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

  @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(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 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 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

 }