Netty源码分析（二）----- ServerBootstrap

2019-09-08 09:46:09来源：博客园阅读 ()

Netty源码分析（二）----- ServerBootstrap

BootStrap在netty的应用程序中负责引导服务器和客户端。netty包含了两种不同类型的引导：
1. 使用服务器的ServerBootStrap，用于接受客户端的连接以及为已接受的连接创建子通道。
2. 用于客户端的BootStrap，不接受新的连接，并且是在父通道类完成一些操作。

一般服务端的代码如下所示：

SimpleServer.java

/**
 * Created by chenhao on 2019/9/4.
 */
public final class SimpleServer {

    public static void main(String[] args) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup();

        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .handler(new SimpleServerHandler())
                    .childHandler(new SimpleServerInitializer())
                    .option(ChannelOption.SO_BACKLOG, 128)
                    .childOption(ChannelOption.SO_KEEPALIVE, true);

            ChannelFuture f = b.bind(8888).sync();

            f.channel().closeFuture().sync();
        } finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

SimpleServerHandler.java

private static class SimpleServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelActive");
    }

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelRegistered");
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        System.out.println("handlerAdded");
    }
}

SimpleServerInitializer.java

public class SimpleServerInitializer extends ChannelInitializer<SocketChannel>{

    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();

        pipeline.addLast("framer", new DelimiterBasedFrameDecoder(8192, Delimiters.lineDelimiter()));
        pipeline.addLast("decoder", new StringDecoder());
        pipeline.addLast("encoder", new StringEncoder());
        pipeline.addLast("handler", new SimpleChatServerHandler());

        System.out.println("SimpleChatClient:" + ch.remoteAddress()+"连接上");
    }
}

在上篇博文（Netty源码分析（一）----- NioEventLoopGroup）中剖析了如下的两行代码内部的构造函数中干了些什么。

EventLoopGroup bossGroup = new NioEventLoopGroup(1);
EventLoopGroup workerGroup = new NioEventLoopGroup();

具体可以见上篇博文，对于如上的两行代码得到的结论是：

１、如果不指定线程数，则线程数为：CPU的核数*２

２、根据线程个数是否为2的幂次方，采用不同策略初始化chooser

３、产生nThreads个NioEventLoop对象保存在children数组中。

可以理解NioEventLoop就是一个线程，线程NioEventLoop中里面有如下几个属性：

1、NioEventLoopGroup　（在父类SingleThreadEventExecutor中）

2、selector

3、provider

4、thread （在父类SingleThreadEventExecutor中）

更通俗点就是： NioEventLoopGroup就是一个线程池，NioEventLoop就是一个线程。NioEventLoopGroup线程池中有N个NioEventLoop线程。

ServerBootstrap类分析

本篇博文将分析如下几行代码里面做了些什么。

ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .handler(new SimpleServerHandler())
                .childHandler(new SimpleServerInitializer())
                .option(ChannelOption.SO_BACKLOG, 128)
                .childOption(ChannelOption.SO_KEEPALIVE, true);

ServerBootstrap类的继承结构如下：

该类的参数，有必要列出：

private final Map<ChannelOption<?>, Object> childOptions = new LinkedHashMap<ChannelOption<?>, Object>();
private final Map<AttributeKey<?>, Object> childAttrs = new LinkedHashMap<AttributeKey<?>, Object>();
private volatile EventLoopGroup childGroup;
private volatile ChannelHandler childHandler;

其父类AbstractBootstrap的参数

private volatile EventLoopGroup group;
private volatile ChannelFactory<? extends C> channelFactory;
private volatile SocketAddress localAddress;
private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();
private final Map<AttributeKey<?>, Object> attrs = new LinkedHashMap<AttributeKey<?>, Object>();
private volatile ChannelHandler handler;

下面主要看下这个链式设置相关的参数。

group(bossGroup, workerGroup)

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;
}

即将workerGroup保存在 ServerBootstrap对象的childGroup属性上。 bossGroup保存在ServerBootstrap对象的group属性上

channel(NioServerSocketChannel.class)

public B channel(Class<? extends C> channelClass) {
    if (channelClass == null) {
        throw new NullPointerException("channelClass");
    }
    return channelFactory(new BootstrapChannelFactory<C>(channelClass));
} 
public B channelFactory(ChannelFactory<? extends C> channelFactory) {
    if (channelFactory == null) {
        throw new NullPointerException("channelFactory");
    }
    if (this.channelFactory != null) {
        throw new IllegalStateException("channelFactory set already");
    }

    this.channelFactory = channelFactory;
    return (B) this;
}

函数功能：设置父类属性channelFactory 为： BootstrapChannelFactory类的对象。其中这里BootstrapChannelFactory对象中包括一个clazz属性为：NioServerSocketChannel.class，从如下该类的构造函数中可以明显的得到这一点。

private static final class BootstrapChannelFactory<T extends Channel> implements ChannelFactory<T> {
    private final Class<? extends T> clazz;

    BootstrapChannelFactory(Class<? extends T> clazz) {
        this.clazz = clazz;
    }

    @Override
    public T newChannel() {
        try {
            return clazz.newInstance();
        } catch (Throwable t) {
            throw new ChannelException("Unable to create Channel from class " + clazz, t);
        }
    }

    @Override
    public String toString() {
        return StringUtil.simpleClassName(clazz) + ".class";
    }
}

并且BootstrapChannelFactory中提供 newChannel()方法，我们可以看到 clazz.newInstance()，主要是通过反射来实例化NioServerSocketChannel.class

handler(new SimpleServerHandler())

public B handler(ChannelHandler handler) {
    if (handler == null) {
        throw new NullPointerException("handler");
    }
    this.handler = handler;
    return (B) this;
}

注意：这里的handler函数的入参类是我们自己提供的。如下，后面的博文中将会分析这个handler将会在哪里以及何时被调用，这里只需要记住这一点即可

private static class SimpleServerHandler extends ChannelInboundHandlerAdapter {
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelActive");
    }

    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelRegistered");
    }

    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        System.out.println("handlerAdded");
    }
}

childHandler(new SimpleServerInitializer())

public ServerBootstrap childHandler(ChannelHandler childHandler) {
    if (childHandler == null) {
        throw new NullPointerException("childHandler");
    }
    this.childHandler = childHandler;
    return this;
}

由最后一句可知，其实就是讲传入的childHandler赋值给ServerBootstrap的childHandler属性。

该函数的主要作用是设置channelHandler来处理客户端的请求的channel的IO。这里我们一般都用ChannelInitializer这个类的实例或则继承自这个类的实例
这里我是通过新建类SimpleChatServerInitializer继承自ChannelInitializer。具体的代码如下：

public class SimpleChatServerInitializer extends ChannelInitializer<SocketChannel>{

    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();

        pipeline.addLast("framer", new DelimiterBasedFrameDecoder(8192, Delimiters.lineDelimiter()));
        pipeline.addLast("decoder", new StringDecoder());
        pipeline.addLast("encoder", new StringEncoder());
        pipeline.addLast("handler", new SimpleChatServerHandler());

        System.out.println("SimpleChatClient:" + ch.remoteAddress()+"连接上");
    }
}

我们再看看ChannelInitializer这个类的继承图可知ChannelInitializer其实就是继承自ChannelHandler的

可知，这个类其实就是往pipeline中添加了很多的channelHandler。

配置ServerBootstrap的option

这里调用的是父类的AbstractBootstrap的option()方法，源码如下：

public <T> B option(ChannelOption<T> option, T value) {
    if (option == null) {
        throw new NullPointerException("option");
    }
    if (value == null) {
        synchronized (options) {
            options.remove(option);
        }
    } else {
        synchronized (options) {
            options.put(option, value);
        }
    }
    return (B) this;
}

其中最重要的一行代码就是：
options.put(option, value);
这里用到了options这个参数，在AbstractBootstrap的定义如下：
private final Map<ChannelOption<?>, Object> options = new LinkedHashMap<ChannelOption<?>, Object>();
可知是私有变量，而且是一个Map集合。这个变量主要是设置TCP连接中的一些可选项,而且这些属性是作用于每一个连接到服务器被创建的channel。

配置ServerBootstrap的childOption

这里调用的是父类的ServerBootstrap的childOption()方法，源码如下：

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;
}