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。

// 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 #2308.         */        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
    
     , Object> attrs = attrs0();    synchronized (attrs) {        for (Entry
     
      , Object> e: attrs.entrySet()) {            @SuppressWarnings("unchecked")            AttributeKey
      
     
    

//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>() {

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。

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。