Package io.netty.channel

Interface ChannelFuture

  • All Superinterfaces:
    Future<java.lang.Void>, java.util.concurrent.Future<java.lang.Void>
    All Known Subinterfaces:
    ChannelProgressiveFuture, ChannelProgressivePromise, ChannelPromise
    All Known Implementing Classes:
    DefaultChannelProgressivePromise, DefaultChannelPromise, DelegatingChannelPromiseNotifier, VoidChannelPromise
    public interface ChannelFuture
extends Future<java.lang.Void>
    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
 @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
 }

    • Method Detail

      • channel

        Channel channel()
        Returns a channel where the I/O operation associated with this future takes place.

      • addListener

        ChannelFuture addListener​(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
        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 interface Future<java.lang.Void>

      • addListeners

        ChannelFuture addListeners​(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
        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 interface Future<java.lang.Void>

      • removeListener

        ChannelFuture removeListener​(GenericFutureListener<? extends Future<? super java.lang.Void>> listener)
        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 interface Future<java.lang.Void>

      • removeListeners

        ChannelFuture removeListeners​(GenericFutureListener<? extends Future<? super java.lang.Void>>... listeners)
        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 interface Future<java.lang.Void>

      • sync

        ChannelFuture sync()
            throws java.lang.InterruptedException
        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 interface Future<java.lang.Void>
        Throws:
        java.lang.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 interface Future<java.lang.Void>

      • await

        ChannelFuture await()
             throws java.lang.InterruptedException
        Description copied from interface: Future
        Waits for this future to be completed.
        Specified by:
        await in interface Future<java.lang.Void>
        Throws:
        java.lang.InterruptedException - if the current thread was interrupted

      • awaitUninterruptibly

        ChannelFuture awaitUninterruptibly()
        Description copied from interface: Future
        Waits for this future to be completed without interruption. This method catches an InterruptedException and discards it silently.
        Specified by:
        awaitUninterruptibly in interface Future<java.lang.Void>