001    /*
002     * Copyright (C) 2007 The Guava Authors
003     *
004     * Licensed under the Apache License, Version 2.0 (the "License");
005     * you may not use this file except in compliance with the License.
006     * You may obtain a copy of the License at
007     *
008     * http://www.apache.org/licenses/LICENSE-2.0
009     *
010     * Unless required by applicable law or agreed to in writing, software
011     * distributed under the License is distributed on an "AS IS" BASIS,
012     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013     * See the License for the specific language governing permissions and
014     * limitations under the License.
015     */
016    
017    package com.google.common.util.concurrent;
018    
019    import com.google.common.annotations.Beta;
020    
021    import java.util.concurrent.Executor;
022    import java.util.concurrent.Future;
023    import java.util.concurrent.RejectedExecutionException;
024    
025    /**
026     * A {@link Future} that accepts completion listeners.  Each listener has an
027     * associated executor, and is invoked using this executor once the future's
028     * computation is {@linkplain Future#isDone() complete}.  If the computation has
029     * already completed when the listener is added, the listener will execute
030     * immediately.
031     *
032     * <p>Common {@code ListenableFuture} implementations include {@link
033     * SettableFuture} and the futures returned by a {@link
034     * ListeningExecutorService} (typically {@link ListenableFutureTask}
035     * instances).
036     *
037     * <p>Usage:
038     * <pre>   {@code
039     *   final ListenableFuture<?> future = myService.async(myRequest);
040     *   future.addListener(new Runnable() {
041     *     public void run() {
042     *       System.out.println("Operation Complete.");
043     *       try {
044     *         System.out.println("Result: " + future.get());
045     *       } catch (Exception e) {
046     *         System.out.println("Error: " + e.message());
047     *       }
048     *     }
049     *   }, executor);}</pre>
050     *
051     * @author Sven Mawson
052     * @author Nishant Thakkar
053     * @since 1
054     */
055    @Beta
056    public interface ListenableFuture<V> extends Future<V> {
057      /**
058       * Registers a listener to be {@linkplain Executor#execute(Runnable) run} on
059       * the given executor.  The listener will run when the {@code Future}'s
060       * computation is {@linkplain Future#isDone() complete} or, if the computation
061       * is already complete, immediately.
062       *
063       * <p>There is no guaranteed ordering of execution of listeners, but any
064       * listener added through this method is guaranteed to be called once the
065       * computation is complete.
066       *
067       * <p>Listeners cannot throw checked exceptions and should not throw {@code
068       * RuntimeException} unless their executors are prepared to handle it.
069       * Listeners that will execute in {@link MoreExecutors#sameThreadExecutor}
070       * should take special care, since they may run during the call to {@code
071       * addListener} or during the call that sets the future's value.
072       *
073       * @param listener the listener to run when the computation is complete
074       * @param executor the executor to run the listener in
075       * @throws NullPointerException if the executor or listener was null
076       * @throws RejectedExecutionException if we tried to execute the listener
077       *         immediately but the executor rejected it.
078       */
079      void addListener(Runnable listener, Executor executor);
080    }