基于Netty构建HTTP访问分为两类，使用浏览器访问的和使用Netty客户端访问。在介绍之前，先简单说一下如何使用Netty实现Http服务的。
Netty的Http服务的流程是：
1、Client向Server发送http请求。
2、Server端对http请求进行解析。
3、Server端向client发送http响应。
4、Client对http响应进行解析。
在网上看到一个很好的流程图，很能说明问题：

其中红色框中的4个类是Netty提供的，它们其实也是一种Handler，其中Encoder继承自ChannelOutboundHandler，Decoder继承自ChannelInboundHandler，它们的作用是：
1、HttpRequestEncoder：对httpRequest进行编码。
2、HttpRequestDecoder：把流数据解析为httpRequest。
3、HttpResponsetEncoder：对httpResponset进行编码。
4、HttpResponseEncoder：把流数据解析为httpResponse。

浏览器访问

HttpServer 类
这个类在浏览器访问和Netty客户端访问时一样的。

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;
import io.netty.handler.codec.http.HttpRequestDecoder;
import io.netty.handler.codec.http.HttpResponseEncoder;

public class HttpServer {

    public void start(int port) throws Exception {
        EventLoopGroup bossGroup = new NioEventLoopGroup(); 
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap(); 
            b.group(bossGroup, workerGroup)
                    .channel(NioServerSocketChannel.class) 
                    .childHandler(new ChannelInitializer<SocketChannel>() { 
                        @Override
                        public void initChannel(SocketChannel ch)
                                throws Exception {
                            // server端发送的是httpResponse，所以要使用HttpResponseEncoder进行编码
                            ch.pipeline().addLast(
                                    new HttpResponseEncoder());
                            // server端接收到的是httpRequest，所以要使用HttpRequestDecoder进行解码
                            ch.pipeline().addLast(
                                    new HttpRequestDecoder());
                            ch.pipeline().addLast(
                                    new HttpServerHandler());
                        }
                    }).option(ChannelOption.SO_BACKLOG, 128) 
                    .childOption(ChannelOption.SO_KEEPALIVE, true); 
            ChannelFuture f = b.bind(port).sync(); 

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

    public static void main(String[] args) throws Exception {
        HttpServer server = new HttpServer();
        server.start(8000);
    }
}

HttpServerHandler 类
直接创建一个response对象，返回值给浏览器。

public class HttpServerHandler extends ChannelInboundHandlerAdapter {
    private ByteBufToBytes reader;
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        FullHttpResponse response = new DefaultFullHttpResponse(
                HTTP_1_1, OK, Unpooled.wrappedBuffer("OK OK OK OK"
                        .getBytes()));
        response.headers().set(CONTENT_TYPE, "text/plain");
        response.headers().set(CONTENT_LENGTH,
                response.content().readableBytes());
        response.headers().set(CONNECTION, HttpHeaderValues.KEEP_ALIVE);
        ctx.write(response);
        ctx.flush();
    }
    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }
    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause)
            throws Exception {
        ctx.close();
    }
}

使用浏览器访问，即可获取OK OK OK OK显示在页面上。

Netty客户端访问

服务端的HttpServer 与浏览器访问的一样，HttpServerHandler 写法如下：

import static io.netty.handler.codec.http.HttpHeaderNames.CONNECTION;
import static io.netty.handler.codec.http.HttpHeaderNames.CONTENT_LENGTH;
import static io.netty.handler.codec.http.HttpHeaderNames.CONTENT_TYPE;
import static io.netty.handler.codec.http.HttpResponseStatus.OK;
import static io.netty.handler.codec.http.HttpVersion.HTTP_1_1;
import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.handler.codec.http.DefaultFullHttpResponse;
import io.netty.handler.codec.http.FullHttpResponse;
import io.netty.handler.codec.http.HttpContent;
import io.netty.handler.codec.http.HttpHeaderValues;
import io.netty.handler.codec.http.HttpRequest;
import io.netty.handler.codec.http.HttpUtil;
public class HttpServerHandler extends ChannelInboundHandlerAdapter {
    private ByteBufToBytes reader;
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        if (msg instanceof HttpRequest) {
            HttpRequest request = (HttpRequest) msg;
            System.out.println("messageType:"+ request.headers().get("messageType"));
            System.out.println("businessType:"+ request.headers().get("businessType"));
            if (HttpUtil.isContentLengthSet(request)) {
                reader = new ByteBufToBytes(
                        (int) HttpUtil.getContentLength(request));
            }
        }
        if (msg instanceof HttpContent) {
            HttpContent httpContent = (HttpContent) msg;
            ByteBuf content = httpContent.content();
            reader.reading(content);
            content.release();
            if (reader.isEnd()) {
                String resultStr = new String(reader.readFull());
                System.out.println("Client said:" + resultStr);
                FullHttpResponse response = new DefaultFullHttpResponse(
                        HTTP_1_1, OK, Unpooled.wrappedBuffer("I am ok"
                                .getBytes()));
                response.headers().set(CONTENT_TYPE, "text/plain");
                response.headers().set(CONTENT_LENGTH,
                        response.content().readableBytes());
                response.headers().set(CONNECTION, HttpHeaderValues.KEEP_ALIVE);
                ctx.write(response);
                ctx.flush();
            }
        }
    }
    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        ctx.flush();
    }
}

还有一个ByteBufToBytes，可以读取NIO的工具类，可以一次性把ByteBuf的数据读取出来，也可以把多次ByteBuf中的数据统一读取出来。

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;
public class ByteBufToBytes {
    private ByteBuf temp;
    private boolean end = true;
    public ByteBufToBytes(int length) {
        temp = Unpooled.buffer(length);
    }
    public void reading(ByteBuf datas) {
        datas.readBytes(temp, datas.readableBytes());
        if (this.temp.writableBytes() != 0) {
            end = false;
        } else {
            end = true;
        }
    }
    public boolean isEnd() {
        return end;
    }
    public byte[] readFull() {
        if (end) {
            byte[] contentByte = new byte[this.temp.readableBytes()];
            this.temp.readBytes(contentByte);
            this.temp.release();
            return contentByte;
        } else {
            return null;
        }
    }
    public byte[] read(ByteBuf datas) {
        byte[] bytes = new byte[datas.readableBytes()];
        datas.readBytes(bytes);
        return bytes;
    }
}

Netty客户端包括三个类HttpClient，HttpClientHandler和ByteBufToBytes。
HttpClient类

import java.net.URI;
import io.netty.bootstrap.Bootstrap;
import io.netty.buffer.Unpooled;
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.NioSocketChannel;
import io.netty.handler.codec.http.DefaultFullHttpRequest;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpMethod;
import io.netty.handler.codec.http.HttpRequestEncoder;
import io.netty.handler.codec.http.HttpResponseDecoder;
import io.netty.handler.codec.http.HttpVersion;
public class HttpClient {
    public void connect(String host, int port) throws Exception {
        EventLoopGroup workerGroup = new NioEventLoopGroup();
        try {
            Bootstrap b = new Bootstrap();
            b.group(workerGroup);
            b.channel(NioSocketChannel.class);
            b.option(ChannelOption.SO_KEEPALIVE, true);
            b.handler(new ChannelInitializer<SocketChannel>() {
                @Override
                public void initChannel(SocketChannel ch) throws Exception {
                    // 客户端接收到的是httpResponse响应，所以要使用HttpResponseDecoder进行解码
                    ch.pipeline().addLast(new HttpResponseDecoder());
                    // 客户端发送的是httprequest，所以要使用HttpRequestEncoder进行编码
                    ch.pipeline().addLast(new HttpRequestEncoder());
                    ch.pipeline().addLast(new HttpClientHandler());
                }
            });
            ChannelFuture f = b.connect(host, port).sync();
            URI uri = new URI("http://127.0.0.1:8000");
            String msg = "Are you ok?";
            DefaultFullHttpRequest request = new DefaultFullHttpRequest(
                    HttpVersion.HTTP_1_1, HttpMethod.POST, uri.toASCIIString(),
                    Unpooled.wrappedBuffer(msg.getBytes()));
            // 构建http请求
            request.headers().set(HttpHeaderNames.HOST, host);
            request.headers().set(HttpHeaderNames.CONNECTION,
                    HttpHeaderNames.CONNECTION);
            request.headers().set(HttpHeaderNames.CONTENT_LENGTH,
                    request.content().readableBytes());
            request.headers().set("messageType", "normal");
            request.headers().set("businessType", "testServerState");
            // 发送http请求
            f.channel().write(request);
            f.channel().flush();
            f.channel().closeFuture().sync();
        } finally {
            workerGroup.shutdownGracefully();
        }
    }
    public static void main(String[] args) throws Exception {
        HttpClient client = new HttpClient();
        client.connect("127.0.0.1", 8000);
    }
}

HttpClientHandler类

import io.netty.buffer.ByteBuf;
import io.netty.channel.ChannelHandlerContext;
import io.netty.channel.ChannelInboundHandlerAdapter;
import io.netty.handler.codec.http.HttpContent;
import io.netty.handler.codec.http.HttpHeaderNames;
import io.netty.handler.codec.http.HttpResponse;
import io.netty.handler.codec.http.HttpUtil;
public class HttpClientHandler extends ChannelInboundHandlerAdapter {   
    private ByteBufToBytes reader;
    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg)
            throws Exception {
        if (msg instanceof HttpResponse) {
            HttpResponse response = (HttpResponse) msg;
            System.out.println("CONTENT_TYPE:"
                    + response.headers().get(HttpHeaderNames.CONTENT_TYPE));
            if (HttpUtil.isContentLengthSet(response)) {
                reader = new ByteBufToBytes(
                        (int) HttpUtil.getContentLength(response));
            }
        }
        if (msg instanceof HttpContent) {
            HttpContent httpContent = (HttpContent) msg;
            ByteBuf content = httpContent.content();
            reader.reading(content);
            content.release();
            if (reader.isEnd()) {
                String resultStr = new String(reader.readFull());
                System.out.println("Server said:" + resultStr);
                ctx.close();
            }
        }
    }
}

最后说几句：

本例中使用Netty4.1.6Final，改写上网上大多例子中的弃用方法。