001 /*
002 * Copyright (C) 2007 Google Inc.
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 import com.google.common.base.Preconditions;
021 import com.google.common.collect.Lists;
022
023 import java.util.Queue;
024 import java.util.concurrent.Executor;
025 import java.util.logging.Level;
026 import java.util.logging.Logger;
027
028 /**
029 * <p>A list of ({@code Runnable}, {@code Executor}) pairs that guarantees
030 * that every {@code Runnable} that is added using the add method will be
031 * executed in its associated {@code Executor} after {@link #run()} is called.
032 * {@code Runnable}s added after {@code run} is called are still guaranteed to
033 * execute.
034 *
035 * @author Nishant Thakkar
036 * @author Sven Mawson
037 * @since 1
038 */
039 @Beta
040 public class ExecutionList implements Runnable {
041
042 // Logger to log exceptions caught when running runnables.
043 private static final Logger LOG =
044 Logger.getLogger(ExecutionList.class.getName());
045
046 // The runnable,executor pairs to execute.
047 private final Queue<RunnableExecutorPair> runnables = Lists.newLinkedList();
048
049 // Boolean we use mark when execution has started. Only accessed from within
050 // synchronized blocks.
051 private boolean executed = false;
052
053 /**
054 * Add the runnable/executor pair to the list of pairs to execute. Executes
055 * the pair immediately if we've already started execution.
056 */
057 public void add(Runnable runnable, Executor executor) {
058
059 // Fail fast on a null. We throw NPE here because the contract of
060 // Executor states that it throws NPE on null listener, so we propagate
061 // that contract up into the add method as well.
062 Preconditions.checkNotNull(runnable, "Runnable was null.");
063 Preconditions.checkNotNull(executor, "Executor was null.");
064
065 boolean executeImmediate = false;
066
067 // Lock while we check state. We must maintain the lock while adding the
068 // new pair so that another thread can't run the list out from under us.
069 // We only add to the list if we have not yet started execution.
070 synchronized (runnables) {
071 if (!executed) {
072 runnables.add(new RunnableExecutorPair(runnable, executor));
073 } else {
074 executeImmediate = true;
075 }
076 }
077
078 // Execute the runnable immediately. Because of scheduling this may end up
079 // getting called before some of the previously added runnables, but we're
080 // ok with that. If we want to change the contract to guarantee ordering
081 // among runnables we'd have to modify the logic here to allow it.
082 if (executeImmediate) {
083 executor.execute(runnable);
084 }
085 }
086
087 /**
088 * Runs this execution list, executing all pairs in the order they were
089 * added. Pairs added after this method has started executing the list will
090 * be executed immediately.
091 */
092 public void run() {
093
094 // Lock while we update our state so the add method above will finish adding
095 // any listeners before we start to run them.
096 synchronized (runnables) {
097 executed = true;
098 }
099
100 // At this point the runnables will never be modified by another
101 // thread, so we are safe using it outside of the synchronized block.
102 while (!runnables.isEmpty()) {
103 runnables.poll().execute();
104 }
105 }
106
107 private static class RunnableExecutorPair {
108 final Runnable runnable;
109 final Executor executor;
110
111 RunnableExecutorPair(Runnable runnable, Executor executor) {
112 this.runnable = runnable;
113 this.executor = executor;
114 }
115
116 void execute() {
117 try {
118 executor.execute(runnable);
119 } catch (RuntimeException e) {
120 // Log it and keep going, bad runnable and/or executor. Don't
121 // punish the other runnables if we're given a bad one. We only
122 // catch RuntimeException because we want Errors to propagate up.
123 LOG.log(Level.SEVERE, "RuntimeException while executing runnable "
124 + runnable + " with executor " + executor, e);
125 }
126 }
127 }
128 }