點事——異步

在JDK1.6提供了Future，FutureTask，ExecutorService等用于支持異步編程，但是Future，FutureTask沒有提供callback機制，只能主動輪詢，通過get去獲取結果。

Guava的ListenableFuture對此做了擴展，支持callback機制。

就callback機制的擴展而言，也并不復雜。看看ListenableFuture與ListenableFutureTask：

public interface ListenableFuture extends Future {
  void addListener(Runnable listener, Executor executor);
}

public class ListenableFutureTask extends FutureTask implements ListenableFuture {

  // The execution list to hold our listeners.
  private final ExecutionList executionList = new ExecutionList();

  //………

  @Override
  public void addListener(Runnable listener, Executor exec) {
    executionList.add(listener, exec);
  }

  /**
   * Internal implementation detail used to invoke the listeners.
   */
  @Override
  protected void done() {
    executionList.execute();
  }
}

callback通過addListener添加，通過override FutureTask的done()函數實現回調。 查看FutureTask會在Task執行完以后調用done()函數，如：

/**
 * Removes and signals all waiting threads, invokes done(), and
 * nulls out callable.
 */
private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {
        if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
            for (;;) {
                Thread t = q.thread;
                if (t != null) {
                    q.thread = null;
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }

    done();

    callable = null;        // to reduce footprint
}

與JDK一樣，ListenableFuture需要通過ExecutorService創建，Guava定義了對應的ExecutorService，并提供MoreExecutors作為工具類，其中實現了ListeningDecorator與ScheduledListeningDecorator，它們都是對應的JDK ExecutorService的裝飾。MoreExecutors提供的工具api非常很多，需要細細瞧瞧。

1.8版本開始，JDK也開始提供了一些更好支持異步編程的Future，典型的有CompletableFuture.