Netty构建Http服务器

之前已经提到了Netty是一个高性能网络通信框架。而Http(超文本传输协议)是目前互联网应用最广泛的一种网络协议了。了Netty对Http提供了非常丰富的支持，让我们可以针对自己的需求实现需要的Http服务器。

Http 协议

Http是建立在TCP之上的应用层协议。它分为请求(Request)和响应(Response)两个部分，其中Request由以下部分组成:

Method Request-URI HTTP-Version CRLF 
Header CRLF 
Header CRLF
Header CRLF
……(n个Header) 
CRLF 
[ message-body ] 

Response由以下部分组成:

HTTP-Version Status-Code Reason-Phrase CRLF     
Header CRLF 
Header CRLF
Header CRLF
……(n个Header) 
CRLF
[ message-body ] 

其中CRLF(Carriage-Return Line-Feed)表示回车换行

编写Netty版本的Http服务器

Http自定义的逻辑控制器HttpHandler.java：

public class HttpHandler extends SimpleChannelInboundHandler<Object> {

    private static final byte[] CONTENT = {'H', 'e', 'l', 'l', 'o', ' ', 'W', 'o', 'r', 'l', 'd'};
    private boolean keepAlive;
    
    public HttpHandler() {
        super();
        System.out.printf("控制器 %s 被创建.\n", this.toString());
    }

    @Override
    public void handlerRemoved(ChannelHandlerContext ctx) throws Exception {
        super.handlerRemoved(ctx);
        System.out.printf("控制器 %s 销毁.\n", this.toString());
    }

    @Override
    public void channelReadComplete(ChannelHandlerContext ctx) throws Exception {
        super.channelReadComplete(ctx);
        System.out.printf("控制器 %s 读取一个包.\n", this.toString());
    }

    @Override
    public void exceptionCaught(ChannelHandlerContext ctx, Throwable cause) throws Exception {
        System.out.printf("控制器 %s 出现异常.\n", this.toString());
        ctx.close();
    }

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (msg instanceof HttpRequest) {

            HttpRequest request = (HttpRequest) msg;

            if (request.getMethod() != HttpMethod.GET) {
                throw new IllegalStateException("请求不是GET请求.");
            }

            if (HttpHeaders.is100ContinueExpected(request)) {
                ctx.write(new DefaultFullHttpResponse(HTTP_1_1, CONTINUE));
            }

            keepAlive = HttpHeaders.isKeepAlive(request);
        }

        if(msg instanceof LastHttpContent){

            // 模拟事务处理
            TimeUnit.SECONDS.sleep(1);

            FullHttpResponse response = new DefaultFullHttpResponse(HTTP_1_1, OK, Unpooled.wrappedBuffer(CONTENT));
            response.headers().set(CONTENT_TYPE, "text/plain");
            response.headers().set(CONTENT_LENGTH, response.content().readableBytes());

            if (!keepAlive) {
                ctx.writeAndFlush(response).addListener(ChannelFutureListener.CLOSE);
            } else {
                response.headers().set(CONNECTION, HttpHeaders.Values.KEEP_ALIVE);
                ctx.writeAndFlush(response);
            }
        }
    }
}

这上面的逻辑非常简单，服务器接收到一个请求之后，发送一个『Hello World』的消息给客户端。

每一个客户端连接到server上会被分配到对应的Handler上处理数据。Netty的设计中把Request分为了HttpRequest和HttpContent两个部分。而由于担心HttpContent内容过长(例如上传文件这种场景)，HttpContent又被分成了普通的HttpContent和LastHttpContent两个部分，这些消息的处理放到Handler中。

Http初始化组件HttpInitializer.java：

public class HttpInitializer extends ChannelInitializer<SocketChannel> {
    @Override
    protected void initChannel(SocketChannel channel) throws Exception {
        // 获取通道
        ChannelPipeline p = channel.pipeline();

        // 添加http加解码器
        p.addLast(new HttpServerCodec());
        p.addLast(new HttpHandler());
    }
}

这里使用Netty自带的Http编解码组件HttpServerCodec对通信数据进行编解码，然后加入自定义的Handler组件，处理自定义的业务逻辑。

Server服务器启动程序HttpServer.java：

public class HttpServer {

    private static final int PORT = 8888;
    private ServerBootstrap serverBootstrap;

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

    public void open() throws InterruptedException {

        serverBootstrap = new ServerBootstrap();
        serverBootstrap.option(ChannelOption.SO_BACKLOG, 1024);
        serverBootstrap.group(bossGroup, workerGroup)
                .channel(NioServerSocketChannel.class)
                .handler(new LoggingHandler(LogLevel.INFO))
                .childHandler(new HttpInitializer());

        Channel ch = serverBootstrap.bind(PORT).sync().channel();

        System.err.printf("访问地址 http://127.0.0.1:%d/'", PORT);

        ch.closeFuture().sync();

    }

    public void close() {
        bossGroup.shutdownGracefully();
        workerGroup.shutdownGracefully();
    }

    public static void main(String args[]) {

        HttpServer server = new HttpServer();

        try {
            server.open();
        } catch (InterruptedException e) {
            System.out.println("出错了!");
        }
        server.close();
    }
}

在main()方法里面会启动服务器，然后Netty就会开始监听8888端口的消息。

安装一个http客户端

写好了Http服务器的server，还需要一个客户端程序来测试Netty。这里我会使用httpie作为HTTP测试客户端。

在OSX上安装：

brew install httpie

在Linux系统上安装：

# Debian-based系列的系统比如:Ubuntu安装
$ apt-get install httpie

# RPM-based系列的系统安装
$ yum install httpie

在命令行使用http命令就可以发出请求。使用--verbose参数可以打印Request的HTTP信息

➜  ~ http --verbose get httpie.org
GET / HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Connection: keep-alive
Host: httpie.org
User-Agent: HTTPie/0.9.2


HTTP/1.1 302 Found
Connection: close
Content-Length: 292
Content-Type: text/html; charset=iso-8859-1
Date: Wed, 20 Apr 2016 07:10:53 GMT
Location: https://github.com/jkbrzt/httpie
Server: Apache/2.2.15 (CentOS)
X-Awesome: Thanks for trying HTTPie :)

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
<html><head>
<title>302 Found</title>
</head><body>
<h1>Found</h1>
<p>The document has moved <a href="https://github.com/jkbrzt/httpie">here</a>.</p>
<hr>
<address>Apache/2.2.15 (CentOS) Server at httpie.org Port 80</address>
</body></html>

Http的server就构建完成了。