Interface ChannelFuture
- All Known Subinterfaces:
ChannelProgressiveFuture
,ChannelProgressivePromise
,ChannelPromise
- All Known Implementing Classes:
AbstractBootstrap.PendingRegistrationPromise
,AbstractChannel.CloseFuture
,CompleteChannelFuture
,DefaultChannelProgressivePromise
,DefaultChannelPromise
,DelegatingChannelPromiseNotifier
,FailedChannelFuture
,Http2CodecUtil.SimpleChannelPromiseAggregator
,SucceededChannelFuture
,VoidChannelPromise
Channel
I/O operation.
All I/O operations in Netty are asynchronous. It means any I/O calls will
return immediately with no guarantee that the requested I/O operation has
been completed at the end of the call. Instead, you will be returned with
a ChannelFuture
instance which gives you the information about the
result or status of the I/O operation.
A ChannelFuture
is either uncompleted or completed.
When an I/O operation begins, a new future object is created. The new future
is uncompleted initially - it is neither succeeded, failed, nor cancelled
because the I/O operation is not finished yet. If the I/O operation is
finished either successfully, with failure, or by cancellation, the future is
marked as completed with more specific information, such as the cause of the
failure. Please note that even failure and cancellation belong to the
completed state.
+---------------------------+ | Completed successfully | +---------------------------+ +----> isDone() = true | +--------------------------+ | | isSuccess() = true | | Uncompleted | | +===========================+ +--------------------------+ | | Completed with failure | | isDone() = false | | +---------------------------+ | isSuccess() = false |----+----> isDone() = true | | isCancelled() = false | | | cause() = non-null | | cause() = null | | +===========================+ +--------------------------+ | | Completed by cancellation | | +---------------------------+ +----> isDone() = true | | isCancelled() = true | +---------------------------+Various methods are provided to let you check if the I/O operation has been completed, wait for the completion, and retrieve the result of the I/O operation. It also allows you to add
ChannelFutureListener
s so you
can get notified when the I/O operation is completed.
Prefer addListener(GenericFutureListener)
to await()
It is recommended to prefer addListener(GenericFutureListener)
to
await()
wherever possible to get notified when an I/O operation is
done and to do any follow-up tasks.
addListener(GenericFutureListener)
is non-blocking. It simply adds
the specified ChannelFutureListener
to the ChannelFuture
, and
I/O thread will notify the listeners when the I/O operation associated with
the future is done. ChannelFutureListener
yields the best
performance and resource utilization because it does not block at all, but
it could be tricky to implement a sequential logic if you are not used to
event-driven programming.
By contrast, await()
is a blocking operation. Once called, the
caller thread blocks until the operation is done. It is easier to implement
a sequential logic with await()
, but the caller thread blocks
unnecessarily until the I/O operation is done and there's relatively
expensive cost of inter-thread notification. Moreover, there's a chance of
dead lock in a particular circumstance, which is described below.
Do not call await()
inside ChannelHandler
The event handler methods in ChannelHandler
are usually called by
an I/O thread. If await()
is called by an event handler
method, which is called by the I/O thread, the I/O operation it is waiting
for might never complete because await()
can block the I/O
operation it is waiting for, which is a dead lock.
// BAD - NEVER DO THIS@Override
public void channelRead(ChannelHandlerContext
ctx, Object msg) {ChannelFuture
future = ctx.channel().close(); future.awaitUninterruptibly(); // Perform post-closure operation // ... } // GOOD@Override
public void channelRead(ChannelHandlerContext
ctx, Object msg) {ChannelFuture
future = ctx.channel().close(); future.addListener(newChannelFutureListener
() { public void operationComplete(ChannelFuture
future) { // Perform post-closure operation // ... } }); }
In spite of the disadvantages mentioned above, there are certainly the cases
where it is more convenient to call await()
. In such a case, please
make sure you do not call await()
in an I/O thread. Otherwise,
BlockingOperationException
will be raised to prevent a dead lock.
Do not confuse I/O timeout and await timeout
The timeout value you specify withFuture.await(long)
,
Future.await(long, TimeUnit)
, Future.awaitUninterruptibly(long)
, or
Future.awaitUninterruptibly(long, TimeUnit)
are not related with I/O
timeout at all. If an I/O operation times out, the future will be marked as
'completed with failure,' as depicted in the diagram above. For example,
connect timeout should be configured via a transport-specific option:
// BAD - NEVER DO THISBootstrap
b = ...;ChannelFuture
f = b.connect(...); f.awaitUninterruptibly(10, TimeUnit.SECONDS); if (f.isCancelled()) { // Connection attempt cancelled by user } else if (!f.isSuccess()) { // You might get a NullPointerException here because the future // might not be completed yet. f.cause().printStackTrace(); } else { // Connection established successfully } // GOODBootstrap
b = ...; // Configure the connect timeout option. b.option(ChannelOption
.CONNECT_TIMEOUT_MILLIS, 10000);ChannelFuture
f = b.connect(...); f.awaitUninterruptibly(); // Now we are sure the future is completed. assert f.isDone(); if (f.isCancelled()) { // Connection attempt cancelled by user } else if (!f.isSuccess()) { f.cause().printStackTrace(); } else { // Connection established successfully }
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Nested Class Summary
Nested classes/interfaces inherited from interface java.util.concurrent.Future
Future.State
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Method Summary
Modifier and TypeMethodDescriptionaddListener
(GenericFutureListener<? extends Future<? super Void>> listener) Adds the specified listener to this future.addListeners
(GenericFutureListener<? extends Future<? super Void>>... listeners) Adds the specified listeners to this future.await()
Waits for this future to be completed.Waits for this future to be completed without interruption.channel()
Returns a channel where the I/O operation associated with this future takes place.boolean
isVoid()
Returnstrue
if thisChannelFuture
is a void future and so not allow to call any of the following methods:addListener(GenericFutureListener)
addListeners(GenericFutureListener[])
await()
Future.await(long, TimeUnit)
()}Future.await(long)
()}awaitUninterruptibly()
sync()
syncUninterruptibly()
removeListener
(GenericFutureListener<? extends Future<? super Void>> listener) Removes the first occurrence of the specified listener from this future.removeListeners
(GenericFutureListener<? extends Future<? super Void>>... listeners) Removes the first occurrence for each of the listeners from this future.sync()
Waits for this future until it is done, and rethrows the cause of the failure if this future failed.Waits for this future until it is done, and rethrows the cause of the failure if this future failed.Methods inherited from interface io.netty.util.concurrent.Future
await, await, awaitUninterruptibly, awaitUninterruptibly, cancel, cause, getNow, isCancellable, isSuccess
Methods inherited from interface java.util.concurrent.Future
exceptionNow, get, get, isCancelled, isDone, resultNow, state
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Method Details
-
channel
Channel channel()Returns a channel where the I/O operation associated with this future takes place. -
addListener
Description copied from interface:Future
Adds the specified listener to this future. The specified listener is notified when this future is done. If this future is already completed, the specified listener is notified immediately.- Specified by:
addListener
in interfaceFuture<Void>
-
addListeners
Description copied from interface:Future
Adds the specified listeners to this future. The specified listeners are notified when this future is done. If this future is already completed, the specified listeners are notified immediately.- Specified by:
addListeners
in interfaceFuture<Void>
-
removeListener
Description copied from interface:Future
Removes the first occurrence of the specified listener from this future. The specified listener is no longer notified when this future is done. If the specified listener is not associated with this future, this method does nothing and returns silently.- Specified by:
removeListener
in interfaceFuture<Void>
-
removeListeners
Description copied from interface:Future
Removes the first occurrence for each of the listeners from this future. The specified listeners are no longer notified when this future is done. If the specified listeners are not associated with this future, this method does nothing and returns silently.- Specified by:
removeListeners
in interfaceFuture<Void>
-
sync
Description copied from interface:Future
Waits for this future until it is done, and rethrows the cause of the failure if this future failed.- Specified by:
sync
in interfaceFuture<Void>
- Throws:
InterruptedException
-
syncUninterruptibly
ChannelFuture syncUninterruptibly()Description copied from interface:Future
Waits for this future until it is done, and rethrows the cause of the failure if this future failed.- Specified by:
syncUninterruptibly
in interfaceFuture<Void>
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await
Description copied from interface:Future
Waits for this future to be completed.- Specified by:
await
in interfaceFuture<Void>
- Throws:
InterruptedException
- if the current thread was interrupted
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awaitUninterruptibly
ChannelFuture awaitUninterruptibly()Description copied from interface:Future
Waits for this future to be completed without interruption. This method catches anInterruptedException
and discards it silently.- Specified by:
awaitUninterruptibly
in interfaceFuture<Void>
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isVoid
boolean isVoid()Returnstrue
if thisChannelFuture
is a void future and so not allow to call any of the following methods:
-