001/*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.util.concurrent;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018
019import com.google.common.annotations.GwtIncompatible;
020import java.util.concurrent.Executor;
021import java.util.logging.Level;
022import java.util.logging.Logger;
023import javax.annotation.Nullable;
024import javax.annotation.concurrent.GuardedBy;
025
026/**
027 * A support class for {@code ListenableFuture} implementations to manage their listeners. An
028 * instance contains a list of listeners, each with an associated {@code Executor}, and guarantees
029 * that every {@code Runnable} that is {@linkplain #add added} will be executed after {@link
030 * #execute()} is called. Any {@code Runnable} added after the call to {@code execute} is still
031 * guaranteed to execute. There is no guarantee, however, that listeners will be executed in the
032 * order that they are added.
033 *
034 * <p>Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown
035 * during {@code Executor.execute} (e.g., a {@code RejectedExecutionException} or an exception
036 * thrown by {@linkplain MoreExecutors#directExecutor direct execution}) will be caught and logged.
037 *
038 * @author Nishant Thakkar
039 * @author Sven Mawson
040 * @since 1.0
041 */
042@GwtIncompatible
043public final class ExecutionList {
044  /** Logger to log exceptions caught when running runnables. */
045  private static final Logger log = Logger.getLogger(ExecutionList.class.getName());
046
047  /**
048   * The runnable, executor pairs to execute.  This acts as a stack threaded through the {@link
049   * RunnableExecutorPair#next} field.
050   */
051  @GuardedBy("this")
052  private RunnableExecutorPair runnables;
053
054  @GuardedBy("this")
055  private boolean executed;
056
057  /** Creates a new, empty {@link ExecutionList}. */
058  public ExecutionList() {}
059
060  /**
061   * Adds the {@code Runnable} and accompanying {@code Executor} to the list of listeners to
062   * execute. If execution has already begun, the listener is executed immediately.
063   *
064   * <p>When selecting an executor, note that {@code directExecutor} is dangerous in some cases. See
065   * the discussion in the {@link ListenableFuture#addListener ListenableFuture.addListener}
066   * documentation.
067   */
068  public void add(Runnable runnable, Executor executor) {
069    // Fail fast on a null. We throw NPE here because the contract of Executor states that it throws
070    // NPE on null listener, so we propagate that contract up into the add method as well.
071    checkNotNull(runnable, "Runnable was null.");
072    checkNotNull(executor, "Executor was null.");
073
074    // Lock while we check state. We must maintain the lock while adding the new pair so that
075    // another thread can't run the list out from under us. We only add to the list if we have not
076    // yet started execution.
077    synchronized (this) {
078      if (!executed) {
079        runnables = new RunnableExecutorPair(runnable, executor, runnables);
080        return;
081      }
082    }
083    // Execute the runnable immediately. Because of scheduling this may end up getting called before
084    // some of the previously added runnables, but we're OK with that. If we want to change the
085    // contract to guarantee ordering among runnables we'd have to modify the logic here to allow
086    // it.
087    executeListener(runnable, executor);
088  }
089
090  /**
091   * Runs this execution list, executing all existing pairs in the order they were added. However,
092   * note that listeners added after this point may be executed before those previously added, and
093   * note that the execution order of all listeners is ultimately chosen by the implementations of
094   * the supplied executors.
095   *
096   * <p>This method is idempotent. Calling it several times in parallel is semantically equivalent
097   * to calling it exactly once.
098   *
099   * @since 10.0 (present in 1.0 as {@code run})
100   */
101  public void execute() {
102    // Lock while we update our state so the add method above will finish adding any listeners
103    // before we start to run them.
104    RunnableExecutorPair list;
105    synchronized (this) {
106      if (executed) {
107        return;
108      }
109      executed = true;
110      list = runnables;
111      runnables = null; // allow GC to free listeners even if this stays around for a while.
112    }
113    // If we succeeded then list holds all the runnables we to execute. The pairs in the stack are
114    // in the opposite order from how they were added so we need to reverse the list to fulfill our
115    // contract.
116    // This is somewhat annoying, but turns out to be very fast in practice. Alternatively, we could
117    // drop the contract on the method that enforces this queue like behavior since depending on it
118    // is likely to be a bug anyway.
119
120    // N.B. All writes to the list and the next pointers must have happened before the above
121    // synchronized block, so we can iterate the list without the lock held here.
122    RunnableExecutorPair reversedList = null;
123    while (list != null) {
124      RunnableExecutorPair tmp = list;
125      list = list.next;
126      tmp.next = reversedList;
127      reversedList = tmp;
128    }
129    while (reversedList != null) {
130      executeListener(reversedList.runnable, reversedList.executor);
131      reversedList = reversedList.next;
132    }
133  }
134
135  /**
136   * Submits the given runnable to the given {@link Executor} catching and logging all {@linkplain
137   * RuntimeException runtime exceptions} thrown by the executor.
138   */
139  private static void executeListener(Runnable runnable, Executor executor) {
140    try {
141      executor.execute(runnable);
142    } catch (RuntimeException e) {
143      // Log it and keep going -- bad runnable and/or executor. Don't punish the other runnables if
144      // we're given a bad one. We only catch RuntimeException because we want Errors to propagate
145      // up.
146      log.log(
147          Level.SEVERE,
148          "RuntimeException while executing runnable " + runnable + " with executor " + executor,
149          e);
150    }
151  }
152
153  private static final class RunnableExecutorPair {
154    final Runnable runnable;
155    final Executor executor;
156    @Nullable RunnableExecutorPair next;
157
158    RunnableExecutorPair(Runnable runnable, Executor executor, RunnableExecutorPair next) {
159      this.runnable = runnable;
160      this.executor = executor;
161      this.next = next;
162    }
163  }
164}