上一节总结了channel代码的架构，了解了从鼎城channel接口的定义以及一层一层最后区分开客户端channel和服务端channel。从中也可以体会到抽象和集成的特点。
本节回顾一下在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。
@Override
void init(Channel channel) throws Exception {
//配置AbstractBootstrap.option
final Map, Object> options = options0();
synchronized (options) {
setChannelOptions(channel, options, logger);
}
//配置AbstractBootstrap.attr
final Map, Object> attrs = attrs0();
synchronized (attrs) {
for (Entry, Object> e: attrs.entrySet()) {
@SuppressWarnings("unchecked")
AttributeKey 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。
pipeline结构图
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
//传递到下一个Handle: ServerBootstrapAcceptor#channelRead()方法
//readBuf.get(i) 获取临时存储的一个channel
pipeline.fireChannelRead(readBuf.get(i));
接着查看ServerBootstrapAcceptor,它是ServerBootstrap的内部类，同时继承自ChannelInboundHandlerAdapter。也是一个ChannelInboundHandler。其channelRead主要做了以下几件事。
- 为客户端
channel的pipeline添加childHandler - 设置客户端TCP相关属性
childOptions和自定义属性childAttrs - workGroup选择NioEventLoop并注册Selector
private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
private final EventLoopGroup childGroup;
private final ChannelHandler childHandler;
private final Entry, Object>[] childOptions;
private final Entry, Object>[] childAttrs;
private final Runnable enableAutoReadTask;
ServerBootstrapAcceptor(
final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
Entry, Object>[] childOptions, Entry, Object>[] childAttrs) {
this.childGroup = childGroup;
this.childHandler = childHandler;
this.childOptions = childOptions;
this.childAttrs = childAttrs;
// Task which is scheduled to re-enable auto-read.
// It's important to create this Runnable before we try to submit it as otherwise the URLClassLoader may
// not be able to load the class because of the file limit it already reached.
//
// See https://github.com/netty/netty/issues/1328
enableAutoReadTask = new Runnable() {
@Override
public void run() {
channel.config().setAutoRead(true);
}
};
}
@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, Object> e: childAttrs) {
child.attr((AttributeKey - 为客户端
channel的pipeline添加childHandler
回顾引导的代码,在配置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。
public class Server {
private static final int PORT = 1111;
public static void main(String[] args) throws Exception {
NioEventLoopGroup bossGroup = new NioEventLoopGroup(1);
bossGroup.setIoRatio(100);
NioEventLoopGroup workerGroup = new NioEventLoopGroup();
ServerBootstrap serverBootstrap = new ServerBootstrap();
try {
serverBootstrap.group(bossGroup, workerGroup)
.channel(NioServerSocketChannel.class)
.option(ChannelOption.SO_REUSEADDR, true)
.childOption(ChannelOption.TCP_NODELAY, true)
.attr(AttributeKey.newInstance("attr"), "attr")
.childAttr(AttributeKey.newInstance("childAttr"), "childAttr")
.handler(new DataServerInitializer())
.childHandler(new DataServerInitializer());
ChannelFuture future = serverBootstrap.bind(PORT).sync();
future.channel().closeFuture().sync();
} finally {
workerGroup.shutdownGracefully();
bossGroup.shutdownGracefully();
}
}
}
class DataServerInitializer extends ChannelInitializer {
@Override
protected void initChannel(SocketChannel ch) throws Exception {
ch.pipeline()
.addLast( new DataServerHandler());
}
}
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。
@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;
}
- workGroup选择NioEventLoop并注册Selector
这个过程在之前学习的《注册Selector》过程一样，不同的地方是unsafe实例不一样。
@Override
public ChannelFuture register(final ChannelPromise promise) {
ObjectUtil.checkNotNull(promise, "promise");
//服务端unsafe是NioMessageUnsafe,客户端的是NioByteUnsafe
promise.channel().unsafe().register(this, promise);
return promise;
}