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/*
* Copyright 2018 The gRPC 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 io.grpc;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import java.lang.Thread.UncaughtExceptionHandler;
import java.util.Queue;
import java.util.concurrent.ConcurrentLinkedQueue;
import java.util.concurrent.Executor;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import javax.annotation.concurrent.ThreadSafe;
/**
* A synchronization context is a queue of tasks that run in sequence. It offers following
* guarantees:
*
*
* - Ordering. Tasks are run in the same order as they are submitted via {@link #execute}
* and {@link #executeLater}.
* - Serialization. Tasks are run in sequence and establish a happens-before relationship
* between them.
* - Non-reentrancy. If a task running in a synchronization context executes or schedules
* another task in the same synchronization context, the latter task will never run
* inline. It will instead be queued and run only after the current task has returned.
*
*
* It doesn't own any thread. Tasks are run from caller's or caller-provided threads.
*
*
Conceptually, it is fairly accurate to think of {@code SynchronizationContext} like a cheaper
* {@code Executors.newSingleThreadExecutor()} when used for synchronization (not long-running
* tasks). Both use a queue for tasks that are run in order and neither guarantee that tasks have
* completed before returning from {@code execute()}. However, the behavior does diverge if locks
* are held when calling the context. So it is encouraged to avoid mixing locks and synchronization
* context except via {@link #executeLater}.
*
*
This class is thread-safe.
*
* @since 1.17.0
*/
@ThreadSafe
@ExperimentalApi("https://github.com/grpc/grpc-java/issues/4984")
public final class SynchronizationContext implements Executor {
private final UncaughtExceptionHandler uncaughtExceptionHandler;
private final Queue queue = new ConcurrentLinkedQueue<>();
private final AtomicReference drainingThread = new AtomicReference<>();
/**
* Creates a SynchronizationContext.
*
* @param uncaughtExceptionHandler handles exceptions thrown out of the tasks. Different from
* what's documented on {@link UncaughtExceptionHandler#uncaughtException}, the thread is
* not terminated when the handler is called.
*/
public SynchronizationContext(UncaughtExceptionHandler uncaughtExceptionHandler) {
this.uncaughtExceptionHandler =
checkNotNull(uncaughtExceptionHandler, "uncaughtExceptionHandler");
}
/**
* Run all tasks in the queue in the current thread, if no other thread is running this method.
* Otherwise do nothing.
*
* Upon returning, it guarantees that all tasks submitted by {@code #executeLater} before it
* have been or will eventually be run, while not requiring any more calls to {@code drain()}.
*/
public final void drain() {
do {
if (!drainingThread.compareAndSet(null, Thread.currentThread())) {
return;
}
try {
Runnable runnable;
while ((runnable = queue.poll()) != null) {
try {
runnable.run();
} catch (Throwable t) {
uncaughtExceptionHandler.uncaughtException(Thread.currentThread(), t);
}
}
} finally {
drainingThread.set(null);
}
// must check queue again here to catch any added prior to clearing drainingThread
} while (!queue.isEmpty());
}
/**
* Adds a task that will be run when {@link #drain} is called.
*
*
This is useful for cases where you want to enqueue a task while under a lock of your own,
* but don't want the tasks to be run under your lock (for fear of deadlock). You can call {@link
* #executeLater} in the lock, and call {@link #drain} outside the lock.
*/
public final void executeLater(Runnable runnable) {
queue.add(checkNotNull(runnable, "runnable is null"));
}
/**
* Adds a task and run it in this synchronization context as soon as possible. The task may run
* inline. If there are tasks that are previously queued by {@link #executeLater} but have not
* been run, this method will trigger them to be run before the given task. This is equivalent to
* calling {@link #executeLater} immediately followed by {@link #drain}.
*/
@Override
public final void execute(Runnable task) {
executeLater(task);
drain();
}
/**
* Throw {@link IllegalStateException} if this method is not called from this synchronization
* context.
*/
public void throwIfNotInThisSynchronizationContext() {
checkState(Thread.currentThread() == drainingThread.get(),
"Not called from the SynchronizationContext");
}
/**
* Schedules a task to be added and run via {@link #execute} after a delay.
*
* @param task the task being scheduled
* @param delay the delay
* @param unit the time unit for the delay
* @param timerService the {@code ScheduledExecutorService} that provides delayed execution
*
* @return an object for checking the status and/or cancel the scheduled task
*/
public final ScheduledHandle schedule(
final Runnable task, long delay, TimeUnit unit, ScheduledExecutorService timerService) {
final ManagedRunnable runnable = new ManagedRunnable(task);
ScheduledFuture future = timerService.schedule(new Runnable() {
@Override
public void run() {
execute(runnable);
}
@Override
public String toString() {
return task.toString() + "(scheduled in SynchronizationContext)";
}
}, delay, unit);
return new ScheduledHandle(runnable, future);
}
private static class ManagedRunnable implements Runnable {
final Runnable task;
boolean isCancelled;
boolean hasStarted;
ManagedRunnable(Runnable task) {
this.task = checkNotNull(task, "task");
}
@Override
public void run() {
// The task may have been cancelled after timerService calls SynchronizationContext.execute()
// but before the runnable is actually run. We must guarantee that the task will not be run
// in this case.
if (!isCancelled) {
hasStarted = true;
task.run();
}
}
}
/**
* Allows the user to check the status and/or cancel a task scheduled by {@link #schedule}.
*
*
This class is NOT thread-safe. All methods must be run from the same {@link
* SynchronizationContext} as which the task was scheduled in.
*/
public static final class ScheduledHandle {
private final ManagedRunnable runnable;
private final ScheduledFuture future;
private ScheduledHandle(ManagedRunnable runnable, ScheduledFuture future) {
this.runnable = checkNotNull(runnable, "runnable");
this.future = checkNotNull(future, "future");
}
/**
* Cancel the task if it's still {@link #isPending pending}.
*/
public void cancel() {
runnable.isCancelled = true;
future.cancel(false);
}
/**
* Returns {@code true} if the task will eventually run, meaning that it has neither started
* running nor been cancelled.
*/
public boolean isPending() {
return !(runnable.hasStarted || runnable.isCancelled);
}
}
}