netty ServerBootStrap解析

netty 抽象BootStrap定义：http://donald-draper.iteye.com/blog/2392492
引言：
前面一篇文章我们看了抽象Bootstrap的定义，先来回顾一下：
   抽象引导程序AbstractBootstrap，内部关联的一个事件循环组EventLoopGroup，一个通道处理器ChannelHandler，一个通道选项集和一个本地Socket地址及一个通道属性集。内部的方法主要配置事件循环组，通道处理，通道选项集，socket地址，及通道属性，通道注册，地址绑定。注册通道到事件循环组，首先由通道工厂创建通道实例，然后初始化通道，初始化工作待子类实现；然后将实际注册工作委托给事件循环组。绑定定socket地址，首先注册通道到事件循环组，待注册完成时，创建一个绑定任务线程完成地址绑定，实际将地址绑定工作委托给通道，并将绑定任务线程交由通道关联的事件循环的事件执行器执行。

今天我们来看服务端Bootstrap：

package io.netty.bootstrap;

import io.netty.channel.Channel;
import io.netty.channel.ChannelConfig;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelFutureListener;
import io.netty.channel.ChannelHandler;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.ServerChannel;
import io.netty.util.AttributeKey;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.concurrent.TimeUnit;

/**
 * {@link Bootstrap} sub-class which allows easy bootstrap of {@link ServerChannel}
 *
 */
public class ServerBootstrap extends AbstractBootstrap<ServerBootstrap, ServerChannel> {
 private static final InternalLogger logger = InternalLoggerFactory.getInstance(ServerBootstrap.class);
    //通道选项及属性集
    private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();
    private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap<AttributeKey<?>, Object>();
    private final ServerBootstrapConfig config = new ServerBootstrapConfig(this);//当前配置
    private volatile EventLoopGroup childGroup;//事件循环组
    private volatile ChannelHandler childHandler;//通道处理器
    public ServerBootstrap() { }
    private ServerBootstrap(ServerBootstrap bootstrap) {
        super(bootstrap);
        childGroup = bootstrap.childGroup;
        childHandler = bootstrap.childHandler;
        synchronized (bootstrap.childOptions) {
            childOptions.putAll(bootstrap.childOptions);
        }
        synchronized (bootstrap.childAttrs) {
            childAttrs.putAll(bootstrap.childAttrs);
        }
    }
}

从上面可看出，服务端Bootstrap虽然继承与抽象Bootstrap，但他有自己的通道选项及属性集，事件循环组和通道处理器。

/**
 * Specify the {@link EventLoopGroup} which is used for the parent (acceptor) and the child (client).
 父监听器和孩子客户端同用一个事件循环组
 */
@Override
public ServerBootstrap group(EventLoopGroup group) {
    return group(group, group);
}

/**
 * Set the {@link EventLoopGroup} for the parent (acceptor) and the child (client). These
 * {@link EventLoopGroup}'s are used to handle all the events and IO for {@link ServerChannel} and
 * {@link Channel}'s.
 设置父监听器和孩子客户端的事件循环，两者用于处理SeverChannel和Channel的所有事件循环
 */
public ServerBootstrap group(EventLoopGroup parentGroup, EventLoopGroup childGroup) {
    super.group(parentGroup);
    if (childGroup == null) {
        throw new NullPointerException("childGroup");
    }
    if (this.childGroup != null) {
        throw new IllegalStateException("childGroup set already");
    }
    this.childGroup = childGroup;
    return this;
}

大胆猜测一下，parentGroup事件循环组用于监听器ServerChannel接受连接,childGroup事件循环组用于当Server通道接收客户端的连接时，产生一个通道用于与客户端交互，childGroup事件循环组用于处理与客户端交互的通道相关事件和IO操作。

/**
 * Allow to specify a {@link ChannelOption} which is used for the {@link Channel} instances once they get created
 * (after the acceptor accepted the {@link Channel}). Use a value of {@code null} to remove a previous set
 * {@link ChannelOption}.
 用于Server通道接收客户端的连接时，产生的通道选项配置
 */
public <T> ServerBootstrap childOption(ChannelOption<T> childOption, T value) {
    if (childOption == null) {
        throw new NullPointerException("childOption");
    }
    if (value == null) {
        synchronized (childOptions) {
            childOptions.remove(childOption);
        }
    } else {
        synchronized (childOptions) {
            childOptions.put(childOption, value);
        }
    }
    return this;
}

从这个方法来看，上面的猜测是正确的。

/**
 * Set the specific {@link AttributeKey} with the given value on every child {@link Channel}. If the value is
 * {@code null} the {@link AttributeKey} is removed
 客户端交互通道属性配置
 */
public <T> ServerBootstrap childAttr(AttributeKey<T> childKey, T value) {
    if (childKey == null) {
        throw new NullPointerException("childKey");
    }
    if (value == null) {
        childAttrs.remove(childKey);
    } else {
        childAttrs.put(childKey, value);
    }
    return this;
}

/**
 * Set the {@link ChannelHandler} which is used to serve the request for the {@link Channel}'s.
配置与客户端交互通道的通道处理器
 */
public ServerBootstrap childHandler(ChannelHandler childHandler) {
    if (childHandler == null) {
        throw new NullPointerException("childHandler");
    }
    this.childHandler = childHandler;
    return this;
}
//验证通道配置是否有效
@Override
public ServerBootstrap validate() {
    super.validate();
    if (childHandler == null) {
        throw new IllegalStateException("childHandler not set");
    }
    if (childGroup == null) {
        logger.warn("childGroup is not set. Using parentGroup instead.");
        childGroup = config.group();
    }
    return this;
}
//获取当前Server引导配置
@Override
public final ServerBootstrapConfig config() {
    return config;
}

下面我们来看初始化通道，这个是重点：

@Override
void init(Channel channel) throws Exception {
    final Map<ChannelOption<?>, Object> options = options0();
    synchronized (options) {
        //设置父Server通道选项
        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();
            //设置父Server通道属性
            channel.attr(key).set(e.getValue());
        }
    }
   //获取Server通道的Channel管道
    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(childOptions.size()));
    }

    synchronized (childAttrs) {
        currentChildAttrs = childAttrs.entrySet().toArray(newAttrArray(childAttrs.size()));
    }

    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) {
	        //将通道处理器添加到通道内部的Channel管道内
                pipeline.addLast(handler);
            }
            
            ch.eventLoop().execute(new Runnable() {
                @Override
                public void run() {
		    //将Server引导配置监听器添加到通道内部的Channel管道内
                    pipeline.addLast(new ServerBootstrapAcceptor(
                            ch, currentChildGroup, currentChildHandler, currentChildOptions, currentChildAttrs));
                }
            });
        }
    });
}

我们来看引导配置监听器，实际为一个Inbound通道处理器

 private static class ServerBootstrapAcceptor extends ChannelInboundHandlerAdapter {
     private final EventLoopGroup childGroup;//与客户端交互通道注册的事件循环组
     private final ChannelHandler childHandler;//与客户端交互通道的通道处理器
     private final Entry<ChannelOption<?>, Object>[] childOptions;//与客户端交互通道的选项配置
     private final Entry<AttributeKey<?>, Object>[] childAttrs;//与客户端交互通道的属性
     private final Runnable enableAutoReadTask;

     ServerBootstrapAcceptor(
             final Channel channel, EventLoopGroup childGroup, ChannelHandler childHandler,
             Entry<ChannelOption<?>, Object>[] childOptions, Entry<AttributeKey<?>, 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.
         // 此任务用于开启通道自动读取配置，将会被所在的事件循环调度。
         // See https://github.com/netty/netty/issues/1328
         enableAutoReadTask = new Runnable() {
             @Override
             public void run() {
	         //开启通道自动读取配置
                 channel.config().setAutoRead(true);
             }
         };
     }

     @Override
     @SuppressWarnings("unchecked")
     public void channelRead(ChannelHandlerContext ctx, Object msg) {
         //与客户端交互通道
         final Channel child = (Channel) msg;
         //配置与客户端交互通道的通道处理器
         child.pipeline().addLast(childHandler);
         //配置与客户端交互通道的选项
         setChannelOptions(child, childOptions, logger);

         for (Entry<AttributeKey<?>, Object> e: childAttrs) {
	    //配置与客户端交互通道的属性
             child.attr((AttributeKey<Object>) e.getKey()).set(e.getValue());
         }

         try {
	    //注册与客户端交互通道到childGroup事件循环组
             childGroup.register(child).addListener(new ChannelFutureListener() {
                 @Override
                 public void operationComplete(ChannelFuture future) throws Exception {
                     if (!future.isSuccess()) {
		         //注册失败，则关闭通道
                         forceClose(child, future.cause());
                     }
                 }
             });
         } catch (Throwable t) {
             forceClose(child, t);
         }
     }
      //关闭通道
     private static void forceClose(Channel child, Throwable t) {
         child.unsafe().closeForcibly();
         logger.warn("Failed to register an accepted channel: {}", child, t);
     }

     @Override
     public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
         final ChannelConfig config = ctx.channel().config();
         if (config.isAutoRead()) {
             // stop accept new connections for 1 second to allow the channel to recover
	     //发生异常，则停止接受连接请求1秒钟，允许通道恢复
             // See https://github.com/netty/netty/issues/1328
             config.setAutoRead(false);
             ctx.channel().eventLoop().schedule(enableAutoReadTask, 1, TimeUnit.SECONDS);
         }
         // still let the exceptionCaught event flow through the pipeline to give the user
         // a chance to do something with it
	 //触发异常
         ctx.fireExceptionCaught(cause);
     }
 }

//ChannelConfig

/**
  * Sets if {@link ChannelHandlerContext#read()} will be invoked automatically so that a user application doesn't
  * need to call it at all. The default value is {@code true}.
  */
 ChannelConfig setAutoRead(boolean autoRead);

回到服务端引导配置:

 ServerBootstrap serverBoot = new ServerBootstrap(); 
            serverBoot.group(bossGroup, workerGroup)
             .channel(NioServerSocketChannel.class) 
	   ...
ChannelFuture f = serverBoot.bind(inetSocketAddress).sync();

结合前一篇定义抽象Bootstrap定义，我们来理一下ServerBootstrap绑定，完成的任务：
Server引导配置绑定socket地址，首先初始化通道，对于Server引导配置，这个通道为
NioServerSocketChannel，初始化通道，即初始化Server通道，从上面的初始化通道方法来看，初始化通道，首先将Server引导配置的父类抽象Bootstrap的选项和属性配置给Server通道，然后将ServerBootstrapAcceptor添加到Server通道内部的Channel管道内，然后将Server通道注册到parentGroup事件循环组中，然后通过Server通道#bind方法完成实际socket地址；
Server引导配置监听器实际为一个Inbound通道处理器，每当有客户端连接请求时，
则创建一个与客户端交互的通道，将child通道选项及属性配置给通道，并将通道注册到childGroup事件循环组，然后将通道处理器添加到与客户端交互的通道内部的Channel管道中。

再来看其他方法：

//创建属性和选项组
@SuppressWarnings("unchecked")
private static Entry<AttributeKey<?>, Object>[] newAttrArray(int size) {
    return new Entry[size];
}
@SuppressWarnings("unchecked")
private static Map.Entry<ChannelOption<?>, Object>[] newOptionArray(int size) {
    return new Map.Entry[size];
}

下面几个方法，没有什么好讲的，很简单：

@Override
@SuppressWarnings("CloneDoesntCallSuperClone")
public ServerBootstrap clone() {
    return new ServerBootstrap(this);
}

/**
 * Return the configured {@link EventLoopGroup} which will be used for the child channels or {@code null}
 * if non is configured yet.
 *
 * @deprecated Use {@link #config()} instead.
 */
@Deprecated
public EventLoopGroup childGroup() {
    return childGroup;
}

final ChannelHandler childHandler() {
    return childHandler;
}

final Map<ChannelOption<?>, Object> childOptions() {
    return copiedMap(childOptions);
}

final Map<AttributeKey<?>, Object> childAttrs() {
    return copiedMap(childAttrs);
}

总结：

      服务端Bootstrap虽然继承与抽象Bootstrap，但他有自己的child通道选项及属性集，事件循环组和通道处理器，这些是用于配置，当Server通道接收客户端的请求，创建与客户端交互的通道。当构造Server引导配置时，如果传递一个事件循环，则Server通道监听器和客户端交互的通道公用一个事件循环组，否则parentGroup事件循环组用于监听器ServerChannel接受连接,childGroup事件循环组用于处理与客户端交互的通道相关事件和IO操作。
      Server引导配置绑定socket地址，首先初始化通道，对于Server引导配置，这个通道为NioServerSocketChannel，初始化通道，即初始化Server通道；初始化Server通道，首先将Server引导配置的父类抽象Bootstrap的选项和属性配置给Server通道，然后添加ServerBootstrapAcceptor到Server通道内部的Channel管道内，然后将Server通道注册到事件循环组parentGroup中，然后通过Server通道#bind方法完成实际socket地址；Server引导配置监听器实际为一个Inbound通道处理器，每当有客户端连接请求时，则创建一个与客户端交互的通道，将child通道选项及属性配置给通道，并将通道注册到childGroup事件循环组，然后将通道处理器添加到与客户端交互的通道内部的Channel管道中。