[编织消息框架][netty源码分析]6 ChannelPipeline 实现类DefaultChannelPipeline职责与实现

ChannelPipeline 负责channel数据进出处理，如数据编解码等。采用拦截思想设计，经过A handler处理后接着交给next handler

ChannelPipeline 并不是直接管理handler 而是通过 context 包装管理，一般以context 命名的是个重量级对象，提供给多层使用 

public interface ChannelPipeline
        extends ChannelInboundInvoker, ChannelOutboundInvoker, Iterable<Entry<String, ChannelHandler>> {
        
    //链表追加handler方法
    ChannelPipeline addLast(String name, ChannelHandler handler);
    ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler);
    
    ///////////////公开获取 ChannelHandler ChannelHandlerContext/////////////////
    ChannelHandler first();
    ChannelHandlerContext firstContext();
    
    ChannelHandler last();
    ChannelHandlerContext lastContext();
    
    //省略部份代码.....
    
    ////////////////ChannelInboundInvoker接口的所有方法 统一返回 ChannelPipeline 对象//////////////////////
    ChannelPipeline fireChannelRegistered();
    ChannelPipeline fireChannelUnregistered();
    ChannelPipeline fireChannelActive();
    ChannelPipeline fireChannelInactive();
    ChannelPipeline fireExceptionCaught(Throwable cause);
    ChannelPipeline fireUserEventTriggered(Object event);
    ChannelPipeline fireChannelRead(Object msg);
    ChannelPipeline fireChannelReadComplete();
    ChannelPipeline fireChannelWritabilityChanged();
    ChannelPipeline flush();
    
    ////////////////ChannelOutboundInvoker接口的所有方法 统一返回 ChannelFuture ChannelOutboundInvoker 对象//////////////////////
    ChannelFuture bind(SocketAddress localAddress);
    ChannelFuture connect(SocketAddress remoteAddress);
    ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress);
    ChannelFuture disconnect();
    ChannelFuture close();
    ChannelFuture deregister();
    ChannelFuture bind(SocketAddress localAddress, ChannelPromise promise);
    ChannelFuture connect(SocketAddress remoteAddress, ChannelPromise promise);
    ChannelFuture connect(SocketAddress remoteAddress, SocketAddress localAddress, ChannelPromise promise);
    ChannelFuture disconnect(ChannelPromise promise);
    ChannelFuture close(ChannelPromise promise);
    ChannelFuture deregister(ChannelPromise promise);
    ChannelOutboundInvoker read();
    ChannelFuture write(Object msg);
    ChannelFuture write(Object msg, ChannelPromise promise);
    ChannelOutboundInvoker flush();
    ChannelFuture writeAndFlush(Object msg, ChannelPromise promise);
    ChannelFuture writeAndFlush(Object msg);
    ChannelPromise newPromise();
    ChannelProgressivePromise newProgressivePromise();
    ChannelFuture newSucceededFuture();
    ChannelFuture newFailedFuture(Throwable cause);
    ChannelPromise voidPromise();
}

DefaultChannelPipeline实现做了代码整理，其中在添加handler时生成context由于代码比较简单不显示出来

public class DefaultChannelPipeline implements ChannelPipeline {
    final AbstractChannelHandlerContext head;
    final AbstractChannelHandlerContext tail;
    
    //绑定返回对象
    private final Channel channel;
    private final ChannelFuture succeededFuture;
    private final VoidChannelPromise voidPromise;
    
    protected DefaultChannelPipeline(Channel channel) {
        this.channel = ObjectUtil.checkNotNull(channel, "channel");
        succeededFuture = new SucceededChannelFuture(channel, null);
        voidPromise =  new VoidChannelPromise(channel, true);

        tail = new TailContext(this);
        head = new HeadContext(this);

        head.next = tail;
        tail.prev = head;
    }
    
    //双向链表追加，在tail之前插入
    private void addLast0(AbstractChannelHandlerContext newCtx) {
        AbstractChannelHandlerContext prev = tail.prev;
        newCtx.prev = prev;
        newCtx.next = tail;
        prev.next = newCtx;
        tail.prev = newCtx;
    }
    //双向链表删除
    private static void remove0(AbstractChannelHandlerContext ctx) {
        AbstractChannelHandlerContext prev = ctx.prev;
        AbstractChannelHandlerContext next = ctx.next;
        prev.next = next;
        next.prev = prev;
    }
    
    //HeadContext ChannelOutboundHandler出站操作通过Unsafe委托处理
    final class HeadContext extends AbstractChannelHandlerContext
            implements ChannelOutboundHandler, ChannelInboundHandler {

        private final Unsafe unsafe;

        HeadContext(DefaultChannelPipeline pipeline) {
            super(pipeline, null, HEAD_NAME, false, true);
            unsafe = pipeline.channel().unsafe();
            setAddComplete();
        }
  
        @Override
        public void bind(
                ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise)
                throws Exception {
            unsafe.bind(localAddress, promise);
        }

        @Override
        public void connect(
                ChannelHandlerContext ctx,
                SocketAddress remoteAddress, SocketAddress localAddress,
                ChannelPromise promise) throws Exception {
            unsafe.connect(remoteAddress, localAddress, promise);
        }

        @Override
        public void write(ChannelHandlerContext ctx, Object msg, ChannelPromise promise) throws Exception {
            unsafe.write(msg, promise);
        }

        @Override
        public void flush(ChannelHandlerContext ctx) throws Exception {
            unsafe.flush();
        }


        @Override
        public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
            invokeHandlerAddedIfNeeded();
            ctx.fireChannelRegistered();
        }

        @Override
        public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
            ctx.fireChannelRead(msg);
        }
        //......
    }
 }
 
    //ChannelHandlerContext 部份代码
    //通过findContextInbound 查找下一个ctx 再通过ctx内部调用handler方法
    @Override
    public ChannelHandlerContext fireChannelRegistered() {
        invokeChannelRegistered(findContextInbound());
        return this;
    }

    static void invokeChannelRegistered(final AbstractChannelHandlerContext next) {
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeChannelRegistered();
        } else {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    next.invokeChannelRegistered();
                }
            });
        }
    }

    private void invokeChannelRegistered() {
        if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelRegistered(this);
            } catch (Throwable t) {
                notifyHandlerException(t);
            }
        } else {
            fireChannelRegistered();
        }
    }
    //数据入站时从头到尾查找ctx
    private AbstractChannelHandlerContext findContextInbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
            ctx = ctx.next;
        } while (!ctx.inbound);
        return ctx;
    }

    　    //数据出站时从尾到头查找ctx
    private AbstractChannelHandlerContext findContextOutbound() {
        AbstractChannelHandlerContext ctx = this;
        do {
            ctx = ctx.prev;
        } while (!ctx.outbound);
        return ctx;
    }

小总：

1.从设计上可以看出，统一返回一个对象能减少大量的学习成本同开发成本

2.追加handler可以绑定一个线程组，在处理比较耗时的handler可以独立绑定线程组

3.从源码上看出：数据入站时如 channelRead(ChannelHandlerContext ctx, Object msg) 当处理完数据要手动执行next ctx action ctx.fireChannelRead(msg) 这点是比如奇怪的，如果开发者忘记调用了链表就断啦

4.DefaultChannelPipeline构造时默认生成head、tail,数据出站时操作顺序是tail ->linkHandler-> head,数据入站时是 head->linkHandler->tail

5.ChannelPipeline用到双向链表技术，大家在研发过程中可参考设计