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大纲

1.Pipeline和Handler的作用和构成

2.ChannelHandler的分类

3.几个特殊的ChannelHandler

4.ChannelHandler的生命周期

5.ChannelPipeline的事件处理

6.关于ChannelPipeline的问题整理

7.ChannelPipeline主要包括三部分内容

8.ChannelPipeline的初始化

9.ChannelPipeline添加ChannelHandler

10.ChannelPipeline删除ChannelHandler

11.Inbound事件的传播

12.Outbound事件的传播

13.ChannelPipeline中异常的传播

14.ChannelPipeline总结

1.Pipeline和Handler的作用和构成

(1)Pipeline和Handler的作用

(2)Pipeline和Handler的构成

(1)Pipeline和Handler的作用

可以在处理复杂的业务逻辑时避免if else的泛滥，可以实现对业务逻辑的模块化处理，不同的逻辑放置到单独的类中进行处理。最后将这些逻辑串联起来，形成一个完整的逻辑处理链。

Netty通过责任链模式来组织代码逻辑，能够支持逻辑的动态添加和删除，能够支持各类协议的扩展。

(2)Pipeline和Handler的构成

在Netty里，一个连接对应着一个Channel。这个Channel的所有处理逻辑都在一个叫ChannelPipeline的对象里，ChannelPipeline是双向链表结构，它和Channel之间是一对一的关系。

ChannelPipeline里的每个结点都是一个ChannelHandlerContext对象，这个ChannelHandlerContext对象能够获得和Channel相关的所有上下文信息。每个ChannelHandlerContext对象都包含一个逻辑处理器ChannelHandler，每个逻辑处理器ChannelHandler都处理一块独立的逻辑。

2.ChannelHandler的分类

ChannelHandler有两大子接口，分别为Inbound和Outbound类型：第一个子接口是ChannelInboundHandler，用于处理读数据逻辑，最重要的方法是channelRead()。第二个子接口是ChannelOutboundHandler，用于处理写数据逻辑，最重要的方法是write()。

这两个子接口默认的实现分别是：ChannelInboundHandlerAdapter和ChannelOutboundHandlerAdapter。它们分别实现了两个子接口的所有功能，在默认情况下会把读写事件传播到下一个Handler。

InboundHandler的事件通常只会传播到下一个InboundHandler，OutboundHandler的事件通常只会传播到下一个OuboundHandler，InboundHandler的执行顺序与实际addLast的添加顺序相同，OutboundHandler的执行顺序与实际addLast的添加顺序相反。

Inbound事件通常由IO线程触发，如TCP链路的建立事件、关闭事件、读事件、异常通知事件等。其触发方法一般带有fire字眼，如下所示：

ctx.fireChannelRegister()、

ctx.fireChannelActive()、

ctx.fireChannelRead()、

ctx.fireChannelReadComplete()、

ctx.fireChannelInactive()。

Outbound事件通常由用户主动发起的网络IO操作触发，如用户发起的连接操作、绑定操作、消息发送等操作。其触发方法一般如：ctx.bind()、ctx.connect()、ctx.write()、ctx.flush()、ctx.read()、ctx.disconnect()、ctx.close()。

3.几个特殊的ChannelHandler

(1)ChannelInboundHandlerAdapter

(2)ChannelOutboundHandlerAdapter

(3)ByteToMessageDecoder

(4)SimpleChannelInboundHandler

(5)MessageToByteEncoder

(1)ChannelInboundHandlerAdapter

ChannelInboundHandlerAdapter主要用于实现ChannelInboundHandler接口的所有方法，这样我们在继承它编写自己的ChannelHandler时就不需要实现ChannelHandler里的每种方法了，从而避免了直接实现ChannelHandler时需要实现其所有方法而导致代码显得冗余和臃肿。

//Handles an I/O event or intercepts an I/O operation, and forwards it to its next handler in its ChannelPipeline.
public interface ChannelHandler {//Gets called after the ChannelHandler was added to the actual context and it's ready to handle events.void handlerAdded(ChannelHandlerContext ctx) throws Exception;//Gets called after the ChannelHandler was removed from the actual context and it doesn't handle events anymore.void handlerRemoved(ChannelHandlerContext ctx) throws Exception;//Gets called if a Throwable was thrown.@Deprecatedvoid exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;//Indicates that the same instance of the annotated ChannelHandler can be added to one or more ChannelPipelines multiple times without a race condition.@Inherited@Documented@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@interface Sharable {// no value}
}//Skeleton implementation of a ChannelHandler.
public abstract class ChannelHandlerAdapter implements ChannelHandler {// Not using volatile because it's used only for a sanity check.boolean added;//Return true if the implementation is Sharable and so can be added to different ChannelPipelines.public boolean isSharable() {Class<?> clazz = getClass();Map<Class<?>, Boolean> cache = InternalThreadLocalMap.get().handlerSharableCache();Boolean sharable = cache.get(clazz);if (sharable == null) {sharable = clazz.isAnnotationPresent(Sharable.class);cache.put(clazz, sharable);}return sharable;}//Do nothing by default, sub-classes may override this method.@Overridepublic void handlerAdded(ChannelHandlerContext ctx) throws Exception {// NOOP}//Do nothing by default, sub-classes may override this method.@Overridepublic void handlerRemoved(ChannelHandlerContext ctx) throws Exception {// NOOP}//Calls ChannelHandlerContext#fireExceptionCaught(Throwable) to forward to the next ChannelHandler in the ChannelPipeline.@Overridepublic void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {ctx.fireExceptionCaught(cause);}
}//ChannelHandler which adds callbacks for state changes. 
//This allows the user to hook in to state changes easily.
public interface ChannelInboundHandler extends ChannelHandler {//The Channel of the ChannelHandlerContext was registered with its EventLoopvoid channelRegistered(ChannelHandlerContext ctx) throws Exception;//The Channel of the ChannelHandlerContext was unregistered from its EventLoopvoid channelUnregistered(ChannelHandlerContext ctx) throws Exception;//The Channel of the ChannelHandlerContext is now activevoid channelActive(ChannelHandlerContext ctx) throws Exception;//The Channel of the ChannelHandlerContext was registered is now inactive and reached its end of lifetime.void channelInactive(ChannelHandlerContext ctx) throws Exception;//Invoked when the current 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 #channelRead(ChannelHandlerContext, Object).//If ChannelOption#AUTO_READ is off, no further attempt to read an inbound data from the current Channel will be made until 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 Channel changed. //You can check the state with Channel#isWritable().void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception;//Gets called if a Throwable was thrown.@Override@SuppressWarnings("deprecation")void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception;
}//Abstract base class for ChannelInboundHandler implementations which provide implementations of all of their methods.
public class ChannelInboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelInboundHandler {//Calls ChannelHandlerContext#fireChannelRegistered() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelRegistered(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelRegistered();}//Calls ChannelHandlerContext#fireChannelUnregistered() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelUnregistered(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelUnregistered();}//Calls ChannelHandlerContext#fireChannelActive() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelActive(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelActive();}//Calls ChannelHandlerContext#fireChannelInactive() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelInactive(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelInactive();}//Calls ChannelHandlerContext#fireChannelRead(Object) to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {ctx.fireChannelRead(msg);}//Calls ChannelHandlerContext#fireChannelReadComplete() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelReadComplete(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelReadComplete();}//Calls ChannelHandlerContext#fireUserEventTriggered(Object) to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void userEventTriggered(ChannelHandlerContext ctx, Object evt) throws Exception {ctx.fireUserEventTriggered(evt);}//Calls ChannelHandlerContext#fireChannelWritabilityChanged() to forward to the next ChannelInboundHandler in the ChannelPipeline.@Overridepublic void channelWritabilityChanged(ChannelHandlerContext ctx) throws Exception {ctx.fireChannelWritabilityChanged();}//Calls ChannelHandlerContext#fireExceptionCaught(Throwable) to forward to the next ChannelHandler in the ChannelPipeline.@Overridepublic void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {ctx.fireExceptionCaught(cause);}
}

(2)ChannelOutboundHandlerAdapter

ChannelOutboundHandlerAdapter主要用于实现ChannelOutboundHandler接口的所有方法，这样我们在继承它编写自己的ChannelHandler时就不需要实现ChannelHandler里的每种方法了，从而避免了直接实现ChannelHandler时需要实现其所有方法而导致代码显得冗余和臃肿。

//ChannelHandler which will get notified for IO-outbound-operations.
public interface ChannelOutboundHandler extends ChannelHandler {//Called once a bind operation is made.void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception;//Called once a connect operation is made.void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) throws Exception;//Called once a disconnect operation is made.void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;//Called once a close operation is made.void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;//Called once a deregister operation is made from the current registered EventLoop.void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception;//Intercepts ChannelHandlerContext#read().void read(ChannelHandlerContext ctx) throws Exception;//Called once a write operation is made. The write operation will write the messages through the ChannelPipeline.//Those are then ready to be flushed to the actual Channel once Channel#flush() is called.void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception;//Called once a flush operation is made. The flush operation will try to flush out all previous written messages that are pending.void flush(ChannelHandlerContext ctx) throws Exception;
}//Skeleton implementation of a ChannelOutboundHandler. This implementation just forwards each method call via the ChannelHandlerContext.
public class ChannelOutboundHandlerAdapter extends ChannelHandlerAdapter implements ChannelOutboundHandler {//Calls ChannelHandlerContext#bind(SocketAddress, ChannelPromise) to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void bind(ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) throws Exception {ctx.bind(localAddress, promise);}//Calls ChannelHandlerContext#connect(SocketAddress, SocketAddress, ChannelPromise) to forward to the next ChannelOutboundHandler in the ChannelPipeline. @Overridepublic void connect(ChannelHandlerContext ctx, SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise) throws Exception {ctx.connect(remoteAddress, localAddress, promise);}//Calls ChannelHandlerContext#disconnect(ChannelPromise) to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void disconnect(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {ctx.disconnect(promise);}//Calls ChannelHandlerContext#close(ChannelPromise) to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void close(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {ctx.close(promise);}//Calls ChannelHandlerContext#deregister(ChannelPromise) to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void deregister(ChannelHandlerContext ctx, ChannelPromise promise) throws Exception {ctx.deregister(promise);}//Calls ChannelHandlerContext#read() to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void read(ChannelHandlerContext ctx) throws Exception {ctx.read();}//Calls ChannelHandlerContext#write(Object, ChannelPromise)} to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {ctx.write(msg, promise);}//Calls ChannelHandlerContext#flush() to forward to the next ChannelOutboundHandler in the ChannelPipeline.@Overridepublic void flush(ChannelHandlerContext ctx) throws Exception {ctx.flush();}
}

(3)ByteToMessageDecoder

基于这个ChannelHandler可以实现自定义解码，而不用关心ByteBuf的强转和解码结果的传递。Netty里的ByteBuf默认下使用的是堆外内存，ByteToMessageDecoder会自动进行内存的释放，不用操心内存管理。我们自定义的ChannelHandler继承了ByteToMessageDecoder后，需要实现decode()方法。

(4)SimpleChannelInboundHandler

基于这个ChannelHandler可以实现每一种指令的处理，不再需要强转、不再有冗长的if else逻辑、不再需要手动传递对象。

同时还可以自动释放没有往下传播的ByteBuf，因为我们编写指令处理ChannelHandler时，可能会编写不用关心的if else判断，然后手动传递无法处理的对象至下一个指令处理器。

//xxxHandler.java
if (packet instanceof xxxPacket) {//进行处理
} else {ctx.fireChannelRead(packet);
}

(5)MessageToByteEncoder

基于这个ChannelHandler可以实现自定义编码，而不用关心ByteBuf的创建，不用把创建完的ByteBuf进行返回。

4.ChannelHandler的生命周期

(1)ChannelHandler回调方法的执行顺序

ChannelHandler回调方法的执行顺序可以称为ChannelHandler的生命周期。

新建连接时ChannelHandler回调方法的执行顺序是：handlerAdded() -> channelRegistered() -> channelActive() -> channelRead() -> channelReadComplete()。

关闭连接时ChannelHandler回调方法的执行顺序是：channelInactive() -> channelUnregistered() -> handlerRemoved()。

接下来是ChannelHandler具体的回调方法说明，其中一二三的顺序可以参考AbstractChannel的内部类AbstractUnsafe的register0()方法。

一.handlerAdded()

检测到新连接后调用"ch.pipeline().addLast(...)"之后的回调，表示当前Channel已成功添加一个ChannelHandler。

二.channelRegistered()

表示当前Channel已和某个NioEventLoop线程建立了绑定关系，已经创建了一个Reactor线程来处理当前这个Channel的读写。

三.channelActive()

当Channel的Pipeline已经添加完所有的ChannelHandler以及绑定好一个NioEventLoop线程，这个Channel对应的连接才算真正被激活，接下来就会回调该方法。

四.channelRead()

服务端每次收到客户端发送的数据时都会回调该方法，表示有数据可读。

五.channelReadComplete()

服务端每次读完一条完整的数据都会回调该方法，表示数据读取完毕。

六.channelInactive()

表示这个连接已经被关闭，该连接在TCP层已经不再是ESTABLISH状态。

七.channelUnregister()

表示与这个连接对应的NioEventLoop线程移除了对这个连接的处理。

八.handlerRemoved()

表示给这个连接添加的所有的ChannelHandler都被移除了。

(2)ChannelHandler回调方法的应用场景

一.handlerAdded()方法与handlerRemoved()方法通常可用于一些资源的申请和释放。

二.channelActive()方法与channelInactive()方法表示的是TCP连接的建立与释放，可用于统计单机连接数或IP过滤。

三.channelRead()方法可用于根据自定义协议进行拆包。每次读到一定数据就累加到一个容器里，然后看看能否拆出完整的包。

四.channelReadComplete()方法可用于实现批量刷新。如果每次向客户端写数据都通过writeAndFlush()方法写数据并刷新到底层，其实并不高效。所以可以把调用writeAndFlush()方法的地方换成调用write()方法，然后再在channelReadComplete()方法里调用ctx.channel().flush()。

5.ChannelPipeline的事件处理

(1)消息读取和发送被Pipeline处理的过程

(2)ChannelPipeline的主要特征

(1)消息读取和发送被Pipeline处理的过程

消息的读取和发送被ChannelPipeline的ChannelHandler链拦截和处理的全过程：

一.首先AbstractNioChannel内部类NioUnsafe的read()方法读取ByteBuf时会触发ChannelRead事件，也就是由NioEventLoop线程调用ChannelPipeline的fireChannelRead()方法将ByteBuf消息传输到ChannelPipeline中。

二.然后ByteBuf消息会依次被HeadContext、xxxChannelHandler、...、TailContext拦截处理。在这个过程中，任何ChannelHandler都可以中断当前的流程，结束消息的传递。

三.接着用户可能会调用ChannelHandlerContext的write()方法发送ByteBuf消息。此时ByteBuf消息会从TailContext开始，途径xxxChannelHandler、...、HeadContext，最终被添加到消息发送缓冲区中等待刷新和发送。在这个过程中，任何ChannelHandler都可以中断当前的流程，中断消息的传递。

(2)ChannelPipeline的主要特征

一.ChannelPipeline支持运行时动态地添加或者删除ChannelHandler

例如业务高峰时对系统做拥塞保护。处于业务高峰期时，则动态地向当前的ChannelPipeline添加ChannelHandler。高峰期过后，再移除ChannelHandler。

二.ChannelPipeline是线程安全的

多个业务线程可以并发操作ChannelPipeline，因为使用了synchronized关键字。但ChannelHandler却不一定是线程安全的，这由用户保证。

6.关于ChannelPipeline的问题整理

一.Netty是如何判断ChannelHandler类型的？

即如何判断一个ChannelHandler是Inbound类型还是Outbound类型？

答：当调用Pipeline去添加一个ChannelHandler结点时，旧版Netty会使用instanceof关键字来判断该结点是Inbound类型还是Outbound类型，并分别用一个布尔类型的变量来进行标识。新版Netty则使用一个整形的executionMask来具体区分详细的Inbound事件和Outbound事件。这个executionMask对应一个16位的二进制数，是哪一种事件就对应哪一个Mask。

//Inbound事件的Mask
MASK_EXEPTION_CAUGHT = 1;
MASK_CHANNEL_REGISTER = 1 << 1;
MASK_CHANNEL_UNREGISTER = 1 << 2;
MASK_CHANNEL_ACTIVE = 1 << 3;
MASK_CHANNEL_INACTIVE = 1 << 4;
MASK_CHANNEL_READ = 1 << 5;
MASK_CHANNEL_READ_COMPLETE = 1 << 6;
MASK_CHANNEL_USER_EVENT_TRIGGERED = 1 << 7;
MASK_CHANNEL_WRITABLITY_CHANGED = 1 << 8;//Outbound事件的Mask
MASK_BIND = 1 << 9;
MASK_CONNECT = 1 << 10;
MASK_DISCONNECT = 1 << 11;
MASK_CLOSE = 1 << 12;
MASK_DEREGISTER = 1 << 13;
MASK_READ = 1 << 14;
MASK_WRITE = 1 << 15;
MASK_FLUSH = 1 << 16;

二.添加ChannelHandler时应遵循什么样的顺序？

答：Inbound类型的事件传播跟添加ChannelHandler的顺序一样，Outbound类型的事件传播跟添加ChannelHandler的顺序相反。

三.用户手动触发事件传播的两种方式有什么区别？

这两种方式是分别是：ctx.writeAndFlush()和ctx.channel().writeAndFlush()。

答：当通过Channel去触发一个事件时，那么该事件会沿整个ChannelPipeline传播。如果是Inbound类型事件，则从HeadContext结点开始向后传播到最后一个Inbound类型的结点。如果是Outbound类型事件，则从TailContext结点开始向前传播到第一个Outbound类型的结点。当通过当前结点去触发一个事件时，那么该事件只会从当前结点开始传播。如果是Inbound类型事件，则从当前结点开始一直向后传播到最后一个Inbound类型的结点。如果是Outbound类型事件，则从当前结点开始一直向前传播到第一个Outbound类型的结点。

7.ChannelPipeline主要包括三部分内容

一.ChannelPipeline的初始化

服务端Channel和客户端Channel在何时初始化ChannelPipeline？在初始化时又做了什么事情？

二.添加和删除ChannelHandler

Netty是如何实现业务逻辑处理器动态编织的？

三.事件和异常的传播

读写事件和异常在ChannelPipeline中的传播。

8.ChannelPipeline的初始化

(1)ChannelPipeline的初始化时机

(2)ChannelPipeline的初始化内容

(3)ChannelPipeline的说明

(1)ChannelPipeline的初始化时机

在服务端启动和客户端连接接入的过程中，在创建NioServerSocketChannel和NioSocketChannel时，会逐层执行父类的构造方法，最后执行到AbstractChannel的构造方法。AbstractChannel的构造方法会将Netty的核心组件创建出来。而核心组件中就包含了DefaultChannelPipeline类型的ChannelPipeline组件。

//A skeletal Channel implementation.
public abstract class AbstractChannel extends DefaultAttributeMap implements Channel { private final Channel parent;private final ChannelId id;private final Unsafe unsafe;private final DefaultChannelPipeline pipeline;...//Creates a new instance.protected AbstractChannel(Channel parent) {this.parent = parent;id = newId();unsafe = newUnsafe();pipeline = newChannelPipeline();}//Returns a new DefaultChannelPipeline instance.protected DefaultChannelPipeline newChannelPipeline() {//创建ChannelPipeline组件return new DefaultChannelPipeline(this);}...
}//The default ChannelPipeline implementation.  
//It is usually created by a Channel implementation when the Channel is created.
public class DefaultChannelPipeline implements ChannelPipeline {final AbstractChannelHandlerContext head;final AbstractChannelHandlerContext tail;private final Channel channel;//保存了Channel的引用...protected DefaultChannelPipeline(Channel channel) {//保存Channel的引用到Pipeline组件的成员变量this.channel = ObjectUtil.checkNotNull(channel, "channel");...tail = new TailContext(this);head = new HeadContext(this);head.next = tail;tail.prev = head;}...
}

(2)ChannelPipeline的初始化内容

ChannelPipeline的初始化主要涉及三部分内容：

一.Pipeline在创建Channel时被创建

二.Pipeline的结点是ChannelHandlerContext

三.Pipeline两大哨兵HeadContext和TailContext

(3)ChannelPipeline的说明

ChannelPipeline中保存了Channel的引用，ChannelPipeline中每个结点都是一个ChannelHandlerContext对象，每个ChannelHandlerContext结点都包裹着一个ChannelHandler执行器，每个ChannelHandlerContext结点都保存了它包裹的执行器ChannelHandler执行操作时所需要的上下文ChannelPipeline。由于ChannelPipeline又保存了Channel的引用，所以每个ChannelHandlerContext结点都可以拿到所有的上下文信息。

ChannelHandlerContext接口多继承自AttributeMap、ChannelInboundInvoker、ChannelOutboundInvoker。

ChannelHandlerContext的关键方法有：channel()、executor()、handler()、pipeline()、alloc()。ChannelHandlerContext默认是由AbstractChannelHandlerContext去实现的，它实现了大部分功能。

ChannelPipeline初始化时会初始化两个结点：HeadContext和TailContext，并构成双向链表。HeadContext结点会比TailContext结点多一个unsafe成员变量。

public class DefaultChannelPipeline implements ChannelPipeline {//ChannelPipeline中每个结点都是一个ChannelHandlerContext对象final AbstractChannelHandlerContext head;final AbstractChannelHandlerContext tail;private final Channel channel;//ChannelPipeline中保存了Channel的引用...protected DefaultChannelPipeline(Channel channel) {//保存Channel的引用到Pipeline组件的成员变量this.channel = ObjectUtil.checkNotNull(channel, "channel");...//ChannelPipeline初始化时会初始化两个结点：HeadContext和TailContext，并构成双向链表tail = new TailContext(this);head = new HeadContext(this);head.next = tail;tail.prev = head;}final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {//HeadContext结点会比TailContext结点多一个unsafe成员变量private final Unsafe unsafe;HeadContext(DefaultChannelPipeline pipeline) {super(pipeline, null, HEAD_NAME, false, true);unsafe = pipeline.channel().unsafe();setAddComplete();}...}final class TailContext extends AbstractChannelHandlerContext implements ChannelInboundHandler {TailContext(DefaultChannelPipeline pipeline) {super(pipeline, null, TAIL_NAME, true, false);setAddComplete();}...}...
}//ChannelHandlerContext默认是由AbstractChannelHandlerContext去实现的，它实现了大部分功能
abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint { //每个ChannelHandlerContext结点都保存了它包裹的执行器ChannelHandler执行操作时所需要的上下文ChannelPipelineprivate final DefaultChannelPipeline pipeline;...AbstractChannelHandlerContext(DefaultChannelPipeline pipeline, EventExecutor executor, String name, boolean inbound, boolean outbound) {this.name = ObjectUtil.checkNotNull(name, "name");this.pipeline = pipeline;this.executor = executor;this.inbound = inbound;this.outbound = outbound;ordered = executor == null || executor instanceof OrderedEventExecutor;}
}public interface ChannelHandlerContext extends AttributeMap, ChannelInboundInvoker, ChannelOutboundInvoker {//Return the Channel which is bound to the ChannelHandlerContext.Channel channel();//Returns the EventExecutor which is used to execute an arbitrary task.EventExecutor executor();//The unique name of the ChannelHandlerContext.//The name was used when then ChannelHandler was added to the ChannelPipeline. //This name can also be used to access the registered ChannelHandler from the ChannelPipeline.String name();//The ChannelHandler that is bound this ChannelHandlerContext.ChannelHandler handler();//Return true if the ChannelHandler which belongs to this context was removed from the ChannelPipeline. //Note that this method is only meant to be called from with in the EventLoop.boolean isRemoved();ChannelHandlerContext fireChannelRegistered();ChannelHandlerContext fireChannelUnregistered();ChannelHandlerContext fireChannelActive();ChannelHandlerContext fireChannelInactive();ChannelHandlerContext fireExceptionCaught(Throwable cause);ChannelHandlerContext fireUserEventTriggered(Object evt);ChannelHandlerContext fireChannelRead(Object msg);ChannelHandlerContext fireChannelReadComplete();ChannelHandlerContext fireChannelWritabilityChanged();ChannelHandlerContext read();ChannelHandlerContext flush();//Return the assigned ChannelPipelineChannelPipeline pipeline();//Return the assigned ByteBufAllocator which will be used to allocate ByteBufs.ByteBufAllocator alloc();...
}

9.ChannelPipeline添加ChannelHandler

(1)常见的客户端代码

(2)ChannelPipeline添加ChannelHandler入口

(3)DefaultChannelPipeline的addLast()方法

(4)检查是否重复添加ChannelHandler结点

(5)创建ChannelHandlerContext结点

(6)添加ChannelHandlerContext结点

(7)回调handerAdded()方法

(8)ChannelPipeline添加ChannelHandler总结

(1)常见的客户端代码

首先用一个拆包器Spliter对二进制数据流进行拆包，然后解码器Decoder会将拆出来的包进行解码，接着业务处理器BusinessHandler会处理解码出来的Java对象，最后编码器Encoder会将业务处理完的结果编码成二进制数据进行输出。

bootstrap.childHandler(new ChannelInitializer<SocketChannel>() {@Overridepublic void initChannel(SocketChannel ch) throws Exception {ChannelPipeline p = ch.pipeline();   p.addLast(newSpliter());p.addLast(new Decoder());p.addLast(new BusinessHandler());p.addLast(new Encoder());          }
});

整个ChannelPipeline的结构如下所示：

这里共有两种不同类型的结点，结点之间通过双向链表连接。一种是ChannelInboundHandler，用来处理Inbound事件，比如读取数据流进行加工处理。一种是ChannelOutboundHandler，用来处理Outbound事件，比如当调用writeAndFlush()方法时就会经过这种类型的Handler。

(2)ChannelPipeline添加ChannelHandler入口

当服务端Channel的Reactor线程轮询到新连接接入的事件时，就会调用AbstractNioChannel的内部类NioUnsafe的read()方法，也就是调用AbstractNioMessageChannel的内部类NioMessageUnsafe的read()方法。

然后会触发执行代码pipeline.fireChannelRead()传播ChannelRead事件，从而最终触发调用ServerBootstrapAcceptor接入器的channelRead()方法。

在ServerBootstrapAcceptor的channelRead()方法中，便会通过执行代码channel.pipeline().addLast()添加ChannelHandler，也就是通过调用DefaultChannelPipeline的addLast()方法添加ChannelHandler。

//SingleThreadEventLoop implementation which register the Channel's to a Selector and so does the multi-plexing of these in the event loop.
public final class NioEventLoop extends SingleThreadEventLoop {Selector selector;private SelectedSelectionKeySet selectedKeys;private boolean needsToSelectAgain;private int cancelledKeys;...@Overrideprotected void run() {for (;;) {...//1.调用select()方法执行一次事件轮询select(wakenUp.getAndSet(false));if (wakenUp.get()) {selector.wakeup();}...//2.处理产生IO事件的ChannelneedsToSelectAgain = false;processSelectedKeys();...//3.执行外部线程放入TaskQueue的任务runAllTasks(ioTime * (100 - ioRatio) / ioRatio);}}private void processSelectedKeys() {if (selectedKeys != null) {//selectedKeys.flip()会返回一个数组processSelectedKeysOptimized(selectedKeys.flip());} else {processSelectedKeysPlain(selector.selectedKeys());}}private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {for (int i = 0;; i ++) {//1.首先取出IO事件final SelectionKey k = selectedKeys[i];if (k == null) {break;}selectedKeys[i] = null;//Help GC//2.然后获取对应的Channel和处理该Channel//默认情况下，这个a就是NioChannel，也就是服务端启动时经过Netty封装的Channelfinal Object a = k.attachment();if (a instanceof AbstractNioChannel) {//网络事件的处理processSelectedKey(k, (AbstractNioChannel) a);} else {//NioTask主要用于当一个SelectableChannel注册到Selector时，执行的一些任务NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;processSelectedKey(k, task);}//3.最后判断是否应该再进行一次轮询if (needsToSelectAgain) {for (;;) {i++;if (selectedKeys[i] == null) {break;}selectedKeys[i] = null;}selectAgain();//selectedKeys.flip()会返回一个数组selectedKeys = this.selectedKeys.flip();i = -1;}}}private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();...try {int readyOps = k.readyOps();...//boss的Reactor线程已经轮询到有ACCEPT事件，即表明有新连接接入//此时将调用Channel的unsafe变量来进行实际操作if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {//调用AbstractNioMessageChannel的NioMessageUnsafe.read()方法//进行新连接接入处理unsafe.read();if (!ch.isOpen()) {return;}}} catch (CancelledKeyException ignored) {unsafe.close(unsafe.voidPromise());}}...
}//AbstractNioChannel base class for Channels that operate on messages.
public abstract class AbstractNioMessageChannel extends AbstractNioChannel {...private final class NioMessageUnsafe extends AbstractNioUnsafe {//临时存放读到的连接NioSocketChannelprivate final List<Object> readBuf = new ArrayList<Object>();@Overridepublic void read() {//断言确保该read()方法必须来自Reactor线程调用assert eventLoop().inEventLoop();//获得Channel对应的Pipelinefinal ChannelPipeline pipeline = pipeline();//获得Channel对应的RecvByteBufAllocator.Handlefinal RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();do {//1.调用NioServerSocketChannel的doReadMessages()方法创建NioSocketChannel//通过JDK的accept()方法去创建JDK Channel，然后把它包装成Netty自定义的Channelint localRead = doReadMessages(readBuf);if (localRead == 0) {break;}} while (allocHandle.continueReading());//控制连接的接入速率，默认一次性读取16个连接//2.设置并绑定NioSocketChannelint size = readBuf.size();for (int i = 0; i < size; i ++) {//调用DefaultChannelPipeline的fireChannelRead()方法pipeline.fireChannelRead(readBuf.get(i));}//3.清理容器并触发DefaultChannelPipeline的fireChannelReadComplete()方法readBuf.clear();pipeline.fireChannelReadComplete();}}...
}//The default ChannelPipeline implementation.  
//It is usually created by a Channel implementation when the Channel is created.
public class DefaultChannelPipeline implements ChannelPipeline {final AbstractChannelHandlerContext head;final AbstractChannelHandlerContext tail;...protected DefaultChannelPipeline(Channel channel) {...tail = new TailContext(this);head = new HeadContext(this);head.next = tail;tail.prev = head;}@Overridepublic final ChannelPipeline fireChannelRead(Object msg) {//从Pipeline的第一个HeadContext处理器开始调用AbstractChannelHandlerContext.invokeChannelRead(head, msg);return this;}final class HeadContext extends AbstractChannelHandlerContext implements ChannelOutboundHandler, ChannelInboundHandler {...@Overridepublic void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {//调用AbstractChannelHandlerContext的fireChannelRead()方法ctx.fireChannelRead(msg);}@Overridepublic ChannelHandler handler() {return this;}...}...
}public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {...//初始化服务端Channel时，会向其Pipeline添加ServerBootstrapAcceptor处理器@Overridevoid init(Channel channel) throws Exception {//1.设置服务端Channel的Option与Attrfinal Map<ChannelOption<?>, Object> options = options0();synchronized (options) {channel.config().setOptions(options);}final Map<AttributeKey<?>, Object> attrs = attrs0();synchronized (attrs) {for (Entry<AttributeKey<?>, Object> e: attrs.entrySet()) {@SuppressWarnings("unchecked")AttributeKey<Object> key = (AttributeKey<Object>) e.getKey();channel.attr(key).set(e.getValue());}}//2.设置客户端Channel的Option与Attrfinal EventLoopGroup currentChildGroup = childGroup;final ChannelHandler currentChildHandler = childHandler;final Entry<ChannelOption<?>, Object>[] currentChildOptions;final Entry<AttributeKey<?>, Object>[] currentChildAttrs;synchronized (childOptions) {currentChildOptions = childOptions.entrySet().toArray(newOptionArray(childOptions.size()));}synchronized (childAttrs) {currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));}//3.配置服务端启动逻辑ChannelPipeline p = channel.pipeline();//p.addLast()用于定义服务端启动过程中需要执行哪些逻辑p.addLast(new ChannelInitializer<Channel>() {@Overridepublic void initChannel(Channel ch) throws Exception {//一.添加用户自定义的Handler，注意这是handler，而不是childHandlerfinal ChannelPipeline pipeline = ch.pipeline();ChannelHandler handler = config.handler();if (handler != null) pipeline.addLast(handler);//二.添加一个特殊的Handler用于接收新连接//自定义的childHandler会作为参数传入连接器ServerBootstrapAcceptorch.eventLoop().execute(new Runnable() {@Overridepublic void run() {//调用DefaultChannelPipeline的addLast()方法pipeline.addLast(new ServerBootstrapAcceptor(currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));}});}});}private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {private final EventLoopGroup childGroup;private final ChannelHandler childHandler;private final Entry<ChannelOption<?>, Object>[] childOptions;private final Entry<AttributeKey<?>, Object>[] childAttrs;...//channelRead()方法在新连接接入时被调用@Override@SuppressWarnings("unchecked")public void channelRead(ChannelHandlerContext ctx, Object msg) {final Channel child = (Channel) msg;//1.给新连接的Channel添加用户自定义的Handler处理器//这里的childHandler其实是一个特殊的Handler: ChannelInitializerchild.pipeline().addLast(childHandler);//2.设置ChannelOption，主要和TCP连接一些底层参数及Netty自身对一个连接的参数有关for (Entry<ChannelOption<?>, Object> e: childOptions) {if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {logger.warn("Unknown channel option: " + e);}}//3.设置新连接Channel的属性for (Entry<AttributeKey<?>, Object> e: childAttrs) {child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());}//4.绑定Reactor线程//childGroup是一个NioEventLoopGroup，所以下面会调用其父类的register()方法childGroup.register(child);}...}...
}

(3)DefaultChannelPipeline的addLast()方法

使用synchronized关键字是为了防止多线程并发操作ChannelPipeline底层的双向链表，添加ChannelHandler结点的过程主要分为4个步骤：

步骤一：判断ChannelHandler是否重复添加

步骤二：创建结点

步骤三：添加结点到链表

步骤四：回调添加完成事件

这个结点便是ChannelHandlerContext，Pipeline里每个结点都是一个ChannelHandlerContext。addLast()方法便是把ChannelHandler包装成一个ChannelHandlerContext，然后添加到链表。

public class DefaultChannelPipeline implements ChannelPipeline {...@Overridepublic final ChannelPipeline addLast(ChannelHandler... handlers) {return addLast(null, handlers);}@Overridepublic final ChannelPipeline addLast(EventExecutorGroup executor, ChannelHandler... handlers) {if (handlers == null) throw new NullPointerException("handlers");for (ChannelHandler h: handlers) {if (h == null) break;addLast(executor, null, h);}return this;}@Overridepublic final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {final AbstractChannelHandlerContext newCtx;synchronized (this) {//1.检查是否有重复的ChannelHandler结点checkMultiplicity(handler);//2.创建ChannelHandlerContext结点newCtx = newContext(group, filterName(name, handler), handler);//3.添加ChannelHandlerContext结点addLast0(newCtx);...}//4.回调用户方法//通过这个方法告诉用户这个ChannelHandler已添加完成，用户在回调方法里可以处理事情了callHandlerAdded0(newCtx);return this;}...
}

(4)检查是否重复添加ChannelHandler结点

Netty使用了一个成员变量added来表示一个ChannelHandler是否已经添加。如果当前要添加的ChannelHandler是非共享的并且已经添加过，那么抛出异常，否则标识该ChannelHandler已添加。

如果一个ChannelHandler支持共享，那么它就可以无限次被添加到ChannelPipeline中。如果要让一个ChannelHandler支持共享，只需要加一个@Sharable注解即可。而ChannelHandlerAdapter的isSharable()方法正是通过判断该ChannelHandler对应的类是否标有@Sharable注解来实现的。

Netty为了性能优化，还使用了ThreadLocal来缓存ChannelHandler是否共享的情况。在高并发海量连接下，每次有新连接添加ChannelHandler都会调用isSharable()方法，从而优化性能。

public class DefaultChannelPipeline implements ChannelPipeline {...private static void checkMultiplicity(ChannelHandler handler) {if (handler instanceof ChannelHandlerAdapter) {ChannelHandlerAdapter h = (ChannelHandlerAdapter) handler;if (!h.isSharable() && h.added) {throw new ChannelPipelineException(h.getClass().getName() +" is not a @Sharable handler, so can't be added or removed multiple times.");}h.added = true;}}...
}//Skeleton implementation of a ChannelHandler.
public abstract class ChannelHandlerAdapter implements ChannelHandler {//Not using volatile because it's used only for a sanity check.boolean added;//Return true if the implementation is Sharable and so can be added to different ChannelPipelines.public boolean isSharable() {//Cache the result of Sharable annotation detection to workaround a condition. //We use a ThreadLocal and WeakHashMap to eliminate the volatile write/reads. //Using different WeakHashMap instances per Thread is good enough for us and the number of Threads are quite limited anyway.Class<?> clazz = getClass();Map<Class<?>, Boolean> cache = InternalThreadLocalMap.get().handlerSharableCache();Boolean sharable = cache.get(clazz);if (sharable == null) {sharable = clazz.isAnnotationPresent(Sharable.class);cache.put(clazz, sharable);}return sharable;}...
}

(5)创建ChannelHandlerContext结点

根据ChannelHandler创建ChannelHandlerContext类型的结点时，会将该ChannelHandler的引用保存到结点的成员变量中。

public class DefaultChannelPipeline implements ChannelPipeline {...    @Overridepublic final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {final AbstractChannelHandlerContext newCtx;synchronized (this) {//1.检查是否有重复的ChannelHandler结点checkMultiplicity(handler);//2.创建ChannelHandlerContext结点newCtx = newContext(group, filterName(name, handler), handler);//3.添加ChannelHandlerContext结点addLast0(newCtx);...}//4.回调用户方法//通过这个方法告诉用户这个ChannelHandler已添加完成，用户在回调方法里可以处理事情了callHandlerAdded0(newCtx);return this;}//给ChannelHandler创建一个唯一性的名字private String filterName(String name, ChannelHandler handler) {if (name == null) {return generateName(handler);}checkDuplicateName(name);return name;}//根据ChannelHandler创建一个ChannelHandlerContext结点private AbstractChannelHandlerContext newContext(EventExecutorGroup group, String name, ChannelHandler handler) {return new DefaultChannelHandlerContext(this, childExecutor(group), name, handler);}...
}final class DefaultChannelHandlerContext extends AbstractChannelHandlerContext {private final ChannelHandler handler;DefaultChannelHandlerContext(DefaultChannelPipeline pipeline, EventExecutor executor, String name, ChannelHandler handler) {super(pipeline, executor, name, isInbound(handler), isOutbound(handler));if (handler == null) {throw new NullPointerException("handler");}this.handler = handler;}...
}abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint {...AbstractChannelHandlerContext(DefaultChannelPipeline pipeline, EventExecutor executor, String name, boolean inbound, boolean outbound) {this.name = ObjectUtil.checkNotNull(name, "name");this.pipeline = pipeline;this.executor = executor;this.inbound = inbound;this.outbound = outbound;ordered = executor == null || executor instanceof OrderedEventExecutor;}...
}

(6)添加ChannelHandlerContext结点

使用尾插法向双向链表添加结点。

public class DefaultChannelPipeline implements ChannelPipeline {...private void addLast0(AbstractChannelHandlerContext newCtx) {AbstractChannelHandlerContext prev = tail.prev;newCtx.prev = prev;newCtx.next = tail;prev.next = newCtx;tail.prev = newCtx;}...
}abstract class AbstractChannelHandlerContext extends DefaultAttributeMap implements ChannelHandlerContext, ResourceLeakHint {volatile AbstractChannelHandlerContext next;volatile AbstractChannelHandlerContext prev;...
}

(7)回调handerAdded()方法

向ChannelPipeline添加完新结点后，会使用CAS修改结点的状态为ADD_COMPLETE表示结点添加完成，然后执行ctx.handler().handlerAdded(ctx)，回调用户在这个要添加的ChannelHandler中实现的handerAdded()方法。

public class DefaultChannelPipeline implements ChannelPipeline {...private void callHandlerAdded0(final AbstractChannelHandlerContext ctx) {//使用CAS修改结点的状态为ADD_COMPLETE表示结点添加完成ctx.setAddComplete();//回调用户在这个要添加的ChannelHandler中实现的handerAdded()方法ctx.handler().handlerAdded(ctx);}...
}//DemoHandler是用户定义的ChannelHandler
public class DemoHandler extends SimpleChannelInboundHandler<...> {@Overridepublic void handlerAdded(ChannelHandlerContext ctx) throws Exception {//这个DemoHandler结点被添加到ChannelPipeline之后，就会回调这里的方法}...
}

最典型的一个回调就是用户代码的ChannelInitializer被添加完成后，会先调用其initChannel()方法将用户自定义的ChannelHandler添加到ChannelPipeline，然后再调用pipeline.remove()方法将自身结点进行删除。

public class NettyServer {private int port;public NettyServer(int port) {this.port = port;}public void start() throws Exception {EventLoopGroup bossGroup = new NioEventLoopGroup();EventLoopGroup workerGroup = new NioEventLoopGroup();try {ServerBootstrap serverBootstrap = new ServerBootstrap();serverBootstrap.group(bossGroup, workerGroup).channel(NioServerSocketChannel.class)//监听端口的ServerSocketChannel.option(ChannelOption.SO_BACKLOG, 128).childOption(ChannelOption.SO_KEEPALIVE, true)//设置一个ChannelInitializer类型的childHandler//新连接接入时，会执行ServerBootstrapAcceptor.channelRead()中的代码"child.pipeline().addLast(childHandler)"//也就是会把这个ChannelInitializer类型的结点会被添加到新连接Channel的Pipeline中//添加完这个结点后会回调ChannelInitializer的handlerAdded()方法//其中会调用ChannelInitializer的initChannel()方法给Pipeline添加真正的结点//执行完initChannel()方法后，就会移除ChannelInitializer这个结点.childHandler(new ChannelInitializer<SocketChannel>() {//处理每个客户端连接的SocketChannel@Overrideprotected void initChannel(SocketChannel socketChannel) throws Exception {socketChannel.pipeline().addLast(new StringDecoder()).addLast(new StringEncoder()).addLast(new NettyServerHandler());//针对网络请求的处理逻辑}});ChannelFuture channelFuture = serverBootstrap.bind(port).sync();//同步等待启动服务器监控端口channelFuture.channel().closeFuture().sync();//同步等待关闭启动服务器的结果} catch (Exception e) {e.printStackTrace();} finally {workerGroup.shutdownGracefully();bossGroup.shutdownGracefully();}}public static void main(String[] args) throws Exception{System.out.println("Starting Netty Server...");int port = 8998;if (args.length > 0) {port = Integer.parseInt(args[0]);}new NettyServer(port).start();}
}@Sharable
public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {...@Overridepublic void handlerAdded(ChannelHandlerContext ctx) throws Exception {if (ctx.channel().isRegistered()) {initChannel(ctx);}}@SuppressWarnings("unchecked")private boolean initChannel(ChannelHandlerContext ctx) throws Exception {if (initMap.putIfAbsent(ctx, Boolean.TRUE) == null) { // Guard against re-entrance.try {initChannel((C) ctx.channel());} catch (Throwable cause) {exceptionCaught(ctx, cause);} finally {remove(ctx);}return true;}return false;}private void remove(ChannelHandlerContext ctx) {try {ChannelPipeline pipeline = ctx.pipeline();if (pipeline.context(this) != null) {//ChannelPipeline删除ChannelHandler结点(ChannelInitializer)pipeline.remove(this);}} finally {initMap.remove(ctx);}}...
}

(8)ChannelPipeline添加ChannelHandler总结

一.判断ChannelHandler是否重复添加的依据是：如果该ChannelHandler不是共享的且已被添加过，则拒绝添加。

二.否则就创建一个ChannelHandlerContext结点(ctx)，并把这个ChannelHandler包装进去，也就是保存ChannelHandler的引用到ChannelHandlerContext的成员变量中。由于创建ctx时保存了ChannelHandler的引用、ChannelPipeline的引用到成员变量，ChannelPipeline又保存了Channel的引用，所以每个ctx都拥有一个Channel的所有信息。

三.接着通过双向链表的尾插法，将这个ChannelHandlerContext结点添加到ChannelPipeline中。

四.最后回调用户在这个要添加的ChannelHandler中实现的handerAdded()方法。