1 /* 2 * Copyright (C) 2007 Google Inc. 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 package com.google.common.util.concurrent; 18 19 import com.google.common.base.Preconditions; 20 import com.google.common.collect.Lists; 21 22 import java.util.List; 23 import java.util.concurrent.Executor; 24 import java.util.logging.Level; 25 import java.util.logging.Logger; 26 27 /** 28 * <p>A list of ({@code Runnable}, {@code Executor}) pairs that guarantees 29 * that every {@code Runnable} that is added using the add method will be 30 * executed in its associated {@code Executor} after {@link #run()} is called. 31 * {@code Runnable}s added after {@code run} is called are still guaranteed to 32 * execute. 33 * 34 * @author Nishant Thakkar 35 * @author Sven Mawson 36 * @since 2009.09.15 <b>tentative</b> 37 */ 38 public class ExecutionList implements Runnable { 39 40 // Logger to log exceptions caught when running runnables. 41 private static final Logger LOG = 42 Logger.getLogger(ExecutionList.class.getName()); 43 44 // The list of runnable,executor pairs to execute. Only modified within 45 // a synchronized block. 46 private final List<RunnableExecutorPair> runnables = Lists.newArrayList(); 47 48 // Boolean we use mark when execution has started. Only accessed from within 49 // synchronized blocks. 50 private boolean executed = false; 51 52 /** 53 * Add the runnable/executor pair to the list of pairs to execute. Executes 54 * the pair immediately if we've already started execution. 55 */ 56 public void add(Runnable runnable, Executor executor) { 57 58 // Fail fast on a null. We throw NPE here because the contract of 59 // Executor states that it throws NPE on null listener, so we propagate 60 // that contract up into the add method as well. 61 Preconditions.checkNotNull(runnable, "Runnable was null."); 62 Preconditions.checkNotNull(executor, "Executor was null."); 63 64 boolean executeImmediate = false; 65 66 // Lock while we check state. We must maintain the lock while adding the 67 // new pair so that another thread can't run the list out from under us. 68 // We only add to the list if we have not yet started execution. 69 synchronized (runnables) { 70 if (!executed) { 71 runnables.add(new RunnableExecutorPair(runnable, executor)); 72 } else { 73 executeImmediate = true; 74 } 75 } 76 77 // Execute the runnable immediately. Because of scheduling this may end up 78 // getting called before some of the previously added runnables, but we're 79 // ok with that. If we want to change the contract to guarantee ordering 80 // among runnables we'd have to modify the logic here to allow it. 81 if (executeImmediate) { 82 executor.execute(runnable); 83 } 84 } 85 86 /** 87 * Runs this execution list, executing all pairs in the order they were 88 * added. Pairs added after this method has started executing the list will 89 * be executed immediately. 90 */ 91 public void run() { 92 93 // Lock while we update our state so the add method above will finish adding 94 // any listeners before we start to run them. 95 synchronized (runnables) { 96 executed = true; 97 } 98 99 // At this point the runnable list will never be modified again, so we are 100 // safe running it outside of the synchronized block. 101 for (RunnableExecutorPair runnableAndExecutor : runnables) { 102 runnableAndExecutor.execute(); 103 } 104 } 105 106 private static class RunnableExecutorPair { 107 final Runnable runnable; 108 final Executor executor; 109 110 RunnableExecutorPair(Runnable runnable, Executor executor) { 111 this.runnable = runnable; 112 this.executor = executor; 113 } 114 115 void execute() { 116 try { 117 executor.execute(runnable); 118 } catch (RuntimeException e) { 119 // Log it and keep going, bad runnable and/or executor. Don't 120 // punish the other runnables if we're given a bad one. We only 121 // catch RuntimeException because we want Errors to propagate up. 122 LOG.log(Level.SEVERE, "RuntimeException while executing runnable " 123 + runnable + " with executor " + executor, e); 124 } 125 } 126 } 127 } 128
