com.google.common.util.concurrent

Class AbstractFuture<V>

java.lang.Object

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

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

All Implemented Interfaces:

ListenableFuture<V>, Future<V>

Direct Known Subclasses:

SettableFuture

@DoNotMock(value="Use Futures.immediate*Future or SettableFuture")
 @GwtCompatible(emulated=true)
public abstract class AbstractFuture<V>
extends FluentFuture<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 afterDone(), which will be invoked automatically when the future completes. Subclasses should rarely override other methods.

Since:

1.0

Author:

Sven Mawson, Luke Sandberg

Constructor Summary

Constructors

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

Method Summary

Modifier and Type	Method and Description
void	addListener(Runnable listener, 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) Attempts to cancel execution of this task.
V	get() Waits if necessary for the computation to complete, and then retrieves its result.
V	get(long timeout, TimeUnit unit) Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.
protected void	interruptTask() Subclasses can override this method to implement interruption of the future's computation.
boolean	isCancelled() Returns true if this task was cancelled before it completed normally.
boolean	isDone() Returns true if this task completed.
protected String	pendingToString() Provide a human-readable explanation of why this future has not yet completed.
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(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).
String	toString() Returns a string representation of the object.
protected boolean	wasInterrupted() Returns true if this future was cancelled with mayInterruptIfRunning set to true.

Methods inherited from class com.google.common.util.concurrent.FluentFuture

addCallback, catching, catchingAsync, from, transform, transformAsync, withTimeout

Methods inherited from class java.lang.Object

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

Constructor Detail

AbstractFuture

protected AbstractFuture()

Constructor for use by subclasses.

Method Detail

get

public V get(long timeout,
             TimeUnit unit)
      throws InterruptedException,
             TimeoutException,
             ExecutionException

Waits if necessary for at most the given time for the computation to complete, and then retrieves its result, if available.

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

Parameters:

timeout - the maximum time to wait

unit - the time unit of the timeout argument

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

InterruptedException - if the current thread was interrupted while waiting

TimeoutException - if the wait timed out

ExecutionException - if the computation threw an exception

get

public V get()
      throws InterruptedException,
             ExecutionException

Waits if necessary for the computation to complete, and then retrieves its result.

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

Returns:

the computed result

Throws:

CancellationException - if the computation was cancelled

InterruptedException - if the current thread was interrupted while waiting

ExecutionException - if the computation threw an exception

isDone

public boolean isDone()

Description copied from interface: java.util.concurrent.Future

Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

Returns:

true if this task completed

isCancelled

public boolean isCancelled()

Description copied from interface: java.util.concurrent.Future

Returns true if this task was cancelled before it completed normally.

Returns:

true if this task was cancelled before it completed

cancel

public boolean cancel(boolean mayInterruptIfRunning)

Attempts to cancel execution of this task. This attempt will fail if the task has already completed, has already been cancelled, or could not be cancelled for some other reason. If successful, and this task has not started when cancel is called, this task should never run. If the task has already started, then the mayInterruptIfRunning parameter determines whether the thread executing this task should be interrupted in an attempt to stop the task.

After this method returns, subsequent calls to Future.isDone() will always return true. Subsequent calls to Future.isCancelled() will always return true if this method returned true.

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.

Rather than override this method to perform additional cancellation work or cleanup, subclasses should override afterDone(), consulting isCancelled() and wasInterrupted() as necessary. This ensures that the work is done even if the future is cancelled without a call to cancel, such as by calling setFuture(cancelledFuture).

Parameters:

mayInterruptIfRunning - true if the thread executing this task should be interrupted; otherwise, in-progress tasks are allowed to complete

Returns:

false if the task could not be cancelled, typically because it has already completed normally; true otherwise

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.

This method is likely to be deprecated. Prefer to override afterDone(), consulting wasInterrupted() to decide whether to interrupt your task.

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(Runnable listener,
                        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.

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 be 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(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 be 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 be 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.

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 interruptTask() method, and the wasInterrupted() method will not return true.

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

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

toString

public String toString()

Description copied from class: java.lang.Object

Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:

 getClass().getName() + '@' + Integer.toHexString(hashCode())
 

Overrides:

toString in class Object

Returns:

a string representation of the object.

pendingToString

@Nullable
protected String pendingToString()

Provide a human-readable explanation of why this future has not yet completed.

Returns:

null if an explanation cannot be provided because the future is done.

Since:

23.0