一個簡單的Java web伺服器實現，比較簡單，基於java.net.Socket和java.net.ServerSocket實現；

程式執行步驟

1. 創建一個ServerSocket對象；
2. 調用ServerSocket對象的accept方法，等待連接，連接成功會返回一個Socket對象，否則一直阻塞等待；
3. 從Socket對象中獲取InputStream和OutputStream位元組流，這兩個流分別對應request請求和response響應；
4. 處理請求：讀取InputStream位元組流信息，轉成字元串形式，並解析，這裡的解析比較簡單，僅僅獲取uri(統一資源標識符)信息;
5. 處理響應：根據解析出來的uri信息，從WEB_ROOT目錄中尋找請求的資源資源文件, 讀取資源文件，並將其寫入到OutputStream位元組流中；
6. 關閉Socket對象；
7. 轉到步驟2，繼續等待連接請求；

代碼實現

伺服器實現：

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錄. 這裡的WEB_ROOT為工作目錄下的webroot目錄
     */
    public static final String WEB_ROOT = System.getProperty("user.dir") + File.separator + "webroot";

    // 關閉服務命令
    private static final String SHUTDOWN_COMMAND = "/SHUTDOWN";

    public static void main(String[] args) {
        HttpServer server = new HttpServer();
        //等待連接請求
        server.await();
    }

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

        // 迴圈等待一個請求
        while (true) {
            Socket socket = null;
            InputStream input = null;
            OutputStream output = null;
            try {
                //等待連接，連接成功後，返回一個Socket對象
                socket = serverSocket.accept();
                input = socket.getInputStream();
                output = socket.getOutputStream();

                // 創建Request對象並解析
                Request request = new Request(input);
                request.parse();
                // 檢查是否是關閉服務命令
                if (request.getUri().equals(SHUTDOWN_COMMAND)) {
                    break;
                }

                // 創建 Response 對象
                Response response = new Response(output);
                response.setRequest(request);
                response.sendStaticResource();

                // 關閉 socket 對象
                socket.close();
                
            } catch (Exception e) {
                e.printStackTrace();
                continue;
            }
        }
    }
}

 Request類：

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;
    }

    //從InputStream中讀取request信息，並從request中獲取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
     * ...
     * 該函數目的就是為了獲取/index.html字元串
     */
    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類：

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 {
            //將web文件寫入到OutputStream位元組流中
            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();
        }
    }
}

結果測試

訪問存在的資源文件(註意存放在工程目錄的webroot文件夾里)：

訪問不存在的資源文件：

關閉伺服器：

參考資料：《深入剖析Tomcat》