netty 服务端启动

创建ServerBootstrap,该类是Netty服务端的启动类
绑定Reactor线程池,EventLoopGroup,实际上就是EventLoop的数组.除了处理IO操作外,用户提交的task和定时任务也是由EventLoop执行.避免了多线程竞争的情况
设置并绑定服务端Channel,对于Nio服务端,使用的是NioServerSocketChannel
链路建立是创建并初始化ChannelPipeline.其本质是一个处理网络事件的职责链
初始化ChannelPipline完成之后,添加并设置ChannelHandler,
绑定并启动监听端口,在绑定之前系统会先进行一些初始化,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
Selector轮询.由Reactor线程NioEventLoop负责调度和执行Selector轮询,选择准备就绪的Channel集合.
当轮询到准备就绪的Channel之后，由Reactor线程执行ChannelPipeline的相应方法，最终调度执行ChannelHandler
执行Netty系统ChannelHandler和用户自定义的ChannelHandler。ChannelPipeline根据网络事件的类型，调度并执行ChannelHandler。

源码分析

服务端从bind 函数启动。bind 函数最后访问到了AbstractBootstrap的 dobind 函数，如下

private ChannelFuture doBind(final SocketAddress localAddress) {
  final ChannelFuture regFuture = initAndRegister();
  //省略其他函数
}

初始化

final ChannelFuture initAndRegister() {
        Channel channel = null;
        try {
            channel = channelFactory.newChannel();
            init(channel);
        //省略其他函数
    }

使用 channelFactory 的 newChannel 构造一个 channel，对于 Nio应用来说，这里是NioServerSocketChannel，其使用了反射来获取实例。
调用 init 函数

init 函数是一个抽象函数，被定义在了ServerBootstrap中

void init(Channel channel) throws Exception {
    final Map<ChannelOption<?>, Object> options = options0();
  ////设置 Socket 参数和 NioServerSocketChannel 的附加属性
    synchronized (options) {
        setChannelOptions(channel, options, logger);
    }

    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());
        }
    }
		//////
    ChannelPipeline p = channel.pipeline();

    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();
            ChannelHandler handler = config.handler();
            if (handler != null) {
                pipeline.addLast(handler);///将 AbstractBootstrap 的 Handler 添加到 ChannelPipeline 中
            }

            ch.eventLoop().execute(new Runnable() {
                @Override
                public void run() {
                  //添加用于注册的 handler
                    pipeline.addLast(new ServerBootstrapAcceptor(
                            ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                }
            });
        }
    });
}

前面一大堆其实都是设置连接参数，对于 Server 来说关键点在于添加了ServerBootstrapAcceptor这是一个内部类，并且其是server 的唯一一个handler。暂且略过不提。

注册

init 后回到initAndRegister函数

final ChannelFuture initAndRegister() {
  //省略其他代码
  ChannelFuture regFuture = config().group().register(channel);
  if (regFuture.cause() != null) {
    if (channel.isRegistered()) {
      channel.close();
    } else {
      channel.unsafe().closeForcibly();
    }
  }
  return regFuture;
}

从当前的执行线程中获取一个并将 channel注册到这个线程中。

对于 NioServerChannel其调用到了SingleThreadEventLoop的 register 函数

@Override
public ChannelFuture register(Channel channel) {
    return register(new DefaultChannelPromise(channel, this));
}

@Override
public ChannelFuture register(final ChannelPromise promise) {
    ObjectUtil.checkNotNull(promise, "promise");
    promise.channel().unsafe().register(this, promise);
    return promise;
}

对于 NioServerChannel 的 unsafe 函数，其在AbstractNioMessageChannel中实现

而 register 函数，则在AbstractUnsafe中定义，并被声明为 final

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);
    }
  }
        }

前面的函数是参数校验，

关键代码为

AbstractChannel.this.eventLoop = eventLoop;

这行代码将 evetloop（即指定的线程）绑定到了 channel 中。

此时调用线程仍然是用户线程，因此会进入

eventLoop.execute(new Runnable() {
    @Override
    public void run() {
        register0(promise);
    }
});

从此刻开始，netty 线程和用户线程就开始并行了。

随后由 eventLoop 来执行register0函数

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);
    }
}

AbstractNioChannel.doRegister

执行 doRegister 函数，其被定义在AbstractNioChannel中

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;
      }
    }
  }
}

其 javaChannel 由NioServerSocketChannel的构造器设置。

这里注册的是 0，表示不监听任何网络操作。

这里注册为 0 的原因有两点

该函数处于AbstractNioChannel，而 clientChannel 和 serverChannel 都是其子类，因此其不能设置特殊的操作
通过 SelectionKey 的 interestOps 可以修改其监听的网络操作

other

pipeline 实际上是一个AbstractChannelHandlerContext的双向链表，并且其默认的 head 和 tail 分别为HeadContext和TailContext。

headContext的 register 的实现为invokeHandlerAddedIfNeeded，即触发第一次注册事件，然而此时其实是不触发的，因为刚刚在之前已经触发过了，随后将在链表中传递注册事件

tailContext 是一个空操作。

初始化 server

除此之外，在初始化 server 时，还添加了一个 ChannelInitializer（在 init 函数中）

查看其实现

@Override
@SuppressWarnings("unchecked")
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();

        // We are done with init the Channel, removing all the state for the Channel now.
        removeState(ctx);
    } else {
        // Called initChannel(...) before which is the expected behavior, so just forward the event.
        ctx.fireChannelRegistered();
    }
}
private boolean initChannel(ChannelHandlerContext ctx) throws Exception {
  if (initMap.add(ctx)) { // 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 {
      ChannelPipeline pipeline = ctx.pipeline();
      if (pipeline.context(this) != null) {
        pipeline.remove(this);
      }
    }
    return true;
  }
  return false;
}

可以看到其在触发 register 事件触发时的默认操作为执行initChannel操作，随后结束移除 init 的上下文这个操作是自定义的，对于 server 来说这个操作为

final ChannelPipeline pipeline = ch.pipeline();
ChannelHandler handler = config.handler();
if (handler != null) {
    pipeline.addLast(handler);
}

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

在这里又提交了一个 task 到 channel 绑定的 eventLoop 中，实际上当前正在进行的 register 也是一个 task，因此这里提交了下一个运行的任务。

需要注意的是这个提交的任务并不一定在下次执行，因为当前是 netty 线程执行，而用户线程可能是并行的。

随后调用 isActive，对于 Server 来说，其实现于NioServerSocketChannel

public boolean isActive() {
  // As java.nio.ServerSocketChannel.isBound() will continue to return true even after the channel was closed
  // we will also need to check if it is open.
  return isOpen() && javaChannel().socket().isBound();
}

随后触发 pipeline 的 active 操作，对于 head 来说在触发了链表的所有 active 操作后，若配置为 autoRead，则进行 channel的 read 操作

private void readIfIsAutoRead() {
    if (channel.config().isAutoRead()) {
        channel.read();
    }
}

随后若当前 channel 注册过，则触发beginRead 函数

此时将调用到AbstractNioChannel 的doBeginRead 操作，其将设置对 channel 添加一个 read 的interestOps。

@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);
    }
}

然而对于NioServerSocketChannel来说，readInterestOps 为OP_ACCEPT

public NioServerSocketChannel(ServerSocketChannel channel) {
    super(null, channel, SelectionKey.OP_ACCEPT);
    config = new NioServerSocketChannelConfig(this, javaChannel().socket());
}

绑定

当触发并行时用户线程回到 bind 函数中

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;
  }
}

确定 initAndRegister 没有发生异常
判断 register 是否完成

注册完成时

若当前 register 已经完成了，则直接进行 bind0

private static void doBind0(
  final ChannelFuture regFuture, final Channel channel,
  final SocketAddress localAddress, final ChannelPromise promise) {

  // This method is invoked before channelRegistered() is triggered.  Give user handlers a chance to set up
  // the pipeline in its channelRegistered() implementation.
  channel.eventLoop().execute(new Runnable() {
    @Override
    public void run() {
      if (regFuture.isSuccess()) {
        channel.bind(localAddress, promise).addListener(ChannelFutureListener.CLOSE_ON_FAILURE);
      } else {
        promise.setFailure(regFuture.cause());
      }
    }
  });
}

这个函数的目的是为了执行 bind，并且对 bind 事件添加失败关闭事件，要注意的是这里是给 channel 绑定的线程提交了一个任务，而非直接执行，bind 函数具体的操作暂且不提。

ChannelFutureListener CLOSE_ON_FAILURE = new ChannelFutureListener() {
  @Override
  public void operationComplete(ChannelFuture future) {
    if (!future.isSuccess()) {
      future.channel().close();
    }
  }
};

注册未完成时

给注册事件添加一个完成时事件

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);
    }
  }
});

最后仍然是执行 doBind0.

doBind0

bind 函数调用到AbstractChannel

@Override
public ChannelFuture bind(SocketAddress localAddress) {
    return pipeline.bind(localAddress);
}

DefaultChannelPipeline 的 bind

@Override
public final ChannelFuture bind(SocketAddress localAddress) {
    return tail.bind(localAddress);
}

TailContext 的 bind由AbstractChannelHandlerContext 实现

public ChannelFuture bind(final SocketAddress localAddress, final ChannelPromise promise) {
    if (localAddress == null) {
        throw new NullPointerException("localAddress");
    }
    if (isNotValidPromise(promise, false)) {
        // cancelled
        return promise;
    }

    final AbstractChannelHandlerContext next = findContextOutbound(MASK_BIND);
    EventExecutor executor = next.executor();
    if (executor.inEventLoop()) {
        next.invokeBind(localAddress, promise);
    } else {
        safeExecute(executor, new Runnable() {
            @Override
            public void run() {
                next.invokeBind(localAddress, promise);
            }
        }, promise, null);
    }
    return promise;
}

当前情况这里运行的线程为 eventLoop

private void invokeBind(SocketAddress localAddress, ChannelPromise promise) {
    if (invokeHandler()) {//期望值为已完成 handler add 事件
        try {
            ((ChannelOutboundHandler) handler()).bind(this, localAddress, promise);
        } catch (Throwable t) {
            notifyOutboundHandlerException(t, promise);
        }
    } else {//递归回bind函数
        bind(localAddress, promise);
    }
}

实际上由于 eventloop 是单线程的，因此执行到这里时，必定已经注册完成了，也就是

调用回 HeadContext

@Override
public void bind(
        ChannelHandlerContext ctx, SocketAddress localAddress, ChannelPromise promise) {
    unsafe.bind(localAddress, promise);
}

由 AbstractUnsafe 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();
    try {
        doBind(localAddress);
    } catch (Throwable t) {
        safeSetFailure(promise, t);
        closeIfClosed();
        return;
    }

    if (!wasActive && isActive()) {
        invokeLater(new Runnable() {
            @Override
            public void run() {
                pipeline.fireChannelActive();
            }
        });
    }

    safeSetSuccess(promise);
}

doBind函数由 NioServerChannel 实现，这里是由 eventLoop 执行的

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

随后若开始doBind前未 active，并且 bind 后active，则提交一个activefire任务到 eventloop 中。

至此服务端启动完成

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