Netty(6)源码-服务端与客户端创建

原生的NIO类图使用有诸多不便，Netty向用户屏蔽了细节，在与用户交界处做了封装。

一、服务端创建时序图

步骤一：创建ServerBootstrap实例

ServerBootstrap是Netty服务端的启动辅助类，它提供了一些列的方法用于设置参数，由于参数太多，使用builder模式。

步骤二：设置并且绑定好Reactor线程池

Netty中的Reactor线程池是EventLoopGroup，它实际上就是EventLoop数组。EventLoop的职责是处理所有注册到本线程多路复用器Selector上的Channel。Selector的轮询操作由绑定的EventLoop线程run方法驱动，在一个循环体内循环执行。值得说明的是，EventLoop的职责不仅仅是处理网络I/O事件，用户自定义的Task和定时任务Task也统一由EventLoop负责处理，这样线程模型就实现了统一。从调度层面看，也不存在从EvenetLoop线程中再启动其他类型的线程用于异步执行另外的任务，这样就避免了多线程的并发操作和锁竞争，提升了I/O线程的处理和调度性能。

步骤三：设置并绑定服务端Channel

服务端需要创建ServerSocketChannel，对原生NIO类库进行了封装，对应是NioServerSocketChannel

对用户而言，不需要关心服务端Channel的底层实现细节和工作原理，只需要指定具体是哪种服务端

Netty的ServerBootstrap提供了channel方法用于指定服务端Channel的类型。Netty通过工厂反射创建NioServerSocketChannel对象。

public B channel(Class<? extends C> channelClass) {
        if (channelClass == null) {
            throw new NullPointerException("channelClass");
        }
        return channelFactory(new ReflectiveChannelFactory<C>(channelClass));
    }

步骤四：链路建立的时候创建并且初始化ChannelPipeline。

它本质上是一个负载处理网络事件的职责链，负载管理和执行ChannelHanler。网络事件以事件流的形式在ChannelPipeline中流转，由ChannelPipeline根据ChannelHandler的执行策略调度ChannelHandler的执行。典型的网络事件如下：

（1）链路注册；

（2）链路激活；

（3）链路断开；

（4）接收到请求消息；

（5）请求消息接收并处理完毕；

（6）发送应答消息；

（7）链路发生异常；

（8）发生用户自定义事件。

步骤五：初始化ChannelPipeline完成之后，添加并设置ChannelHandler。

ChannelHandler是Netty提供给用户定制和扩展的关键接口。利用ChannelHandler用户可以完成大多数的功能定制，例如消息编解码、心跳、安全认证、TSL/SSL认证、流量控制和流量整形等。Netty同时也提供了大量的系统ChannelHandler供用户使用，比较实用的系统ChannelHandler总结如下：

（1）系统编解码框架——ByteToMessageCodec；

（2）通用基于长度的半包解码器——LengthFieldBasedFrameDecoder;

（3）码流日志打印Handler——LoggingHandler；

（4）SSL安全认证Handler——SslHandler；

（5）链路空闲检测Handler——IdleStateHandler；

（6）流量整形Handler——ChannelTrafficShapingHandler;

（7）Base64编解码——Base64Decoder和Base64Encoder。

.childHandler(new ChannelInitializer<SocketChannel>() {  
            @Override  
            public void initChannel(SocketChannel ch)  
                throws Exception {  
                ch.pipeline().addLast(  
                    new EchoServerHandler());  
            }  
            }); 

步骤6：绑定并启动监听端口。

在绑定监听端口之前系统会做一系列的初始化和检测工作，完成之后，会启动监听端口，并将ServerSocketChannel注册到Selector上监听客户端连接，相关代码如下。

   public ChannelFuture bind(SocketAddress localAddress) {
        validate();
        if (localAddress == null) {
            throw new NullPointerException("localAddress");
        }
        return doBind(localAddress);
    }

步骤7：Selector轮询。

由Reactor线程NioEventLoop负责调度和执行Selector轮询操作，选择准备就绪的Channel集合，相关代码如下

private void select(boolean oldWakenUp) throws IOException {
        Selector selector = this.selector;  
        try {  
 
                //此处代码省略...  
                int selectedKeys = selector.select(timeoutMillis);  
                selectCnt ++;  
                //此处代码省略...  

步骤8：当轮询到准备就绪的Channel之后，就由Reactor线程NioEventLoop执行ChannelPipeline的相应方法，最终调度并执行ChannelHandler

步骤9：执行Netty系统ChannelHandler和用户添加定制的ChannelHandler。

ChannelPipeline根据网络事件的类型，调度并执行ChannelHandler，相关代码如下。

    public ChannelHandlerContext fireChannelRead(final Object msg) {
        invokeChannelRead(findContextInbound(), msg);
        return this;
    }

二、Netty服务端创建源码分析

1. 创建线程组：

　　通常会创建两个EventLoopGroup，也可以只创建一个并共享。

        EventLoopGroup bossGroup = new NioEventLoopGroup();
        EventLoopGroup workerGroup = new NioEventLoopGroup();

NioEventLoopGroup实际就是Reactor线程池，负责调度和执行具体的任务：

　　client接入

　　网络读写事件处理

　　用户自定义任务

　　定时任务

通过ServerBootstrap的group方法将2个EventLoopGroup实例传入：

    public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
        //1. 调用父类的group方法传入parentGroup
        super.group(parentGroup);
        //2. 设置childGroup
        if (childGroup == null) {
            throw new NullPointerException("childGroup");
        }
        if (this.childGroup != null) {
            throw new IllegalStateException("childGroup set already");
        }
        this.childGroup = childGroup;
        return this;
    }

如果只传一个参数，则2个线程池会被重用。

    public ServerBootstrap group(EventLoopGroup group) {
        return group(group, group);
    }

2. 设置服务端Channel用于端口监听和客户端链路接入:

根据传入的channel class创建对应的服务端Channel，调用的是ReflectiveChannelFactory

public class ReflectiveChannelFactory<T extends Channel> implements ChannelFactory<T> {

    private final Class<? extends T> clazz;

    public ReflectiveChannelFactory(Class<? extends T> clazz) {
        if (clazz == null) {
            throw new NullPointerException("clazz");
        }
        this.clazz = clazz;
    }

    @Override
    public T newChannel() {
        try {
            return clazz.newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }

    @Override
    public String toString() {
        return StringUtil.simpleClassName(clazz) + ".class";
    }
}

3. 下面要设定服务端TCP参数：

Netty使用一个LinkedHashMap来保存

private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();

主要参数是TCP的backlog参数，底层C对应的接口为：

int listen(int fd, int backlog);

backlog指定了内核为此套接字接口排队的最大连接个数，内核要为套接字维护2个队列：未连接队列和已连接队列，根据TCP三路握手的三个分节分隔这2个队列。

服务器处于listen状态时，收到客户端syn分节(connect)时在未完成队列中创建一个新的条目，然后用三路握手的第二个分节即服务器的syn响应客户端，此条目在第三个分节到达前(客户端对服务器syn的ack)一直保留在未完成队列中，如果三路握手完成，该条目将从未完成连接队列搬到已完成队列尾部。

当进程调用accept时，从已完成队列中的头部取出一个条目给进程，当已完成队列为空时进程将进入睡眠，直到有条目在已完成队列中才唤醒。backlog则用来规定2个队列总和的最大值，大多数实现值为5，但是在高并发场景中显然不够，比如Lighttpd中此值达到128*8。需要设置此值更大原因是未完成连接队列可能因为客户端syn的到达以及等待握手第三个分节的到达延时而增大。Netty默认是100，用户可以调整。

4. 下面可以为启动辅助类和其父类分别指定Handler。

本质区别就是：

• ServerBootstrap中的Handler是NioServerSocketChannel使用的，所有连接该监听端口的客户端都会执行它；
• 父类AbstractBootstrap中的Handler是个工厂类，它为每个新接入的客户端都创建一个新的Handler；

5. 服务端启动的最后一步，就是绑定本地端口，启动服务：

    private ChannelFuture doBind(final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
        if (regFuture.cause() != null) {
            return regFuture;
        }

        if (regFuture.isDone()) {
            // At this point we know that the registration was complete and successful.
            ChannelPromise promise = channel.newPromise();
            doBind0(regFuture, channel, localAddress, promise);
            return promise;
        } else {
            // Registration future is almost always fulfilled already, but just in case it's not.
            final PendingRegistrationPromise promise = new PendingRegistrationPromise(channel);
            regFuture.addListener(new ChannelFutureListener() {
                @Override
                public void operationComplete(ChannelFuture future) throws Exception {
                    Throwable cause = future.cause();
                    if (cause != null) {
                        // Registration on the EventLoop failed so fail the ChannelPromise directly to not cause an
                        // IllegalStateException once we try to access the EventLoop of the Channel.
                        promise.setFailure(cause);
                    } else {
                        // Registration was successful, so set the correct executor to use.
                        // See https://github.com/netty/netty/issues/2586
                        promise.registered();

                        doBind0(regFuture, channel, localAddress, promise);
                    }
                }
            });
            return promise;
        }
    }

看红色标注的initAndRegister方法:

调用 channelFactory.newChannel()常见NioServerSocketChannel

然后调用init(channel)方法，由具体子类实现，这里实现的子类是ServerBootrap.

    final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();
            init(channel);
        } catch (Throwable t) {
            if (channel != null) {
                // channel can be null if newChannel crashed (eg SocketException("too many open files"))
                channel.unsafe().closeForcibly();
            }
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }

        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        // If we are here and the promise is not failed, it's one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop's task queue for later execution.
        //    i.e. It's safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regFuture;
    }

下面关注init方法，主要完成了以下功能：

(1). 设置Socket参数和NioServerSocketChannel的附加属性，代码如下:

        final 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).将AbstractBootstrap的Handler添加到NioServerSocketChannel的ChannelPipeline中，将用于服务端注册的Handler ServerBootstrapAcceptor添加到ChannelPipeline中，代码如下：

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

                // We add this handler via the EventLoop as the user may have used a ChannelInitializer as handler.
                // In this case the initChannel(...) method will only be called after this method returns. Because
                // of this we need to ensure we add our handler in a delayed fashion so all the users handler are
                // placed in front of the ServerBootstrapAcceptor.
                ch.eventLoop().execute(new Runnable() {
                    @Override
                    public void run() {
                        pipeline.addLast(new ServerBootstrapAcceptor(
                                currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                    }
                });
            }
        });

到此处，Netty服务端监听的相关资源已经初始化完毕，就剩下最后一步-

6. 注册NioServerSocketChannel到Reactor线程的多路复用器上，然后轮询客户端连接事件。

在分析注册代码之前，我们先通过下图看看目前NioServerSocketChannel的ChannelPipeline的组成：

通过Debug最终发现在AbstractChannel类中的register方法上：

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);
                }
            }
        }

首先判断是否是NioEventLoop自身发起的操作，如果是，则不存在并发操作，直接执行Channel注册；如果由其它线程发起，则封装成一个Task放入消息队列中异步执行。此处，由于是由ServerBootstrap所在线程执行的注册操作，所以会将其封装成Task投递到NioEventLoop中执行：

        private void register0(ChannelPromise promise) {
            try {
                // check if the channel is still open as it could be closed in the mean time when the register
                // call was outside of the eventLoop
                if (!promise.setUncancellable() || !ensureOpen(promise)) {
                    return;
                }
                boolean firstRegistration = neverRegistered;
                doRegister();
                neverRegistered = false;
                registered = true;

                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();

                safeSetSuccess(promise);
                pipeline.fireChannelRegistered();
                // Only fire a channelActive if the channel has never been registered. This prevents firing
                // multiple channel actives if the channel is deregistered and re-registered.
                if (isActive()) {
                    if (firstRegistration) {
                        pipeline.fireChannelActive();
                    } else if (config().isAutoRead()) {
                        // This channel was registered before and autoRead() is set. This means we need to begin read
                        // again so that we process inbound data.
                        //
                        // See https://github.com/netty/netty/issues/4805
                        beginRead();
                    }
                }
            } catch (Throwable t) {
                // Close the channel directly to avoid FD leak.
                closeForcibly();
                closeFuture.setClosed();
                safeSetFailure(promise, t);
            }
        }

将NioServerSocketChannel注册到NioEventLoop的Selector上，代码如下：

    protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(eventLoop().selector, 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;
                }
            }
        }
    }

大伙儿可能会很诧异，应该注册OP_ACCEPT（16）到多路复用器上，怎么注册0呢？0表示只注册，不监听任何网络操作。这样做的原因如下：

• 注册方法是多态的，它既可以被NioServerSocketChannel用来监听客户端的连接接入，也可以用来注册SocketChannel，用来监听网络读或者写操作；
• 通过SelectionKey的interestOps(int ops)方法可以方便的修改监听操作位。所以，此处注册需要获取SelectionKey并给AbstractNioChannel的成员变量selectionKey赋值。

注册成功之后，触发ChannelRegistered事件，方法如下：

                doRegister();
                neverRegistered = false;
                registered = true;

                // Ensure we call handlerAdded(...) before we actually notify the promise. This is needed as the
                // user may already fire events through the pipeline in the ChannelFutureListener.
                pipeline.invokeHandlerAddedIfNeeded();

                safeSetSuccess(promise);
                pipeline.fireChannelRegistered();

Netty的HeadHandler不需要处理ChannelRegistered事件，所以，直接调用下一个Handler，当ChannelRegistered事件传递到TailHandler后结束，TailHandler也不关心ChannelRegistered事件。

ChannelRegistered事件传递完成后，判断ServerSocketChannel监听是否成功，如果成功，需要出发NioServerSocketChannel的ChannelActive事件，判断方法即isActive().

 　　　　　　　　　pipeline.fireChannelRegistered();
                // Only fire a channelActive if the channel has never been registered. This prevents firing
                // multiple channel actives if the channel is deregistered and re-registered.
                if (isActive()) {
                    if (firstRegistration) {
                        pipeline.fireChannelActive();
                    } else if (config().isAutoRead()) {
                        // This channel was registered before and autoRead() is set. This means we need to begin read
                        // again so that we process inbound data.
                        //
                        // See https://github.com/netty/netty/issues/4805
                        beginRead();
                    }
                }

其中isActive()是一个多态方法，如果是服务端则，判断监听是否启动；如果是客户端，判断TCP连接是否完成。ChannelActive事件在ChannelPipeline()传递，完成之后根据配置决定是否自动触发Channel的读操作。

读方法最终到：

        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());
            }
        }

由于不同类型的Channel对读的准备工作不同，因此doBeginRead也是个多态方法。

对于NIO通信，无论是客户端，还是服务端，都要设置网络监听操作位为自己感兴趣的，对于NioServerSocketChannel感兴趣的是OP_ACCEPT(16)，于是修改操作位：

 @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);
        }
    }

某些情况下，当前监听的操作类型和Channel关心的网络事件是一致的，不需要重复注册，所以增加了&的判断。JDK SelectionKey有4种操作类型，分别为：

(1) OP_READ = 1 <<0;

(2) OP_WRTE = 1 <<2;

(3) OP_CONNECT = 1 <<3;

(4) OP_ACCEPT = 1 <<4;

此时，服务器监听启动部分源码已经分析结束。

三、客户端接入源码分析

负责处理网络读写、连接和客户端请求介入的Reactor线程就是NioEventLoop，下面分析如何处理新的客户端接入。

当多路复用器检测到新的Channel时候，默认执行processSelectedKeysOptimized方法.

    private void processSelectedKeys() {
        if (selectedKeys != null) {
            processSelectedKeysOptimized(selectedKeys.flip());
        } else {
            processSelectedKeysPlain(selector.selectedKeys());
        }
    }

由于Channel的Attachment是NioServerSocketChannel，所以执行processSelectedKey方法。

            if (a instanceof AbstractNioChannel) {
                processSelectedKey(k, (AbstractNioChannel) a);
            } else {
                @SuppressWarnings("unchecked")
                NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                processSelectedKey(k, task);
            }

继续点入该方法debug，由于监听的是连接操作，会执行unsafe.read()方法。由于不同的Channel执行不同的操作，所以NioUnsafe被设计为接口。

debug发现使用的是:

其read()方法如下所示：

 @Override
        public void read() {
            assert eventLoop().inEventLoop();
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            allocHandle.reset(config);

            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    do {
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) {
                            break;
                        }
                      　//代码省略...
    }

对doReadMessages方法进行分析，发现它实际就是接受新的客户端连接并且创建NioSocketChannel:

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

        try {
            if (ch != null) {
                buf.add(new NioSocketChannel(this, ch));
                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;
    }

接收到新的连接之后，触发ChannelPipeLine的ChannelRead方法，代码如下：

                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false;
                    pipeline.fireChannelRead(readBuf.get(i));
                }

于是触发pipeLine调用链，事件在ChannelPipeline中传递，执行ServerBootstrapAcceptor中的channelRead方法，代码如下：

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

            child.pipeline().addLast(childHandler); //(1)

            for (Entry<ChannelOption<?>, Object> e: childOptions) {
                try {
                    if (!child.config().setOption((ChannelOption<Object>) e.getKey(), e.getValue())) {
                        logger.warn("Unknown channel option: " + e);
                    }
                } catch (Throwable t) {
                    logger.warn("Failed to set a channel option: " + child, t);
                }
            }
　　　　　　　　//(2)
            for (Entry<AttributeKey<?>, Object> e: childAttrs) {
                child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
            }
　　　　　　　　//(3)
            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);
            }
        }

上面方法主要分为3个步骤：

(1) 加入childHandler到客户端SocketChannel的ChannelPipeline中

(2) 设置SocketChannel的TCP参数

(3) 注册SocketChannel到多路复用器

注意这里register注册也是注册操作位为0.

执行完注册后，紧接着会触发ChannelReadComplete事件。

Netty的Header和Tailer本身不关于这个事件，因此ChannelReadComplete是直接透传， 执行完ChannelReadComplete后，接着执行PipeLine的read()方法，最终到HeadHandler的read()方法。read()方法在前面说过，会修改操作位，此时这里debug发现把操作位修改OP_READ。

此时，客户端连接处理完成，可以进行网络读写等I/O操作。

四、Netty客户端创建流程

1、用户线程创建Bootstrap

Bootstrap b = new Bootstrap();

Bootstrap是Socket客户端创建工具类，通过API设置创建客户端相关的参数，异步发起客户端连接。

2、创建处理客户端连接、IO读写的Reactor线程组NioEventLoopGroup

EventLoopGroup group = new NioEventLoopGroup();

3、通过Bootstrap的ChannelFactory和用户指定的Channel类型创建用于客户端连接的NioSocketChannel

b.group(group).channel(NioSocketChannel.class).option(ChannelOption.TCP_NODELAY, true)

　此处的NioSocketChannel类似于Java NIO提供的SocketChannel。

4、创建默认的channel Handler pipeline

b.group(group).channel(NioSocketChannel.class).option(ChannelOption.TCP_NODELAY, true)
 .handler(new ChannelInitializer<SocketChannel>()
  {
       @Override
       public void initChannel(SocketChannel ch) throws Exception
       {
         ch.pipeline().addLast(new HelloClientHandler());
        }
  });

用于调度和执行网络事件。

5、异步发起TCP连接

 // 发起异步连接操作
 ChannelFuture f = b.connect(host, port).sync();

SocketChannel执行connect()操作后有以下三种结果：

• 连接成功，然会true;
• 暂时没有连接上，服务器端没有返回ACK应答，连接结果不确定，返回false。此种结果下，需要将NioSocketChannel中的selectionKey设置为OP_CONNECT，监听连接结果；
• 接连失败，直接抛出I/O异常

6、注册对应的网络监听状态位到多路复用器

7、由多路复用器在I/O中轮询个Channel，处理连接结果

8、如果连接成功，设置Future结果，发送连接成功事件，触发ChannelPipeline执行

9、由ChannelPipeline调度执行系统和用户的ChannelHandler，执行业务逻辑

五、客户端创建源码分析

Netty客户端创建流程非常繁琐，这里只针对关键步骤进行分析。

5.1 客户端连接辅助类Bootstrap

1. 设置I/O线程组：只需要一个线程组EventLoopGroup

2. TCP参数设置：

    public <T> B option(ChannelOption<T> option, T value) {
        if (option == null) {
            throw new NullPointerException("option");
        }
        if (value == null) {
            synchronized (options) {
                options.remove(option);
            }
        } else {
            synchronized (options) {
                options.put(option, value);
            }
        }
        return (B) this;
    }

主要TCP参数如下：

• (1) SO_TIMEOUT: 控制读取操作将阻塞多少毫秒，如果返回值为0，计时器就被禁止了，该线程将被无限期阻塞。
• (2) SO_SNDBUF: 套接字使用的发送缓冲区大小
• (3) SO_RCVBUF: 套接字使用的接收缓冲区大小
• (4) SO_REUSEADDR : 是否允许重用端口
• (5) CONNECT_TIMEOUT_MILLIS: 客户端连接超时时间，原生NIO不提供该功能，Netty使用的是自定义连接超时定时器检测和超时控制
• (6) TCP_NODELAY : 是否使用Nagle算法

3. channel接口

同样使用反射创建NioSocketChannel

4. 设置Handler接口

Bootstrap为了简化Handler的编排，提供了ChannelInitializer，当TCP链路注册成功后，调用initChannel接口:

    public final void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        // Normally this method will never be called as handlerAdded(...) should call initChannel(...) and remove
        // the handler.
        if (initChannel(ctx)) {
            // we called initChannel(...) so we need to call now pipeline.fireChannelRegistered() to ensure we not
            // miss an event.
            ctx.pipeline().fireChannelRegistered();
        } else {
            // Called initChannel(...) before which is the expected behavior, so just forward the event.
            ctx.fireChannelRegistered();
        }
    }

其中InitChannel为抽象接口，即下面红色标注的代码，用户便是在这个方法中设置ChannelHandler:

    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) {
                // Explicitly call exceptionCaught(...) as we removed the handler before calling initChannel(...).
                // We do so to prevent multiple calls to initChannel(...).
                exceptionCaught(ctx, cause);
            } finally {
                remove(ctx);
            }
            return true;
        }
        return false;
    }

5.2 客户端连接操作

1. 首先要创建和初始化NioSocketChannel

    private ChannelFuture doResolveAndConnect(final SocketAddress remoteAddress, final SocketAddress localAddress) {
        final ChannelFuture regFuture = initAndRegister();
        final Channel channel = regFuture.channel();
　　　　  //.....

创建之后，初始化，然后在注册：

final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();
            init(channel);
        } catch (Throwable t) {
            if (channel != null) {
                // channel can be null if newChannel crashed (eg SocketException("too many open files"))
                channel.unsafe().closeForcibly();
            }
            // as the Channel is not registered yet we need to force the usage of the GlobalEventExecutor
            return new DefaultChannelPromise(channel, GlobalEventExecutor.INSTANCE).setFailure(t);
        }

        ChannelFuture regFuture = config().group().register(channel);
        if (regFuture.cause() != null) {
            if (channel.isRegistered()) {
                channel.close();
            } else {
                channel.unsafe().closeForcibly();
            }
        }

        // If we are here and the promise is not failed, it's one of the following cases:
        // 1) If we attempted registration from the event loop, the registration has been completed at this point.
        //    i.e. It's safe to attempt bind() or connect() now because the channel has been registered.
        // 2) If we attempted registration from the other thread, the registration request has been successfully
        //    added to the event loop's task queue for later execution.
        //    i.e. It's safe to attempt bind() or connect() now:
        //         because bind() or connect() will be executed *after* the scheduled registration task is executed
        //         because register(), bind(), and connect() are all bound to the same thread.

        return regFuture;
    }

2. 创建完成后，连接操作会异步执行，最终调用到HeadContext的connect方法.

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

的connect操作如下：

 if (doConnect(remoteAddress, localAddress)) {
                    fulfillConnectPromise(promise, wasActive);
                } else {
//...

3. doConnect三种可能结果

• 连接成功，然会true;
• 暂时没有连接上，服务器端没有返回ACK应答，连接结果不确定，返回false。此种结果下，需要将NioSocketChannel中的selectionKey设置为OP_CONNECT，监听连接结果；
• 接连失败，直接抛出I/O异常

异步返回之后，需要判断连接结果，如果成功，则触发ChannelActive事件。最终会将NioSocketChannel中的selectionKey设置为SelectionKey.OP_READ，用于监听网络读操作。

5.3 异步连接结果通知

NioEventLoop的Selector轮询客户端连接Channel，当服务端返回应答后，进行判断。依旧是NioEventLoop中的processSelectedKey方法：

 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();
            }

下面分析finishConnect方法：

        @Override
        public final void finishConnect() {
            // Note this method is invoked by the event loop only if the connection attempt was
            // neither cancelled nor timed out.

            assert eventLoop().inEventLoop();

            try {
                boolean wasActive = isActive();
                doFinishConnect(); //判断SocketChannel的连接结果，true表示成功
                fulfillConnectPromise(connectPromise, wasActive); //触发链路激活
            } catch (Throwable t) {
                fulfillConnectPromise(connectPromise, annotateConnectException(t, requestedRemoteAddress));
            } finally {
                // Check for null as the connectTimeoutFuture is only created if a connectTimeoutMillis > 0 is used
                // See https://github.com/netty/netty/issues/1770
                if (connectTimeoutFuture != null) {
                    connectTimeoutFuture.cancel(false);
                }
                connectPromise = null;
            }
        }

fulfillConnectPromise方法则触发链路激活事件，并由ChannelPipeline进行传播:

        private void fulfillConnectPromise(ChannelPromise promise, boolean wasActive) {
            if (promise == null) {
                // Closed via cancellation and the promise has been notified already.
                return;
            }

            // Get the state as trySuccess() may trigger an ChannelFutureListener that will close the Channel.
            // We still need to ensure we call fireChannelActive() in this case.
            boolean active = isActive();

            // trySuccess() will return false if a user cancelled the connection attempt.
            boolean promiseSet = promise.trySuccess();

            // Regardless if the connection attempt was cancelled, channelActive() event should be triggered,
            // because what happened is what happened.
            if (!wasActive && active) {
                pipeline().fireChannelActive();
            }

            // If a user cancelled the connection attempt, close the channel, which is followed by channelInactive().
            if (!promiseSet) {
                close(voidPromise());
            }
        }

跟之前类似，将网络监听修改为读操作。

5.4 客户端连接超时机制

由Netty自己实现的客户端超时机制，在AbstractNioChannel的connect方法中：

 public final void connect(
                final SocketAddress remoteAddress, final SocketAddress localAddress, final ChannelPromise promise) {
            if (!promise.setUncancellable() || !ensureOpen(promise)) {
                return;
            }

            try {
                if (connectPromise != null) {
                    // Already a connect in process.
                    throw new ConnectionPendingException();
                }

                boolean wasActive = isActive();
                if (doConnect(remoteAddress, localAddress)) {
                    fulfillConnectPromise(promise, wasActive);
                } else {
                    connectPromise = promise;
                    requestedRemoteAddress = remoteAddress;

                    // Schedule connect timeout.
                    int connectTimeoutMillis = config().getConnectTimeoutMillis();
                    if (connectTimeoutMillis > 0) {
                        connectTimeoutFuture = eventLoop().schedule(new Runnable() {
                            @Override
                            public void run() {
                                ChannelPromise connectPromise = AbstractNioChannel.this.connectPromise;
                                ConnectTimeoutException cause =
                                        new ConnectTimeoutException("connection timed out: " + remoteAddress);
                                if (connectPromise != null && connectPromise.tryFailure(cause)) {
                                    close(voidPromise());
                                }
                            }
                        }, connectTimeoutMillis, TimeUnit.MILLISECONDS);
                    }

                    promise.addListener(new ChannelFutureListener() {
                        @Override
                        public void operationComplete(ChannelFuture future) throws Exception {
                            if (future.isCancelled()) {
                                if (connectTimeoutFuture != null) {
                                    connectTimeoutFuture.cancel(false);
                                }
                                connectPromise = null;
                                close(voidPromise());
                            }
                        }
                    });
                }
            } catch (Throwable t) {
                promise.tryFailure(annotateConnectException(t, remoteAddress));
                closeIfClosed();
            }
        }

一旦超时定时器执行，则说明客户端超时，构造异常，将异常结果设置到connectPromise中，同时关闭客户端句柄。

如果在超时之前获取结果，则直接删除定时器，防止其被触发。