【Netty】ChannelPipeline和ChannelHandler（一）

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简介

前文学习Netty的ByteBuf数据容器。本文开始学习ChannelPipeline和ChannelHandler，它们的角色非常类似于流水线以及工人。

Channel的生命周期

Channel接口定义了一个简单但是强大的状态模型，该模型与ChannelInboundHandler API紧密联系。

ChannelInboundHandler源码如下

public interface ChannelInboundHandler extends ChannelHandler {

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered with its {@link EventLoop}
     */
    void channelRegistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was unregistered from its {@link EventLoop}
     */
    void channelUnregistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} is now active
     */
    void channelActive(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered is now inactive and reached its
     * end of lifetime.
     */
    void channelInactive(ChannelHandlerContext ctx) throws Exception;

    /**
     * Invoked when the current {@link Channel} has read a message from the peer.
     */
    void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception;

    /**
     * Invoked when the last message read by the current read operation has been consumed by
     * {@link #channelRead(ChannelHandlerContext, Object)}.  If {@link ChannelOption#AUTO_READ} is off, no further
     * attempt to read an inbound data from the current {@link Channel} will be made until
     * {@link ChannelHandlerContext#read()} is called.
     */
    void channelReadComplete(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if an user event was triggered.
     */
    void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception;

    /**
     * Gets called once the writable state of a {@link Channel} changed. You can check the state with
     * {@link Channel#isWritable()}.
     */
    void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if a {@link Throwable} was thrown.
     */
    @Override
    @SuppressWarnings("deprecation")
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}

其中标记Channel生命周期状态的方法如下：

以下列出Channel的4个状态：

ChannelUnregistered：Channel已经被创建，但还未注册到EventLoop
ChannelRegistered：Channel已经被注册到了EventLoop
ChannelActive：Channel处于活动状态（已经连接到它的远程节点）。它现在可以接收和发送数据了
ChannelInactive：Channel没有连接到远程节点

Channel的正常生命周期如下图所示

当这些状态发生改变时，将会生成对应的事件。这些事件将会被转发给ChannelPipeline中的ChannelHandler，其可以随后对它们做出响应。

ChannelHandler的生命周期

在ChannelHandler被添加到ChannelPipeline中或者被从ChannelPipeline中移除时会调用这些操作，这些方法中的每一个都接受一个ChannelHandlerContext参数。

ChannelHandler源码如下：

public interface ChannelHandler {

    /**
     * Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
     */
    void handlerAdded(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called after the {@link ChannelHandler} was removed from the actual context and it doesn't handle events
     * anymore.
     */
    void handlerRemoved(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if a {@link Throwable} was thrown.
     *
     * @deprecated if you want to handle this event you should implement {@link ChannelInboundHandler} and
     * implement the method there.
     */
    @Deprecated
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;

    /**
     * Indicates that the same instance of the annotated {@link ChannelHandler}
     * can be added to one or more {@link ChannelPipeline}s multiple times
     * without a race condition.
     * 

     * If this annotation is not specified, you have to create a new handler
     * instance every time you add it to a pipeline because it has unshared
     * state such as member variables.
     * 

     * This annotation is provided for documentation purpose, just like
     * the JCIP annotations.
     */
    @Inherited
    @Documented
    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @interface Sharable {
        // no value
    }
}

ChannelHandler的生命周期方法解析：

handlerAdded：当把ChannelHandler添加到ChannelPipeline中时被调用
handlerRemoved：当从ChannelPipeline中移除ChannelHandler时被调用
exceptionCaught：当处理过程中在ChannelPipeline中有错误产生时被调用

Netty定义了下面两个重要的ChannelHandler子接口：

ChannelInboundHandler——处理入站数据以及各种状态变化
ChannelOutboundHandler——处理出站数据并且允许拦截所有的操作

ChannelHandler之ChannelInboundHandler

ChannelInboundHandler接口生命周期中的方法，当接受到数据或者其对应的Channel的状态发生变化则会调用方法

ChannelInboundHandler源码如下：

public interface ChannelInboundHandler extends ChannelHandler {

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered with its {@link EventLoop}
     */
    void channelRegistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was unregistered from its {@link EventLoop}
     */
    void channelUnregistered(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} is now active
     */
    void channelActive(ChannelHandlerContext ctx) throws Exception;

    /**
     * The {@link Channel} of the {@link ChannelHandlerContext} was registered is now inactive and reached its
     * end of lifetime.
     */
    void channelInactive(ChannelHandlerContext ctx) throws Exception;

    /**
     * Invoked when the current {@link Channel} has read a message from the peer.
     */
    void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception;

    /**
     * Invoked when the last message read by the current read operation has been consumed by
     * {@link #channelRead(ChannelHandlerContext, Object)}.  If {@link ChannelOption#AUTO_READ} is off, no further
     * attempt to read an inbound data from the current {@link Channel} will be made until
     * {@link ChannelHandlerContext#read()} is called.
     */
    void channelReadComplete(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if an user event was triggered.
     */
    void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception;

    /**
     * Gets called once the writable state of a {@link Channel} changed. You can check the state with
     * {@link Channel#isWritable()}.
     */
    void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception;

    /**
     * Gets called if a {@link Throwable} was thrown.
     */
    @Override
    @SuppressWarnings("deprecation")
    void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}

当某个ChannelInboundHandler的实现重写channelRead()方法时，它将负责显式地释放与池化ByteBuf实例相关的内存，Netty为此提供了一个实用方法ReferenceCountUtil.release()。

@ChannelHandler.Sharable
//扩展了ChannelInboundHandlerAdapter
public class DiscardHandler extends ChannelInboundHandlerAdapter{

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        //丢弃已接收的消息
        ReferenceCountUtil.release(msg);
    }
}

当你不想处理这些释放资源等操作的话，Netty提供了SimpleChannelInboundHandler会自动释放资源，因此无需显式释放，代码如下：

public class MySimpleHandler
    extends SimpleChannelInboundHandler