<!--jar包的依赖-->
    <dependency>
      <groupId>jakarta.servlet</groupId>
      <artifactId>jakarta.servlet-api</artifactId>
      <version>5.0.0</version>
      <scope>provided</scope>
    </dependency>

<!--jsp的依赖-->
    <dependency>
      <groupId>jakarta.servlet.jsp</groupId>
      <artifactId>jakarta.servlet.jsp-api</artifactId>
      <version>3.0.0</version>
      <scope>provided</scope>
    </dependency>

MyTomcat类代码如下 : （服务端；自定义的Tomcat)

package tomcat;

import java.io.*;
import java.net.ServerSocket;
import java.net.Socket;

/**
    服务器端
 */
public class MyTomcat {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(8080);
        System.out.println("Tomcat在8080端口进行监听...");
        System.out.println("--------------------------------------");

        while (!serverSocket.isClosed()) {
            //获取Socket对象
            Socket socket = serverSocket.accept();

            //接收来自浏览器端的信息
            InputStream inputStream = socket.getInputStream();
            BufferedReader bufferedReader =
                    new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));

            String content = null;  //局部变量在使用前必须赋初值。

            while ((content = bufferedReader.readLine()) != null) {
                //判断content是否是空串儿
                if (content.length() == 0) {
                    break;
                }
                System.out.println(content);
            }

            //服务器向浏览器回送消息
            OutputStream outputStream = socket.getOutputStream();
            //设置一个HTTP响应包的响应头
            // "/r/n"表示换行
            String respHeader =
                    "HTTP/1.1 200\r\n" +
                            "Content-Type: text/html;charset=utf-8\r\n\r\n";
            //设置HTTP响应的响应体
            String respBody = respHeader + "<h1>你好！</h1>";
            /*
                注意这里不能使用字符包装流来写数据!!!。
             */
            outputStream.write(respBody.getBytes());

            System.out.println("--------------------------------------");
            System.out.println(respBody);

            //释放资源
            outputStream.flush();
            outputStream.close();
            inputStream.close();
            socket.close();
//            serverSocket.close();
        }
    }
}

在浏览器地址栏访问本机8080端口。
login.html代码如下 :

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>login</title>
    <style>
        table, tr, td {
            border:2px cornflowerblue solid;
            border-collapse: collapse;
            padding: 10px;
            background-color: lightcyan
        }
        #tr01 {
            text-align: center
        }
    </style>
</head>
<body>
<form action="/Cyan_Tomcat/login" methods="get">
    <table>
        <tr>
            <th colspan="2">User Logging</th>
        </tr>
        <tr>
            <td>Username: </td>
            <td><input type="text" name="username"/></td>
        </tr>
        <tr>
            <td>Password: </td>
            <td><input type="password" name="password"/></td>
        </tr>
        <tr id="tr01">
            <td><input type="submit" value="Submit"/></td>
            <td><input type="reset" value="Reset"/></td>
        </tr>
    </table>
</form>
</body>
</html>

LoginServlet类代码如下 :

package servlet;

import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.PrintWriter;

/**
 * @author : Cyan_RA9
 * @version : 21.0
 */
@WebServlet(urlPatterns = {"/login"})
public class LoginServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("LoginServlet's doPost is invoked~");

        resp.setContentType("text/html; charset=utf-8");
        PrintWriter writer = resp.getWriter();

        req.setCharacterEncoding("utf-8");
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        if ("Cyan".equals(username) && "123".equals(password)) {
            writer.print("<h1>登录成功！</h1>");
        } else {
            writer.print("<h1>登录失败！请重新尝试！</h1>");
        }
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doPost(req, resp);
    }
}

运行效果 :

如果以Web工程配置好的Tomcat运行，就会按照LoginServlet类的代码逻辑来处理业务，如下图所示 : (GIF)

编辑

如果以自定义的Tomcat运行，就会以MyTomcat类中的代码逻辑来处理业务。
如下图所示 : (GIF)

编辑

二、多线程模型的实现

1.思路分析 :

当服务器端接收到浏览器端的HTTP请求后，启动一个新的线程，令该线程持有浏览器对应的Socket对象，完成线程和浏览器的对接。
可通过实现Runnable接口的方式定义线程类HttpRequestHandler，线程对象用于处理来自浏览器的HTTP请求。

2.处理HTTP请求 :

线程类HttpRequestHandler类代码如下 :

package tomcat.handler;

import tomcat.http.CyanServletRequest;
import tomcat.http.CyanServletResponse;
import tomcat.servlet.CyanLoginServlet;

import java.io.*;
import java.net.Socket;

public class HttpRequestHandler implements Runnable {
    private Socket socket;

    public HttpRequestHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            //接收客户端的信息
            InputStream inputStream = socket.getInputStream();

/*  以下代码已在CyanServletRequest类中实现
            BufferedReader bufferedReader =
                    new BufferedReader(new InputStreamReader(inputStream, "utf-8"));

            //BIO，每次请求都对应一个新的线程
            System.out.println("当前线程 = " + Thread.currentThread().getName());

            String content = null;  //局部变量在使用前必须赋初值。

            while ((content = bufferedReader.readLine()) != null) {
                //判断是否读到了空字符串
                if (content.length() == 0) {
                    break;
                }
                System.out.println(content);
            }
*/
            //Die first and live second
            //获取客户端的信息（利用了CyanServletRequest中封装好的方法）
            //以下代码已在CyanLoginServlet中实现
/*            CyanServletRequest cyanServletRequest = new CyanServletRequest(inputStream);
            String username = cyanServletRequest.getParameter("username");
            String password = cyanServletRequest.getParameter("password");
            System.out.println("username = " + username);
            System.out.println("password = " + password);
            System.out.println(cyanServletRequest);*/

            //给客户端回送信息
            //以下代码已在CyanLoginServlet类中实现。
/*            CyanServletResponse cyanServletResponse = new CyanServletResponse(socket.getOutputStream());
            String resp = CyanServletResponse.respHeader + "<h1>CyanServletResponse!</h1>";
            OutputStream outputStream = cyanServletResponse.getOutputStream();
            outputStream.write(resp.getBytes());
            outputStream.flush();
            outputStream.close();*/

            CyanServletRequest cyanServletRequest = new CyanServletRequest(inputStream);
            CyanServletResponse cyanServletResponse = new CyanServletResponse(socket.getOutputStream());

            CyanLoginServlet cyanLoginServlet = new CyanLoginServlet();
            cyanLoginServlet.doPost(cyanServletRequest, cyanServletResponse);

            //释放资源
            inputStream.close();
            socket.close();


/* 以下代码已在CyanServletResponse类中实现 :
            String respHeader = "HTTP/1.1 200\r\n" +
                    "Content-Type: text/html;charset=utf-8\r\n\r\n";
            String respHttp = respHeader + "<h1>Cyan_RA9</h1>";

            System.out.println("-----------------------------------------------");
            System.out.println("回送的信息如下：(回显)");
            System.out.println(respHttp);

            OutputStream outputStream = socket.getOutputStream();
            outputStream.write(respHttp.getBytes());

            //释放资源
            outputStream.flush();
            outputStream.close();
            inputStream.close();
            socket.close(); */
        } catch (IOException e) {
            throw new RuntimeException(e);
        } finally {
            //确保Socket关闭
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

3.自定义Tomcat :

在MyTomcat_EX类中实现线程的分发。
MyTomcat_EX类代码如下 :

package tomcat;

import tomcat.handler.HttpRequestHandler;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.net.ServerSocket;
import java.net.Socket;

/**
 * @author : Cyan_RA9
 * @version : 21.0
 */
public class MyTomcat_EX {
    public static void main(String[] args) throws IOException {
        ServerSocket serverSocket = new ServerSocket(8080);
        System.out.println("MyTomcat_EX在8080端口进行监听...");

        while (!serverSocket.isClosed()) {
            Socket socket = serverSocket.accept();
            HttpRequestHandler httpRequestHandler = new HttpRequestHandler(socket);
            Thread thread = new Thread(httpRequestHandler);

            thread.start();
        }
    }
}

运行效果 : (GIF)

编辑

三、自定义Servlet规范

1. HTTP请求和响应 :

1° CyanServletRequest

CyanServletRequest类的作用等同于原始的HttpServletRequest，该类用于封装HTTP请求中的数据，eg : method, URI, 以及表单数据的参数列表等。
CyanServletRequest类代码如下 :

package tomcat.http;

import java.io.*;
import java.util.HashMap;

/**
 * @author : Cyan_RA9
 * @version : 21.0
 * @function : like the original HttpServletRequest.
 */
public class CyanServletRequest {
    private String method;
    private String URI;
    private HashMap<String, String> parametersMapping = new HashMap<>();
    private InputStream inputStream;

    /*
        此处传入的InputStream对象是和Socket关联的InputStream.
     */
    public CyanServletRequest(InputStream inputStream) {
        this.inputStream = inputStream;

        //完成对HTTP请求数据的封装
        this.init();
    }

    private void init() {
        System.out.println("\nCyanServletRequest's init is invoked~");
        try {
            //注意转换流的形参列表
            BufferedReader bufferedReader =
                    new BufferedReader(new InputStreamReader(inputStream, "utf-8"));

            //首先读取HTTP请求的请求行
            /* eg :  GET /Cyan/cyan.html HTTP/1.1　*/
            String requestLine = bufferedReader.readLine();
            String[] requestLineArr = requestLine.split(" ");
            //获取method
            method = requestLineArr[0];
            //获取URI
            int index = requestLineArr[1].indexOf("?");
            if (index == -1) {      //if判断成立，说明请求行中没有参数列表
                URI = requestLineArr[1];
            } else {
                URI = requestLineArr[1].substring(0, index);
                //获取参数列表
                String parameters = requestLineArr[1].substring(index + 1);
                String[] parameterPairs = parameters.split("&");
                //兼容性处理，防止?后啥都没有。
                if (null != parameterPairs && !"".equals(parameterPairs)) {
                    for (String parameterPair : parameterPairs) {
                        String[] parameter = parameterPair.split("=");
                        if (parameter.length == 2) { //判断是否为一对完整的"name=value".
                            parametersMapping.put(parameter[0],parameter[1]);
                        }
                    }
                }
            }

            //!!! 直接关闭Socket关联的InputStream，会引起Socket的关闭。
            //inputStream.close();
        } catch (UnsupportedEncodingException e) {
            throw new RuntimeException(e);
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public String getMethod() {
        return method;
    }

    public void setMethod(String method) {
        this.method = method;
    }

    public String getURI() {
        return URI;
    }

    public void setURI(String URI) {
        this.URI = URI;
    }

    //重要
    public String getParameter(String name) {
        if (parametersMapping.containsKey(name)) {      //注意此处API的使用！
            return parametersMapping.get(name);
        } else {
            return null;
        }
    }

    @Override
    public String toString() {
        return "CyanServletRequest{" +
                "method='" + method + '\'' +
                ", URI='" + URI + '\'' +
                ", parametersMapping=" + parametersMapping +
                '}';
    }
}

CyanServletRequest类测试，运行效果如下GIF :

编辑

2° CyanServletResponse

CyanServletResponse类的作用等同于原始的HttpServletResponse，用于封装HTTP响应的相关信息。
CyanServletResponse类代码如下 :

package tomcat.http;

import java.io.OutputStream;

/**
 * @author : Cyan_RA9
 * @version : 21.0
 * @function : like the original HttpServletResponse
 */
public class CyanServletResponse {
    private OutputStream outputStream;
    //设置一个HTTP响应头
    public static final String respHeader = "HTTP/1.1 200\r\n" +
            "Content-Type: text/html;charset=utf-8\r\n\r\n";

    //传入与Socket关联的OutputStream对象
    public CyanServletResponse(OutputStream outputStream) {
        this.outputStream = outputStream;
    }

    public OutputStream getOutputStream() {
        return outputStream;
    }
}

运行测试（如下GIF图）：

编辑

2.Servlet规范 :

1° CyanServlet

CyanServlet仅保留原生Servlet的init, destroy, service方法，其中，service方法供将来CyanServlet的抽象实现类CyanHttpServlet去重写。注意，service方法的形参列表，要使用自定义的CyanServletRequest 和 CyanServletResponse.
CyanServlet接口，代码如下 :

package tomcat.servlet;

import tomcat.http.CyanServletRequest;
import tomcat.http.CyanServletResponse;

import java.io.IOException;

public interface CyanServlet {
    void init() throws Exception;

    void service(CyanServletRequest req, CyanServletResponse resp) throws IOException;

    void destroy();
}

2° CyanHttpServlet

CyanHttpServlet的作用，类似于原生的HttpServlet；在CyanHttpServlet中实现CyanServlet接口中的service方法，在service方法中，要对HTTP请求的method类型进行判断。

CyanHttpServlet抽象类代码如下 :

package tomcat.servlet;

import tomcat.http.CyanServletRequest;
import tomcat.http.CyanServletResponse;

import java.io.IOException;

public abstract class CyanHttpServlet implements CyanServlet{
    @Override
    public void service(CyanServletRequest req, CyanServletResponse resp) throws IOException {
        //忽略大小写
        if ("GET".equalsIgnoreCase(req.getMethod())) {
            this.deGet(req, resp);
        } else if ("POST".equalsIgnoreCase(req.getMethod())) {
            this.doPost(req, resp);
        }
    }

    //模板设计模式
    public abstract void deGet(CyanServletRequest req, CyanServletResponse resp);
    public abstract void doPost(CyanServletRequest req, CyanServletResponse resp);
}

3° CyanLoginServlet

CyanLoginServlet是一个简单的servlet实例，用于继承CyanHttpServlet抽象类，并实现CyanHttpServlet类中的doGet和doPost抽象方法。之后，在HttpRequestHandler线程类中先尝试直接调用CyanLoginServlet实例。
CyanLoginServlet代码如下 :

package tomcat.servlet;

import tomcat.http.CyanServletRequest;
import tomcat.http.CyanServletResponse;

import java.io.IOException;
import java.io.OutputStream;

public class CyanLoginServlet extends CyanHttpServlet{
    @Override
    public void deGet(CyanServletRequest req, CyanServletResponse resp) {
        doPost(req, resp);
    }

    @Override
    public void doPost(CyanServletRequest req, CyanServletResponse resp) {
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        //获取与当前Socket相关联的OutputStream对象
        OutputStream outputStream = resp.getOutputStream();
        String respInfo = CyanServletResponse.respHeader + "<h1>username = " +
                username + "</h1><br/> " + "<h1>password = " + password + "</h1>" + "<br/>" +
                "<h3>Cyan_RA9</h3>";
        try {
            outputStream.write(respInfo.getBytes());
            outputStream.flush();
            outputStream.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    @Override
    public void init() throws Exception {

    }

    @Override
    public void destroy() {

    }
}

此外，还需要更新HttpServletHandler类中的类型，将已封装好的代码注释掉，HttpServletHandler类已更新，在上文“多线程模型实现”的HTTP请求处理模块。
运行效果如下图所示 :

编辑

3.容器实现 :

1° 思路分析

Tomcat中维护有至少两个大的HashMap容器。以web.xml配置文件的方式为例，其中一个HashMap容器，key存放<url-pattern>，value存放<servlet-name>；另一个HashMap容器，key存放<servlet-name>，value存放通过<servlet-class>来反射生成的servlet实例。

2° web.xml配置文件

web.xml配置文件如下 :

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<!--xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0"-->
<!--IDEA报错——
  因为这是我们自定义的servlet，IDEA无法识别；无所谓！继续用！-->
<web-app>
  <display-name>Archetype Created Web Application</display-name>
  <servlet>
    <servlet-name>CyanLoginServlet</servlet-name>
    <servlet-class>tomcat.servlet.CyanLoginServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>CyanLoginServlet</servlet-name>
    <url-pattern>/cyanLogin</url-pattern>
  </servlet-mapping>
</web-app>

3° 最终版自定义Tomcat

在MyTomcat_Pro类中定义两个CurrentHashMap对象；定义init方法完成对两个CurrentHashMap对象的初始化（使用Dom4J读取web.xml配置文件）。
首先，需要在Maven的pom.xml配置文件中，引入dom4j依赖，如下图所示 :

编辑

然后，将web.xml配置文件拷贝到/target/classes/目录下一份，如下图所示 :

编辑

MyTomcat_Pro代码如下 :

package tomcat;

import org.dom4j.Document;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;
import tomcat.handler.HttpRequestHandler;
import tomcat.servlet.CyanHttpServlet;

import java.io.File;
import java.io.IOException;
import java.lang.reflect.Constructor;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;

/**
 * The final own custom Tomcat.
 */
public class MyTomcat_Pro {
    //Tomcat维护的第一个容器
        //String --> <servlet-name>
        //CyanHttpServlet --> 可存放它的子类(即各种servlet实例)
    public static final ConcurrentHashMap<String, CyanHttpServlet> servletMapping =
            new ConcurrentHashMap<>();

    //Tomcat维护的第二个容器
        //String --> <url-patterns>
        //String --> <servlet-name>
    public static final ConcurrentHashMap<String, String> servletURLMapping =
            new ConcurrentHashMap<>();

    public static void main(String[] args) {
        MyTomcat_Pro myTomcat_pro = new MyTomcat_Pro();
        myTomcat_pro.init();
        myTomcat_pro.run();
    }

    public void run() {
        try {
            ServerSocket serverSocket = new ServerSocket(8080);
            System.out.println("MyTomcat_Pro在8080端口进行监听...");

            while (!serverSocket.isClosed()) {
                Socket socket = serverSocket.accept();
                Thread thread = new Thread(new HttpRequestHandler(socket));
                thread.start();
            }

        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    public void init() {
        String path = MyTomcat_Pro.class.getResource("/").getPath();
        //path = "/javaProject/Servlet/Cyan_Tomcat/target/classes/"

        //使用Dom4J技术解析web.xml文件
        SAXReader saxReader = new SAXReader();
        try {
            //注意文件名
            Document document = saxReader.read(new File(path + "web.xml"));
            System.out.println("document = " + document);
            //获取根元素<web-app>
            Element rootElement = document.getRootElement();
            //得到根元素下面的所有子元素
            List<Element> elements = rootElement.elements();
            //遍历并判断
            for (Element element : elements) {
                if ("servlet".equals(element.getName())) {
                    Element servlet_name = element.element("servlet-name");
                    Element servlet_class = element.element("servlet-class");

                    //反射机制创建servlet实例 (注意getText()方法的使用！)
                    Class<?> clazz = Class.forName(servlet_class.getText().trim());
                    Constructor<?> constructor = clazz.getConstructor();
                    CyanHttpServlet o = (CyanHttpServlet) constructor.newInstance();
                    servletMapping.put(servlet_name.getText(), o);
                } else if ("servlet-mapping".equals(element.getName())) {
                    Element url_pattern = element.element("url-pattern");
                    Element servlet_name = element.element("servlet-name");

                    servletURLMapping.put(url_pattern.getText(), servlet_name.getText());
                }
            }

        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}

4° 最终版自定义线程类

package tomcat.handler;

import tomcat.MyTomcat_Pro;
import tomcat.http.CyanServletRequest;
import tomcat.http.CyanServletResponse;
import tomcat.servlet.CyanHttpServlet;
import tomcat.servlet.CyanLoginServlet;

import java.io.*;
import java.net.Socket;

public class HttpRequestHandler implements Runnable {
    private Socket socket;

    public HttpRequestHandler(Socket socket) {
        this.socket = socket;
    }

    @Override
    public void run() {
        try {
            //接收客户端的信息
            CyanServletRequest cyanServletRequest = new CyanServletRequest(socket.getInputStream());
            CyanServletResponse cyanServletResponse = new CyanServletResponse(socket.getOutputStream());

            String uri = cyanServletRequest.getURI();
            System.out.println("uri = " + uri);
            String servlet_name = MyTomcat_Pro.servletURLMapping.get(uri);
            /*
                这里的servlet_name可能为空。
                解决方式一 : 将CurrentHashMap替换为HashMap
                解决方式二 : 增加一个是否为null的判断。
             */
            if (servlet_name == null) {
                servlet_name = "";
            }

            //多态 --> 动态绑定
            CyanHttpServlet cyanHttpServlet = MyTomcat_Pro.servletMapping.get(servlet_name);
            if (cyanHttpServlet != null) {  //判断是否正常得到servlet实例
                cyanHttpServlet.service(cyanServletRequest, cyanServletResponse);
            } else {
                //如果没有找到servlet，返回404
                String resp = CyanServletResponse.respHeader + "<h1>404 Not Found!</h1>";
                OutputStream outputStream = cyanServletResponse.getOutputStream();
                outputStream.write(resp.getBytes());
                outputStream.flush();
                outputStream.close();
            }

            //释放资源
            socket.close();
        } catch (IOException e) {
            throw new RuntimeException(e);
        } finally {
            //确保Socket关闭
            if (socket != null) {
                try {
                    socket.close();
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
        }
    }
}

5° 容器启动测试

如下图所示（GIF）：

编辑

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JavaWeb 手写Tomcat底层机制

一、Tomcat底层整体架构

1.简介 :

2.分析图 :

3.基于Socket开发服务端的流程 :

4.打通服务器端和客户端的数据通道 :