「How Tomcat works」这本书的第一个例子是读取文件并输出，如果读取的文件符合 HTTP 协议格式，那么就可以输出到浏览器并展示。

按照这个思路就可以实现一个简单的 HTTP 服务器，服务器的页面以 HTTP 格式保存在磁盘上，虽然这个 demo 并没有什么实际的用途，但是对于理解 HTTP 协议及 HTTP 服务器的实现有很大帮助，本文记录下实现，希望能有所启发。

如何实现

因为将协议数据全部保存到文件中，因此代码中并不对 HTTP 协议做任何处理。

Request

Request 只需要从请求的输入流中解析出 uri 即可，用于定位要访问的页面

import java.io.IOException;
import java.io.InputStream;

/**
 * @author : Ares
 * @createTime : Aug 21, 2012 9:48:45 PM
 * @version : 1.0
 * @description :
 */
public class Request {
	private InputStream input;
	private String uri;

	public Request(InputStream input) {
		this.input = input;
	}

	public void parse() {
		// Read a set of characters from the socket
		StringBuffer request = new StringBuffer(2048);
		int i;
		byte[] buffer = new byte[2048];
		try {
			i = input.read(buffer);
		} catch (IOException e) {
			e.printStackTrace();
			i = -1;
		}
		for (int j = 0; j < i; j++) {
			request.append((char) buffer[j]);
		}
		System.out.print(request.toString());
		uri = parseUri(request.toString());
	}

	private String parseUri(String requestString) {
		int index1, index2;
		index1 = requestString.indexOf(' ');
		if (index1 != -1) {
			index2 = requestString.indexOf(' ', index1 + 1);
			if (index2 > index1)
				return requestString.substring(index1 + 1, index2);
		}
		return null;
	}

	public String getUri() {
		return uri;
	}
}

Response

Response 负责将对应的内容输出到输出流，为了实现简单它还负责了读取文件的职责。

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * @author : Ares
 * @createTime : Aug 21, 2012 9:51:37 PM
 * @version : 1.0
 * @description :
 * 
 * HTTP Response = Status-Line (( general-header | response-header |
 * entity-header ) CRLF) CRLF [ message-body ] Status-Line = HTTP-Version SP
 * Status-Code SP Reason-Phrase CRLF
 * 
 */
public class Response {
	private static final int BUFFER_SIZE = 1024;
	Request request;
	OutputStream output;

	public Response(OutputStream output) {
		this.output = output;
	}

	public void setRequest(Request request) {
		this.request = request;
	}

	public void sendStaticResource() throws IOException {
		byte[] bytes = new byte[BUFFER_SIZE];
		FileInputStream fis = null;
		try {
			File file = new File(HttpServer.WEB_ROOT, request.getUri());
			if (file.exists()) {
				fis = new FileInputStream(file);
				int ch = fis.read(bytes, 0, BUFFER_SIZE);
				while (ch != -1) {
					output.write(bytes, 0, ch);
					ch = fis.read(bytes, 0, BUFFER_SIZE);
				}
			} else {
				// file not found
				String errorMessage = "HTTP/1.1 404 File Not Found\r\n"
						+ "Content-Type: text/html\r\n"
						+ "Content-Length: 23\r\n" + "\r\n" 
						+ "<h1>File Not Found</h1>";
				output.write(errorMessage.getBytes());
			}
		} catch (Exception e) {
			// thrown if cannot instantiate a File object
			System.out.println(e.toString());
		} finally {
			if (fis != null){
				fis.close();
			}
		}
	}
}

HttpServer

核心的 HttpServer 负责启动 ServerSocket 并绑定到指定端口上，监听到请求后创建 Socket，获取 InputStream 构建 Request 对象，完成 Request 对象解析和 Response 对象构建，之后读取文件并输出。

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.InetAddress;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * @author : Ares
 * @createTime : Aug 21, 2012 9:45:01 PM
 * @version : 1.0
 * @description :
 */
public class HttpServer {

	/**
	 * WEB_ROOT is the directory where our HTML and other files reside. For this
	 * package, WEB_ROOT is the "webroot" directory under the working directory.
	 * The working directory is the location in the file system from where the
	 * java command was invoked.
	 */
	public static final String WEB_ROOT = System.getProperty("user.dir")
			+ File.separator + "webroot";

	// shutdown command
	private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

  // default host
	private static final String DEFAULT_HOST = "127.0.0.1";

	// the shutdown command received
	private boolean shutdown = false;

	public static void main(String[] args) {
		String host = DEFAULT_HOST;
		if (args.length > 0 && args[0].equals("-s")) {
			host = NetUtils.getFirstLocalIp();
		}
    System.out.println("host: " + host);
		HttpServer server = new HttpServer();
		server.await(host);
	}

	public void await(String host) {
		ServerSocket serverSocket = null;
		int port = 8080;
		try {
			serverSocket = new ServerSocket(port, 1, InetAddress.getByName(host));
		} catch (IOException e) {
			e.printStackTrace();
			System.exit(1);
		}
		// Loop waiting for a request
		while (!shutdown) {
			Socket socket = null;
			InputStream input = null;
			OutputStream output = null;

			try {
				socket = serverSocket.accept();
				input = socket.getInputStream();
				output = socket.getOutputStream();
				// create Request object and parse
				Request request = new Request(input);
				request.parse();

				// create Response object
				Response response = new Response(output);
				response.setRequest(request);
				response.sendStaticResource();

				// Close the socket
				socket.close();
				// check if the previous URI is a shutdown command
				shutdown = request.getUri().equals(SHUTDOWN_COMMAND);
			} catch (Exception e) {
				e.printStackTrace();
				continue;
			}
		}
	}
}

NetUtils

为了获取到网卡 IP 地址，需要用到一个 NetUtils 工具类

import java.net.InetAddress;
import java.net.NetworkInterface;
import java.net.SocketException;
import java.util.ArrayList;
import java.util.Enumeration;
import java.util.List;

public class NetUtils {
    public static String getFirstLocalIp() {
        List<String> allNoLoopbackAddresses = getAllLocalIp();
        if (allNoLoopbackAddresses.isEmpty()) {
            throw new IllegalStateException("Sorry, seems you don't have a network card :( ");
        }
        return allNoLoopbackAddresses.get(allNoLoopbackAddresses.size() - 1);
    }

    public static List<String> getAllLocalIp() {
        List<String> noLoopbackAddresses = new ArrayList<>();
        List<InetAddress> allInetAddresses = getAllLocalAddress();

        for (InetAddress address : allInetAddresses) {
            if (!address.isLoopbackAddress() && !address.isLinkLocalAddress()) {
                noLoopbackAddresses.add(address.getHostAddress());
            }
        }

        return noLoopbackAddresses;
    }

    public static List<InetAddress> getAllLocalAddress() {
        try {
            Enumeration<NetworkInterface> networkInterfaces = NetworkInterface.getNetworkInterfaces();
            List<InetAddress> addresses = new ArrayList<>();

            while (networkInterfaces.hasMoreElements()) {
                NetworkInterface networkInterface = networkInterfaces.nextElement();
                Enumeration<InetAddress> inetAddresses = networkInterface.getInetAddresses();
                while (inetAddresses.hasMoreElements()) {
                    InetAddress inetAddress = inetAddresses.nextElement();
                    addresses.add(inetAddress);
                }
            }

            return addresses;
        } catch (SocketException e) {
            throw new RuntimeException(e.getMessage(), e);
        }
    }
}

如何运行

将上述代码放到一个目录下，执行 javac HttpServer.java 编译，不出意外的会生成所有的字节码，所有文件如下

1 2	HttpServer.class NetUtils.class Request.class Response.class webroot HttpServer.java NetUtils.java Request.java Response.java

其中 webroot 是一个目录，因为程序中定义了资源文件的存放路径为

1	System.getProperty("user.dir") + File.separator + "webroot"

如果不设置 user.dir 系统属性，那么默认是在当前目录下。

接下来在 webroot 目录下创建 HTTP 协议文件，如下是我定义的一个 json 格式的返回数据文件 tip.json

HTTP/1.1 200 SUCCESS
Content-Type: text/json;charset=utf-8
Content-Length: 48

{
    "code": 0,
    "msg": "记得写周报"
}

执行 java HttpServer 命令启动服务

1 2	➜ 1 java HttpServer host: 127.0.0.1

然后在浏览器中输入 http://127.0.0.1:8080/tip.json，你会看到一个 json 格式的输出

如果想对外提供服务，你可以输入 java HttpServe -s 启动服务，然后就会看到服务绑定到了一个对外的 IP

1 2	➜ 1 java HttpServer -s host: 192.168.1.4

HTTP 协议格式

这里说的是 HTTP 1.1 的协议格式，它使用的是文本格式

如下是一个 HTTP 响应的格式

它主要包含 3 部分数据：

状态行。包含协议版本号、code、状态
响应报头。这里有响应内容的格式，响应内容的长度
响应报文主体。这是真正的业务内容

每一行数据使用 \r\n 作为结尾。

gorden5566的博客

将HTTP协议格式的文本输出到浏览器