Netty5服务端源码解析

Netty5源码解析

今天让我来总结下netty5的服务端代码。

服务端（ServerBootstrap）
示例代码如下：

import io.netty.bootstrap.ServerBootstrap;
import io.netty.channel.ChannelFuture;
import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelOption;
import io.netty.channel.EventLoopGroup;
import io.netty.channel.nio.NioEventLoopGroup;
import io.netty.channel.socket.SocketChannel;
import io.netty.channel.socket.nio.NioServerSocketChannel;

/**
 * Created by yaojiafeng on 16/1/17.
 */
public class SimpleServer {

    public void bind(int port) throws Exception {
        // 配置服务端的NIO线程组
        EventLoopGroup bossGroup = new NioEventLoopGroup(1);
        EventLoopGroup workerGroup = new NioEventLoopGroup(1);
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class)
                    .option(ChannelOption.SO_BACKLOG, 1024)
                    .childHandler(new ChildChannelHandler());
            // 绑定端口，同步等待成功
            ChannelFuture f = b.bind(port).sync();

            // 等待服务端监听端口关闭
            f.channel().closeFuture().sync();
        } finally {
            // 优雅退出，释放线程池资源
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }

    private class ChildChannelHandler extends ChannelInitializer<SocketChannel> {
        @Override
        protected void initChannel(SocketChannel arg0) throws Exception {
            arg0.pipeline().addLast(new SimpleServerHandler());
        }
    }

    /**
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception {
        int port = 8081;
        if (args != null && args.length > 0) {
            try {
                port = Integer.valueOf(args[0]);
            } catch (NumberFormatException e) {
                // 采用默认值
            }
        }
        new SimpleServer().bind(port);
    }
}

1.1. 设置EventLoopGroup
首先创建2个EventLoopGroup，一个parentGroup（用于接受新连接），childGroup（用于执行读写事件），NioEventLoopGroup内部根据设置的nEventLoops参数创建对应大小的NioEventLoop数组，并且每个NioEventLoop默认使用ForkJoinPool的一个线程，所以NioEventLoop称为单线程事件循环。

1.2. 构造ServerBootstrap
构造ServerBootstrap对象，并设置EventLoopGroup，channel（NioServerSocketChannel服务端套接字），一些option例如ChannelOption.SO_BACKLOG，childHandler（客户端连接后在管道链设置的ChannelHandler）

1.3. 同步绑定端口

b.bind(port).sync()

1.3.1 validate方法
validate方法验证parentGroup和channelFactory不能为null

1.3.2 initAndRegister方法
刚方法内部使用channelFactory通过反射构造NioServerSocketChannel的实例对象，NioServerSocketChannel实例对象构造内部主要包含ServerSocketChannel，DefaultChannelId（标识唯一性），Unsafe（所有IO操作都在这个类里）,DefaultChannelPipeline(通道处理器管道链,自定义的ChannelHandler都在这里),NioServerSocketChannelConfig(一些配置信息)。
构造完调用init初始化NioServerSocketChannel，包括设置自定义的ChannelHandler，ServerBootstrapAcceptor（专门用于接受客户端新连接时，初始化NioSocketChannel并注册进childGroup进行读写监听）。

ChannelFuture regFuture = group().register(channel)

异步注册NioServerSocketChannel到parentGroup里的NioEventLoop。
因为注册过程是在NioEventLoop异步执行的，这里直接先分析register方法

1.3.3 异步register

channel.unsafe().register(this, promise)

注册的时候会调用以上方法，委派给Unsafe的register方法，内部会给NioServerSocketChannel的字段eventLoop初始化（NioServerSocketChannel关联唯一的一个NioEventLoop），然后会调用

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

这个会开启NioEventLoop的事件循环线程，并放task到taskQueue里，作为异步执行register0方法。

register0方法会调用外部类（NioServerSocketChannel）的doRegister方法,

protected void doRegister() throws Exception {
        boolean selected = false;
        for (;;) {
            try {
                selectionKey = javaChannel().register(((NioEventLoop) eventLoop().unwrap()).selector, 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.
                    ((NioEventLoop) eventLoop().unwrap()).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;
                }
            }
        }
    }

这里使用了NIO的API，把NioServerSocketChannel里的ServerSocketChannel注册到NioServerSocketChannel关联的NioEventLoop里的selector。
接下来的safeSetSuccess会把Main线程设置的监听器，设置bind任务。

1.3.4 执行bind操作

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());
                }
            }
        });
    }

注册成功的情况下，执行bind操作（NioServerSocketChannel的bind方法），一路追踪到

unsafe.bind(localAddress, promise);

unsafe的bind方法，内部调用NioServerSocketChannel的doBind方法

protected void doBind(SocketAddress localAddress) throws Exception {
        javaChannel().socket().bind(localAddress, config.getBacklog());
    }

并且设置pipeline.fireChannelActive()任务，fireChannelActive任务会调用channel.read()方法，内部会调用到unsafe.beginRead()方法，最终调用的是NioServerSocketChannel的doBeginRead方法，重新设置SelectionKey的感兴趣的事件readInterestOp（NioServerSocketChannel构造的时候确定的为SelectionKey.OP_ACCEPT），开始接收新连接。

protected void doBeginRead() throws Exception {
        // Channel.read() or ChannelHandlerContext.read() was called
        if (inputShutdown) {
            return;
        }

        final SelectionKey selectionKey = this.selectionKey;
        if (!selectionKey.isValid()) {
            return;
        }

        readPending = true;

        final int interestOps = selectionKey.interestOps();
        if ((interestOps & readInterestOp) == 0) {
            selectionKey.interestOps(interestOps | readInterestOp);
        }
    }

1.4. NioEventLoop事件循环接受新连接
NioEventLoop不停的通过ForkJoinPool执行它的asRunnable任务（通过每次执行任务将要完成时，重新把asRunnable设置到ForkJoinPool里）。
从asRunnable的run方法开始，内部先执行selector的select操作，然后先调用processSelectedKeys()方法，获取到激活的selectedKeys数组，这里如果有新连接进来，那么就有一个SelectionKey，获取它的attachment（NioServerSocketChannel）,然后调用processSelectedKey方法。

private static void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
        final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
        if (!k.isValid()) {
            // close the channel if the key is not valid anymore
            unsafe.close(unsafe.voidPromise());
            return;
        }

        try {
            int readyOps = k.readyOps();
            // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
            // to a spin loop
            if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
                if (!ch.isOpen()) {
                    // Connection already closed - no need to handle write.
                    return;
                }
            }
            if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                // Call forceFlush which will also take care of clear the OP_WRITE once there is nothing left to write
                ch.unsafe().forceFlush();
            }
            if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                // remove OP_CONNECT as otherwise Selector.select(..) will always return without blocking
                // See https://github.com/netty/netty/issues/924
                int ops = k.interestOps();
                ops &= ~SelectionKey.OP_CONNECT;
                k.interestOps(ops);

                unsafe.finishConnect();
            }
        } catch (CancelledKeyException ignored) {
            unsafe.close(unsafe.voidPromise());
        }
    }

1.4.1 执行获取新连接方法

if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                unsafe.read();
                if (!ch.isOpen()) {
                    // Connection already closed - no need to handle write.
                    return;
                }
            }

当readyOps等于SelectionKey.OP_ACCEPT调用unsafe.read()，这里调用到了AbstractNioMessageChannel的内部类NioMessageUnsafe的read方法。
read方法会循环接受新连接，一次默认能接受16个连接，具体调用doReadMessages方法。

protected int doReadMessages(List<Object> buf) throws Exception {
        SocketChannel ch = javaChannel().accept();

        try {
            if (ch != null) {
                buf.add(new NioSocketChannel(this, ch));
                return 1;
            }
        } catch (Throwable t) {
            logger.warn("Failed to create a new channel from an accepted socket.", t);

            try {
                ch.close();
            } catch (Throwable t2) {
                logger.warn("Failed to close a socket.", t2);
            }
        }

        return 0;
    }

这里调用NIO的API，accept方法获取SocketChannel，并封装成NioSocketChannel（NioSocketChannel的构造字段和NioServerSocketChannel类似，只是NioSocketChannel默认的感兴趣事件为SelectionKey.OP_READ）。
接受连接完成，循环调用pipeline.fireChannelRead()方法。

1.4.2 ServerBootstrapAcceptor的channelRead方法
上面的管道调用fireChannelRead方法，通过责任链方式依次调用ChannelHandler的channelRead方法，最重要的就是ServerBootstrapAcceptor的channelRead方法。
它这个方法设置了childHandler到NioSocketChannel（新连接）的管道链里，然后又是异步注册NioSocketChannel到childGroup里的NioEventLoop里，注册过程和前面1.3章节的大体一致，也是启动了childGroup里的NioEventLoop的事件循环异步注册。只是因为是NioSocketChannel一些实现的方法不一样，执行的代码有点差别，最终注册完成也会调用pipeline的fireChannelActive()方法。

1.4.3 fireChannelActive方法

public ChannelPipeline fireChannelActive() {
        head.fireChannelActive();

        if (channel.config().isAutoRead()) {
            channel.read();
        }

        return this;
    }

channel的read方法最终委派调用到unsafe.beginRead()方法，然后又是NioSocketChannel的doBeginRead方法，重新设置SelectionKey的感兴趣事件为SelectionKey.OP_READ（NioSocketChannel的默认值）。到这里连接已经建立，并且开启了客户端连接读事件的监听。

PS：上面的SimpleServerHandler代码如下：

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerAdapter;
import io.netty.channel.ChannelHandlerContext;

/**
 * Created by yaojiafeng on 16/1/17.
 */
public class SimpleServerHandler extends ChannelHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        ByteBuf body = (ByteBuf) msg;
        byte[] bytes = new byte[body.readableBytes()];
        body.readBytes(bytes);
        System.out.println(new String(bytes));

        ByteBuf resp = Unpooled.copiedBuffer(bytes);
        ctx.writeAndFlush(resp);
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) {
        ctx.close();
    }
}