/*
* 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 java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* <p>An abstract implementation of the {@link Future} interface. This class
* is an abstraction of {@link java.util.concurrent.FutureTask} to support use
* for tasks other than {@link Runnable}s. It uses an
* {@link AbstractQueuedSynchronizer} to deal with concurrency issues and
* guarantee thread safety. It could be used as a base class to
* {@code FutureTask}, or any other implementor of the {@code Future} interface.
*
* <p>This class implements all methods in {@code Future}. Subclasses should
* provide a way to set the result of the computation through the protected
* methods {@link #set(Object)}, {@link #setException(Throwable)}, or
* {@link #cancel()}. If subclasses want to implement cancellation they can
* override the {@link #cancel(boolean)} method with a real implementation, the
* default implementation doesn't support cancellation.
*
* <p>The state changing methods all return a boolean indicating success or
* failure in changing the future's state. Valid states are running,
* completed, failed, or cancelled. Because this class does not implement
* cancellation it is left to the subclass to distinguish between created
* and running tasks.
*
* @author Sven Mawson
* @since 2009.09.15 <b>tentative</b>
*/
public abstract class AbstractFuture<V> implements Future<V> {
/** Synchronization control for AbstractFutures. */
private final Sync<V> sync = new Sync<V>();
/*
* Blocks until either the task completes or the timeout expires. Uses the
* sync blocking-with-timeout support provided by AQS.
*/
public V get(long timeout, TimeUnit unit) throws InterruptedException,
TimeoutException, ExecutionException {
return sync.get(unit.toNanos(timeout));
}
/*
* Blocks until the task completes or we get interrupted. Uses the
* interruptible blocking support provided by AQS.
*/
public V get() throws InterruptedException, ExecutionException {
return sync.get();
}
/*
* Checks if the sync is not in the running state.
*/
public boolean isDone() {
return sync.isDone();
}
/*
* Checks if the sync is in the cancelled state.
*/
public boolean isCancelled() {
return sync.isCancelled();
}
/*
* Default implementation of cancel that never cancels the future.
* Subclasses should override this to implement cancellation if desired.
*/
public boolean cancel(boolean mayInterruptIfRunning) {
return false;
}
/**
* Subclasses should invoke this method to set the result of the computation
* to {@code value}. This will set the state of the future to
* {@link AbstractFuture.Sync#COMPLETED} and call {@link #done()} if the
* state was successfully changed.
*
* @param value the value that was the result of the task.
* @return true if the state was successfully changed.
*/
protected boolean set(V value) {
boolean result = sync.set(value);
if (result) {
done();
}
return result;
}
/**
* Subclasses should invoke this method to set the result of the computation
* to an error, {@code throwable}. This will set the state of the future to
* {@link AbstractFuture.Sync#COMPLETED} and call {@link #done()} if the
* state was successfully changed.
*
* @param throwable the exception that the task failed with.
* @return true if the state was successfully changed.
* @throws Error if the throwable was an {@link Error}.
*/
protected boolean setException(Throwable throwable) {
boolean result = sync.setException(throwable);
if (result) {
done();
}
// If it's an Error, we want to make sure it reaches the top of the
// call stack, so we rethrow it.
if (throwable instanceof Error) {
throw (Error) throwable;
}
return result;
}
/**
* Subclasses should invoke this method to mark the future as cancelled.
* This will set the state of the future to {@link
* AbstractFuture.Sync#CANCELLED} and call {@link #done()} if the state was
* successfully changed.
*
* @return true if the state was successfully changed.
*/
protected final boolean cancel() {
boolean result = sync.cancel();
if (result) {
done();
}
return result;
}
/*
* Called by the success, failed, or cancelled methods to indicate that the
* value is now available and the latch can be released. Subclasses can
* use this method to deal with any actions that should be undertaken when
* the task has completed.
*/
protected void done() {
// Default implementation does nothing.
}
/**
* <p>Following the contract of {@link AbstractQueuedSynchronizer} we create a
* private subclass to hold the synchronizer. This synchronizer is used to
* implement the blocking and waiting calls as well as to handle state changes
* in a thread-safe manner. The current state of the future is held in the
* Sync state, and the lock is released whenever the state changes to either
* {@link #COMPLETED} or {@link #CANCELLED}.
*
* <p>To avoid races between threads doing release and acquire, we transition
* to the final state in two steps. One thread will successfully CAS from
* RUNNING to COMPLETING, that thread will then set the result of the
* computation, and only then transition to COMPLETED or CANCELLED.
*
* <p>We don't use the integer argument passed between acquire methods so we
* pass around a -1 everywhere.
*/
static final class Sync<V> extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 0L;
/* Valid states. */
static final int RUNNING = 0;
static final int COMPLETING = 1;
static final int COMPLETED = 2;
static final int CANCELLED = 4;
private V value;
private ExecutionException exception;
/*
* Acquisition succeeds if the future is done, otherwise it fails.
*/
@Override
protected int tryAcquireShared(int ignored) {
if (isDone()) {
return 1;
}
return -1;
}
/*
* We always allow a release to go through, this means the state has been
* successfully changed and the result is available.
*/
@Override
protected boolean tryReleaseShared(int finalState) {
setState(finalState);
return true;
}
/**
* Blocks until the task is complete or the timeout expires. Throws a
* {@link TimeoutException} if the timer expires, otherwise behaves like
* {@link #get()}.
*/
V get(long nanos) throws TimeoutException, CancellationException,
ExecutionException, InterruptedException {
// Attempt to acquire the shared lock with a timeout.
if (!tryAcquireSharedNanos(-1, nanos)) {
throw new TimeoutException("Timeout waiting for task.");
}
return getValue();
}
/**
* Blocks until {@link #complete(Object, Throwable, int)} has been
* successfully called. Throws a {@link CancellationException} if the task
* was cancelled, or a {@link ExecutionException} if the task completed with
* an error.
*/
V get() throws CancellationException, ExecutionException,
InterruptedException {
// Acquire the shared lock allowing interruption.
acquireSharedInterruptibly(-1);
return getValue();
}
/**
* Implementation of the actual value retrieval. Will return the value
* on success, an exception on failure, a cancellation on cancellation, or
* an illegal state if the synchronizer is in an invalid state.
*/
private V getValue() throws CancellationException, ExecutionException {
int state = getState();
switch (state) {
case COMPLETED:
if (exception != null) {
throw exception;
} else {
return value;
}
case CANCELLED:
throw new CancellationException("Task was cancelled.");
default:
throw new IllegalStateException(
"Error, synchronizer in invalid state: " + state);
}
}
/**
* Checks if the state is {@link #COMPLETED} or {@link #CANCELLED}.
*/
boolean isDone() {
return (getState() & (COMPLETED | CANCELLED)) != 0;
}
/**
* Checks if the state is {@link #CANCELLED}.
*/
boolean isCancelled() {
return getState() == CANCELLED;
}
/**
* Transition to the COMPLETED state and set the value.
*/
boolean set(V v) {
return complete(v, null, COMPLETED);
}
/**
* Transition to the COMPLETED state and set the exception.
*/
boolean setException(Throwable t) {
return complete(null, t, COMPLETED);
}
/**
* Transition to the CANCELLED state.
*/
boolean cancel() {
return complete(null, null, CANCELLED);
}
/**
* Implementation of completing a task. Either {@code v} or {@code t} will
* be set but not both. The {@code finalState} is the state to change to
* from {@link #RUNNING}. If the state is not in the RUNNING state we
* return {@code false}.
*
* @param v the value to set as the result of the computation.
* @param t the exception to set as the result of the computation.
* @param finalState the state to transition to.
*/
private boolean complete(V v, Throwable t, int finalState) {
if (compareAndSetState(RUNNING, COMPLETING)) {
this.value = v;
this.exception = t == null ? null : new ExecutionException(t);
releaseShared(finalState);
return true;
}
// The state was not RUNNING, so there are no valid transitions.
return false;
}
}
}