/*
* Copyright 2012 The Netty Project
*
* The Netty Project licenses this file to you under the Apache License,
* version 2.0 (the "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at:
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
* License for the specific language governing permissions and limitations
* under the License.
*/
package io.netty.channel;
import io.netty.bootstrap.Bootstrap;
import io.netty.util.concurrent.BlockingOperationException;
import io.netty.util.concurrent.Future;
import io.netty.util.concurrent.GenericFutureListener;
import java.util.concurrent.TimeUnit;
/**
* The result of an asynchronous {@link Channel} I/O operation.
* <p>
* 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 {@link ChannelFuture} instance which gives you the information about the
* result or status of the I/O operation.
* <p>
* A {@link ChannelFuture} is either <em>uncompleted</em> or <em>completed</em>.
* 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.
* <pre>
* +---------------------------+
* | Completed successfully |
* +---------------------------+
* +----> isDone() = <b>true</b> |
* +--------------------------+ | | isSuccess() = <b>true</b> |
* | Uncompleted | | +===========================+
* +--------------------------+ | | Completed with failure |
* | isDone() = <b>false</b> | | +---------------------------+
* | isSuccess() = false |----+----> isDone() = <b>true</b> |
* | isCancelled() = false | | | cause() = <b>non-null</b> |
* | cause() = null | | +===========================+
* +--------------------------+ | | Completed by cancellation |
* | +---------------------------+
* +----> isDone() = <b>true</b> |
* | isCancelled() = <b>true</b> |
* +---------------------------+
* </pre>
*
* 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 {@link ChannelFutureListener}s so you
* can get notified when the I/O operation is completed.
*
* <h3>Prefer {@link #addListener(GenericFutureListener)} to {@link #await()}</h3>
*
* It is recommended to prefer {@link #addListener(GenericFutureListener)} to
* {@link #await()} wherever possible to get notified when an I/O operation is
* done and to do any follow-up tasks.
* <p>
* {@link #addListener(GenericFutureListener)} is non-blocking. It simply adds
* the specified {@link ChannelFutureListener} to the {@link ChannelFuture}, and
* I/O thread will notify the listeners when the I/O operation associated with
* the future is done. {@link 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.
* <p>
* By contrast, {@link #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 {@link #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.
*
* <h3>Do not call {@link #await()} inside {@link ChannelHandler}</h3>
* <p>
* The event handler methods in {@link ChannelHandler} is usually called by
* an I/O thread. If {@link #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 {@link #await()} can block the I/O
* operation it is waiting for, which is a dead lock.
* <pre>
* // BAD - NEVER DO THIS
* {@code @Override}
* public void channelRead({@link ChannelHandlerContext} ctx, GoodByeMessage msg) {
* {@link ChannelFuture} future = ctx.channel().close();
* future.awaitUninterruptibly();
* // Perform post-closure operation
* // ...
* }
*
* // GOOD
* {@code @Override}
* public void channelRead({@link ChannelHandlerContext} ctx, GoodByeMessage msg) {
* {@link ChannelFuture} future = ctx.channel().close();
* future.addListener(new {@link ChannelFutureListener}() {
* public void operationComplete({@link ChannelFuture} future) {
* // Perform post-closure operation
* // ...
* }
* });
* }
* </pre>
* <p>
* In spite of the disadvantages mentioned above, there are certainly the cases
* where it is more convenient to call {@link #await()}. In such a case, please
* make sure you do not call {@link #await()} in an I/O thread. Otherwise,
* {@link BlockingOperationException} will be raised to prevent a dead lock.
*
* <h3>Do not confuse I/O timeout and await timeout</h3>
*
* The timeout value you specify with {@link #await(long)},
* {@link #await(long, TimeUnit)}, {@link #awaitUninterruptibly(long)}, or
* {@link #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:
* <pre>
* // BAD - NEVER DO THIS
* {@link Bootstrap} b = ...;
* {@link 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
* {@link Bootstrap} b = ...;
* // Configure the connect timeout option.
* <b>b.option({@link ChannelOption}.CONNECT_TIMEOUT_MILLIS, 10000);</b>
* {@link 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
* }
* </pre>
*/
public interface ChannelFuture extends Future<Void> {
/**
* Returns a channel where the I/O operation associated with this
* future takes place.
*/
Channel channel();
@Override
ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> listener);
@Override
ChannelFuture addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
@Override
ChannelFuture removeListener(GenericFutureListener<? extends Future<? super Void>> listener);
@Override
ChannelFuture removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
@Override
ChannelFuture sync() throws InterruptedException;
@Override
ChannelFuture syncUninterruptibly();
@Override
ChannelFuture await() throws InterruptedException;
@Override
ChannelFuture awaitUninterruptibly();
}