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/*
* 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 static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Strings.isNullOrEmpty;
import static com.google.common.base.Throwables.throwIfUnchecked;
import static com.google.common.util.concurrent.Futures.getDone;
import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
import static java.util.concurrent.atomic.AtomicReferenceFieldUpdater.newUpdater;
import com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Ascii;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotMock;
import com.google.errorprone.annotations.ForOverride;
import com.google.j2objc.annotations.ReflectionSupport;
import java.security.AccessController;
import java.security.PrivilegedActionException;
import java.security.PrivilegedExceptionAction;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Executor;
import java.util.concurrent.Future;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
import java.util.concurrent.locks.LockSupport;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.Nullable;
/**
* An abstract implementation of {@link ListenableFuture}, intended for advanced users only. More
* common ways to create a {@code ListenableFuture} include instantiating a {@link SettableFuture},
* submitting a task to a {@link ListeningExecutorService}, and deriving a {@code Future} from an
* existing one, typically using methods like {@link Futures#transform(ListenableFuture,
* com.google.common.base.Function, java.util.concurrent.Executor) Futures.transform} and {@link
* Futures#catching(ListenableFuture, Class, com.google.common.base.Function,
* java.util.concurrent.Executor) Futures.catching}.
*
* This class implements all methods in {@code ListenableFuture}. Subclasses should provide a way
* to set the result of the computation through the protected methods {@link #set(Object)}, {@link
* #setFuture(ListenableFuture)} and {@link #setException(Throwable)}. Subclasses may also override
* {@link #afterDone()}, which will be invoked automatically when the future completes. Subclasses
* should rarely override other methods.
*
* @author Sven Mawson
* @author Luke Sandberg
* @since 1.0
*/
@SuppressWarnings("ShortCircuitBoolean") // we use non-short circuiting comparisons intentionally
@DoNotMock("Use Futures.immediate*Future or SettableFuture")
@GwtCompatible(emulated = true)
@ReflectionSupport(value = ReflectionSupport.Level.FULL)
public abstract class AbstractFuture extends FluentFuture {
// NOTE: Whenever both tests are cheap and functional, it's faster to use &, | instead of &&, ||
private static final boolean GENERATE_CANCELLATION_CAUSES =
Boolean.parseBoolean(
System.getProperty("guava.concurrent.generate_cancellation_cause", "false"));
/**
* A less abstract subclass of AbstractFuture. This can be used to optimize setFuture by ensuring
* that {@link #get} calls exactly the implementation of {@link AbstractFuture#get}.
*/
abstract static class TrustedFuture extends AbstractFuture {
@CanIgnoreReturnValue
@Override
public final V get() throws InterruptedException, ExecutionException {
return super.get();
}
@CanIgnoreReturnValue
@Override
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
return super.get(timeout, unit);
}
@Override
public final boolean isDone() {
return super.isDone();
}
@Override
public final boolean isCancelled() {
return super.isCancelled();
}
@Override
public final void addListener(Runnable listener, Executor executor) {
super.addListener(listener, executor);
}
@CanIgnoreReturnValue
@Override
public final boolean cancel(boolean mayInterruptIfRunning) {
return super.cancel(mayInterruptIfRunning);
}
}
// Logger to log exceptions caught when running listeners.
private static final Logger log = Logger.getLogger(AbstractFuture.class.getName());
// A heuristic for timed gets. If the remaining timeout is less than this, spin instead of
// blocking. This value is what AbstractQueuedSynchronizer uses.
private static final long SPIN_THRESHOLD_NANOS = 1000L;
private static final AtomicHelper ATOMIC_HELPER;
static {
AtomicHelper helper;
try {
helper = new UnsafeAtomicHelper();
} catch (Throwable unsafeFailure) {
// catch absolutely everything and fall through to our 'SafeAtomicHelper'
// The access control checks that ARFU does means the caller class has to be AbstractFuture
// instead of SafeAtomicHelper, so we annoyingly define these here
try {
helper =
new SafeAtomicHelper(
newUpdater(Waiter.class, Thread.class, "thread"),
newUpdater(Waiter.class, Waiter.class, "next"),
newUpdater(AbstractFuture.class, Waiter.class, "waiters"),
newUpdater(AbstractFuture.class, Listener.class, "listeners"),
newUpdater(AbstractFuture.class, Object.class, "value"));
} catch (Throwable atomicReferenceFieldUpdaterFailure) {
// Some Android 5.0.x Samsung devices have bugs in JDK reflection APIs that cause
// getDeclaredField to throw a NoSuchFieldException when the field is definitely there.
// For these users fallback to a suboptimal implementation, based on synchronized. This will
// be a definite performance hit to those users.
log.log(Level.SEVERE, "UnsafeAtomicHelper is broken!", unsafeFailure);
log.log(Level.SEVERE, "SafeAtomicHelper is broken!", atomicReferenceFieldUpdaterFailure);
helper = new SynchronizedHelper();
}
}
ATOMIC_HELPER = helper;
// Prevent rare disastrous classloading in first call to LockSupport.park.
// See: https://bugs.openjdk.java.net/browse/JDK-8074773
@SuppressWarnings("unused")
Class ensureLoaded = LockSupport.class;
}
/**
* Waiter links form a Treiber stack, in the {@link #waiters} field.
*/
private static final class Waiter {
static final Waiter TOMBSTONE = new Waiter(false /* ignored param */);
@Nullable volatile Thread thread;
@Nullable volatile Waiter next;
/**
* Constructor for the TOMBSTONE, avoids use of ATOMIC_HELPER in case this class is loaded
* before the ATOMIC_HELPER. Apparently this is possible on some android platforms.
*/
Waiter(boolean unused) {}
Waiter() {
// avoid volatile write, write is made visible by subsequent CAS on waiters field
ATOMIC_HELPER.putThread(this, Thread.currentThread());
}
// non-volatile write to the next field. Should be made visible by subsequent CAS on waiters
// field.
void setNext(Waiter next) {
ATOMIC_HELPER.putNext(this, next);
}
void unpark() {
// This is racy with removeWaiter. The consequence of the race is that we may spuriously call
// unpark even though the thread has already removed itself from the list. But even if we did
// use a CAS, that race would still exist (it would just be ever so slightly smaller).
Thread w = thread;
if (w != null) {
thread = null;
LockSupport.unpark(w);
}
}
}
/**
* Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted
* nodes. This is an O(n) operation in the common case (and O(n^2) in the worst), but we are saved
* by two things.
*
* - This is only called when a waiting thread times out or is interrupted. Both of which should
* be rare.
*
- The waiters list should be very short.
*
*/
private void removeWaiter(Waiter node) {
node.thread = null; // mark as 'deleted'
restart:
while (true) {
Waiter pred = null;
Waiter curr = waiters;
if (curr == Waiter.TOMBSTONE) {
return; // give up if someone is calling complete
}
Waiter succ;
while (curr != null) {
succ = curr.next;
if (curr.thread != null) { // we aren't unlinking this node, update pred.
pred = curr;
} else if (pred != null) { // We are unlinking this node and it has a predecessor.
pred.next = succ;
if (pred.thread == null) { // We raced with another node that unlinked pred. Restart.
continue restart;
}
} else if (!ATOMIC_HELPER.casWaiters(this, curr, succ)) { // We are unlinking head
continue restart; // We raced with an add or complete
}
curr = succ;
}
break;
}
}
/** Listeners also form a stack through the {@link #listeners} field. */
private static final class Listener {
static final Listener TOMBSTONE = new Listener(null, null);
final Runnable task;
final Executor executor;
// writes to next are made visible by subsequent CAS's on the listeners field
@Nullable Listener next;
Listener(Runnable task, Executor executor) {
this.task = task;
this.executor = executor;
}
}
/** A special value to represent {@code null}. */
private static final Object NULL = new Object();
/** A special value to represent failure, when {@link #setException} is called successfully. */
private static final class Failure {
static final Failure FALLBACK_INSTANCE =
new Failure(
new Throwable("Failure occurred while trying to finish a future.") {
@Override
public synchronized Throwable fillInStackTrace() {
return this; // no stack trace
}
});
final Throwable exception;
Failure(Throwable exception) {
this.exception = checkNotNull(exception);
}
}
/** A special value to represent cancellation and the 'wasInterrupted' bit. */
private static final class Cancellation {
// constants to use when GENERATE_CANCELLATION_CAUSES = false
static final Cancellation CAUSELESS_INTERRUPTED;
static final Cancellation CAUSELESS_CANCELLED;
static {
if (GENERATE_CANCELLATION_CAUSES) {
CAUSELESS_CANCELLED = null;
CAUSELESS_INTERRUPTED = null;
} else {
CAUSELESS_CANCELLED = new Cancellation(false, null);
CAUSELESS_INTERRUPTED = new Cancellation(true, null);
}
}
final boolean wasInterrupted;
@Nullable final Throwable cause;
Cancellation(boolean wasInterrupted, @Nullable Throwable cause) {
this.wasInterrupted = wasInterrupted;
this.cause = cause;
}
}
/** A special value that encodes the 'setFuture' state. */
private static final class SetFuture implements Runnable {
final AbstractFuture owner;
final ListenableFuture future;
SetFuture(AbstractFuture owner, ListenableFuture future) {
this.owner = owner;
this.future = future;
}
@Override
public void run() {
if (owner.value != this) {
// nothing to do, we must have been cancelled, don't bother inspecting the future.
return;
}
Object valueToSet = getFutureValue(future);
if (ATOMIC_HELPER.casValue(owner, this, valueToSet)) {
complete(owner);
}
}
}
// TODO(lukes): investigate using the @Contended annotation on these fields when jdk8 is
// available.
/**
* This field encodes the current state of the future.
*
* The valid values are:
*
* - {@code null} initial state, nothing has happened.
*
- {@link Cancellation} terminal state, {@code cancel} was called.
*
- {@link Failure} terminal state, {@code setException} was called.
*
- {@link SetFuture} intermediate state, {@code setFuture} was called.
*
- {@link #NULL} terminal state, {@code set(null)} was called.
*
- Any other non-null value, terminal state, {@code set} was called with a non-null argument.
*
*/
private volatile Object value;
/** All listeners. */
private volatile Listener listeners;
/** All waiting threads. */
private volatile Waiter waiters;
/**
* Constructor for use by subclasses.
*/
protected AbstractFuture() {}
// Gets and Timed Gets
//
// * Be responsive to interruption
// * Don't create Waiter nodes if you aren't going to park, this helps reduce contention on the
// waiters field.
// * Future completion is defined by when #value becomes non-null/non SetFuture
// * Future completion can be observed if the waiters field contains a TOMBSTONE
// Timed Get
// There are a few design constraints to consider
// * We want to be responsive to small timeouts, unpark() has non trivial latency overheads (I
// have observed 12 micros on 64 bit linux systems to wake up a parked thread). So if the
// timeout is small we shouldn't park(). This needs to be traded off with the cpu overhead of
// spinning, so we use SPIN_THRESHOLD_NANOS which is what AbstractQueuedSynchronizer uses for
// similar purposes.
// * We want to behave reasonably for timeouts of 0
// * We are more responsive to completion than timeouts. This is because parkNanos depends on
// system scheduling and as such we could either miss our deadline, or unpark() could be delayed
// so that it looks like we timed out even though we didn't. For comparison FutureTask respects
// completion preferably and AQS is non-deterministic (depends on where in the queue the waiter
// is). If we wanted to be strict about it, we could store the unpark() time in the Waiter node
// and we could use that to make a decision about whether or not we timed out prior to being
// unparked.
/**
* {@inheritDoc}
*
* The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
* current thread is interrupted during the call, even if the value is already available.
*
* @throws CancellationException {@inheritDoc}
*/
@CanIgnoreReturnValue
@Override
public V get(long timeout, TimeUnit unit)
throws InterruptedException, TimeoutException, ExecutionException {
// NOTE: if timeout < 0, remainingNanos will be < 0 and we will fall into the while(true) loop
// at the bottom and throw a timeoutexception.
long remainingNanos = unit.toNanos(timeout); // we rely on the implicit null check on unit.
if (Thread.interrupted()) {
throw new InterruptedException();
}
Object localValue = value;
if (localValue != null & !(localValue instanceof SetFuture)) {
return getDoneValue(localValue);
}
// we delay calling nanoTime until we know we will need to either park or spin
final long endNanos = remainingNanos > 0 ? System.nanoTime() + remainingNanos : 0;
long_wait_loop:
if (remainingNanos >= SPIN_THRESHOLD_NANOS) {
Waiter oldHead = waiters;
if (oldHead != Waiter.TOMBSTONE) {
Waiter node = new Waiter();
do {
node.setNext(oldHead);
if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
while (true) {
LockSupport.parkNanos(this, remainingNanos);
// Check interruption first, if we woke up due to interruption we need to honor that.
if (Thread.interrupted()) {
removeWaiter(node);
throw new InterruptedException();
}
// Otherwise re-read and check doneness. If we loop then it must have been a spurious
// wakeup
localValue = value;
if (localValue != null & !(localValue instanceof SetFuture)) {
return getDoneValue(localValue);
}
// timed out?
remainingNanos = endNanos - System.nanoTime();
if (remainingNanos < SPIN_THRESHOLD_NANOS) {
// Remove the waiter, one way or another we are done parking this thread.
removeWaiter(node);
break long_wait_loop; // jump down to the busy wait loop
}
}
}
oldHead = waiters; // re-read and loop.
} while (oldHead != Waiter.TOMBSTONE);
}
// re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
// waiter.
return getDoneValue(value);
}
// If we get here then we have remainingNanos < SPIN_THRESHOLD_NANOS and there is no node on the
// waiters list
while (remainingNanos > 0) {
localValue = value;
if (localValue != null & !(localValue instanceof SetFuture)) {
return getDoneValue(localValue);
}
if (Thread.interrupted()) {
throw new InterruptedException();
}
remainingNanos = endNanos - System.nanoTime();
}
String futureToString = toString();
// It's confusing to see a completed future in a timeout message; if isDone() returns false,
// then we know it must have given a pending toString value earlier. If not, then the future
// completed after the timeout expired, and the message might be success.
if (isDone()) {
throw new TimeoutException(
"Waited " + timeout + " " + Ascii.toLowerCase(unit.toString())
+ " but future completed as timeout expired");
}
throw new TimeoutException(
"Waited " + timeout + " " + Ascii.toLowerCase(unit.toString()) + " for " + futureToString);
}
/**
* {@inheritDoc}
*
*
The default {@link AbstractFuture} implementation throws {@code InterruptedException} if the
* current thread is interrupted during the call, even if the value is already available.
*
* @throws CancellationException {@inheritDoc}
*/
@CanIgnoreReturnValue
@Override
public V get() throws InterruptedException, ExecutionException {
if (Thread.interrupted()) {
throw new InterruptedException();
}
Object localValue = value;
if (localValue != null & !(localValue instanceof SetFuture)) {
return getDoneValue(localValue);
}
Waiter oldHead = waiters;
if (oldHead != Waiter.TOMBSTONE) {
Waiter node = new Waiter();
do {
node.setNext(oldHead);
if (ATOMIC_HELPER.casWaiters(this, oldHead, node)) {
// we are on the stack, now wait for completion.
while (true) {
LockSupport.park(this);
// Check interruption first, if we woke up due to interruption we need to honor that.
if (Thread.interrupted()) {
removeWaiter(node);
throw new InterruptedException();
}
// Otherwise re-read and check doneness. If we loop then it must have been a spurious
// wakeup
localValue = value;
if (localValue != null & !(localValue instanceof SetFuture)) {
return getDoneValue(localValue);
}
}
}
oldHead = waiters; // re-read and loop.
} while (oldHead != Waiter.TOMBSTONE);
}
// re-read value, if we get here then we must have observed a TOMBSTONE while trying to add a
// waiter.
return getDoneValue(value);
}
/**
* Unboxes {@code obj}. Assumes that obj is not {@code null} or a {@link SetFuture}.
*/
private V getDoneValue(Object obj) throws ExecutionException {
// While this seems like it might be too branch-y, simple benchmarking proves it to be
// unmeasurable (comparing done AbstractFutures with immediateFuture)
if (obj instanceof Cancellation) {
throw cancellationExceptionWithCause("Task was cancelled.", ((Cancellation) obj).cause);
} else if (obj instanceof Failure) {
throw new ExecutionException(((Failure) obj).exception);
} else if (obj == NULL) {
return null;
} else {
@SuppressWarnings("unchecked") // this is the only other option
V asV = (V) obj;
return asV;
}
}
@Override
public boolean isDone() {
final Object localValue = value;
return localValue != null & !(localValue instanceof SetFuture);
}
@Override
public boolean isCancelled() {
final Object localValue = value;
return localValue instanceof Cancellation;
}
/**
* {@inheritDoc}
*
*
If a cancellation attempt succeeds on a {@code Future} that had previously been {@linkplain
* #setFuture set asynchronously}, then the cancellation will also be propagated to the delegate
* {@code Future} that was supplied in the {@code setFuture} call.
*
*
Rather than override this method to perform additional cancellation work or cleanup,
* subclasses should override {@link #afterDone}, consulting {@link #isCancelled} and {@link
* #wasInterrupted} as necessary. This ensures that the work is done even if the future is
* cancelled without a call to {@code cancel}, such as by calling {@code
* setFuture(cancelledFuture)}.
*/
@CanIgnoreReturnValue
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
Object localValue = value;
boolean rValue = false;
if (localValue == null | localValue instanceof SetFuture) {
// Try to delay allocating the exception. At this point we may still lose the CAS, but it is
// certainly less likely.
Object valueToSet =
GENERATE_CANCELLATION_CAUSES
? new Cancellation(
mayInterruptIfRunning, new CancellationException("Future.cancel() was called."))
: (mayInterruptIfRunning
? Cancellation.CAUSELESS_INTERRUPTED
: Cancellation.CAUSELESS_CANCELLED);
AbstractFuture abstractFuture = this;
while (true) {
if (ATOMIC_HELPER.casValue(abstractFuture, localValue, valueToSet)) {
rValue = true;
// We call interuptTask before calling complete(), which is consistent with
// FutureTask
if (mayInterruptIfRunning) {
abstractFuture.interruptTask();
}
complete(abstractFuture);
if (localValue instanceof SetFuture) {
// propagate cancellation to the future set in setfuture, this is racy, and we don't
// care if we are successful or not.
ListenableFuture futureToPropagateTo = ((SetFuture) localValue).future;
if (futureToPropagateTo instanceof TrustedFuture) {
// If the future is a TrustedFuture then we specifically avoid calling cancel()
// this has 2 benefits
// 1. for long chains of futures strung together with setFuture we consume less stack
// 2. we avoid allocating Cancellation objects at every level of the cancellation
// chain
// We can only do this for TrustedFuture, because TrustedFuture.cancel is final and
// does nothing but delegate to this method.
AbstractFuture trusted = (AbstractFuture) futureToPropagateTo;
localValue = trusted.value;
if (localValue == null | localValue instanceof SetFuture) {
abstractFuture = trusted;
continue; // loop back up and try to complete the new future
}
} else {
// not a TrustedFuture, call cancel directly.
futureToPropagateTo.cancel(mayInterruptIfRunning);
}
}
break;
}
// obj changed, reread
localValue = abstractFuture.value;
if (!(localValue instanceof SetFuture)) {
// obj cannot be null at this point, because value can only change from null to non-null.
// So if value changed (and it did since we lost the CAS), then it cannot be null and
// since it isn't a SetFuture, then the future must be done and we should exit the loop
break;
}
}
}
return rValue;
}
/**
* Subclasses can override this method to implement interruption of the future's computation. The
* method is invoked automatically by a successful call to {@link #cancel(boolean) cancel(true)}.
*
*
The default implementation does nothing.
*
*
This method is likely to be deprecated. Prefer to override {@link #afterDone}, consulting
* {@link #wasInterrupted} to decide whether to interrupt your task.
*
* @since 10.0
*/
protected void interruptTask() {}
/**
* Returns true if this future was cancelled with {@code mayInterruptIfRunning} set to {@code
* true}.
*
* @since 14.0
*/
protected final boolean wasInterrupted() {
final Object localValue = value;
return (localValue instanceof Cancellation) && ((Cancellation) localValue).wasInterrupted;
}
/**
* {@inheritDoc}
*
* @since 10.0
*/
@Override
public void addListener(Runnable listener, Executor executor) {
checkNotNull(listener, "Runnable was null.");
checkNotNull(executor, "Executor was null.");
Listener oldHead = listeners;
if (oldHead != Listener.TOMBSTONE) {
Listener newNode = new Listener(listener, executor);
do {
newNode.next = oldHead;
if (ATOMIC_HELPER.casListeners(this, oldHead, newNode)) {
return;
}
oldHead = listeners; // re-read
} while (oldHead != Listener.TOMBSTONE);
}
// If we get here then the Listener TOMBSTONE was set, which means the future is done, call
// the listener.
executeListener(listener, executor);
}
/**
* Sets the result of this {@code Future} unless this {@code Future} has already been cancelled or
* set (including {@linkplain #setFuture set asynchronously}). When a call to this method returns,
* the {@code Future} is guaranteed to be {@linkplain #isDone done} only if the call was
* accepted (in which case it returns {@code true}). If it returns {@code false}, the {@code
* Future} may have previously been set asynchronously, in which case its result may not be known
* yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
* method, only by a call to {@link #cancel}.
*
* @param value the value to be used as the result
* @return true if the attempt was accepted, completing the {@code Future}
*/
@CanIgnoreReturnValue
protected boolean set(@Nullable V value) {
Object valueToSet = value == null ? NULL : value;
if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
complete(this);
return true;
}
return false;
}
/**
* Sets the failed result of this {@code Future} unless this {@code Future} has already been
* cancelled or set (including {@linkplain #setFuture set asynchronously}). When a call to this
* method returns, the {@code Future} is guaranteed to be {@linkplain #isDone done} only if
* the call was accepted (in which case it returns {@code true}). If it returns {@code false}, the
* {@code Future} may have previously been set asynchronously, in which case its result may not be
* known yet. That result, though not yet known, cannot be overridden by a call to a {@code set*}
* method, only by a call to {@link #cancel}.
*
* @param throwable the exception to be used as the failed result
* @return true if the attempt was accepted, completing the {@code Future}
*/
@CanIgnoreReturnValue
protected boolean setException(Throwable throwable) {
Object valueToSet = new Failure(checkNotNull(throwable));
if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
complete(this);
return true;
}
return false;
}
/**
* Sets the result of this {@code Future} to match the supplied input {@code Future} once the
* supplied {@code Future} is done, unless this {@code Future} has already been cancelled or set
* (including "set asynchronously," defined below).
*
*
If the supplied future is {@linkplain #isDone done} when this method is called and the call
* is accepted, then this future is guaranteed to have been completed with the supplied future by
* the time this method returns. If the supplied future is not done and the call is accepted, then
* the future will be set asynchronously. Note that such a result, though not yet known,
* cannot be overridden by a call to a {@code set*} method, only by a call to {@link #cancel}.
*
*
If the call {@code setFuture(delegate)} is accepted and this {@code Future} is later
* cancelled, cancellation will be propagated to {@code delegate}. Additionally, any call to
* {@code setFuture} after any cancellation will propagate cancellation to the supplied {@code
* Future}.
*
*
Note that, even if the supplied future is cancelled and it causes this future to complete,
* it will never trigger interruption behavior. In particular, it will not cause this future to
* invoke the {@link #interruptTask} method, and the {@link #wasInterrupted} method will not
* return {@code true}.
*
* @param future the future to delegate to
* @return true if the attempt was accepted, indicating that the {@code Future} was not previously
* cancelled or set.
* @since 19.0
*/
@Beta
@CanIgnoreReturnValue
protected boolean setFuture(ListenableFuture future) {
checkNotNull(future);
Object localValue = value;
if (localValue == null) {
if (future.isDone()) {
Object value = getFutureValue(future);
if (ATOMIC_HELPER.casValue(this, null, value)) {
complete(this);
return true;
}
return false;
}
SetFuture valueToSet = new SetFuture(this, future);
if (ATOMIC_HELPER.casValue(this, null, valueToSet)) {
// the listener is responsible for calling completeWithFuture, directExecutor is appropriate
// since all we are doing is unpacking a completed future which should be fast.
try {
future.addListener(valueToSet, directExecutor());
} catch (Throwable t) {
// addListener has thrown an exception! SetFuture.run can't throw any exceptions so this
// must have been caused by addListener itself. The most likely explanation is a
// misconfigured mock. Try to switch to Failure.
Failure failure;
try {
failure = new Failure(t);
} catch (Throwable oomMostLikely) {
failure = Failure.FALLBACK_INSTANCE;
}
// Note: The only way this CAS could fail is if cancel() has raced with us. That is ok.
boolean unused = ATOMIC_HELPER.casValue(this, valueToSet, failure);
}
return true;
}
localValue = value; // we lost the cas, fall through and maybe cancel
}
// The future has already been set to something. If it is cancellation we should cancel the
// incoming future.
if (localValue instanceof Cancellation) {
// we don't care if it fails, this is best-effort.
future.cancel(((Cancellation) localValue).wasInterrupted);
}
return false;
}
/**
* Returns a value that satisfies the contract of the {@link #value} field based on the state of
* given future.
*
* This is approximately the inverse of {@link #getDoneValue(Object)}
*/
private static Object getFutureValue(ListenableFuture future) {
Object valueToSet;
if (future instanceof TrustedFuture) {
// Break encapsulation for TrustedFuture instances since we know that subclasses cannot
// override .get() (since it is final) and therefore this is equivalent to calling .get()
// and unpacking the exceptions like we do below (just much faster because it is a single
// field read instead of a read, several branches and possibly creating exceptions).
Object v = ((AbstractFuture) future).value;
if (v instanceof Cancellation) {
// If the other future was interrupted, clear the interrupted bit while preserving the cause
// this will make it consistent with how non-trustedfutures work which cannot propagate the
// wasInterrupted bit
Cancellation c = (Cancellation) v;
if (c.wasInterrupted) {
v =
c.cause != null
? new Cancellation(/* wasInterrupted= */ false, c.cause)
: Cancellation.CAUSELESS_CANCELLED;
}
}
return v;
} else {
// Otherwise calculate valueToSet by calling .get()
try {
Object v = getDone(future);
valueToSet = v == null ? NULL : v;
} catch (ExecutionException exception) {
valueToSet = new Failure(exception.getCause());
} catch (CancellationException cancellation) {
valueToSet = new Cancellation(false, cancellation);
} catch (Throwable t) {
valueToSet = new Failure(t);
}
}
return valueToSet;
}
/** Unblocks all threads and runs all listeners. */
private static void complete(AbstractFuture future) {
Listener next = null;
outer: while (true) {
future.releaseWaiters();
// We call this before the listeners in order to avoid needing to manage a separate stack data
// structure for them.
// afterDone() should be generally fast and only used for cleanup work... but in theory can
// also be recursive and create StackOverflowErrors
future.afterDone();
// push the current set of listeners onto next
next = future.clearListeners(next);
future = null;
while (next != null) {
Listener curr = next;
next = next.next;
Runnable task = curr.task;
if (task instanceof SetFuture) {
SetFuture setFuture = (SetFuture) task;
// We unwind setFuture specifically to avoid StackOverflowErrors in the case of long
// chains of SetFutures
// Handling this special case is important because there is no way to pass an executor to
// setFuture, so a user couldn't break the chain by doing this themselves. It is also
// potentially common if someone writes a recursive Futures.transformAsync transformer.
future = setFuture.owner;
if (future.value == setFuture) {
Object valueToSet = getFutureValue(setFuture.future);
if (ATOMIC_HELPER.casValue(future, setFuture, valueToSet)) {
continue outer;
}
}
// other wise the future we were trying to set is already done.
} else {
executeListener(task, curr.executor);
}
}
break;
}
}
/**
* Callback method that is called exactly once after the future is completed.
*
*
If {@link #interruptTask} is also run during completion, {@link #afterDone} runs after it.
*
*
The default implementation of this method in {@code AbstractFuture} does nothing. This is
* intended for very lightweight cleanup work, for example, timing statistics or clearing fields.
* If your task does anything heavier consider, just using a listener with an executor.
*
* @since 20.0
*/
@Beta
@ForOverride
protected void afterDone() {}
/**
* Returns the exception that this {@code Future} completed with. This includes completion through
* a call to {@link #setException} or {@link #setFuture setFuture}{@code (failedFuture)} but not
* cancellation.
*
* @throws RuntimeException if the {@code Future} has not failed
*/
final Throwable trustedGetException() {
return ((Failure) value).exception;
}
/**
* If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts)
* the given future (if available).
*/
final void maybePropagateCancellationTo(@Nullable Future related) {
if (related != null & isCancelled()) {
related.cancel(wasInterrupted());
}
}
/** Releases all threads in the {@link #waiters} list, and clears the list. */
private void releaseWaiters() {
Waiter head;
do {
head = waiters;
} while (!ATOMIC_HELPER.casWaiters(this, head, Waiter.TOMBSTONE));
for (Waiter currentWaiter = head;
currentWaiter != null;
currentWaiter = currentWaiter.next) {
currentWaiter.unpark();
}
}
/**
* Clears the {@link #listeners} list and prepends its contents to {@code onto}, least recently
* added first.
*/
private Listener clearListeners(Listener onto) {
// We need to
// 1. atomically swap the listeners with TOMBSTONE, this is because addListener uses that to
// to synchronize with us
// 2. reverse the linked list, because despite our rather clear contract, people depend on us
// executing listeners in the order they were added
// 3. push all the items onto 'onto' and return the new head of the stack
Listener head;
do {
head = listeners;
} while (!ATOMIC_HELPER.casListeners(this, head, Listener.TOMBSTONE));
Listener reversedList = onto;
while (head != null) {
Listener tmp = head;
head = head.next;
tmp.next = reversedList;
reversedList = tmp;
}
return reversedList;
}
// TODO(user) move this up into FluentFuture, or parts as a default method on ListenableFuture?
@Override
public String toString() {
StringBuilder builder = new StringBuilder().append(super.toString()).append("[status=");
if (isCancelled()) {
builder.append("CANCELLED");
} else if (isDone()) {
addDoneString(builder);
} else {
String pendingDescription;
try {
pendingDescription = pendingToString();
} catch (RuntimeException e) {
// Don't call getMessage or toString() on the exception, in case the exception thrown by the
// subclass is implemented with bugs similar to the subclass.
pendingDescription = "Exception thrown from implementation: " + e.getClass();
}
// The future may complete during or before the call to getPendingToString, so we use null
// as a signal that we should try checking if the future is done again.
if (!isNullOrEmpty(pendingDescription)) {
builder.append("PENDING, info=[").append(pendingDescription).append("]");
} else if (isDone()) {
addDoneString(builder);
} else {
builder.append("PENDING");
}
}
return builder.append("]").toString();
}
/**
* Provide a human-readable explanation of why this future has not yet completed.
*
* @return null if an explanation cannot be provided because the future is done.
* @since 23.0
*/
@Nullable
protected String pendingToString() {
Object localValue = value;
if (localValue instanceof SetFuture) {
return "setFuture=[" + ((SetFuture) localValue).future + "]";
} else if (this instanceof ScheduledFuture) {
return "remaining delay=["
+ ((ScheduledFuture) this).getDelay(TimeUnit.MILLISECONDS)
+ " ms]";
}
return null;
}
private void addDoneString(StringBuilder builder) {
try {
V value = getDone(this);
builder.append("SUCCESS, result=[").append(value).append("]");
} catch (ExecutionException e) {
builder.append("FAILURE, cause=[").append(e.getCause()).append("]");
} catch (CancellationException e) {
builder.append("CANCELLED"); // shouldn't be reachable
} catch (RuntimeException e) {
builder.append("UNKNOWN, cause=[").append(e.getClass()).append(" thrown from get()]");
}
}
/**
* Submits the given runnable to the given {@link Executor} catching and logging all
* {@linkplain RuntimeException runtime exceptions} thrown by the executor.
*/
private static void executeListener(Runnable runnable, Executor executor) {
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);
}
}
private abstract static class AtomicHelper {
/** Non volatile write of the thread to the {@link Waiter#thread} field. */
abstract void putThread(Waiter waiter, Thread newValue);
/** Non volatile write of the waiter to the {@link Waiter#next} field. */
abstract void putNext(Waiter waiter, Waiter newValue);
/** Performs a CAS operation on the {@link #waiters} field. */
abstract boolean casWaiters(AbstractFuture future, Waiter expect, Waiter update);
/** Performs a CAS operation on the {@link #listeners} field. */
abstract boolean casListeners(AbstractFuture future, Listener expect, Listener update);
/** Performs a CAS operation on the {@link #value} field. */
abstract boolean casValue(AbstractFuture future, Object expect, Object update);
}
/**
* {@link AtomicHelper} based on {@link sun.misc.Unsafe}.
*
*
Static initialization of this class will fail if the {@link sun.misc.Unsafe} object cannot
* be accessed.
*/
private static final class UnsafeAtomicHelper extends AtomicHelper {
static final sun.misc.Unsafe UNSAFE;
static final long LISTENERS_OFFSET;
static final long WAITERS_OFFSET;
static final long VALUE_OFFSET;
static final long WAITER_THREAD_OFFSET;
static final long WAITER_NEXT_OFFSET;
static {
sun.misc.Unsafe unsafe = null;
try {
unsafe = sun.misc.Unsafe.getUnsafe();
} catch (SecurityException tryReflectionInstead) {
try {
unsafe =
AccessController.doPrivileged(
new PrivilegedExceptionAction() {
@Override
public sun.misc.Unsafe run() throws Exception {
Class k = sun.misc.Unsafe.class;
for (java.lang.reflect.Field f : k.getDeclaredFields()) {
f.setAccessible(true);
Object x = f.get(null);
if (k.isInstance(x)) {
return k.cast(x);
}
}
throw new NoSuchFieldError("the Unsafe");
}
});
} catch (PrivilegedActionException e) {
throw new RuntimeException("Could not initialize intrinsics", e.getCause());
}
}
try {
Class abstractFuture = AbstractFuture.class;
WAITERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("waiters"));
LISTENERS_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("listeners"));
VALUE_OFFSET = unsafe.objectFieldOffset(abstractFuture.getDeclaredField("value"));
WAITER_THREAD_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("thread"));
WAITER_NEXT_OFFSET = unsafe.objectFieldOffset(Waiter.class.getDeclaredField("next"));
UNSAFE = unsafe;
} catch (Exception e) {
throwIfUnchecked(e);
throw new RuntimeException(e);
}
}
@Override
void putThread(Waiter waiter, Thread newValue) {
UNSAFE.putObject(waiter, WAITER_THREAD_OFFSET, newValue);
}
@Override
void putNext(Waiter waiter, Waiter newValue) {
UNSAFE.putObject(waiter, WAITER_NEXT_OFFSET, newValue);
}
/** Performs a CAS operation on the {@link #waiters} field. */
@Override
boolean casWaiters(AbstractFuture future, Waiter expect, Waiter update) {
return UNSAFE.compareAndSwapObject(future, WAITERS_OFFSET, expect, update);
}
/** Performs a CAS operation on the {@link #listeners} field. */
@Override
boolean casListeners(AbstractFuture future, Listener expect, Listener update) {
return UNSAFE.compareAndSwapObject(future, LISTENERS_OFFSET, expect, update);
}
/** Performs a CAS operation on the {@link #value} field. */
@Override
boolean casValue(AbstractFuture future, Object expect, Object update) {
return UNSAFE.compareAndSwapObject(future, VALUE_OFFSET, expect, update);
}
}
/** {@link AtomicHelper} based on {@link AtomicReferenceFieldUpdater}. */
private static final class SafeAtomicHelper extends AtomicHelper {
final AtomicReferenceFieldUpdater waiterThreadUpdater;
final AtomicReferenceFieldUpdater waiterNextUpdater;
final AtomicReferenceFieldUpdater waitersUpdater;
final AtomicReferenceFieldUpdater listenersUpdater;
final AtomicReferenceFieldUpdater valueUpdater;
SafeAtomicHelper(
AtomicReferenceFieldUpdater waiterThreadUpdater,
AtomicReferenceFieldUpdater waiterNextUpdater,
AtomicReferenceFieldUpdater waitersUpdater,
AtomicReferenceFieldUpdater listenersUpdater,
AtomicReferenceFieldUpdater valueUpdater) {
this.waiterThreadUpdater = waiterThreadUpdater;
this.waiterNextUpdater = waiterNextUpdater;
this.waitersUpdater = waitersUpdater;
this.listenersUpdater = listenersUpdater;
this.valueUpdater = valueUpdater;
}
@Override
void putThread(Waiter waiter, Thread newValue) {
waiterThreadUpdater.lazySet(waiter, newValue);
}
@Override
void putNext(Waiter waiter, Waiter newValue) {
waiterNextUpdater.lazySet(waiter, newValue);
}
@Override
boolean casWaiters(AbstractFuture future, Waiter expect, Waiter update) {
return waitersUpdater.compareAndSet(future, expect, update);
}
@Override
boolean casListeners(AbstractFuture future, Listener expect, Listener update) {
return listenersUpdater.compareAndSet(future, expect, update);
}
@Override
boolean casValue(AbstractFuture future, Object expect, Object update) {
return valueUpdater.compareAndSet(future, expect, update);
}
}
/**
* {@link AtomicHelper} based on {@code synchronized} and volatile writes.
*
* This is an implementation of last resort for when certain basic VM features are broken (like
* AtomicReferenceFieldUpdater).
*/
private static final class SynchronizedHelper extends AtomicHelper {
@Override
void putThread(Waiter waiter, Thread newValue) {
waiter.thread = newValue;
}
@Override
void putNext(Waiter waiter, Waiter newValue) {
waiter.next = newValue;
}
@Override
boolean casWaiters(AbstractFuture future, Waiter expect, Waiter update) {
synchronized (future) {
if (future.waiters == expect) {
future.waiters = update;
return true;
}
return false;
}
}
@Override
boolean casListeners(AbstractFuture future, Listener expect, Listener update) {
synchronized (future) {
if (future.listeners == expect) {
future.listeners = update;
return true;
}
return false;
}
}
@Override
boolean casValue(AbstractFuture future, Object expect, Object update) {
synchronized (future) {
if (future.value == expect) {
future.value = update;
return true;
}
return false;
}
}
}
private static CancellationException cancellationExceptionWithCause(
@Nullable String message, @Nullable Throwable cause) {
CancellationException exception = new CancellationException(message);
exception.initCause(cause);
return exception;
}
}