netty源码分析之-Future、ChannelFuture与ChannelPromise详解（3）

对于jdk底层已经有对Future的实现，用来执行异步操作并且提供相应对结果操作的方法。但是，在netty内部也同样实现了自己的Future，并且继承了jdk中的Future接口，提供了一些额外的方法来针对在netty中相关的异步I/O操作来进行处理

jdk中的Future

该接口表示的是异步计算的结果，提供若干方法来监测计算是否完成、等待计算完成、获取计算的结果。下面举例其使用方法：

{
  interface ArchiveSearcher { String search(String target); }
  class App {
    ExecutorService executor = ...
    ArchiveSearcher searcher = ...
    void showSearch(final String target)
        throws InterruptedException {
      Future future
        = executor.submit(new Callable() {
          public String call() {
              return searcher.search(target);
          }});
      displayOtherThings(); // do other things while searching
      try {
        displayText(future.get()); // use future
      } catch (ExecutionException ex) { cleanup(); return; }
    }
  }}

future的get来获取异步计算的结果，该方法会阻塞直到计算完成

上述submit方法也可以被以下替换：

{
  FutureTask<String> future =
    new FutureTask<String>(new Callable<String>() {
      public String call() {
        return searcher.search(target);
    }});
  executor.execute(future);}

---

netty中的Future

public interface Future<V> extends java.util.concurrent.Future<V>

下面是一些比较重要方法的定义，其中addListener方法非常重要：

1. cause方法表示如果I/O操作失败，返回异常信息
2. cancel方法的boolean参数表示是否对已经开始执行的操作进行中断
3. isSuccess方法表示I/O操作是否已经成功的完成。对于上述jdk中Future申明的isDone方法，只能知道I/O是否结束，有可能是成功完成、被取消、异常中断。netty中Future的此isSuccess方法能够很好的判断出操作是否正真地成功完成
4. sync方法阻塞直到future完成操作，如果操作失败会重新抛出异常
5. addListener方法会添加特定的监听器到future，这些监听器会在future isDone返回true的时候立刻被通知。这是netty中很重要的扩展方法，这里用到了观察者模式

addListener方法传入的监听器会实现以下接口，也就是被通知的时候operationComplete方法会被调用：

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;
}

为什么future中有get方法来获取异步的结果，这里又扩展了监听器这种方法。如果使用get，我们会考虑到底在什么时候使用，因为该方法会阻塞后续逻辑代码，如果我们使用监听器，毫无疑问，会更加优雅地在合理的时间来处理我们的逻辑代码

---

ChannelFuture

netty中的ChannelFuture继承来netty中的自己的Future

public interface ChannelFuture extends Future<Void>

ChannelFuture表示Channel中异步I/O操作的结果，在netty中所有的I/O操作都是异步的，I/O的调用会直接返回，可以通过ChannelFuture来获取I/O操作的结果状态。对于多种状态的表示如下：

                                       +---------------------------+
 *                                      | 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      |
 *                                      +---------------------------+

需要注意的是failure和cancellation都会表示操作完成，但是对应的状态是不同的。与Future类似，可以通过添加ChannelFutureListener监听器，当I/O操作完成的时候来通知调用。相比于wait()方式也更推荐这种方式来获取结果状态或者执行后续操作。
此外，不建议在ChannelHandler中调用await（），因为ChannelHandler中事件驱动的方法被一个I/O线程调用，可能一直不回完成，那么await（）也可能被I/O线程调用，同样会一直block，因此会产生死锁。例如：

 //永远不要这样做
 * // BAD - NEVER DO THIS
 * public void channelRead({@link ChannelHandlerContext} ctx, Object msg) {
 *     {@link ChannelFuture} future = ctx.channel().close();
 *     future.awaitUninterruptibly();
 *     // Perform post-closure operation
 *     // ...
 * }

 //而应该这样做：
 * // GOOD
 * 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
 *             // ...
 *         }
 *     });
 * }

对于I/O超时和await（）超时的区别：

 //永远不要这样做
 * // 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
 * }
//当awaitUninterruptibly也就是await超时之后，ChannelFuture对应的连接是可能没有完成，那么执行后续的操作就会异常

 //而应该这样做
 * // GOOD
 * {@link Bootstrap} b = ...;
 * // Configure the connect timeout option.
 * b.option({@link ChannelOption}.CONNECT_TIMEOUT_MILLIS, 10000);
 * {@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
 * }
 //当通过option的方式添加超时时间，如果超时则会被当做failure结果返回，同时再调用awaitUninterruptibly的时候一定是future已经操作完成

ChannelFuture中需要注意的是添加了channel方法来获取Channel：

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

JDK所提供的Future只能通过手工方式检查执行结果，而这个操作是会阻塞的；Netty则对ChannelFuture进行来增强，通过ChannelFutureListener以回调的方式来获取执行结果，去除来手工检查阻塞的操作。需要注意的是ChannelFutureListener的operationComplete方法是由I/O线程执行的，因此要注意的是不要在这里执行耗时操作，否则需要通过另外的线程或线程池来执行

ChannelPromise

ChannelPromise是一种可写的特殊ChannelFuture

public interface ChannelPromise extends ChannelFuture, Promise<Void>

对于Promise：

public interface Promise<V> extends Future<V>

定义了可以标识Future成功或者失败的方法，并且每一个Future只能够被标识一次，如果成功将会去通知之前所定义的listeners

    /**
     * Marks this future as a success and notifies all
     * listeners.
     *
     * If it is success or failed already it will throw an {@link IllegalStateException}.
     */
    Promise setSuccess(V result);

    /**
     * Marks this future as a success and notifies all
     * listeners.
     *
     * @return {@code true} if and only if successfully marked this future as
     *         a success. Otherwise {@code false} because this future is
     *         already marked as either a success or a failure.
     */
    boolean trySuccess(V result);

    /**
     * Marks this future as a failure and notifies all
     * listeners.
     *
     * If it is success or failed already it will throw an {@link IllegalStateException}.
     */
    Promise setFailure(Throwable cause);

    /**
     * Marks this future as a failure and notifies all
     * listeners.
     *
     * @return {@code true} if and only if successfully marked this future as
     *         a failure. Otherwise {@code false} because this future is
     *         already marked as either a success or a failure.
     */
    boolean tryFailure(Throwable cause);

    /**
     * Make this future impossible to cancel.
     *
     * @return {@code true} if and only if successfully marked this future as uncancellable or it is already done
     *         without being cancelled.  {@code false} if this future has been cancelled already.
     */
    boolean setUncancellable();

在DefaultChannelPromise默认实现中，当表示为成功时会通知相应listeners

@Override
    public ChannelPromise setSuccess(Void result) {
        super.setSuccess(result);
        return this;
    }

在setSuccess方法中：

private void notifyListenersNow() {
        ...
        for (;;) {
            if (listeners instanceof DefaultFutureListeners) {
                notifyListeners0((DefaultFutureListeners) listeners);
            } else {
                notifyListener0(this, (GenericFutureListener) listeners);
            }
            synchronized (this) {
                if (this.listeners == null) {
                    // Nothing can throw from within this method, so setting notifyingListeners back to false does not
                    // need to be in a finally block.
                    notifyingListeners = false;
                    return;
                }
                listeners = this.listeners;
                this.listeners = null;
            }
        }
    }