netty源碼分析-ChannelFuture

概述

    對于netty中的channelfuture來說它主要是一個基于對java 中future的一個更新 對于jdk中的future來說函數執行結果是通過get()方法進行擷取的 。

   然而jdk中future 的get（）方法是有兩種形式一種是非阻塞（在結果為計算完成時傳回空）的直接擷取傳回值一種是通過阻塞的形式等待傳回值。雖然在通過isdone方法可以擷取方法是否執行完成但是逾時異常也都是執行完成。這無疑來說是一種缺陷。

  在netty的channelfuture引入了監聽器的方法其涉及的主要原理是觀察者模式（也叫釋出訂閱模式）通過引入監聽器就可以第一時間獲得傳回值且不用進行線程阻塞等待。

/*
 * 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.//io操作的解果都是異步的
 * <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.//所有的io操作都是異步的，這意味着任何io操作都會直接的被傳回結果當請求進來       *  的時候。你會獲得一個channelfuture對象通過他可以獲得io操作的結果
 * <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.//當io操作開始的時候 ，一個新的future對象将會被建立 這個新的future對象 不是處于完成狀态，也不是成功或者失敗或者取消的狀态 因為io操作還沒有完成。如果io操作完成 且成功 ，失敗 ，取消，那麼這個future的狀态就被标志為對應的成功，失敗，取消
 * <pre>
在jdk的future中isDone()方法僅僅代表着方法是否完成但是對于方法執行是否成功沒有辨別是以netty中的channelfuture對future做了一個拓展判斷執行是否成功或者是否出現異常
 *                                      +---------------------------+
 *                                      | 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      |
 *                                      +---------------------------+
 * </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.
 //channelfuture提供給你不同的方法 确認io操作是否完成 ，比如等待完成 或者添加監聽器在方法完成後你可以被通知
 *
 * <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.
//開發者建議使用監聽器的方式而不是等待的方式去獲得結果 這樣可以在任何情況下當io操作完成都可以被通知并進行下一步的任務
 * <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} are 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 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, Object msg) {
 *     {@link ChannelFuture} future = ctx.channel().close();
 *     future.awaitUninterruptibly();
 *     // Perform post-closure operation
 *     // ...
 * }
 *
 * // GOOD
 * {@code @Override}
 * public void channelRead({@link ChannelHandlerContext} ctx, Object 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();

//添加監聽器的方法
    @Override
    ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> listener);

      

//GenericFutureListener接口當future操作執行完畢調用該方法進行通知
package io.netty.util.concurrent;

import java.util.EventListener;

/**
 * Listens to the result of a {@link Future}.  The result of the asynchronous operation is notified once this listener
 * is added by calling {@link Future#addListener(GenericFutureListener)}.
 */
public interface GenericFutureListener<F extends Future<?>> extends EventListener {

    /**
     * Invoked when the operation associated with the {@link Future} has been completed.
     *
     * @param future  the source {@link Future} which called this callback
     */
    void operationComplete(F future) throws Exception;
}
      

@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();

    /**
     * Returns {@code true} if this {@link ChannelFuture} is a void future and so not allow to call any of the
     * following methods:
     * <ul>
     *     <li>{@link #addListener(GenericFutureListener)}</li>
     *     <li>{@link #addListeners(GenericFutureListener[])}</li>
     *     <li>{@link #await()}</li>
     *     <li>{@link #await(long, TimeUnit)} ()}</li>
     *     <li>{@link #await(long)} ()}</li>
     *     <li>{@link #awaitUninterruptibly()}</li>
     *     <li>{@link #sync()}</li>
     *     <li>{@link #syncUninterruptibly()}</li>
     * </ul>
     */
    boolean isVoid();
}