Netty 异步模型

简介

Netty中的 I/O 操作是异步的, 包括 Bind、Write、Connect 等操作会简单的返回一个ChannelFuture。
调用者不能立刻获得结果, 而是通过Future-Listener 机制, 用户可以方便的主动获取或者通过通知机制获得IO操作结果。
Netty的异步模型是建立在future和callback之上的。callback就是回调。
Future的核心思想是: 假设一个方法func(), 其计算过程可能很耗时, 等待func()返回不合适。那么就可以在调用func()的时候, 立马返回一个Future, 后续可以通过Future去监控方法func()的处理过程(即： Future-Listener机制)

Future说明

表示异步的结果, 可以通过它提供的方法来检测执行是否完成, 比如检索计算等。
ChannelFuture是一个继承了Future类的接口, public interface ChannelFuture extends Future {}。可以添加监听器, 当监听的事件发生时, 就会通知到监听器。

ChannelFuture类注释

/**

 * The result of an asynchronous {@link Channel} I/O operation.

 * 异步I/O操作的执行结果

 * <p>

 * All I/O operations in Netty are asynchronous.

 * Netty中的所有 I/O操作都是异步的

 * 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.

 * 这意味着 任意 I/O 调用都会直接返回, 但是不能保证请求的I/O 操作在被调用前能够完成。

 *

 * 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.

 * 但是, 会有一个能提供该I/O操作的结果或状态的ChannelFuture类实例被返回。

 * <p>

 *

 * A {@link ChannelFuture} is either <em>uncompleted</em> or <em>completed</em>.

 * channelFuture的状态可以是未完成的也可以是完成的。

 *

 * When an I/O operation begins, a new future object is created.

 * 当一个 I/O 操作开始, 一个新的对象被创建。

 *

 * The new future is uncompleted initially - it is neither succeeded, failed, nor

 * cancelled because the I/O operation is not finished yet.

 * 新的future一开始是未完成状态, 它不是成功的, 失败的, 或取消的, 因为 I/O 操作并没有完成。

 *

 * 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.

 * 如果 I/O 操作是成功完成的, 失败的 或是 被取消状态, future就被标记为带有特定信息的完成状

 * 态, 比如导致失败的原因。

 *

 * Please note that even failure and cancellation belong to the completed state.

 * 请记住, 即使是 失败 和 取消 都是完成状态

 * <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.

 * 有许多方法提供给你来检查 此I/O 操作是否完成, 在等待完成, 并取回 I/O 操作的结果。

 * It also allows you to add {@link ChannelFutureListener}s so you

 * can get notified when the I/O operation is completed.

 * 它还允许你添加 ChannelFuture监听器, 所以你能够在I/O 操作完成时被通知。

 *

 * <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.

 * 当一个 I/O 操作被完成 并且 有接下来的任务要做时, 推荐使用 addListener(添加监听器)而不是

 * await() 方法, 因为使用监听器的方式可以被通知。

 * <p>

 * {@link #addListener(GenericFutureListener)} is non-blocking.

 * addListener 方法是非阻塞的

 *

 * 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.

 * 它仅仅添加了特定的ChannelFutureListener到ChannelFuture中, 并且 I/O 线程会在 与future

 * 相关联的 I/O 操作完成时通知监听器。

 *

 * {@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.

 * 由于本身不阻塞, ChannelFutureListener(监听器) 能提供最好的效用 和 最好的资源利用率, 但

 * 是如果内没有习惯于事件启动编程模型, 实现一系列逻辑时可能会比较tricky。

 *

 * <p>

 * By contrast, {@link #await()} is a blocking operation.

 * 相比较之下, await() 方法是一个阻塞操作。

 * Once called, the caller thread blocks until the operation is done.

 * 一旦被调用, 在操作结束之前, 调用者线程会一直阻塞。

 * 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.

 * 以await() 方法实现一系列的逻辑会相对简单, 但是调用者线程在I/O操作间有不必要的阻塞 以及

 * 线程内部通信代价很高。

 * Moreover, there's a chance of dead lock in a particular circumstance, which is

 * described below.

 * 此外, 在特殊情况下, 还有可能会产生死锁, 描述如下。

 *

 * <h3>Do not call {@link #await()} inside {@link ChannelHandler}</h3>

 * 不要在 ChannelHandler 中调用 await() 方法

 * <p>

 * The event handler methods in {@link ChannelHandler} are usually called by

 * an I/O thread.

 * ChannelHandler中的事件处理方法通常是由 I/O 线程调用的。

 *

 * 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.

 * 如果 await() 方法是被一个事件处理方法以 I/O 线程的形式调用的, 该 I/O 操作会因为await()

 * 方法阻塞了此 正在被等待的 I/O 操作, 从而导致死锁。

 * <pre>

 * // BAD - NEVER DO THIS 千万别做以下操作

 * {@code @Override}

 * public void channelRead({@link ChannelHandlerContext} ctx, Object msg) {

 *     {@link ChannelFuture} future = ctx.channel().close();

 *     future.awaitUninterruptibly();

 *     // Perform post-closure operation

 *     // ...

 * }

 *

 * // GOOD 好的操作

 * {@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()}.

 * 尽管await()方法的缺点已经在上列出, 还是肯定会有使用它跟方便的情况

 * In such a case, please make sure you do not call {@link #await()} in an I/O

 * thread.

 * 在此情况下, 请确保你没有在I/O线程中调用await()

 * Otherwise, {@link BlockingOperationException} will be raised to prevent a dead

 * lock.

 * 此外 BlockingOperationException(阻塞操作异常) 会被抛出来预防死锁

 *

 * <h3>Do not confuse I/O timeout and await timeout</h3>

 * 不要将 I/O 超时 和 await 超时 弄混

 *

 * 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.

 * 你使用 await(long), await(long, TimeUnit), awaitUninterruptibly(long) 或

 * awaitUninterruptibly(long, TimeUnit)方法时的延时与 I/O 延迟无关。

 *

 * If an I/O operation times out, the future will be marked as

 * 'completed with failure,' as depicted in the diagram above.

 * 如果 I/O 操作延时, 该future 会被标记为 完成且失败, 就像途中描述的那样。

 *

 * For example, connect timeout should be configured via a transport-specific

 * option:

 * 比如, 连接事件应当通过特定的传输选项配置

 * <pre>

 * // BAD - NEVER DO THIS 不要做以下操作

 * {@link Bootstrap} b = ...;

 * {@link ChannelFuture} f = b.connect(...);

 * f.awaitUninterruptibly(10, TimeUnit.SECONDS);

 * if (f.isCancelled()) {

 *     // Connection attempt cancelled by user

 *     // 连接请求被用户取消

 * } else if (!f.isSuccess()) {

 *     // You might get a NullPointerException here because the future

 *     // might not be completed yet.

 *     // 你可能会得到一个空指针, 因为该future没有被完成。

 *     f.cause().printStackTrace();

 * } else {

 *     // Connection established successfully

 *     // 连接建立成功

 * }

 *

 * // GOOD 好的操作

 * {@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.

 * // 此时我们可以确认该future已完成

 * assert f.isDone();

 *

 * if (f.isCancelled()) {

 *     // Connection attempt cancelled by user

 *     // 连接请求被用户取消

 * } else if (!f.isSuccess()) {

 *     f.cause().printStackTrace();

 * } else {

 *     // Connection established successfully

 *     // 成功建立连接

 * }

 * </pre>

 */

工作原理示意图

inBound: 入栈
outBound: 出栈
说明:
1. 在使用Netty进行编程时, 拦截操作和转换出入栈数据只需要提供callback 或利用future即可。
2. 这使得链式操作简单、高效, 并有利于编写可重用的、通用的代码。
3. Netty 框架的目标就是让你的业务逻辑从网络基础应用编码中分离出来。

Future-Listener机制

当Future对象刚刚创建时, 处于非完成状态, 调用者可以通过返回的ChannelFuture来获取操作执行的状态, 注册监听函数来执行完成后的操作。
常见操作
- 通过 isDone 方法来判断当前操作是否完成;
- 通过 isSuccess 方法来判断已完成的当前操作是否成功;
- 通过 getCause 方法来获取已完成的当前操作失败的原因;
- 通过 isCancelled 方法来判断已完成的当前操作是否被取消；
- 通过 addListener 方法来注册监听器, 当操作已完成(isDone 方法返回完成), 将会通知指定的监听器; 如果 Future 对象已完成, 则通知指定的监听器

巴特西

Netty 异步模型

简介

Future说明

ChannelFuture类注释

工作原理示意图

Future-Listener机制

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