一个简单的Java web服务器实现，比较简单，基于java.net.Socket和java.net.ServerSocket实现；

程序执行步骤

1. 创建一个ServerSocket对象；
2. 调用ServerSocket对象的accept方法，等待连接，连接成功会返回一个Socket对象，否则一直阻塞等待；
3. 从Socket对象中获取InputStream和OutputStream字节流，这两个流分别对应request请求和response响应；
4. 处理请求：读取InputStream字节流信息，转成字符串形式，并解析，这里的解析比较简单，仅仅获取uri(统一资源标识符)信息;
5. 处理响应：根据解析出来的uri信息，从WEB_ROOT目录中寻找请求的资源资源文件, 读取资源文件，并将其写入到OutputStream字节流中；
6. 关闭Socket对象；
7. 转到步骤2，继续等待连接请求；

代码实现

服务器实现：

package ex01.pyrmont;

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

public class HttpServer {

    /**
     * WEB_ROOT是HTML和其它文件存放的目录. 这里的WEB_ROOT为工作目录下的webroot目录
     */
    public static final String WEB_ROOT = System.getProperty("user.dir") + File.separator + "webroot";

    // 关闭服务命令
    private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

    public static void main(String[] args) {
        HttpServer server = new HttpServer();
        //等待连接请求
        server.await();
    }

    public void await() {
        ServerSocket serverSocket = null;
        int port = 8080;
        try {
            //服务器套接字对象
            serverSocket = new ServerSocket(port, 1, InetAddress.getByName("127.0.0.1"));
        } catch (IOException e) {
            e.printStackTrace();
            System.exit(1);
        }

        // 循环等待一个请求
        while (true) {
            Socket socket = null;
            InputStream input = null;
            OutputStream output = null;
            try {
                //等待连接，连接成功后，返回一个Socket对象
                socket = serverSocket.accept();
                input = socket.getInputStream();
                output = socket.getOutputStream();

                // 创建Request对象并解析
                Request request = new Request(input);
                request.parse();
                // 检查是否是关闭服务命令
                if (request.getUri().equals(SHUTDOWN_COMMAND)) {
                    break;
                }

                // 创建 Response 对象
                Response response = new Response(output);
                response.setRequest(request);
                response.sendStaticResource();

                // 关闭 socket 对象
                socket.close();

            } catch (Exception e) {
                e.printStackTrace();
                continue;
            }
        }
    }
}

Request类：

package ex01.pyrmont;

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

public class Request {

    private InputStream input;
    private String uri;

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

    //从InputStream中读取request信息，并从request中获取uri值
    public void parse() {
        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());
    }

    /**
     *
     * requestString形式如下：
     * GET /index.html HTTP/1.1
     * Host: localhost:8080
     * Connection: keep-alive
     * Cache-Control: max-age=0
     * ...
     * 该函数目的就是为了获取/index.html字符串
     */
    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类：

package ex01.pyrmont;

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

/*
  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 {
            //将web文件写入到OutputStream字节流中
            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();
        }
    }
}

结果测试

访问存在的资源文件(注意存放在工程目录的webroot文件夹里)：

访问不存在的资源文件：

关闭服务器：

参考资料：《深入剖析Tomcat》