ServerBootstrap与Bootstrap分别是netty中服务端与客户端的引导类，主要负责服务端与客户端初始化、配置及启动引导等工作，接下来我们就通过netty源码中的示例对ServerBootstrap与Bootstrap的源码进行一个简单的分析。首先我们知道这两个类都继承自AbstractBootstrap类

接下来我们就通过netty源码中ServerBootstrap的实例入手对其进行一个简单的分析。

// Configure the server. EventLoopGroup bossGroup = new NioEventLoopGroup(1); EventLoopGroup workerGroup = new NioEventLoopGroup(); final EchoServerHandler serverHandler = new EchoServerHandler(); try { //初始化一个服务端引导类 ServerBootstrap b = new ServerBootstrap(); b.group(bossGroup, workerGroup) //设置线程组 .channel(NioServerSocketChannel.class)//设置ServerSocketChannel的IO模型 分为epoll与Nio .option(ChannelOption.SO_BACKLOG, 100)//设置option参数，保存成一个LinkedHashMap<ChannelOption<?>, Object>() .handler(new LoggingHandler(LogLevel.INFO))//这个hanlder 只专属于 ServerSocketChannel 而不是 SocketChannel。 .childHandler(new ChannelInitializer<SocketChannel>() { //这个handler 将会在每个客户端连接的时候调用。供 SocketChannel 使用。 @Override public void initChannel(SocketChannel ch) throws Exception { ChannelPipeline p = ch.pipeline(); if (sslCtx != null) { p.addLast(sslCtx.newHandler(ch.alloc())); } //p.addLast(new LoggingHandler(LogLevel.INFO)); p.addLast(serverHandler); } }); // Start the server. 启动服务 ChannelFuture f = b.bind(PORT).sync(); // Wait until the server socket is closed. f.channel().closeFuture().sync(); } finally { // Shut down all event loops to terminate all threads. bossGroup.shutdownGracefully(); workerGroup.shutdownGracefully(); }

接下来我们主要从服务端的socket在哪里初始化与哪里accept连接这两个问题入手对netty服务端启动的流程进行分析；

 我们首先要知道，netty服务的启动其实可以分为以下四步：

创建服务端Channel

初始化服务端Channel

注册Selector

端口绑定

一、创建服务端Channel

1、服务端Channel的创建，主要为以下流程

我们通过跟踪代码能够看到

final ChannelFuture regFuture = initAndRegister();// 初始化并创建 NioServerSocketChannel

我们在initAndRegister()中可以看到channel的初始化。

channel = channelFactory.newChannel(); // 通过 反射工厂创建一个 NioServerSocketChannel

 我进一步看newChannel()中的源码，在ReflectiveChannelFactory这个反射工厂中，通过clazz这个类的反射创建了一个服务端的channel。

@Override

    public T newChannel() {

        try {

            return clazz.getConstructor().newInstance();//反射创建

        } catch (Throwable t) {

            throw new ChannelException("Unable to create Channel from class " + clazz, t);

        }

    }

既然通过反射，我们就要知道clazz类是什么，那么我我们来看下channelFactory这个工厂类是在哪里初始化的，初始化的时候我们传入了哪个channel。

这里我们需要看下demo实例中初始化ServerBootstrap时.channel(NioServerSocketChannel.class)这里的具体实现，我们看下源码

public B channel(Class<? extends C> channelClass) {

        if (channelClass == null) {

            throw new NullPointerException("channelClass");

        }

        return channelFactory(new ReflectiveChannelFactory<C>(channelClass));

    }

通过上面的代码我可以直观的看出正是在这里我们通过NioServerSocketChannel这个类构造了一个反射工厂。

那么到这里就很清楚了，我们创建的Channel就是一个NioServerSocketChannel，那么具体的创建我们就需要看下这个类的构造函数。首先我们看下一个NioServerSocketChannel创建的具体流程

首先是newsocket(),我们先看下具体的代码，在NioServerSocketChannel的构造函数中我们创建了一个jdk原生的ServerSocketChannel

/**

    * Create a new instance

    */

    public NioServerSocketChannel() {

        this(newSocket(DEFAULT_SELECTOR_PROVIDER));//传入默认的SelectorProvider

    }

    private static ServerSocketChannel newSocket(SelectorProvider provider) {

        try {

            /**

            *  Use the {@link SelectorProvider} to open {@link SocketChannel} and so remove condition in

            *  {@link SelectorProvider#provider()} which is called by each ServerSocketChannel.open() otherwise.

            *

            *  See <a href="https://github.com/netty/netty/issues/2308">#2308</a>.

            */

            return provider.openServerSocketChannel();//可以看到创建的是jdk底层的ServerSocketChannel

        } catch (IOException e) {

            throw new ChannelException(

                    "Failed to open a server socket.", e);

        }

    }

第二步是通过NioServerSocketChannelConfig配置服务端Channel的构造函数，在代码中我们可以看到我们把NioServerSocketChannel这个类传入到了NioServerSocketChannelConfig的构造函数中进行配置

/** * Create a new instance using the given {@link ServerSocketChannel}. */ public NioServerSocketChannel(ServerSocketChannel channel) { super(null, channel, SelectionKey.OP_ACCEPT);//调用父类构造函数,传入创建的channel config = new NioServerSocketChannelConfig(this, javaChannel().socket()); }

第三步在父类AbstractNioChannel的构造函数中把创建服务端的Channel设置为非阻塞模式

/** * Create a new instance * * @param parent the parent {@link Channel} by which this instance was created. May be {@code null} * @param ch the underlying {@link SelectableChannel} on which it operates * @param readInterestOp the ops to set to receive data from the {@link SelectableChannel} */ protected AbstractNioChannel(Channel parent, SelectableChannel ch, int readInterestOp) { super(parent); this.ch = ch;//这个ch就是传入的通过jdk创建的Channel this.readInterestOp = readInterestOp; try { ch.configureBlocking(false);//设置为非阻塞 } 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); } }

第四步调用AbstractChannel这个抽象类的构造函数设置Channel的id（每个Channel都有一个id，唯一标识）,unsafe（tcp相关底层操作），pipeline（逻辑链）等，而不管是服务的Channel还是客户端的Channel都继承自这个抽象类，他们也都会有上述相应的属性。我们看下AbstractChannel的构造函数

/** * Creates a new instance. * * @param parent * the parent of this channel. {@code null} if there's no parent. */ protected AbstractChannel(Channel parent) { this.parent = parent; id = newId();//创建Channel唯一标识 unsafe = newUnsafe();//netty封装的TCP 相关操作类 pipeline = newChannelPipeline();//逻辑链 }

 2、初始化服务端创建的Channel

init(channel);// 初始化这个 NioServerSocketChannel

我们首先列举下init(channel)中具体都做了哪了些功能：

设置ChannelOptions、ChannelAttrs ,配置服务端Channel的相关属性；

设置ChildOptions、ChildAttrs，配置每个新连接的Channel的相关属性；

Config handler，配置服务端pipeline;

add ServerBootstrapAcceptor,添加连接器，对accpet接受到的新连接进行处理，添加一个nio线程；

那么接下来我们通过代码，对每一步设置进行一下分析：

首先是在SeverBootstrap的init()方法中对ChannelOptions、ChannelAttrs 的配置的关键代码

final Map<ChannelOption<?>, Object> options = options0();//拿到你设置的option synchronized (options) { setChannelOptions(channel, options, logger);//设置NioServerSocketChannel相应的TCP参数，其实这一步就是把options设置到channel的config中 } 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()); } }

然后是对ChildOptions、ChildAttrs配置的关键代码

//可以看到两个都是局部变量，会在下面设置pipeline时用到 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)); }

第三步对服务端Channel的handler进行配置

p.addLast(new ChannelInitializer<Channel>() { @Override public void initChannel(final Channel ch) throws Exception { final ChannelPipeline pipeline = ch.pipeline(); ChannelHandler handler = config.handler();//拿到我们自定义的hanler if (handler != null) { pipeline.addLast(handler); } ch.eventLoop().execute(new Runnable() { @Override public void run() { pipeline.addLast(new ServerBootstrapAcceptor( ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } }); } });

第四步添加ServerBootstrapAcceptor连接器，这个是netty向服务端Channel自定义添加的一个handler，用来处理新连接的添加与属性配置，我们来看下关键代码

ch.eventLoop().execute(new Runnable() { @Override public void run() { //在这里会把我们自定义的ChildGroup、ChildHandler、ChildOptions、ChildAttrs相关配置传入到ServerBootstrapAcceptor构造函数中，并绑定到新的连接上 pipeline.addLast(new ServerBootstrapAcceptor( ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs)); } });

三、注册Selector

一个服务端的Channel创建完毕后，下一步就是要把它注册到一个事件轮询器Selector上，在initAndRegister()中我们把上面初始化的Channel进行注册

ChannelFuture regFuture = config().group().register(channel);//注册我们已经初始化过的Channel

而这个register具体实现是在AbstractChannel中的AbstractUnsafe抽象类中的

/** * 1、先是一系列的判断。 * 2、判断当前线程是否是给定的 eventLoop 线程。注意：这点很重要，Netty 线程模型的高性能取决于对于当前执行的Thread 的身份的确定。如果不在当前线程，那么就需要很多同步措施（比如加锁），上下文切换等耗费性能的操作。 * 3、异步（因为我们这里直到现在还是 main 线程在执行，不属于当前线程）的执行 register0 方法。 */ @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;//绑定线程 if (eventLoop.inEventLoop()) { register0(promise);//实际的注册过程 } 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); } } }

首先我们对整个注册的流程做一个梳理

接下来我们进入register0()方法看下注册过程的具体实现

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();//jdk channel的底层注册 neverRegistered = false; registered = true; // 触发绑定的handler事件 // 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); } }

AbstractNioChannel中doRegister()的具体实现就是把jdk底层的channel绑定到eventLoop的selecor上

@Override protected void doRegister() throws Exception { boolean selected = false; for (;;) { try { //把channel注册到eventLoop上的selector上 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; } } } }

到这里netty就把服务端的channel注册到了指定的selector上，下面就是服务端口的邦迪

三、端口绑定

首先我们梳理下netty中服务端口绑定的流程

我们来看下AbstarctUnsafe中bind()方法的具体实现

@Override public final void bind(final SocketAddress localAddress, final ChannelPromise promise) { assertEventLoop(); if (!promise.setUncancellable() || !ensureOpen(promise)) { return; } // See: https://github.com/netty/netty/issues/576 if (Boolean.TRUE.equals(config().getOption(ChannelOption.SO_BROADCAST)) && localAddress instanceof InetSocketAddress && !((InetSocketAddress) localAddress).getAddress().isAnyLocalAddress() && !PlatformDependent.isWindows() && !PlatformDependent.maybeSuperUser()) { // Warn a user about the fact that a non-root user can't receive a // broadcast packet on *nix if the socket is bound on non-wildcard address. logger.warn( "A non-root user can't receive a broadcast packet if the socket " + "is not bound to a wildcard address; binding to a non-wildcard " + "address (" + localAddress + ") anyway as requested."); } boolean wasActive = isActive();//判断绑定是否完成 try { doBind(localAddress);//底层jdk绑定端口 } catch (Throwable t) { safeSetFailure(promise, t); closeIfClosed(); return; } if (!wasActive && isActive()) { invokeLater(new Runnable() { @Override public void run() { pipeline.fireChannelActive();//触发ChannelActive事件 } }); } safeSetSuccess(promise); }

在doBind(localAddress)中netty实现了jdk底层端口的绑定

@Override protected void doBind(SocketAddress localAddress) throws Exception { if (PlatformDependent.javaVersion() >= 7) { javaChannel().bind(localAddress, config.getBacklog()); } else { javaChannel().socket().bind(localAddress, config.getBacklog()); } }

在 pipeline.fireChannelActive()中会触发pipeline中的channelActive()方法

@Override public void channelActive(ChannelHandlerContext ctx) throws Exception { ctx.fireChannelActive(); readIfIsAutoRead(); }

在channelActive中首先会把ChannelActive事件往下传播，然后调用readIfIsAutoRead()方法出触发channel的read事件，而它最终调用AbstractNioChannel中的doBeginRead()方法

@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);//readInterestOp为 SelectionKey.OP_ACCEPT } }

 在doBeginRead()方法，netty会把accept事件注册到Selector上。

到此我们对netty服务端的启动流程有了一个大致的了解，整体可以概括为下面四步：

1、channelFactory.newChannel()，其实就是创建jdk底层channel，并初始化id、piepline等属性；

2、init(channel)，添加option、attr等属性，并添加ServerBootstrapAcceptor连接器；

3、config().group().register(channel)，把jdk底层的channel注册到eventLoop上的selector上；

4、doBind0(regFuture, channel, localAddress, promise)，完成服务端端口的监听，并把accept事件注册到selector上；

以上就是对netty服务端启动流程进行的一个简单分析，有很多细节没有关注与深入，其中如有不足与不正确的地方还望指出与海涵。