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
017package com.google.common.util.concurrent;
018
019import com.google.common.annotations.VisibleForTesting;
020import com.google.common.base.Preconditions;
021
022import java.util.concurrent.Executor;
023import java.util.logging.Level;
024import java.util.logging.Logger;
025
026import javax.annotation.Nullable;
027import javax.annotation.concurrent.GuardedBy;
028
029/**
030 * <p>A list of listeners, each with an associated {@code Executor}, that
031 * guarantees that every {@code Runnable} that is {@linkplain #add added} will
032 * be executed after {@link #execute()} is called. Any {@code Runnable} added
033 * after the call to {@code execute} is still guaranteed to execute. There is no
034 * guarantee, however, that listeners will be executed in the order that they
035 * are added.
036 *
037 * <p>Exceptions thrown by a listener will be propagated up to the executor.
038 * Any exception thrown during {@code Executor.execute} (e.g., a {@code
039 * RejectedExecutionException} or an exception thrown by {@linkplain
040 * MoreExecutors#directExecutor direct execution}) will be caught and
041 * logged.
042 *
043 * @author Nishant Thakkar
044 * @author Sven Mawson
045 * @since 1.0
046 */
047public final class ExecutionList {
048  // Logger to log exceptions caught when running runnables.
049  @VisibleForTesting static final Logger log = Logger.getLogger(ExecutionList.class.getName());
050
051  /**
052   * The runnable, executor pairs to execute.  This acts as a stack threaded through the 
053   * {@link RunnableExecutorPair#next} field.
054   */
055  @GuardedBy("this")
056  private RunnableExecutorPair runnables;
057  @GuardedBy("this")
058  private boolean executed;
059
060  /** Creates a new, empty {@link ExecutionList}. */
061  public ExecutionList() {}
062
063  /**
064   * Adds the {@code Runnable} and accompanying {@code Executor} to the list of
065   * listeners to execute. If execution has already begun, the listener is
066   * executed immediately.
067   *
068   * <p>Note: For fast, lightweight listeners that would be safe to execute in
069   * any thread, consider {@link MoreExecutors#directExecutor}. For heavier
070   * listeners, {@code directExecutor()} carries some caveats: First, the
071   * thread that the listener runs in depends on whether the {@code
072   * ExecutionList} has been executed at the time it is added. In particular,
073   * listeners may run in the thread that calls {@code add}. Second, the thread
074   * that calls {@link #execute} may be an internal implementation thread, such
075   * as an RPC network thread, and {@code directExecutor()} listeners may
076   * run in this thread. Finally, during the execution of a {@code
077   * directExecutor} listener, all other registered but unexecuted
078   * listeners are prevented from running, even if those listeners are to run
079   * in other executors.
080   */
081  public void add(Runnable runnable, Executor executor) {
082    // Fail fast on a null.  We throw NPE here because the contract of
083    // Executor states that it throws NPE on null listener, so we propagate
084    // that contract up into the add method as well.
085    Preconditions.checkNotNull(runnable, "Runnable was null.");
086    Preconditions.checkNotNull(executor, "Executor was null.");
087
088    // Lock while we check state.  We must maintain the lock while adding the
089    // new pair so that another thread can't run the list out from under us.
090    // We only add to the list if we have not yet started execution.
091    synchronized (this) {
092      if (!executed) {
093        runnables = new RunnableExecutorPair(runnable, executor, runnables);
094        return;
095      }
096    }
097    // Execute the runnable immediately. Because of scheduling this may end up
098    // getting called before some of the previously added runnables, but we're
099    // OK with that.  If we want to change the contract to guarantee ordering
100    // among runnables we'd have to modify the logic here to allow it.
101    executeListener(runnable, executor);
102  }
103
104  /**
105   * Runs this execution list, executing all existing pairs in the order they
106   * were added. However, note that listeners added after this point may be
107   * executed before those previously added, and note that the execution order
108   * of all listeners is ultimately chosen by the implementations of the
109   * supplied executors.
110   *
111   * <p>This method is idempotent. Calling it several times in parallel is
112   * semantically equivalent to calling it exactly once.
113   *
114   * @since 10.0 (present in 1.0 as {@code run})
115   */
116  public void execute() {
117    // Lock while we update our state so the add method above will finish adding
118    // any listeners before we start to run them.
119    RunnableExecutorPair list;
120    synchronized (this) {
121      if (executed) {
122        return;
123      }
124      executed = true;
125      list = runnables;
126      runnables = null;  // allow GC to free listeners even if this stays around for a while.
127    }
128    // If we succeeded then list holds all the runnables we to execute.  The pairs in the stack are
129    // in the opposite order from how they were added so we need to reverse the list to fulfill our
130    // contract.
131    // This is somewhat annoying, but turns out to be very fast in practice.  Alternatively, we 
132    // could drop the contract on the method that enforces this queue like behavior since depending
133    // on it is likely to be a bug anyway.
134    
135    // N.B. All writes to the list and the next pointers must have happened before the above 
136    // synchronized block, so we can iterate the list without the lock held here.
137    RunnableExecutorPair reversedList = null;
138    while (list != null) {
139      RunnableExecutorPair tmp = list;
140      list = list.next;
141      tmp.next = reversedList;
142      reversedList = tmp;
143    }
144    while (reversedList != null) {
145      executeListener(reversedList.runnable, reversedList.executor);
146      reversedList = reversedList.next;
147    }
148  }
149
150  /**
151   * Submits the given runnable to the given {@link Executor} catching and logging all 
152   * {@linkplain RuntimeException runtime exceptions} thrown by the executor.
153   */
154  private static void executeListener(Runnable runnable, Executor executor) {
155    try {
156      executor.execute(runnable);
157    } catch (RuntimeException e) {
158      // Log it and keep going, bad runnable and/or executor.  Don't
159      // punish the other runnables if we're given a bad one.  We only
160      // catch RuntimeException because we want Errors to propagate up.
161      log.log(Level.SEVERE, "RuntimeException while executing runnable "
162          + runnable + " with executor " + executor, e);
163    }
164  }
165
166  private static final class RunnableExecutorPair {
167    final Runnable runnable;
168    final Executor executor;
169    @Nullable RunnableExecutorPair next;
170
171    RunnableExecutorPair(Runnable runnable, Executor executor, RunnableExecutorPair next) {
172      this.runnable = runnable;
173      this.executor = executor;
174      this.next = next;
175    }
176  }
177}