Netty接收数据流程（源码）

Netty接收数据主线：

多路复用器(Selector)接收到OP_READ事件
处理OP_READ事件：NioSocketChannel.NioSocketChannelUnsafe.read()
  分配一个初始1024字节的byte buffer来接收数据
  从Channel接收数据到byte buffer
  记录实际接收数据大小，调整下次分配byte buffer大小
  触发pipeline.fireChannelRead(byteBuf)把读取到的数据传播出去
  判断接收byte buffer 是否满载而归：是，尝试继续读取直到没有数据或满16次；否，结束本轮读取，等待下次OP_READ事件

源码：

NioEventLoop:

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            final EventLoop eventLoop;
            try {
                eventLoop = ch.eventLoop();
            } catch (Throwable ignored) {
                // If the channel implementation throws an exception because there is no event loop, we ignore this
                // because we are only trying to determine if ch is registered to this event loop and thus has authority
                // to close ch.
                return;
            }
            // Only close ch if ch is still registered to this EventLoop. ch could have deregistered from the event loop
            // and thus the SelectionKey could be cancelled as part of the deregistration process, but the channel is
            // still healthy and should not be closed.
            // See https://github.com/netty/netty/issues/5125
            if (eventLoop == this) {
                // close the channel if the key is not valid anymore
                unsafe.close(unsafe.voidPromise());
            }
            return;
        }

        try {
            int readyOps = k.readyOps();
            // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
            // the NIO JDK channel implementation may throw a NotYetConnectedException.
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }

            // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }

            // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
            // to a spin loop
            //处理读请求
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }

然后进去到AbstractNioByteChannel中：

@Override
        public final void read() {
            final ChannelConfig config = config();
            if (shouldBreakReadReady(config)) {
                clearReadPending();
                return;
            }
            final ChannelPipeline pipeline = pipeline();
            final ByteBufAllocator allocator = config.getAllocator();
            final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
            allocHandle.reset(config);

            ByteBuf byteBuf = null;
            boolean close = false;
            try {
                do {
                    byteBuf = allocHandle.allocate(allocator);
                    allocHandle.lastBytesRead(doReadBytes(byteBuf));
                    if (allocHandle.lastBytesRead() <= 0) {
                        // nothing was read. release the buffer.
                        byteBuf.release();
                        byteBuf = null;
                        close = allocHandle.lastBytesRead() < 0;
                        if (close) {
                            // There is nothing left to read as we received an EOF.
                            readPending = false;
                        }
                        break;
                    }

                    allocHandle.incMessagesRead(1);
                    readPending = false;
                    pipeline.fireChannelRead(byteBuf);
                    byteBuf = null;
                } while (allocHandle.continueReading());

                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();

                if (close) {
                    closeOnRead(pipeline);
                }
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close, allocHandle);
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    removeReadOp();
                }
            }
        }
    }

pipeline.fireChannelRead(byteBuf);进入到DefaultChannelPipeline中

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

继续到AbstractChannelHandlerContext：

    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);
                }
            });
        }
    }

继续到AbstractChannelHandlerContext：

    private void invokeChannelRead(Object msg) {
        if (invokeHandler()) {
            try {
                ((ChannelInboundHandler) handler()).channelRead(this, msg);
            } catch (Throwable t) {
                invokeExceptionCaught(t);
            }
        } else {
            fireChannelRead(msg);
        }
    }

可以看到最终回调用channelRead方法

服务端代码示例：

• 一个信息处理器(handler): 这个实现是服务端的业务逻辑部分，当连接创建后和接收信息后的处理类。
• 服务器: 主要通过ServerBootstrap设置服务器的监听端口等启动部分。

EchoServerHandler

通过继承ChannelInboundHandlerAdapter，这个类提供了默认的ChannelInboundHandler实现，只需覆盖以下的方法:

• channelRead() - 每个消息入站都会调用
• channelReadComplete() - 通知处理器最后的channelRead()是当前批处理中的最后一条消息时调用
• exceptionCaught() - 捕获到异常时调用

@ChannelHandler.Sharable // 标识这类的实例之间可以在 channel 里面共享
public class EchoServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        ByteBuf in = (ByteBuf) msg;
        System.out.println("Server received: " + in.toString(CharsetUtil.UTF_8));
        ctx.write(in); // 将所接收的消息返回给发送者
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.writeAndFlush(Unpooled.EMPTY_BUFFER) // 冲刷所有待审消息到远程节点。关闭通道后，操作完成
            .addListener(ChannelFutureListener.CLOSE);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        cause.printStackTrace();
        ctx.close();
    }
}

EchoServer

创建ServerBootstrap实例来引导服务器，本服务端分配了一个NioEventLoopGroup实例来处理事件的处理，如接受新的连接和读/写数据，然后绑定本地端口，分配EchoServerHandler实例给Channel，这样服务器初始化完成，可以使用了。

public class EchoServer {

    private final int port;

    public EchoServer(int port) {
        this.port = port;
    }

    public void start() throws Exception {
        NioEventLoopGroup group = new NioEventLoopGroup(); // 创建 EventLoopGroup

        try {
            ServerBootstrap bootstrap = new ServerBootstrap(); // 创建 ServerBootstrap
            bootstrap.group(group)
                    .channel(NioServerSocketChannel.class) // 指定使用 NIO 的传输 Channel
                    .localAddress(new InetSocketAddress(port)) // 设置 socket 地址使用所选的端口
                    .childHandler(new ChannelInitializer() { // 添加 EchoServerHandler 到 Channel 的 ChannelPipeline
                        @Override
                        protected void initChannel(SocketChannel socketChannel) throws Exception {
                            socketChannel.pipeline().addLast(new EchoServerHandler());
                        }
                    });

            ChannelFuture future = bootstrap.bind().sync(); // 绑定的服务器;sync 等待服务器关闭
            System.out.println(EchoServer.class.getName() + " started and listen on " + future.channel().localAddress());
            future.channel().closeFuture().sync(); // 关闭 channel 和 块，直到它被关闭
        } finally {
            group.shutdownGracefully().sync(); // 关闭 EventLoopGroup，释放所有资源。
        }
    }

    public static void main(String[] args) throws Exception {
        int port = 4567;
        if (args.length == 1) {
            port = Integer.parseInt(args[0]);
        }
        new EchoServer(port).start(); // 设计端口、启动服务器
    }
}