/*
* Copyright (C) 2007 The Guava Authors
*
* 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.base.Preconditions;
import com.google.common.collect.Lists;
import java.util.Queue;
import java.util.concurrent.Executor;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* <p>A list of listeners, each with an associated {@code Executor}, that
* guarantees that every {@code Runnable} that is {@linkplain #add added} will
* be executed after {@link #execute()} is called. Any {@code Runnable} added
* after the call to {@code execute} is still guaranteed to execute. There is no
* guarantee, however, that listeners will be executed in the order that they
* are added.
*
* <p>Exceptions thrown by a listener will be propagated up to the executor.
* Any exception thrown during {@code Executor.execute} (e.g., a {@code
* RejectedExecutionException} or an exception thrown by {@linkplain
* MoreExecutors#sameThreadExecutor inline execution}) will be caught and
* logged.
*
* @author Nishant Thakkar
* @author Sven Mawson
* @since 1.0
*/
public final class ExecutionList {
// Logger to log exceptions caught when running runnables.
private static final Logger log =
Logger.getLogger(ExecutionList.class.getName());
// The runnable,executor pairs to execute.
private final Queue<RunnableExecutorPair> runnables = Lists.newLinkedList();
// Boolean we use mark when execution has started. Only accessed from within
// synchronized blocks.
private boolean executed = false;
/** Creates a new, empty {@link ExecutionList}. */
public ExecutionList() {
}
/**
* Adds the {@code Runnable} and accompanying {@code Executor} to the list of
* listeners to execute. If execution has already begun, the listener is
* executed immediately.
*
* <p>Note: For fast, lightweight listeners that would be safe to execute in
* any thread, consider {@link MoreExecutors#sameThreadExecutor}. For heavier
* listeners, {@code sameThreadExecutor()} carries some caveats: First, the
* thread that the listener runs in depends on whether the {@code
* ExecutionList} has been executed at the time it is added. In particular,
* listeners may run in the thread that calls {@code add}. Second, the thread
* that calls {@link #execute} may be an internal implementation thread, such
* as an RPC network thread, and {@code sameThreadExecutor()} listeners may
* run in this thread. Finally, during the execution of a {@code
* sameThreadExecutor} listener, all other registered but unexecuted
* listeners are prevented from running, even if those listeners are to run
* in other executors.
*/
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) {
new RunnableExecutorPair(runnable, executor).execute();
}
}
/**
* Runs this execution list, executing all existing pairs in the order they
* were added. However, note that listeners added after this point may be
* executed before those previously added, and note that the execution order
* of all listeners is ultimately chosen by the implementations of the
* supplied executors.
*
* <p>This method is idempotent. Calling it several times in parallel is
* semantically equivalent to calling it exactly once.
*
* @since 10.0 (present in 1.0 as {@code run})
*/
public void execute() {
// Lock while we update our state so the add method above will finish adding
// any listeners before we start to run them.
synchronized (runnables) {
if (executed) {
return;
}
executed = true;
}
// At this point the runnables will never be modified by another
// thread, so we are safe using it outside of the synchronized block.
while (!runnables.isEmpty()) {
runnables.poll().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);
}
}
}
}