com.google.common.util.concurrent

Class AbstractFuture<V>

java.lang.Object

com.google.common.util.concurrent.AbstractFuture<V>

All Implemented Interfaces:

ListenableFuture<V>, java.util.concurrent.Future<V>

Direct Known Subclasses:

AbstractFuture.TrustedFuture, MoreExecutors.ScheduledListeningDecorator.NeverSuccessfulListenableFutureTask, TestingExecutors.NoOpScheduledExecutorService.NeverScheduledFuture

@GwtCompatible(emulated=true)
public abstract class AbstractFuture<V>
extends java.lang.Object
implements ListenableFuture<V>

An abstract implementation of ListenableFuture, intended for advanced users only. More common ways to create a ListenableFuture include instantiating a SettableFuture, submitting a task to a ListeningExecutorService, and deriving a Future from an existing one, typically using methods like Futures.transform and Futures.catching.

This class implements all methods in ListenableFuture. Subclasses should provide a way to set the result of the computation through the protected methods set(Object), setFuture(ListenableFuture) and setException(Throwable). Subclasses may also override interruptTask(), which will be invoked automatically if a call to cancel(true) succeeds in canceling the future. Subclasses should rarely override other methods.

Since:

1.0

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
private static class	AbstractFuture.AtomicHelper
private static class	AbstractFuture.Cancellation A special value to represent cancellation and the 'wasInterrupted' bit.
private static class	AbstractFuture.Failure A special value to represent failure, when setException(java.lang.Throwable) is called successfully.
private static class	AbstractFuture.Listener Listeners also form a stack through the listeners field.
private static class	AbstractFuture.SafeAtomicHelper AbstractFuture.AtomicHelper based on AtomicReferenceFieldUpdater.
private static class	AbstractFuture.SetFuture<V> A special value that encodes the 'setFuture' state.
private static class	AbstractFuture.SynchronizedHelper AbstractFuture.AtomicHelper based on synchronized and volatile writes.
(package private) static class	AbstractFuture.TrustedFuture<V> A less abstract subclass of AbstractFuture.
private static class	AbstractFuture.UnsafeAtomicHelper AbstractFuture.AtomicHelper based on Unsafe.
private static class	AbstractFuture.Waiter Waiter links form a Treiber stack, in the waiters field.

Field Summary

Fields

Modifier and Type	Field and Description
private static AbstractFuture.AtomicHelper	ATOMIC_HELPER
private static boolean	GENERATE_CANCELLATION_CAUSES
private AbstractFuture.Listener	listeners All listeners.
private static java.util.logging.Logger	log
private static java.lang.Object	NULL A special value to represent null.
private static long	SPIN_THRESHOLD_NANOS
private java.lang.Object	value This field encodes the current state of the future.
private AbstractFuture.Waiter	waiters All waiting threads.

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	AbstractFuture() Constructor for use by subclasses.

Method Summary

Modifier and Type	Method and Description
void	addListener(java.lang.Runnable listener, java.util.concurrent.Executor executor) Registers a listener to be run on the given executor.
protected void	afterDone() Callback method that is called exactly once after the future is completed.
boolean	cancel(boolean mayInterruptIfRunning)
private static java.util.concurrent.CancellationException	cancellationExceptionWithCause(java.lang.String message, java.lang.Throwable cause)
private AbstractFuture.Listener	clearListeners(AbstractFuture.Listener onto) Clears the listeners list and prepends its contents to onto, least recently added first.
private static void	complete(AbstractFuture<?> future) Unblocks all threads and runs all listeners.
private static void	executeListener(java.lang.Runnable runnable, java.util.concurrent.Executor executor) Submits the given runnable to the given Executor catching and logging all runtime exceptions thrown by the executor.
V	get()
V	get(long timeout, java.util.concurrent.TimeUnit unit)
private V	getDoneValue(java.lang.Object obj) Unboxes obj.
private static java.lang.Object	getFutureValue(ListenableFuture<?> future) Returns a value, suitable for storing in the value field.
protected void	interruptTask() Subclasses can override this method to implement interruption of the future's computation.
boolean	isCancelled()
boolean	isDone()
(package private) void	maybePropagateCancellation(java.util.concurrent.Future<?> related) If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts) the given future (if available).
private void	releaseWaiters() Releases all threads in the waiters list, and clears the list.
private void	removeWaiter(AbstractFuture.Waiter node) Marks the given node as 'deleted' (null waiter) and then scans the list to unlink all deleted nodes.
protected boolean	set(V value) Sets the result of this Future unless this Future has already been cancelled or set (including set asynchronously).
protected boolean	setException(java.lang.Throwable throwable) Sets the failed result of this Future unless this Future has already been cancelled or set (including set asynchronously).
protected boolean	setFuture(ListenableFuture<? extends V> future) Sets the result of this Future to match the supplied input Future once the supplied Future is done, unless this Future has already been cancelled or set (including "set asynchronously," defined below).
(package private) java.lang.Throwable	trustedGetException() Returns the exception that this Future completed with.
protected boolean	wasInterrupted() Returns true if this future was cancelled with mayInterruptIfRunning set to true.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

GENERATE_CANCELLATION_CAUSES

private static final boolean GENERATE_CANCELLATION_CAUSES

log

private static final java.util.logging.Logger log

SPIN_THRESHOLD_NANOS

private static final long SPIN_THRESHOLD_NANOS

See Also:

Constant Field Values

ATOMIC_HELPER

private static final AbstractFuture.AtomicHelper ATOMIC_HELPER

NULL

private static final java.lang.Object NULL

A special value to represent null.

value

private volatile java.lang.Object value

This field encodes the current state of the future.

The valid values are:

• null initial state, nothing has happened.
• AbstractFuture.Cancellation terminal state, cancel was called.
• AbstractFuture.Failure terminal state, setException was called.
• AbstractFuture.SetFuture intermediate state, setFuture was called.
• NULL terminal state, set(null) was called.
• Any other non-null value, terminal state, set was called with a non-null argument.

listeners

private volatile AbstractFuture.Listener listeners

All listeners.

waiters

private volatile AbstractFuture.Waiter waiters

All waiting threads.

Constructor Detail

AbstractFuture

protected AbstractFuture()

Constructor for use by subclasses.

Method Detail

removeWaiter

private void removeWaiter(AbstractFuture.Waiter node)

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.

get

public V get(long timeout,
             java.util.concurrent.TimeUnit unit)
      throws java.lang.InterruptedException,
             java.util.concurrent.TimeoutException,
             java.util.concurrent.ExecutionException

The default AbstractFuture implementation throws InterruptedException if the current thread is interrupted before or during the call, even if the value is already available.

Specified by:

get in interface java.util.concurrent.Future<V>

Throws:

java.lang.InterruptedException - if the current thread was interrupted before or during the call (optional but recommended).

java.util.concurrent.CancellationException

java.util.concurrent.TimeoutException

java.util.concurrent.ExecutionException

get

public V get()
      throws java.lang.InterruptedException,
             java.util.concurrent.ExecutionException

The default AbstractFuture implementation throws InterruptedException if the current thread is interrupted before or during the call, even if the value is already available.

Specified by:

get in interface java.util.concurrent.Future<V>

Throws:

java.lang.InterruptedException - if the current thread was interrupted before or during the call (optional but recommended).

java.util.concurrent.CancellationException

java.util.concurrent.ExecutionException

getDoneValue

private V getDoneValue(java.lang.Object obj)
                throws java.util.concurrent.ExecutionException

Unboxes obj. Assumes that obj is not null or a AbstractFuture.SetFuture.

Throws:

java.util.concurrent.ExecutionException

isDone

public boolean isDone()

Specified by:

isDone in interface java.util.concurrent.Future<V>

isCancelled

public boolean isCancelled()

Specified by:

isCancelled in interface java.util.concurrent.Future<V>

cancel

public boolean cancel(boolean mayInterruptIfRunning)

If a cancellation attempt succeeds on a Future that had previously been set asynchronously, then the cancellation will also be propagated to the delegate Future that was supplied in the setFuture call.

Specified by:

cancel in interface java.util.concurrent.Future<V>

interruptTask

protected void interruptTask()

Subclasses can override this method to implement interruption of the future's computation. The method is invoked automatically by a successful call to cancel(true).

The default implementation does nothing.

Since:

10.0

wasInterrupted

protected final boolean wasInterrupted()

Returns true if this future was cancelled with mayInterruptIfRunning set to true.

Since:

14.0

addListener

public void addListener(java.lang.Runnable listener,
                        java.util.concurrent.Executor executor)

Registers a listener to be run on the given executor. The listener will run when the Future's computation is complete or, if the computation is already complete, immediately.

There is no guaranteed ordering of execution of listeners, but any listener added through this method is guaranteed to be called once the computation is complete.

Exceptions thrown by a listener will be propagated up to the executor. Any exception thrown during Executor.execute (e.g., a RejectedExecutionException or an exception thrown by direct execution) will be caught and logged.

Note: For fast, lightweight listeners that would be safe to execute in any thread, consider MoreExecutors.directExecutor(). Otherwise, avoid it. Heavyweight directExecutor listeners can cause problems, and these problems can be difficult to reproduce because they depend on timing. For example:

• The listener may be executed by the caller of addListener. That caller may be a UI thread or other latency-sensitive thread. This can harm UI responsiveness.
• The listener may be executed by the thread that completes this Future. That thread may be an internal system thread such as an RPC network thread. Blocking that thread may stall progress of the whole system. It may even cause a deadlock.
• The listener may delay other listeners, even listeners that are not themselves directExecutor listeners.

This is the most general listener interface. For common operations performed using listeners, see Futures. For a simplified but general listener interface, see addCallback().

Memory consistency effects: Actions in a thread prior to adding a listener happen-before its execution begins, perhaps in another thread.

Specified by:

addListener in interface ListenableFuture<V>

Parameters:

listener - the listener to run when the computation is complete

executor - the executor to run the listener in

Since:

10.0

set

protected boolean set(@Nullable
                      V value)

Sets the result of this Future unless this Future has already been cancelled or set (including set asynchronously). When a call to this method returns, the Future is guaranteed to be done only if the call was accepted (in which case it returns true). If it returns false, the Future may have previously been set asynchronously, in which case its result may not be known yet. That result, though not yet known, cannot by overridden by a call to a set* method, only by a call to cancel(boolean).

Parameters:

value - the value to be used as the result

Returns:

true if the attempt was accepted, completing the Future

setException

protected boolean setException(java.lang.Throwable throwable)

Sets the failed result of this Future unless this Future has already been cancelled or set (including set asynchronously). When a call to this method returns, the Future is guaranteed to be done only if the call was accepted (in which case it returns true). If it returns false, the Future may have previously been set asynchronously, in which case its result may not be known yet. That result, though not yet known, cannot by overridden by a call to a set* method, only by a call to cancel(boolean).

Parameters:

throwable - the exception to be used as the failed result

Returns:

true if the attempt was accepted, completing the Future

setFuture

@Beta
protected boolean setFuture(ListenableFuture<? extends V> future)

Sets the result of this Future to match the supplied input Future once the supplied Future is done, unless this Future has already been cancelled or set (including "set asynchronously," defined below).

If the supplied future is 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 by overridden by a call to a set* method, only by a call to cancel(boolean).

If the call setFuture(delegate) is accepted and this Future is later cancelled, cancellation will be propagated to delegate. Additionally, any call to setFuture after any cancellation will propagate cancellation to the supplied Future.

Parameters:

future - the future to delegate to

Returns:

true if the attempt was accepted, indicating that the Future was not previously cancelled or set.

Since:

19.0

getFutureValue

private static java.lang.Object getFutureValue(ListenableFuture<?> future)

Returns a value, suitable for storing in the value field. From the given future, which is assumed to be done.

This is approximately the inverse of getDoneValue(Object)

complete

private static void complete(AbstractFuture<?> future)

Unblocks all threads and runs all listeners.

afterDone

@Beta
protected void afterDone()

Callback method that is called exactly once after the future is completed.

If interruptTask() is also run during completion, afterDone() runs after it.

The default implementation of this method in 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

trustedGetException

final java.lang.Throwable trustedGetException()

Returns the exception that this Future completed with. This includes completion through a call to setException or setFuture(failedFuture) but not cancellation.

Throws:

java.lang.RuntimeException - if the Future has not failed

maybePropagateCancellation

final void maybePropagateCancellation(@Nullable
                                      java.util.concurrent.Future<?> related)

If this future has been cancelled (and possibly interrupted), cancels (and possibly interrupts) the given future (if available).

This method should be used only when this future is completed. It is designed to be called from done.

releaseWaiters

private void releaseWaiters()

Releases all threads in the waiters list, and clears the list.

clearListeners

private AbstractFuture.Listener clearListeners(AbstractFuture.Listener onto)

Clears the listeners list and prepends its contents to onto, least recently added first.

executeListener

private static void executeListener(java.lang.Runnable runnable,
                                    java.util.concurrent.Executor executor)

Submits the given runnable to the given Executor catching and logging all runtime exceptions thrown by the executor.

cancellationExceptionWithCause

private static java.util.concurrent.CancellationException cancellationExceptionWithCause(@Nullable
                                                                                         java.lang.String message,
                                                                                         @Nullable
                                                                                         java.lang.Throwable cause)