netty底层是事件驱动的异步库 但是可以await或者sync(本质是future超时机制)同步返回 但是官方 Prefer addListener(GenericFutureListener) to await()
Interface ChannelFuture
- All Superinterfaces:
- java.util.concurrent.Future<java.lang.Void>
- All Known Subinterfaces:
- ChannelProgressiveFuture, ChannelProgressivePromise, ChannelPromise
- All Known Implementing Classes:
- DefaultChannelProgressivePromise, DefaultChannelPromise
public interface ChannelFuture extends Future<java.lang.Void>
The result of an asynchronousChannel
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 addChannelFutureListener
s so you can get notified when the I/O operation is completed.Prefer
It is recommended to preferaddListener(GenericFutureListener)
toawait()
addListener(GenericFutureListener)
toawait()
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 specifiedChannelFutureListener
to theChannelFuture
, 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 withawait()
, 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()
insideChannelHandler
The event handler methods in
ChannelHandler
are usually called by an I/O thread. Ifawait()
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 becauseawait()
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 callawait()
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)
, orFuture.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 } // 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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