@Override
    public final ChannelPipeline addLast(EventExecutorGroup group, String name, ChannelHandler handler) {
        final AbstractChannelHandlerContext newCtx;
        synchronized (this) {
            checkMultiplicity(handler);
            newCtx = newContext(group, filterName(name, handler), handler);
            addLast0(newCtx);

            // If the registered is false it means that the channel was not registered on an eventloop yet.
            // In this case we add the context to the pipeline and add a task that will call
            // ChannelHandler.handlerAdded(...) once the channel is registered.
            if (!registered) {
                newCtx.setAddPending();
                callHandlerCallbackLater(newCtx, true);
                return this;
            }

            EventExecutor executor = newCtx.executor();
            if (!executor.inEventLoop()) {
                newCtx.setAddPending();
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        callHandlerAdded0(newCtx);
                    }
                });
                return this;
            }
        }
        callHandlerAdded0(newCtx);
        return this;
    }
    private void addLast0(AbstractChannelHandlerContext newCtx) {
        AbstractChannelHandlerContext prev = tail.prev;
        newCtx.prev = prev;
        newCtx.next = tail;
        prev.next = newCtx;
        tail.prev = newCtx;
    }

1.2.2 I/O事件執(zhí)行過程分析

我們以一個(gè)I/O讀事件作為一個(gè)代表對(duì)ChannelPipeline的執(zhí)行過程進(jìn)行分析，ChannelPipeline中對(duì)讀事件的執(zhí)行方法是fireChannelRead(Object msg)。
通過代碼分析，我們可以看到會(huì)直接執(zhí)行到AbstractChannelHandlerContext類的方法invokeChannelRead(final AbstractChannelHandlerContext next, Object msg)，入?yún)轭^指針head和對(duì)象msg。

    @Override
    public final ChannelPipeline fireChannelRead(Object msg) {
        AbstractChannelHandlerContext.invokeChannelRead(head, msg);
        return this;
    }

在invokeChannelRead的方法中，還是會(huì)根據(jù)executor.inEventLoop()方法，根據(jù)用戶的線程設(shè)置，最終調(diào)用到對(duì)應(yīng)handler的channelRead方法。

    static void invokeChannelRead(final AbstractChannelHandlerContext next, Object msg) {
        final Object m = next.pipeline.touch(ObjectUtil.checkNotNull(msg, "msg"), next);
        EventExecutor executor = next.executor();
        if (executor.inEventLoop()) {
            next.invokeChannelRead(m);
        } else {
            executor.execute(new Runnable() {
                @Override
                public void run() {
                    next.invokeChannelRead(m);
                }
            });
        }
    }

    private void invokeChannelRead(Object msg) {
    //to detect if {@link ChannelHandler#handlerAdded(ChannelHandlerContext)} was called yet. If not return {@code false} and if called or could not detect return {@code true}.
        if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelRead(this, msg);
            } catch (Throwable t) {
                notifyHandlerException(t);
            }
        } else {
            fireChannelRead(msg);
        }
    }

2. ChannelHandler

先看一些類的繼承圖：

image.png

2.1 ChannelHandler中的方法

Netty定義了良好的類型層次結(jié)構(gòu)來表示不同的處理程序類型，所有的類型的父類是ChannelHandler。ChannelHandler提供了在其生命周期內(nèi)添加或從ChannelPipeline中刪除的方法。
1). handlerAdded，ChannelHandler添加到實(shí)際上下文中準(zhǔn)備處理事件
2). handlerRemoved，將ChannelHandler從實(shí)際上下文中刪除，不再處理事件
3). exceptionCaught，處理拋出的異常
2、ChannelInboundHandler
ChannelInboundHandler提供了一些方法再接收數(shù)據(jù)或Channel狀態(tài)改變時(shí)被調(diào)用。下面是ChannelInboundHandler的一些方法： 1). channelRegistered，ChannelHandlerContext的Channel被注冊(cè)到EventLoop； 2). channelUnregistered，ChannelHandlerContext的Channel從EventLoop中注銷 3). channelActive，ChannelHandlerContext的Channel已激活 4). channelInactive，ChannelHanderContxt的Channel結(jié)束生命周期 5). channelRead，從當(dāng)前Channel的對(duì)端讀取消息 6). channelReadComplete，消息讀取完成后執(zhí)行 7). userEventTriggered，一個(gè)用戶事件被觸發(fā) 8). channelWritabilityChanged，改變通道的可寫狀態(tài)，可以使用Channel.isWritable()檢查 9). exceptionCaught，重寫父類ChannelHandler的方法，處理異常.
舉一個(gè)最常用的MessageToMessageDecoder作為例子，執(zhí)行decode將msg對(duì)象進(jìn)行轉(zhuǎn)換后，如果想繼續(xù)在Pipeline中繼續(xù)傳遞下去，必須顯示地去執(zhí)行ctx.fireChannelRead方法，會(huì)通過AbstractChannelHandlerContext繼續(xù)輪轉(zhuǎn)到下一個(gè)ChannelHandler去執(zhí)行。

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        CodecOutputList out = CodecOutputList.newInstance();
        try {
            if (acceptInboundMessage(msg)) {
                @SuppressWarnings("unchecked")
                I cast = (I) msg;
                try {
                    decode(ctx, cast, out);
                } finally {
                    ReferenceCountUtil.release(cast);
                }
            } else {
                out.add(msg);
            }
        } catch (Exception e) {
            throw new DecoderException(e);
        } finally {
            int size = out.size();
            for (int i = 0; i < size; i ++) {
                ctx.fireChannelRead(out.getUnsafe(i));
            }
            out.recycle();
        }
    }