【Netty】如何接入新连接

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前文再续，书接上一回【 NioEventLoop 】。

在研究NioEventLoop执行过程的时候，检测IO事件（包括新连接），处理IO事件，执行所有任务三个过程。其中检测IO事件中通过持有的selector去轮询事件，检测出新连接。这里复用同一段代码。

Channel的设计

在开始分析前，先了解一下Channel的设计

顶层Channel接口定义了socket事件如读、写、连接、绑定等事件，并使用AbstractChannel作为骨架实现了这些方法。查看器成员变量，发现大多数通用的组件，都被定义在这里

第二层AbstractNioChannel定义了以NIO，即Selector的方式进行读写事件的监听。其成员变量保存了selector相关的一些属性。

第三层内容比较多，定义了服务端channel（左边继承了AbstractNioMessageChannel的NioServerSocketChannel）以及客户端channel（右边继承了AbstractNioByteChannel的NioSocketChannel）。

如何接入新连接?

本文开始探索一下Netty是如何接入新连接？主要分为四个部分

1.检测新连接

2.创建NioSocketChannel

3.分配线程和注册Selector

4.向Selector注册读事件

1.检测新连接

Netty服务端在启动的时候会绑定一个bossGroup，即NioEventLoop,在 bind() 绑定端口的时候注册accept（新连接接入）事件。扫描到该事件后，便处理。因此入口从： NioEventLoop#processSelectedKeys() 开始。

private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        //省略代码
        // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
        // to a spin loop
        //如果当前NioEventLoop是workGroup 则可能是OP_READ，bossGroup是OP_ACCEPT
        if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {

            //新连接接入以及读事件处理入口
            unsafe.read();
        }
      }

关键的新连接接入以及读事件处理入口 unsafe.read();

a).这里的 unsafe 是在Channel创建过程的时候，调用了父类 AbstractChannel#AbstractChannel() 的构造方法，和 pipeline 一起初始化的。

protected AbstractChannel(Channel parent) {
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }

服务端：

unsafe 为 NioServerSockeChanne l的父类AbstractNioMessageChannel#newUnsafe()创建,可以看到对应的是AbstractNioMessageChannel的内部类 NioMessageUnsafe ;

客户端：

unsafe为 NioSocketChannel 的的父类AbstractNioUnsafe#newUnsafe()创建的话，它对应的是AbstractNioByteChannel的内部类 NioByteUnsafe

b).unsafe.read()

NioMessageUnsafe.read() 中主要的操作如下：

1.循环调用jdk底层的代码创建channel，并用netty的NioSocketChannel包装起来，代表新连接成功接入一个通道。

2.将所有获取到的channel存储到一个容器当中，检测接入的连接数，默认是一次接16个连接

3.遍历容器中的channel,依次调用方法fireChannelRead，4.fireChannelReadComplete，fireExceptionCaught来触发对应的传播事件。

private final class NioMessageUnsafe extends AbstractNioUnsafe {
        //临时存储读到的连接
        private final List<Object> readBuf = new ArrayList<Object>();

        @Override
        public void read() {
            assert eventLoop().inEventLoop();
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();

            //服务端接入速率处理器
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            allocHandle.reset(config);

            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    //while循环调用doReadMessages()创建新连接对象
                    do {
                        //获取jdk底层的channel,并加入readBuf容器
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) {
                            break;
                        }
                        if (localRead < 0) {
                            closed = true;
                            break;
                        }
                        //把读到的连接做一个累加totalMessages，默认最多累计读取16个连接，结束循环
                        allocHandle.incMessagesRead(localRead);
                        
                    } while (allocHandle.continueReading());
                } catch (Throwable t) {
                    exception = t;
                }
                
                //触发readBuf容器内所有的传播事件:ChannelRead 读事件
                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false;
                    pipeline.fireChannelRead(readBuf.get(i));
                }
                //清空容器
                readBuf.clear();
                allocHandle.readComplete();
                //触发传播事件：ChannelReadComplete，所有的读事件完成
                pipeline.fireChannelReadComplete();

                if (exception != null) {
                    closed = closeOnReadError(exception);
                    //触发传播事件：exceptionCaught，触发异常
                    pipeline.fireExceptionCaught(exception);
                }

                if (closed) {
                    inputShutdown = true;
                    if (isOpen()) {
                        close(voidPromise());
                    }
                }
            } 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();
                }
            }
        }
    }

而这一段关键代码逻辑中 int localRead = doReadMessages(readBuf); 它创建jdk底层channel并且用NioSocketChannel包装起来，将该channel添加到传入的容器保存起来，同时返回一个计数。

protected int doReadMessages(List<Object> buf) throws Exception {
        SocketChannel ch = SocketUtils.accept(javaChannel());

        try {
            if (ch != null) {
  //将jdk底层的channel封装到netty的channel，并存储到传入的容器当中
                //this为服务端channel
                buf.add(new NioSocketChannel(this, ch));
 //成功和创建 客户端接入的一条通道，并返回
                return 1;
            }
        } catch (Throwable t) {
            logger.warn("Failed to create a new channel from an accepted socket.", t);

            try {
                ch.close();
            } catch (Throwable t2) {
                logger.warn("Failed to close a socket.", t2);
            }
        }

        return 0;
    }

2.创建NioSocketChannel

通过检测IO事件轮询新连接，当前成功检测到连接接入事件之后，会调用 NioServerSocketChannel#doReadMessages() 方法，进行创建 NioSocketChannel ,即客户端channel的过程。

下面就来了解一下 NioSocketChannel 的主要工作：

.查看原代码做了两件事，调用父类构造方法，实例化一个NioSocketChannelConfig。

public NioSocketChannel(Channel parent, SocketChannel socket) {
        super(parent, socket);
        //实例化一个NioSocketChannelConfig
        config = new NioSocketChannelConfig(this, socket.socket());
    }

1）、查看NioSocketChannel父类构造方法,主要是 保存客户端注册的读事件、channel为成员变量，以及设置阻塞模式为非阻塞。

public NioSocketChannel(Channel parent, SocketChannel socket) {
        super(parent, socket);
        //实例化一个NioSocketChannelConfig
        config = new NioSocketChannelConfig(this, socket.socket());
    }
    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        //传入感兴趣的读事件：客户端channel的读事件
        super(parent, ch, SelectionKey.OP_READ);
    }

    protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent);
        //保存客户端channel为成员变量
        this.ch = ch;
        //保存感兴趣的读事件为成员变量
        this.readInterestOp = readInterestOp;
        try {
            //配置阻塞模式为非阻塞
            ch.configureBlocking(false);
        } catch (IOException e) {
            try {
                ch.close();
            } catch (IOException e2) {
                if (logger.isWarnEnabled()) {
                    logger.warn(
                            "Failed to close a partially initialized socket.", e2);
                }
            }

            throw new ChannelException("Failed to enter non-blocking mode.", e);
        }
    }

最后调用父类的构造方法，是设置 该客户端channel对应的服务端channel，以及channel的id和两大组件unsafe和pipeline

protected AbstractChannel(Channel parent) {
        //parent为创建次客户端channel的服务端channel（服务端启动过程中通过反射创建的）
        this.parent = parent;
        id = newId();
        unsafe = newUnsafe();
        pipeline = newChannelPipeline();
    }

2）、再看NioSocketChannelConfig实例化。主要是保存了javaSocket，并且通过 setTcpNoDelay(true); 禁止了tcp的Nagle算法，目的是为了尽量让小的数据包整合成大的发送出去,降低延时.

private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
            super(channel, javaSocket);
            calculateMaxBytesPerGatheringWrite();
        }

    public DefaultSocketChannelConfig(SocketChannel channel, Socket javaSocket) {
        super(channel);
        if (javaSocket == null) {
            throw new NullPointerException("javaSocket");
        }
        //保存socket
        this.javaSocket = javaSocket;

        // Enable TCP_NODELAY by default if possible.
        if (PlatformDependent.canEnableTcpNoDelayByDefault()) {
            try {
                //禁止Nagle算法,目的是为了让小的数据包尽量集合成大的数据包发送出去
                setTcpNoDelay(true);
            } catch (Exception e) {
                // Ignore.
            }
        }
    }

3.分配线程和注册Selector

服务端启动初始化的时候 ServerBootstrap#init() ，主要做了一些参数的配置。其中对于 childGroup,childOptions,childAttrs,childHandler 等参数被进行了单独配置。作为参数和 ServerBootstrapAcceptor 一起，被当作一个特殊的handle，封装到pipeline中。 ServerBootstrapAcceptor 中的 eventLoop 为 workGroup 。

public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
  //省略了很多代码.............
    @Override
    void init(Channel channel) throws Exception {

        //配置AbstractBootstrap.option
        final Map<ChannelOption<?>, Object> options = options0();
        synchronized (options) {
            setChannelOptions(channel, options, logger);
        }

        //配置AbstractBootstrap.attr
        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());
            }
        }
        //配置pipeline
        ChannelPipeline p = channel.pipeline();

        //获取ServerBootstrapAcceptor配置参数
        final EventLoopGroup currentChildGroup = childGroup;
        final ChannelHandler currentChildHandler = childHandler;

        final Entry<ChannelOption<?>, Object>[] currentChildOptions;
        final Entry<AttributeKey<?>, Object>[] currentChildAttrs;
        synchronized (childOptions) {
            currentChildOptions = childOptions.entrySet().toArray(newOptionArray(0));
        }
        synchronized (childAttrs) {
            currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(0));
        }

        p.addLast(new ChannelInitializer<Channel>() {
            @Override
            public void initChannel(final Channel ch) throws Exception {
                final ChannelPipeline pipeline = ch.pipeline();
                //配置AbstractBootstrap.handler
                ChannelHandler handler = config.handler();
                if (handler != null) {
                    pipeline.addLast(handler);
                }

                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        //配置ServerBootstrapAcceptor,作为Handle紧跟HeadContext
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });
    }

//省略了很多代码.............
}

可见，整个服务端pipeline的结构如下图所示。 bossGroup 控制IO事件的检测与处理，整个 bossGroup 对应的pipeline只包括头（ HeadContext ）尾( TailContext )以及中部的 ServerBootstrap.ServerBootstrapAcceptor 。

当新连接接入的时候 AbstractNioMessageChannel.NioMessageUnsafe#read() 方法被调用，最终调用 fireChannelRead() ，方法来触发下一个Handler的 channelRead 方法。而这个Handler正是 ServerBootstrapAcceptor

它是ServerBootstrap的内部类，同时继承自 ChannelInboundHandlerAdapter 。也是一个 ChannelInboundHandler 。其中channelRead主要做了以下几件事。

1.为客户端channel的pipeline添加childHandler

2.设置客户端TCP相关属性childOptions和自定义属性childAttrs

3.workGroup选择NioEventLoop并注册Selector

1）、为客户端channel的pipeline添加childHandler

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;
        private final Runnable enableAutoReadTask;

        ServerBootstrapAcceptor(
                final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
                Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, Object>[] childAttrs) {
            this.childGroup = childGroup;
            this.childHandler = childHandler;
            this.childOptions = childOptions;
            this.childAttrs = childAttrs;

       //省略了一些代码。。。。。 
        @Override
        @SuppressWarnings("unchecked")
        public void channelRead(ChannelHandlerContext ctx, Object msg) {
            //该channel为客户端接入时创建的channel
            final Channel child = (Channel) msg;

            //添加childHandler
            child.pipeline().addLast(childHandler);

            //设置TCP相关属性：childOptions
            setChannelOptions(child, childOptions, logger);

            //设置自定义属性:childAttrs
            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }

            try {
                //选择NioEventLoop并注册Selector
                childGroup.register(child)
                        .addListener(new ChannelFutureListener() {
                    @Override
                    public void operationComplete(ChannelFuture future) throws Exception {
                        if (!future.isSuccess()) {
                            forceClose(child, future.cause());
                        }
                    }
                });
            } catch (Throwable t) {
                forceClose(child, t);
            }
        }
      //省略了一些代码。。。。。
    }

客户端 channel 的pipeline添加 childHandler ，在服务端EchoServer创建流程中，childHandler的时候,使用了 ChannelInitializer 的一个自定义实例。并且覆盖了其 initChannel 方法，改方法获取到pipeline并添加具体的Handler。查看 ChannelInitializer 具体的添加逻辑, handlerAdded 方法。其实在 initChannel 逻辑中，首先是 回调到用户代码执行 initChannel ,用户代码执行添加Handler的添加操作，之后将ChannelInitializer自己从pipeline中删除 。

public abstract class ChannelInitializer<C extends Channel> extends ChannelInboundHandlerAdapter {

 @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        if (ctx.channel().isRegistered()) {
            // This should always be true with our current DefaultChannelPipeline implementation.
            // The good thing about calling initChannel(...) in handlerAdded(...) is that there will be no ordering
            // surprises if a ChannelInitializer will add another ChannelInitializer. This is as all handlers
            // will be added in the expected order.

            //初始化Channel
            if (initChannel(ctx)) {

                // We are done with init the Channel, removing the initializer now.
                removeState(ctx);
            }
        }
    }

    private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
        if (initMap.add(ctx)) { // Guard against re-entrance.
            try {
                //回调到用户代码
                initChannel((C) ctx.channel());
            } catch (Throwable cause) {
                // Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
                // We do so to prevent multiple calls to initChannel(...).
                exceptionCaught(ctx, cause);
            } finally {
                ChannelPipeline pipeline = ctx.pipeline();
                if (pipeline.context(this) != null) {
                    //删除本身
                    pipeline.remove(this);
                }
            }
            return true;
        }
        return false;
    }

}

2）、设置客户端TCP相关属性childOptions和自定义属性childAttrs

这点在 ServerBootstrapAcceptor#init() 方法中已经体现

3）、workGroup选择NioEventLoop并注册Selector

这要从 AbstractBootstrap#initAndRegister() 方法开始，然后跟踪源码会来到 AbstractUnsafe#register() 方法

protected abstract class AbstractUnsafe implements Unsafe {
      //省略了一些代码。。。。。
  @Override
        public final void register(EventLoop eventLoop, final ChannelPromise promise) {
            if (eventLoop == null) {
                throw new NullPointerException("eventLoop");
            }
            if (isRegistered()) {
                promise.setFailure(new IllegalStateException("registered to an event loop already"));
                return;
            }
            if (!isCompatible(eventLoop)) {
                promise.setFailure(
                        new IllegalStateException("incompatible event loop type: " + eventLoop.getClass().getName()));
                return;
            }

            AbstractChannel.this.eventLoop = eventLoop;

            if (eventLoop.inEventLoop()) {
                register0(promise);
            } else {
                try {
                    eventLoop.execute(new Runnable() {
                        @Override
                        public void run() {
                            register0(promise);
                        }
                    });
                } catch (Throwable t) {
                    logger.warn(
                            "Force-closing a channel whose registration task was not accepted by an event loop: {}",
                            AbstractChannel.this, t);
                    closeForcibly();
                    closeFuture.setClosed();
                    safeSetFailure(promise, t);
                }
            }
        }
      //省略了一些代码。。。。。
}

最后调用 AbstractNioUnsafe#doRegister() 方法通过jdk的 javaChannel().register 完成注册功能。

protected abstract class AbstractNioUnsafe extends AbstractUnsafe implements NioUnsafe {
      //省略了一些代码。。。。。
  @Override
    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().unwrappedSelector(), 0, this);
                return;
            } catch (CancelledKeyException e) {
                if (!selected) {
                    // Force the Selector to select now as the "canceled" SelectionKey may still be
                    // cached and not removed because no Select.select(..) operation was called yet.
                    eventLoop().selectNow();
                    selected = true;
                } else {
                    // We forced a select operation on the selector before but the SelectionKey is still cached
                    // for whatever reason. JDK bug ?
                    throw e;
                }
            }
        }
    }
      //省略了一些代码。。。。。
}

4.向Selector注册读事件

a）、入口： ServerBootstrap.ServerBootstrapAcceptor#channelRead()#childGroup.register() ;

public void channelRead(ChannelHandlerContext ctx, Object msg) {
            final Channel child = (Channel) msg;

            child.pipeline().addLast(childHandler);

            setChannelOptions(child, childOptions, logger);

            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }

            try {
                childGroup.register(child).addListener(new ChannelFutureListener() {
                    @Override
                    public void operationComplete(ChannelFuture future) throws Exception {
                        if (!future.isSuccess()) {
                            forceClose(child, future.cause());
                        }
                    }
                });
            } catch (Throwable t) {
                forceClose(child, t);
            }
        }

b）、实际上调用了 AbstractChannel.AbstractUnsafe#register0() ，触发了通道激活事件;

//触发通道激活事件，调用HeadContent的
   pipeline.fireChannelActive();

c）、 pipeline 的头部开始，即 DefaultChannelPipeline.HeadContext#channelActive() 从而触发了 readIfIsAutoRead() ;

@Override
  public void channelActive(ChannelHandlerContext ctx) {
            ctx.fireChannelActive();

            readIfIsAutoRead();
  }

d）、读事件将从尾部的TailContent#read()被触发，从而依次执行ctx.read()，从尾部开始，每个outboundHandler的read()事件都被触发。直到头部。

@Override
    public final ChannelPipeline read() {
        tail.read();
        return this;
    }


    @Override
    public ChannelHandlerContext read() {
        //获取最近的outboundhandler
        final AbstractChannelHandlerContext next = findContextOutbound();
        EventExecutor executor = next.executor();

        //并依次执行其read方法
        if (executor.inEventLoop()) {
            next.invokeRead();
        } else {
            Tasks tasks = next.invokeTasks;
            if (tasks == null) {
                next.invokeTasks = tasks = new Tasks(next);
            }
            executor.execute(tasks.invokeReadTask);
        }

        return this;
    }

e）、进入头部HeadContext#read()，并且最终更改了selectionKey,向selector注册了读事件

HeadContext#read()

@Override
        public void read(ChannelHandlerContext ctx) {
            unsafe.beginRead();
        }

AbstractChannel#beginRead()

@Override
        public final void beginRead() {
            assertEventLoop();

            if (!isActive()) {
                return;
            }

            try {
                doBeginRead();
            } catch (final Exception e) {
                invokeLater(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.fireExceptionCaught(e);
                    }
                });
                close(voidPromise());
            }
        }

AbstractNioMessageChannel#doBeginRead

@Override
    protected void doBeginRead() throws Exception {
        if (inputShutdown) {
            return;
        }
        super.doBeginRead();
    }

AbstractNioChannel#doBeginRead()

@Override
    protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return;
        }

        readPending = true;

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

总结

Netty如何接入新连接基本流程如上所述，如果有误，还望各位指正。建议先从前两篇看起比较好理解点。

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