The result of an asynchronous
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.
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
@Overridepublic void channelRead(
ChannelHandlerContext ctx, Object msg) {
ChannelFuture future = ctx.channel().close();
future.awaitUninterruptibly();
// Perform post-closure operation
// ...
}
// GOOD
@Overridepublic void channelRead(
ChannelHandlerContext ctx, Object msg) {
ChannelFuture future = ctx.channel().close();
future.addListener(new
ChannelFutureListener() {
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 with
#await(long),
#await(long,TimeUnit),
#awaitUninterruptibly(long), or
#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 THIS
Bootstrap 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
}
// GOOD
Bootstrap 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
}