netty源码分析(四)Netty提供的Future与ChannelFuture优势分析与源码讲解

上一节我们讲到netty启动服务类AbstractBootstrap的doBind的方法：

private ChannelFuture doBind(final SocketAddress localAddress) {
final ChannelFuture regFuture = initAndRegister();
...略
}

这里边有一个重要的类ChannelFuture ：

最红他们的接口会来到jdk的Future接口，Future代表了一个异步处理的结果。
/**
* A {@code Future} represents the result of an asynchronous
* computation.  Methods are provided to check if the computation is
* complete, to wait for its completion, and to retrieve the result of
* the computation.  The result can only be retrieved using method
* {@code get} when the computation has completed, blocking if
* necessary until it is ready.  Cancellation is performed by the
* {@code cancel} method.  Additional methods are provided to
* determine if the task completed normally or was cancelled. Once a
* computation has completed, the computation cannot be cancelled.
* If you would like to use a {@code Future} for the sake
* of cancellability but not provide a usable result, you can
* declare types of the form {@code Future<?>} and
* return {@code null} as a result of the underlying task.
译：
一个Future代表一个一步计算的结果，它提供了一些方法检查是否计算完毕，比如等待计算完毕，获取计算结果的方法。当计算完毕之后只能通过get方法获取结果，或者一直阻塞等待计算的完成。取消操作通过cancle方法来取消，另外还提供了检测是正常的完成还是被取消的方法，当一个计算完成后，不能进行取消操作。如果你想用Future实现取消，但是却没有一个可用的结果，你可以声明很多Future的类型，然后返回一个null的结果给当前任务。

* <p>
* <b>Sample Usage</b> (Note that the following classes are all
* made-up.)
* <pre> {@code
* interface ArchiveSearcher { String search(String target); }
* class App {
*   ExecutorService executor = ...//线程池
*   ArchiveSearcher searcher = ...//搜索接口
*   void showSearch(final String target)
*       throws InterruptedException {
*     Future<String> future
*       = executor.submit(new Callable<String>() {//创建一个Callable给线程池，Callable是有返回结果的。
*         public String call() {
*             return searcher.search(target);
*         }});
*     displayOtherThings(); // do other things while searching  中间可以做其他的事情，submit不会阻塞。
*     try {
*       displayText(future.get()); // use future  使用future的get拿到线程的处理结果，此处是阻塞的方式。
*     } catch (ExecutionException ex) { cleanup(); return; }
*   }
* }}</pre>
*
* The {@link FutureTask} class is an implementation of {@code Future} that
* implements {@code Runnable}, and so may be executed by an {@code Executor}.
* FutureTask是Future的实现类，并且实现了Runnable接口，因此可以被Executor执行。
* For example, the above construction with {@code submit} could be replaced by:
* 之前的方式可以被下边的FutureTask的方式替换。
*  <pre> {@code
* FutureTask<String> future =
*   new FutureTask<String>(new Callable<String>() {
*     public String call() {
*       return searcher.search(target);
*   }});
* executor.execute(future);}</pre>
*
* <p>Memory consistency effects: Actions taken by the asynchronous computation
* <a href="package-summary.html#MemoryVisibility"> <i>happen-before</i></a>
* actions following the corresponding {@code Future.get()} in another thread.
* 内存一致性影响：异步计算的动作的完成，发生在Future.get()之前。
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's {@code get} method
*/
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled();
boolean isDone();
V get() throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;}

io.netty.util.concurrent.Future对java.util.concurrent.Future进行了扩展：

public interface Future<V> extends java.util.concurrent.Future<V> {
boolean isSuccess();//是否计算成功
boolean isCancellable();//可以被取消
Throwable cause();//原因
Future<V> addListener(GenericFutureListener<? extends Future<? super V>> listener);//添加一个监听器
Future<V> addListeners(GenericFutureListener<? extends Future<? super V>>... listeners);//添加多个监听器
Future<V> removeListener(GenericFutureListener<? extends Future<? super V>> listener);//移除一个监听器
Future<V> removeListeners(GenericFutureListener<? extends Future<? super V>>... listeners);//移除多个监听器
Future<V> sync() throws InterruptedException;//等待结果返回
Future<V> syncUninterruptibly();//等待结果返回，不能被中断
Future<V> await() throws InterruptedException;//等待结果返回
Future<V> awaitUninterruptibly();//等待结果返回，不能被中断
boolean await(long timeout, TimeUnit unit) throws InterruptedException;
boolean await(long timeoutMillis) throws InterruptedException;
boolean awaitUninterruptibly(long timeout, TimeUnit unit);
boolean awaitUninterruptibly(long timeoutMillis);
V getNow();//立刻返回，没有计算完毕，返回null，需要配合isDone()方法判定是不是已经完成，因为runnable没有返回结果，
//而callable有返回结果
boolean cancel(boolean mayInterruptIfRunning);  //取消
}

提到一个监听器GenericFutureListener的封装，一碰到XXXlistener，都会用到监听器模式：

/**
* 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)}.
* 监听Future的结果，当一个监听器被注册后，结果的异步操作会被注册的监听器监听。
*/
public interface GenericFutureListener<F extends Future<?>> extends EventListener {

/**
* Invoked when the operation associated with the {@link Future} has been completed.
*
* 和Future的完成计算相关的事件，次方法会被调用。
*/
void operationComplete(F future) throws Exception;
}

监听器对Future的扩展起到了很灵活的作用，当某个计算完毕，会触发相应的时间，得到Future的结果，因为jdk的get方法我们知道什么时候去掉，调早了需要等待，调晚了浪费了一段时间，还有isDone里边有2种情况，无法区分到底是正常的io完毕返回的true还是被取消之后返回的true，所有到了netty的Future里边加了一个isSuccess()方法，只有正常的io处理结束isSuccess()才返回true。

接下来我们会走一下ChannelFuture的源码的doc：

/**
* The result of an asynchronous {@link Channel} I/O operation.
* Channel的异步io操作的结果。
* <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.
* netty中所有的i/o都是异步的，意味着很多i/o操作被调用过后会立刻返回，并且不能保证i/o请求操作被调用后计算已经完毕，
* 替代它的是返回一个当前i/o操作状态和结果信息的ChannelFuture实例。
* <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.
* 一个ChannelFuture要么是完成的，要么是未完成的。当一个i/o操作开始的时候，会创建一个future 对象，future 初始化的时候是为完成的状态，
* 既不是是成功的，或者失败的，也不是取消的，因为i/o操作还没有完成，如果一个i/o不管是成功，还是失败，或者被取消，future 会被标记一些特殊
* 的信息，比如失败的原因，请注意即使是失败和取消也属于完成状态。
* <pre>
*                                      +---------------------------+
*                                      | 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提供了很多方法让你检查i/o操作是否完成、等待完成、获取i/o操作的结果，他也允许你添加ChannelFutureListener
* 因此可以在i/o操作完成的时候被通知。
* <h3>Prefer {@link #addListener(GenericFutureListener)} to {@link #await()}</h3>
* 建议使用addListener(GenericFutureListener)，而不使用await()
* 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.
* 推荐优先使用addListener(GenericFutureListener)，不是await()在可能的情况下，这样就能在i/o操作完成的时候收到通知，并且可以去做
* 后续的任务处理。
* <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.
* addListener(GenericFutureListener)本身是非阻塞的，他会添加一个指定的ChannelFutureListener到ChannelFuture
* 并且i/o线程在完成对应的操作将会通知监听器，ChannelFutureListener也会提供最好的性能和资源利用率，因为他永远不会阻塞，但是如果
* 不是基于事件编程，他可能在顺序逻辑存在棘手的问题。
* <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.
*相反的，await()是一个阻塞的操作，一旦被调用，调用者线程在操作完成之前是阻塞的，实现顺序的逻辑比较容易，但是他让调用者线程等待是没有必要
* 的，会造成资源的消耗，更多可能性会造成死锁，接下来会介绍。
* <h3>Do not call {@link #await()} inside {@link ChannelHandler}</h3>
* 不要再ChannelHandler里边调用await()方法
* <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.
* ChannelHandler里边的时间处理器通常会被i/o线程调用，如果await()被一个时间处理方法调用，并且是一个i/o线程，那么这个i/o操作将永远不会
* 完成，因为await()是会阻塞i/o操作，这是一个死锁。
* <pre>
* // BAD - NEVER DO THIS 不推荐的使用方式
* {@code @Override}
* public void channelRead({@link ChannelHandlerContext} ctx, Object msg) {
*     {@link ChannelFuture} future = ctx.channel().close();
*     future.awaitUninterruptibly();//不要使用await的 方式
*     // Perform post-closure operation
*     // ...
* }
*
* // GOOD
* {@code @Override} //推荐使用的方式
* public void channelRead({@link ChannelHandlerContext} ctx, Object msg) {
*     {@link ChannelFuture} future = ctx.channel().close();
*     future.addListener(new {@link ChannelFutureListener}() {//使用时间的方式
*         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.
* 尽管出现了上面提到的这些缺陷，但是在某些情况下更方便，在这种情况下，请确保不要再i/o线程里边调用await()方法，
* 否则会出现BlockingOperationException异常，导致死锁。
* <h3>Do not confuse I/O timeout and await timeout</h3>
*不要将i/o超时和等待超时混淆。
* 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:
* 使用await(long)、await(long, TimeUnit)、awaitUninterruptibly(long)、awaitUninterruptibly(long, TimeUnit)设置的超时时间
* 和i/o超时没有任何关系，如果一个i/o操作超时，future 将被标记为失败的完成状态，比如连接超时通过一些选项来配置：
* <pre>
* // BAD - NEVER DO THIS //不推荐的方式
* {@link Bootstrap} b = ...;
* {@link ChannelFuture} f = b.connect(...);
* f.awaitUninterruptibly(10, TimeUnit.SECONDS);//不真正确的等待超时
* if (f.isCancelled()) {
*     // Connection attempt cancelled by user
* } else if (!f.isSuccess()) {
*     // You might get a NullPointerException here because the future//不能确保future 的完成。
*     // might not be completed yet.
*     f.cause().printStackTrace();
* } else {
*     // Connection established successfully
* }
*
* // GOOD//推荐的方式
* {@link Bootstrap} b = ...;
* // Configure the connect timeout option.
* <b>b.option({@link ChannelOption}.CONNECT_TIMEOUT_MILLIS, 10000);</b>//配置连接超时
* {@link ChannelFuture} f = b.connect(...);
* f.awaitUninterruptibly();
*
* // Now we are sure the future is completed.//确保future 一定是完成了。
* 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.
* 返回和当前future 相关联的i/o操作的channel
*/
Channel channel();
ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> listener);
ChannelFuture addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
ChannelFuture removeListener(GenericFutureListener<? extends Future<? super Void>> listener);
ChannelFuture removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
ChannelFuture sync() throws InterruptedException;
ChannelFuture syncUninterruptibly();
ChannelFuture await() throws InterruptedException;
ChannelFuture awaitUninterruptibly();
}

下一接介绍initAndRegister( )方法。