public interface ChannelFuture extends Future<java.lang.Void>
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 ChannelFutureListeners so you
can get notified when the I/O operation is completed.
addListener(GenericFutureListener) to await()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.
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@Overridepublic void channelRead(ChannelHandlerContextctx, Object msg) {ChannelFuturefuture = ctx.channel().close(); future.awaitUninterruptibly(); // Perform post-closure operation // ... } // GOOD@Overridepublic void channelRead(ChannelHandlerContextctx, Object msg) {ChannelFuturefuture = ctx.channel().close(); future.addListener(newChannelFutureListener() { public void operationComplete(ChannelFuturefuture) { // 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.
Future.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 THISBootstrapb = ...;ChannelFuturef = 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 } // GOODBootstrapb = ...; // Configure the connect timeout option. b.option(ChannelOption.CONNECT_TIMEOUT_MILLIS, 10000);ChannelFuturef = 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 }
| 限定符和类型 | 方法和说明 |
|---|---|
ChannelFuture |
addListener(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
Adds the specified listener to this future.
|
ChannelFuture |
addListeners(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
Adds the specified listeners to this future.
|
ChannelFuture |
await()
Waits for this future to be completed.
|
ChannelFuture |
awaitUninterruptibly()
Waits for this future to be completed without
interruption.
|
Channel |
channel()
Returns a channel where the I/O operation associated with this
future takes place.
|
boolean |
isVoid()
Returns
true if this ChannelFuture 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()
|
ChannelFuture |
removeListener(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
Removes the first occurrence of the specified listener from this future.
|
ChannelFuture |
removeListeners(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
Removes the first occurrence for each of the listeners from this future.
|
ChannelFuture |
sync()
Waits for this future until it is done, and rethrows the cause of the failure if this future
failed.
|
ChannelFuture |
syncUninterruptibly()
Waits for this future until it is done, and rethrows the cause of the failure if this future
failed.
|
await, await, awaitUninterruptibly, awaitUninterruptibly, cancel, cause, getNow, isCancellable, isSuccessChannel channel()
ChannelFuture addListener(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
FutureChannelFuture addListeners(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
FutureChannelFuture removeListener(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
FutureChannelFuture removeListeners(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
FutureChannelFuture sync() throws java.lang.InterruptedException
Futurejava.lang.InterruptedExceptionChannelFuture syncUninterruptibly()
FutureChannelFuture await() throws java.lang.InterruptedException
Futurejava.lang.InterruptedException - if the current thread was interruptedChannelFuture awaitUninterruptibly()
FutureInterruptedException and
discards it silently.boolean isVoid()
true if this ChannelFuture is a void future and so not allow to call any of the
following methods: