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Guava is a suite of core and expanded libraries that include
utility classes, google's collections, io classes, and much
much more.
Guava has only one code dependency - javax.annotation,
per the JSR-305 spec.
/*
* Copyright (C) 2008 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 java.util.ArrayDeque;
import java.util.Queue;
import java.util.concurrent.Executor;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.concurrent.GuardedBy;
/**
* Executor ensuring that all Runnables submitted are executed in order,
* using the provided Executor, and serially such that no two will ever
* be running at the same time.
*
* TODO(user): The tasks are given to the underlying executor as a single
* task, which means the semantics of the executor may be changed, e.g. the
* executor may have an afterExecute method that runs after every task
*
* TODO(user): What happens in case of shutdown or shutdownNow? Should
* TaskRunner check for interruption?
*
* TODO(user): It would be nice to provide a handle to individual task
* results using Future. Maybe SerializingExecutorService?
*
* @author JJ Furman
*/
final class SerializingExecutor implements Executor {
private static final Logger log =
Logger.getLogger(SerializingExecutor.class.getName());
/** Underlying executor that all submitted Runnable objects are run on. */
private final Executor executor;
/** A list of Runnables to be run in order. */
@GuardedBy("internalLock")
private final Queue waitQueue = new ArrayDeque();
/**
* We explicitly keep track of if the TaskRunner is currently scheduled to
* run. If it isn't, we start it. We can't just use
* waitQueue.isEmpty() as a proxy because we need to ensure that only one
* Runnable submitted is running at a time so even if waitQueue is empty
* the isThreadScheduled isn't set to false until after the Runnable is
* finished.
*/
@GuardedBy("internalLock")
private boolean isThreadScheduled = false;
/** The object that actually runs the Runnables submitted, reused. */
private final TaskRunner taskRunner = new TaskRunner();
/**
* Creates a SerializingExecutor, running tasks using {@code executor}.
*
* @param executor Executor in which tasks should be run. Must not be null.
*/
public SerializingExecutor(Executor executor) {
Preconditions.checkNotNull(executor, "'executor' must not be null.");
this.executor = executor;
}
private final Object internalLock = new Object() {
@Override public String toString() {
return "SerializingExecutor lock: " + super.toString();
}
};
/**
* Runs the given runnable strictly after all Runnables that were submitted
* before it, and using the {@code executor} passed to the constructor. .
*/
@Override
public void execute(Runnable r) {
Preconditions.checkNotNull(r, "'r' must not be null.");
boolean scheduleTaskRunner = false;
synchronized (internalLock) {
waitQueue.add(r);
if (!isThreadScheduled) {
isThreadScheduled = true;
scheduleTaskRunner = true;
}
}
if (scheduleTaskRunner) {
boolean threw = true;
try {
executor.execute(taskRunner);
threw = false;
} finally {
if (threw) {
synchronized (internalLock) {
// It is possible that at this point that there are still tasks in
// the queue, it would be nice to keep trying but the error may not
// be recoverable. So we update our state and propogate so that if
// our caller deems it recoverable we won't be stuck.
isThreadScheduled = false;
}
}
}
}
}
/**
* Task that actually runs the Runnables. It takes the Runnables off of the
* queue one by one and runs them. After it is done with all Runnables and
* there are no more to run, puts the SerializingExecutor in the state where
* isThreadScheduled = false and returns. This allows the current worker
* thread to return to the original pool.
*/
private class TaskRunner implements Runnable {
@Override
public void run() {
boolean stillRunning = true;
try {
while (true) {
Preconditions.checkState(isThreadScheduled);
Runnable nextToRun;
synchronized (internalLock) {
nextToRun = waitQueue.poll();
if (nextToRun == null) {
isThreadScheduled = false;
stillRunning = false;
break;
}
}
// Always run while not holding the lock, to avoid deadlocks.
try {
nextToRun.run();
} catch (RuntimeException e) {
// Log it and keep going.
log.log(Level.SEVERE, "Exception while executing runnable "
+ nextToRun, e);
}
}
} finally {
if (stillRunning) {
// An Error is bubbling up, we should mark ourselves as no longer
// running, that way if anyone tries to keep using us we won't be
// corrupted.
synchronized (internalLock) {
isThreadScheduled = false;
}
}
}
}
}
}