1. netty可以做的事情

• netty可以作為RPC的通訊框架
• netty作為長連線伺服器(websockt)
• netty也可以作為http伺服器(不是基於servlet規範)

2. netty的Hello World(Http伺服器)

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

public class TestServer {

    public static void main(String[] args) {
        // bossGroup 不斷的從客戶端接受連結，但不會做任何處理，直接轉給 workerGroup
        EventLoopGroup bossGroup = new NioEventLoopGroup();
        // workerGroup 從 bossGroup 接收到連結，然後獲取引數等進行處理
        EventLoopGroup workerGroup = new
 
 NioEventLoopGroup();

        ServerBootstrap serverBootstrap = new ServerBootstrap();
        serverBootstrap.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .childHandler(new TestServerInitializer());
        try {
            ChannelFuture channelFuture = serverBootstrap.bind(8899
 
).sync();
            channelFuture.channel().closeFuture().sync();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            bossGroup.shutdownGracefully();
            workerGroup.shutdownGracefully();
        }
    }
}

import io.netty.channel.ChannelInitializer;
import io.netty.channel.ChannelPipeline;
import io.netty.channel.socket.SocketChannel;
import io.netty.handler.codec.http.HttpServerCodec;

public class TestServerInitializer extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel ch) throws Exception {
        ChannelPipeline pipeline = ch.pipeline();
        pipeline.addLast("httpServerCodec",new HttpServerCodec());
        pipeline.addLast("testHttpServerHandler",new TestHttpServerHandler());
    }
}

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.handler.codec.http.DefaultFullHttpResponse;
import io.netty.handler.codec.http.FullHttpResponse;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpObject;
import io.netty.handler.codec.http.HttpRequest;
import io.netty.handler.codec.http.HttpResponseStatus;
import io.netty.handler.codec.http.HttpVersion;
import io.netty.util.CharsetUtil;

public class TestHttpServerHandler extends SimpleChannelInboundHandler<HttpObject> {

    /**
     * 讀取客戶端發起的請求，並向客戶端返回響應的方法
     * @param ctx
     * @param msg
     * @throws Exception
     */
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, HttpObject msg) {
        if (msg instanceof HttpRequest) {
            ByteBuf content = Unpooled.copiedBuffer("Hello Word", CharsetUtil.UTF_8);
            FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, content);
            response.headers().set(HttpHeaderNames.CONTENT_TYPE, "text/plain");
            response.headers().set(HttpHeaderNames.CONTENT_LENGTH, content.readableBytes());
            ctx.writeAndFlush(response);
        }
    }
}

3. netty使用的基本流程

• 啟動一個Bootstrap伺服器，Bootstrap裡面會關聯兩個事件迴圈組(bossGroup、workerGroup)
• bossGroup 用於獲取連結
• workerGroup 用於處理連結
• 伺服器啟動時關聯一個處理器(ChannelHandler)
• 在ChannelHandler中新增若干個自定義或者netty提供的處理器
• 在自定義的處理器(ChannelHandler)中返回響應
• 整個請求處理完畢

netty處理請求進入的類一般是xxxInbound(ChannelInboundHandler)
netty處理響應輸出的類一般是xxxOutbound(ChannelOutboundHandler)

4. TestHttpServerHandler分析

我們複寫一下父類中的一些方法，這些方法在事件發生時會被回撥

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.SimpleChannelInboundHandler;
import io.netty.handler.codec.http.DefaultFullHttpResponse;
import io.netty.handler.codec.http.FullHttpResponse;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpObject;
import io.netty.handler.codec.http.HttpRequest;
import io.netty.handler.codec.http.HttpResponseStatus;
import io.netty.handler.codec.http.HttpVersion;
import io.netty.util.CharsetUtil;

public class TestHttpServerHandler extends SimpleChannelInboundHandler<HttpObject> {

    /**
     * 讀取客戶端發起的請求，並向客戶端返回響應的方法
     * @param ctx
     * @param msg
     * @throws Exception
     */
    @Override
    protected void channelRead0(ChannelHandlerContext ctx, HttpObject msg) {
        if (msg instanceof HttpRequest) {
            ByteBuf content = Unpooled.copiedBuffer("Hello Word", CharsetUtil.UTF_8);
            FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK, content);
            response.headers().set(HttpHeaderNames.CONTENT_TYPE, "text/plain");
            response.headers().set(HttpHeaderNames.CONTENT_LENGTH, content.readableBytes());
            ctx.writeAndFlush(response);
        }
    }

    // 3. 管道活躍
    @Override
    public void channelActive(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelActive");
        super.channelActive(ctx);
    }

    // 2. 管道註冊
    @Override
    public void channelRegistered(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelRegistered");
        super.channelRegistered(ctx);
    }

    // 1. 新的管道新增
    @Override
    public void handlerAdded(ChannelHandlerContext ctx) throws Exception {
        System.out.println("handlerAdded");
        super.handlerAdded(ctx);
    }

    // 4. 管道不活躍
    @Override
    public void channelInactive(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelInactive");
        super.channelInactive(ctx);
    }

    // 5. 管道取消註冊
    @Override
    public void channelUnregistered(ChannelHandlerContext ctx) throws Exception {
        System.out.println("channelUnregistered");
        super.channelUnregistered(ctx);
    }
}

我們使用curl訪問，發現控制檯列印資訊如下：

curl "http://localhost:8899"
handlerAdded
channelRegistered
channelActive
channelInactive
channelUnregistered

• handlerAdded在父級介面(ChannelHandler)中的doc如下：

/**
     * Gets called after the {@link ChannelHandler} was added to the actual context and it's ready to handle events.
     */
    void handlerAdded(ChannelHandlerContext ctx) throws Exception;

意思是handlerAdded將在ChannelHandler被新增到真實的上下文後被呼叫，這時表示上下文已經準備好處理事件了。

• channelRegistered在父級介面(ChannelInboundHandler)中的doc如下：

The {@link Channel} of the {@link ChannelHandlerContext} was registered with its {@link EventLoop}

意思是ChannelHandlerContext(上下文)的Channel(管道)註冊了它的EventLoop(事件迴圈)

• channelActive在父級介面(ChannelInboundHandler)中的doc如下：

The {@link Channel} of the {@link ChannelHandlerContext} is now active

意思是ChannelHandlerContext(上下文)的Channel(通道)現在處於活躍狀態

• channelInactive在父級介面(ChannelInboundHandler)中的doc如下：

The {@link Channel} of the {@link ChannelHandlerContext} was registered is now inactive and reached its end of lifetime.

意思是ChannelHandlerContext(上下文)註冊的Channel(通道)當前處於不活躍狀態，並且到達了它生命週期的末尾

• channelUnregistered在父級介面(ChannelInboundHandler)中的doc如下：

The {@link Channel} of the {@link ChannelHandlerContext} was unregistered from its {@link EventLoop}

意思是ChannelHandlerContext(上下文)的Channel(通道)從它的EventLoop(事件迴圈)中取消了註冊

我們使用curl請求這五個方法都會被呼叫，但是當我們使用瀏覽器去請求時channelInactive和channelUnregistered可能不會被呼叫，但當我們關閉瀏覽器時這兩個方法就會被呼叫。這是應該目前的瀏覽器時基於HTTP1.1協議，請求完畢時並不會馬上關閉連線，當瀏覽器關閉時連線斷開，服務端就會呼叫以上兩個方法來完成通道的關閉。