如何用Netty实现高效的HTTP服务器

下面就让我来详细讲解“如何用Netty实现高效的HTTP服务器”的完整攻略。

1. 引言

Netty是一个高性能、异步的网络编程框架，使用它可以轻松地开发TCP、UDP、HTTP等各种协议的客户端和服务器端。本文将主要讲解如何使用Netty实现高效的HTTP服务器。

2. 环境准备

在开始本篇攻略之前，需要准备如下环境：
1. JDK 8 或以上版本
2. Netty 4.1.x

3. 编写HTTP服务器

下面是一个基本的HTTP服务器的示例。首先我们需要继承ChannelInboundHandlerAdapter类，然后重写channelRead()方法来处理HTTP请求的逻辑。

public class HttpServerHandler extends ChannelInboundHandlerAdapter {

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) {
        if (msg instanceof FullHttpRequest) {
            FullHttpRequest request = (FullHttpRequest) msg;
            // 处理HTTP请求的逻辑
            // ...
        }
    }

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

然后我们需要编写一个包含上述HttpServerHandler的引导程序。这个引导程序的作用是创建一个ServerBootstrap对象并进行一些初始化操作。

public class HttpServer {

    public static void main(String[] args) throws Exception {
        EventLoopGroup group = new NioEventLoopGroup();
        try {
            ServerBootstrap b = new ServerBootstrap();
            b.group(group)
                .channel(NioServerSocketChannel.class)
                .localAddress(new InetSocketAddress(8080))
                .childHandler(new ChannelInitializer<SocketChannel>() {
                    @Override
                    public void initChannel(SocketChannel ch)
                            throws Exception {
                        ch.pipeline().addLast(new HttpServerCodec());
                        ch.pipeline().addLast(new HttpObjectAggregator(65536));
                        ch.pipeline().addLast(new HttpServerHandler());
                    }
                });
            ChannelFuture f = b.bind().sync();
            f.channel().closeFuture().sync();
        } finally {
            group.shutdownGracefully().sync();
        }
    }
}

在这个引导程序中，我们使用了ServerBootstrap创建了一个NioServerSocketChannel监听8080端口的Socket。然后我们添加了三个ChannelHandler，分别是：
- HttpServerCodec：这个ChannelHandler负责HTTP协议的编码和解码。
- HttpObjectAggregator：这个ChannelHandler将HTTP请求进行聚合。
- HttpServerHandler：这个ChannelHandler是我们自己实现的类，负责处理来自客户端的HTTP请求。

4. 示例

下面是两条示例说明，帮助你更好地理解如何使用Netty实现高效的HTTP服务器。

示例1：返回静态资源

下面的示例演示了如何使用Netty返回一个静态资源（比如html页面）。

public class HttpServerHandler extends ChannelInboundHandlerAdapter {

    private static final String HTTP_DATE_FORMAT = "EEE, dd MMM yyyy HH:mm:ss zzz";
    private static final String HTTP_DATE_GMT_TIMEZONE = "GMT";
    private static final int HTTP_CACHE_SECONDS = 60;

    @Override
    public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
        if (msg instanceof FullHttpRequest) {
            FullHttpRequest request = (FullHttpRequest) msg;
            String uri = request.uri();
            if ("/".equals(uri)) {
                uri = "/index.html";
            }
            String path = "/path/to/static/resources" + uri;
            File file = new File(path);
            if (file.exists()) {
                RandomAccessFile raf = new RandomAccessFile(file, "r");
                long fileLength = raf.length();

                HttpResponse response = new DefaultHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
                HttpUtil.setContentLength(response, fileLength);
                setContentTypeHeader(response, file);
                setDateAndCacheHeaders(response, file);
                if (HttpUtil.isKeepAlive(request)) {
                    HttpUtil.setKeepAlive(response, true);
                }

                ctx.write(response);
                ChannelFuture sendFileFuture;
                DefaultFileRegion defaultFileRegion = new DefaultFileRegion(raf.getChannel(), 0, fileLength);

                sendFileFuture = ctx.write(defaultFileRegion, ctx.newProgressivePromise());
                ctx.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT);

                if (!HttpUtil.isKeepAlive(request)) {
                    sendFileFuture.addListener(ChannelFutureListener.CLOSE);
                }
            } else {
                // 返回404错误
                FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.NOT_FOUND);
                ctx.write(response);
            }
        }
    }

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

    private static void setDateHeader(HttpResponse response) {
        SimpleDateFormat dateFormatter = new SimpleDateFormat(HTTP_DATE_FORMAT, Locale.US);
        dateFormatter.setTimeZone(TimeZone.getTimeZone(HTTP_DATE_GMT_TIMEZONE));

        Calendar time = new GregorianCalendar();
        response.headers().set(HttpHeaderNames.DATE, dateFormatter.format(time.getTime()));
    }

    private static void setDateAndCacheHeaders(HttpResponse response, File fileToCache) {
        SimpleDateFormat dateFormatter = new SimpleDateFormat(HTTP_DATE_FORMAT, Locale.US);
        dateFormatter.setTimeZone(TimeZone.getTimeZone(HTTP_DATE_GMT_TIMEZONE));

        // Add default cache headers.
        Calendar time = new GregorianCalendar();
        response.headers().set(HttpHeaderNames.DATE, dateFormatter.format(time.getTime()));
        time.add(Calendar.SECOND, HTTP_CACHE_SECONDS);
        response.headers().set(HttpHeaderNames.EXPIRES, dateFormatter.format(time.getTime()));
        response.headers().set(HttpHeaderNames.CACHE_CONTROL, "private, max-age=" + HTTP_CACHE_SECONDS);
        response.headers().set(
                HttpHeaderNames.LAST_MODIFIED, dateFormatter.format(new Date(fileToCache.lastModified())));
    }

    private static void setContentTypeHeader(HttpResponse response, File file) {
        MimetypesFileTypeMap mimeTypesMap = new MimetypesFileTypeMap();
        response.headers().set(
                HttpHeaderNames.CONTENT_TYPE,
                mimeTypesMap.getContentType(file.getPath()));
    }
}

在这个示例中，我们使用了Netty提供的DefaultFileRegion来返回静态资源。

示例2：接收表单数据

下面的示例演示了如何使用Netty接收表单数据并返回处理结果。

public class HttpServerHandler extends SimpleChannelInboundHandler<FullHttpRequest> {

    private static final HttpDataFactory factory =
            new DefaultHttpDataFactory(DefaultHttpDataFactory.MINSIZE); // Disk if size exceed MINSIZE
    private HttpPostRequestDecoder httpDecoder;

    @Override
    protected void channelRead0(ChannelHandlerContext ctx, FullHttpRequest request) throws Exception {
        if (HttpUtil.is100ContinueExpected(request)) {
            ctx.write(new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.CONTINUE));
        }

        if (!request.decoderResult().isSuccess()) {
            FullHttpResponse response = new DefaultFullHttpResponse(
                    HttpVersion.HTTP_1_1, HttpResponseStatus.BAD_REQUEST,
                    Unpooled.copiedBuffer("Invalid Request", CharsetUtil.UTF_8));
            HttpUtil.setContentLength(response, response.content().readableBytes());
            ctx.write(response);
            return;
        }

        if (httpDecoder == null) {
            httpDecoder = new HttpPostRequestDecoder(factory, request);
        }

        if (request instanceof LastHttpContent) {
            httpDecoder.offer(request);
            List<InterfaceHttpData> postData =
                    httpDecoder.getBodyHttpDatas();

            // 打印表单数据
            for (InterfaceHttpData data: postData) {
                System.out.println(data.toString());
            }

            FullHttpResponse response = new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK);
            response.content().writeBytes("处理结果".getBytes());
            HttpUtil.setContentLength(response, response.content().readableBytes());
            ctx.write(response);
            httpDecoder.destroy();
            httpDecoder = null;

            if (HttpUtil.isKeepAlive(request)) {
                ctx.write(new DefaultFullHttpResponse(HttpVersion.HTTP_1_1, HttpResponseStatus.OK));
            } else {
                ctx.writeAndFlush(LastHttpContent.EMPTY_LAST_CONTENT).addListener(ChannelFutureListener.CLOSE);
            }
        }
    }

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

在这个示例中，我们使用了Netty提供的HttpPostRequestDecoder来处理表单数据。我们需要在channelRead0()方法中按照如下流程进行处理：
1. 如果请求是100 Continue，则返回HttpResponseStatus.CONTINUE。
2. 如果请求解析出现错误，则返回HttpResponseStatus.BAD_REQUEST。
3. 如果请求是最后一块内容，则调用HttpPostRequestDecoder.getBodyHttpDatas()进行数据处理，并返回结果。

2021年11月4日 更新