    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.base.Preconditions;
    020    import com.google.common.collect.Lists;
    021
    022    import java.util.Queue;
    023    import java.util.concurrent.Executor;
    024    import java.util.logging.Level;
    025    import java.util.logging.Logger;
    026
    027    /**
    028     * <p>A list of listeners, each with an associated {@code Executor}, that
    029     * guarantees that every {@code Runnable} that is {@linkplain #add added} will
    030     * be executed after {@link #execute()} is called. Any {@code Runnable} added
    031     * after the call to {@code execute} is still guaranteed to execute. There is no
    032     * guarantee, however, that listeners will be executed in the order that they
    033     * are added.
    034     *
    035     * <p>Exceptions thrown by a listener will be propagated up to the executor.
    036     * Any exception thrown during {@code Executor.execute} (e.g., a {@code
    037     * RejectedExecutionException} or an exception thrown by {@linkplain
    038     * MoreExecutors#sameThreadExecutor inline execution}) will be caught and
    039     * logged.
    040     *
    041     * @author Nishant Thakkar
    042     * @author Sven Mawson
    043     * @since 1.0
    044     */
    045    public final class ExecutionList {
    046
    047      // Logger to log exceptions caught when running runnables.
    048      private static final Logger log =
    049          Logger.getLogger(ExecutionList.class.getName());
    050
    051      // The runnable,executor pairs to execute.
    052      private final Queue<RunnableExecutorPair> runnables = Lists.newLinkedList();
    053
    054      // Boolean we use mark when execution has started.  Only accessed from within
    055      // synchronized blocks.
    056      private boolean executed = false;
    057
    058      /** Creates a new, empty {@link ExecutionList}. */
    059      public ExecutionList() {
    060      }
    061
    062      /**
    063       * Adds the {@code Runnable} and accompanying {@code Executor} to the list of
    064       * listeners to execute. If execution has already begun, the listener is
    065       * executed immediately.
    066       *
    067       * <p>Note: For fast, lightweight listeners that would be safe to execute in
    068       * any thread, consider {@link MoreExecutors#sameThreadExecutor}. For heavier
    069       * listeners, {@code sameThreadExecutor()} carries some caveats: First, the
    070       * thread that the listener runs in depends on whether the {@code
    071       * ExecutionList} has been executed at the time it is added. In particular,
    072       * listeners may run in the thread that calls {@code add}. Second, the thread
    073       * that calls {@link #execute} may be an internal implementation thread, such
    074       * as an RPC network thread, and {@code sameThreadExecutor()} listeners may
    075       * run in this thread. Finally, during the execution of a {@code
    076       * sameThreadExecutor} listener, all other registered but unexecuted
    077       * listeners are prevented from running, even if those listeners are to run
    078       * in other executors.
    079       */
    080      public void add(Runnable runnable, Executor executor) {
    081        // Fail fast on a null.  We throw NPE here because the contract of
    082        // Executor states that it throws NPE on null listener, so we propagate
    083        // that contract up into the add method as well.
    084        Preconditions.checkNotNull(runnable, "Runnable was null.");
    085        Preconditions.checkNotNull(executor, "Executor was null.");
    086
    087        boolean executeImmediate = false;
    088
    089        // Lock while we check state.  We must maintain the lock while adding the
    090        // new pair so that another thread can't run the list out from under us.
    091        // We only add to the list if we have not yet started execution.
    092        synchronized (runnables) {
    093          if (!executed) {
    094            runnables.add(new RunnableExecutorPair(runnable, executor));
    095          } else {
    096            executeImmediate = true;
    097          }
    098        }
    099
    100        // Execute the runnable immediately. Because of scheduling this may end up
    101        // getting called before some of the previously added runnables, but we're
    102        // OK with that.  If we want to change the contract to guarantee ordering
    103        // among runnables we'd have to modify the logic here to allow it.
    104        if (executeImmediate) {
    105          new RunnableExecutorPair(runnable, executor).execute();
    106        }
    107      }
    108
    109      /**
    110       * Runs this execution list, executing all existing pairs in the order they
    111       * were added. However, note that listeners added after this point may be
    112       * executed before those previously added, and note that the execution order
    113       * of all listeners is ultimately chosen by the implementations of the
    114       * supplied executors.
    115       *
    116       * <p>This method is idempotent. Calling it several times in parallel is
    117       * semantically equivalent to calling it exactly once.
    118       *
    119       * @since 10.0 (present in 1.0 as {@code run})
    120       */
    121      public void execute() {
    122        // Lock while we update our state so the add method above will finish adding
    123        // any listeners before we start to run them.
    124        synchronized (runnables) {
    125          if (executed) {
    126            return;
    127          }
    128          executed = true;
    129        }
    130
    131        // At this point the runnables will never be modified by another
    132        // thread, so we are safe using it outside of the synchronized block.
    133        while (!runnables.isEmpty()) {
    134          runnables.poll().execute();
    135        }
    136      }
    137
    138      private static class RunnableExecutorPair {
    139        final Runnable runnable;
    140        final Executor executor;
    141
    142        RunnableExecutorPair(Runnable runnable, Executor executor) {
    143          this.runnable = runnable;
    144          this.executor = executor;
    145        }
    146
    147        void execute() {
    148          try {
    149            executor.execute(runnable);
    150          } catch (RuntimeException e) {
    151            // Log it and keep going, bad runnable and/or executor.  Don't
    152            // punish the other runnables if we're given a bad one.  We only
    153            // catch RuntimeException because we want Errors to propagate up.
    154            log.log(Level.SEVERE, "RuntimeException while executing runnable "
    155                + runnable + " with executor " + executor, e);
    156          }
    157        }
    158      }
    159    }