Interface ChannelFuture

All Known Implementing Classes:
AbstractChannel.ChannelCloseFuture, ChannelRunnableWrapper, CompleteChannelFuture, DefaultChannelFuture, FailedChannelFuture, SslHandler.SSLEngineInboundCloseFuture, SucceededChannelFuture
public interface ChannelFuture
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    |    |    | getCause() = non-null     |
|    getCause() = 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(ChannelFutureListener) to await()

It is recommended to prefer addListener(ChannelFutureListener) to await() wherever possible to get notified when an I/O operation is done and to do any follow-up tasks.

addListener(ChannelFutureListener) 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 is often called by an I/O thread unless an ExecutionHandler is in the ChannelPipeline. 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 be 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 messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
    if (e.getMessage() instanceof GoodByeMessage) {
        ChannelFuture future = e.getChannel().close();
        future.awaitUninterruptibly();
        // Perform post-closure operation
        // ...
    }
}

// GOOD
@Override
public void messageReceived(ChannelHandlerContext ctx, MessageEvent e) {
    if (e.getMessage() instanceof GoodByeMessage) {
        ChannelFuture future = e.getChannel().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, IllegalStateException 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
ClientBootstrap 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.getCause().printStackTrace();
} else {
    // Connection established successfully
}

// GOOD
ClientBootstrap b = ...;
// Configure the connect timeout option.
b.setOption("connectTimeoutMillis", 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.getCause().printStackTrace();
} else {
    // Connection established successfully
}

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    addListener(ChannelFutureListener listener)
    Adds the specified listener to this future.
    ChannelFuture
    await()
    Waits for this future to be completed.
    boolean
    await(long timeoutMillis)
    Waits for this future to be completed within the specified time limit.
    boolean
    await(long timeout, TimeUnit unit)
    Waits for this future to be completed within the specified time limit.
    ChannelFuture
    awaitUninterruptibly()
    Waits for this future to be completed without interruption.
    boolean
    awaitUninterruptibly(long timeoutMillis)
    Waits for this future to be completed within the specified time limit without interruption.
    boolean
    awaitUninterruptibly(long timeout, TimeUnit unit)
    Waits for this future to be completed within the specified time limit without interruption.
    boolean
    cancel()
    Cancels the I/O operation associated with this future and notifies all listeners if canceled successfully.
    Throwable
    getCause()
    Returns the cause of the failed I/O operation if the I/O operation has failed.
    Channel
    getChannel()
    Returns a channel where the I/O operation associated with this future takes place.
    boolean
    isCancelled()
    Returns true if and only if this future was cancelled by a cancel() method.
    boolean
    isDone()
    Returns true if and only if this future is complete, regardless of whether the operation was successful, failed, or cancelled.
    boolean
    isSuccess()
    Returns true if and only if the I/O operation was completed successfully.
    void
    removeListener(ChannelFutureListener listener)
    Removes the specified listener from this future.
    boolean
    setFailure(Throwable cause)
    Marks this future as a failure and notifies all listeners.
    boolean
    setProgress(long amount, long current, long total)
    Notifies the progress of the operation to the listeners that implements ChannelFutureProgressListener.
    boolean
    setSuccess()
    Marks this future as a success and notifies all listeners.
    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.

  • Method Details

    • getChannel

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

    • isDone

      boolean isDone()
      Returns true if and only if this future is complete, regardless of whether the operation was successful, failed, or cancelled.

    • isCancelled

      boolean isCancelled()
      Returns true if and only if this future was cancelled by a cancel() method.

    • isSuccess

      boolean isSuccess()
      Returns true if and only if the I/O operation was completed successfully.

    • getCause

      Throwable getCause()
      Returns the cause of the failed I/O operation if the I/O operation has failed.
      Returns:
      the cause of the failure. null if succeeded or this future is not completed yet.

    • cancel

      boolean cancel()
      Cancels the I/O operation associated with this future and notifies all listeners if canceled successfully.
      Returns:
      true if and only if the operation has been canceled. false if the operation can't be canceled or is already completed.

    • setSuccess

      boolean setSuccess()
      Marks this future as a success and notifies all listeners.
      Returns:
      true if and only if successfully marked this future as a success. Otherwise false because this future is already marked as either a success or a failure.

    • setFailure

      boolean setFailure(Throwable cause)
      Marks this future as a failure and notifies all listeners.
      Returns:
      true if and only if successfully marked this future as a failure. Otherwise false because this future is already marked as either a success or a failure.

    • setProgress

      boolean setProgress(long amount, long current, long total)
      Notifies the progress of the operation to the listeners that implements ChannelFutureProgressListener. Please note that this method will not do anything and return false if this future is complete already.
      Returns:
      true if and only if notification was made.

    • addListener

      void addListener(ChannelFutureListener listener)
      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.

    • removeListener

      void removeListener(ChannelFutureListener listener)
      Removes 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.

    • sync

      ChannelFuture sync() throws InterruptedException
      Waits for this future until it is done, and rethrows the cause of the failure if this future failed. If the cause of the failure is a checked exception, it is wrapped with a new ChannelException before being thrown.
      Throws:
      InterruptedException

    • syncUninterruptibly

      ChannelFuture syncUninterruptibly()
      Waits for this future until it is done, and rethrows the cause of the failure if this future failed. If the cause of the failure is a checked exception, it is wrapped with a new ChannelException before being thrown.

    • await

      ChannelFuture await() throws InterruptedException
      Waits for this future to be completed.
      Throws:
      InterruptedException - if the current thread was interrupted

    • awaitUninterruptibly

      ChannelFuture awaitUninterruptibly()
      Waits for this future to be completed without interruption. This method catches an InterruptedException and discards it silently.

    • await

      boolean await(long timeout, TimeUnit unit) throws InterruptedException
      Waits for this future to be completed within the specified time limit.
      Returns:
      true if and only if the future was completed within the specified time limit
      Throws:
      InterruptedException - if the current thread was interrupted

    • await

      boolean await(long timeoutMillis) throws InterruptedException
      Waits for this future to be completed within the specified time limit.
      Returns:
      true if and only if the future was completed within the specified time limit
      Throws:
      InterruptedException - if the current thread was interrupted

    • awaitUninterruptibly

      boolean awaitUninterruptibly(long timeout, TimeUnit unit)
      Waits for this future to be completed within the specified time limit without interruption. This method catches an InterruptedException and discards it silently.
      Returns:
      true if and only if the future was completed within the specified time limit

    • awaitUninterruptibly

      boolean awaitUninterruptibly(long timeoutMillis)
      Waits for this future to be completed within the specified time limit without interruption. This method catches an InterruptedException and discards it silently.
      Returns:
      true if and only if the future was completed within the specified time limit