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/*
* Copyright (C) 2007 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.util.concurrent;
import com.google.common.annotations.Beta;
import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;
import java.util.List;
import java.util.concurrent.Executor;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* A list of ({@code Runnable}, {@code Executor}) pairs that guarantees
* that every {@code Runnable} that is added using the add method will be
* executed in its associated {@code Executor} after {@link #run()} is called.
* {@code Runnable}s added after {@code run} is called are still guaranteed to
* execute.
*
* @author Nishant Thakkar
* @author Sven Mawson
* @since 1
*/
@Beta
public class ExecutionList implements Runnable {
// Logger to log exceptions caught when running runnables.
private static final Logger LOG =
Logger.getLogger(ExecutionList.class.getName());
// The list of runnable,executor pairs to execute. Only modified within
// a synchronized block.
private final List runnables = Lists.newArrayList();
// Boolean we use mark when execution has started. Only accessed from within
// synchronized blocks.
private boolean executed = false;
/**
* Add the runnable/executor pair to the list of pairs to execute. Executes
* the pair immediately if we've already started execution.
*/
public void add(Runnable runnable, Executor executor) {
// Fail fast on a null. We throw NPE here because the contract of
// Executor states that it throws NPE on null listener, so we propagate
// that contract up into the add method as well.
Preconditions.checkNotNull(runnable, "Runnable was null.");
Preconditions.checkNotNull(executor, "Executor was null.");
boolean executeImmediate = false;
// Lock while we check state. We must maintain the lock while adding the
// new pair so that another thread can't run the list out from under us.
// We only add to the list if we have not yet started execution.
synchronized (runnables) {
if (!executed) {
runnables.add(new RunnableExecutorPair(runnable, executor));
} else {
executeImmediate = true;
}
}
// Execute the runnable immediately. Because of scheduling this may end up
// getting called before some of the previously added runnables, but we're
// ok with that. If we want to change the contract to guarantee ordering
// among runnables we'd have to modify the logic here to allow it.
if (executeImmediate) {
executor.execute(runnable);
}
}
/**
* Runs this execution list, executing all pairs in the order they were
* added. Pairs added after this method has started executing the list will
* be executed immediately.
*/
public void run() {
// Lock while we update our state so the add method above will finish adding
// any listeners before we start to run them.
synchronized (runnables) {
executed = true;
}
// At this point the runnable list will never be modified again, so we are
// safe running it outside of the synchronized block.
for (RunnableExecutorPair runnableAndExecutor : runnables) {
runnableAndExecutor.execute();
}
}
private static class RunnableExecutorPair {
final Runnable runnable;
final Executor executor;
RunnableExecutorPair(Runnable runnable, Executor executor) {
this.runnable = runnable;
this.executor = executor;
}
void execute() {
try {
executor.execute(runnable);
} catch (RuntimeException e) {
// Log it and keep going, bad runnable and/or executor. Don't
// punish the other runnables if we're given a bad one. We only
// catch RuntimeException because we want Errors to propagate up.
LOG.log(Level.SEVERE, "RuntimeException while executing runnable "
+ runnable + " with executor " + executor, e);
}
}
}
}