1、简要概述web容器原理

1. servlet生命周期方法：init(), service(), destory()
   大部分的框架，如struts, springmvc等都是基于servlet作为主入口，实际上是WEB请求的一个入口。
2. J2EE中有两个非常重要的对象：HttpRequest,HttpResponse,但是这两个对象从来不是由程序员单独创建的，而是由web容器自动创建的。

1.1 从模拟tomcat开始

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;

public class SimpleServer {
    //模拟tomcat
    public static void main(String[] args) {
        int port = 8080;
        try (ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("Server is listening on port " + port);
            while (true) {
                Socket clientSocket = serverSocket.accept();
                // 开启新线程处理客户端，支持多连接
                new Thread(() -> handleClient(clientSocket)).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void handleClient(Socket clientSocket) {
        System.out.println("New client connected：" + clientSocket.getRemoteSocketAddress());
        BufferedReader in = null;
        PrintWriter out = null;
        try {
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            out = new PrintWriter(clientSocket.getOutputStream(), true);

            String clientMessage;
            while ((clientMessage = in.readLine()) != null) {
                System.out.println("Client says: " + clientMessage);
                out.println(clientMessage);
            }

        } catch (SocketException e) {
            // 【重点】捕获客户端强制断开连接异常
            String msg = e.getMessage();
            if (msg.contains("中止了一个已建立的连接") || msg.contains("Connection aborted")) {
                System.out.println("WARN：客户端强制断开连接");
            } else {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (in != null) in.close();
                if (out != null) out.close();
                if (!clientSocket.isClosed()) {
                    clientSocket.close();
                }
                System.out.println("Client disconnected");
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

浏览器输入：localhost:8080,查看控制台输出

"D:\Program Files\Java\jdk-21\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2026.1\lib\idea_rt.jar=55073" -Dfile.encoding=UTF-8 -Dsun.stdout.encoding=UTF-8 -Dsun.stderr.encoding=UTF-8 -classpath D:\project\deepseek\DNTomcat\out\production\DNTomcat SimpleServer
Server is listening on port 8080
New client connected：/[0:0:0:0:0:0:0:1]:55087
Client says: GET / HTTP/1.1
Client says: Host: localhost:8080
Client says: Connection: keep-alive
Client says: Cache-Control: max-age=0
Client says: sec-ch-ua: "Not;A=Brand";v="8", "Chromium";v="150", "Google Chrome";v="150"
Client says: sec-ch-ua-mobile: ?0
Client says: sec-ch-ua-platform: "Windows"
Client says: Upgrade-Insecure-Requests: 1
Client says: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/150.0.0.0 Safari/537.36
Client says: Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Client says: Sec-Fetch-Site: none
Client says: Sec-Fetch-Mode: navigate
Client says: Sec-Fetch-User: ?1
Client says: Sec-Fetch-Dest: document
Client says: Accept-Encoding: gzip, deflate, br, zstd
New client connected：/[0:0:0:0:0:0:0:1]:55089
New client connected：/[0:0:0:0:0:0:0:1]:55088
Client says: Accept-Language: zh-CN,zh;q=0.9
Client says: 
WARN：客户端强制断开连接
Client disconnected
Client says: GET / HTTP/1.1
Client says: Host: localhost:8080
Client says: Connection: keep-alive
Client says: Cache-Control: max-age=0
Client says: sec-ch-ua: "Not;A=Brand";v="8", "Chromium";v="150", "Google Chrome";v="150"
Client says: sec-ch-ua-mobile: ?0
Client says: sec-ch-ua-platform: "Windows"
Client says: Upgrade-Insecure-Requests: 1
Client says: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/150.0.0.0 Safari/537.36
Client says: Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Client says: Sec-Fetch-Site: none
Client says: Sec-Fetch-Mode: navigate
Client says: Sec-Fetch-User: ?1
Client says: Sec-Fetch-Dest: document
Client says: Accept-Encoding: gzip, deflate, br, zstd
Client says: Accept-Language: zh-CN,zh;q=0.9
Client says: 
WARN：客户端强制断开连接
Client disconnected
New client connected：/[0:0:0:0:0:0:0:1]:55090
Client says: GET / HTTP/1.1
Client says: Host: localhost:8080
Client says: Connection: keep-alive
Client says: Cache-Control: max-age=0
Client says: sec-ch-ua: "Not;A=Brand";v="8", "Chromium";v="150", "Google Chrome";v="150"
Client says: sec-ch-ua-mobile: ?0
Client says: sec-ch-ua-platform: "Windows"
Client says: Upgrade-Insecure-Requests: 1
Client says: User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/150.0.0.0 Safari/537.36
Client says: Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Client says: Sec-Fetch-Site: none
Client says: Sec-Fetch-Mode: navigate
Client says: Sec-Fetch-User: ?1
Client says: Sec-Fetch-Dest: document
Client says: Accept-Encoding: gzip, deflate, br, zstd
Client says: Accept-Language: zh-CN,zh;q=0.9
Client says: 
WARN：客户端强制断开连接
Client disconnected
New client connected：/[0:0:0:0:0:0:0:1]:55091
Client disconnected
Client disconnected

从上面的clientMessage参数可以获取到请求的数据，包括我们日常开发中常见的url,method，Connection: keep-alive等

2. 自定义Http的servlet

整体设计思路

1.基于当前 SimpleServer 简易 TCP 服务改造，模拟 HTTP 1.0 短连接；
2.自定义 HttpServletRequest：解析客户端 HTTP 请求行、请求头、请求参数；
3.自定义 HttpServletResponse：封装输出流、响应状态码、响应头、响应正文；
4.抽象 HttpServlet 父类，提供 doGet() 模板方法；
5.改造 handleClient 读取原始 TCP 字节，封装请求对象，调用 doGet 输出 HTTP 响应。

2.1. 模拟 HttpServletRequest

package http.impl;

import java.io.BufferedReader;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

/**
 * 模拟 HttpServletRequest，解析HTTP请求行、请求头、GET参数
 */
public class HttpServletRequest {

    // 请求行：GET /demo?name=test HTTP/1.1
    private String method;
    private String requestUri;
    private String httpVersion;
    // 纯路径
    private String path;
    // GET查询参数
    private Map<String, String> parameterMap = new HashMap<>();
    // 请求头集合
    private Map<String, String> headerMap = new HashMap<>();
    // 原始输入流
    private BufferedReader reader;

    public HttpServletRequest(BufferedReader reader) throws IOException {
        this.reader = reader;
        parseRequestLine();
        parseHeaders();
    }

    // 解析第一行 请求行 GET /xxx HTTP/1.1
    private void parseRequestLine() throws IOException {
        String line = reader.readLine();
        if (line == null) return;
        String[] parts = line.split(" ");
        this.method = parts[0];
        String uri = parts[1];
        this.httpVersion = parts[2];

        // 拆分路径与GET参数
        if (uri.contains("?")) {
            String[] uriArr = uri.split("\\?");
            this.path = uriArr[0];
            parseQueryParams(uriArr[1]);
        } else {
            this.path = uri;
        }
        this.requestUri = uri;
    }

    // 解析URL拼接的GET参数
    private void parseQueryParams(String queryStr) {
        String[] params = queryStr.split("&");
        for (String param : params) {
            String[] kv = param.split("=");
            if (kv.length == 2) {
                parameterMap.put(kv[0], kv[1]);
            }
        }
    }

    // 解析所有请求头（空行停止）
    private void parseHeaders() throws IOException {
        String line;
        while ((line = reader.readLine()) != null && !line.isEmpty()) {
            String[] header = line.split(":", 2);
            headerMap.put(header[0].trim(), header[1].trim());
        }
    }

    // ========== 对外提供API ==========
    public String getMethod() {
        return method;
    }

    public String getRequestURI() {
        return requestUri;
    }

    public String getPathInfo() {
        return path;
    }

    public String getParameter(String name) {
        return parameterMap.get(name);
    }

    public Map<String, String> getParameterMap() {
        return parameterMap;
    }

    public String getHeader(String name) {
        return headerMap.get(name);
    }
}

2.2 模拟 HttpServletResponse

package http.impl;

import java.io.PrintWriter;

/**
 * 模拟 HttpServletResponse，输出标准HTTP响应报文
 */
public class HttpServletResponse {
    private PrintWriter out;
    private int status = 200;
    private String msg = "OK";
    private StringBuilder body = new StringBuilder();

    public HttpServletResponse(PrintWriter out) {
        this.out = out;
    }

    // 设置响应状态码
    public void setStatus(int code, String message) {
        this.status = code;
        this.msg = message;
    }

    // 向响应体写入内容
    public void write(String content) {
        body.append(content);
    }

    // 一次性输出完整HTTP报文到客户端
    public void flush() {
        // 响应首行 HTTP/1.1 200 OK
        out.printf("HTTP/1.1 %d %s\r\n", status, msg);
        // 基础响应头
        out.println("Content-Type: text/html;charset=utf-8");
        out.println("Server: SimpleJavaMockTomcat");
        out.println("Connection: close");
        out.println(); // 空行分隔头与正文
        // 响应正文
        out.print(body);
        out.flush();
    }
}

2.3 抽象 HttpServlet 模板父类

package http.impl;

public abstract class HttpServlet {
    /**
     * 子类重写此方法处理GET请求
     */
    protected abstract void doGet(HttpServletRequest request, HttpServletResponse response);

    /**
     * 统一分发请求
     */
    public void service(HttpServletRequest request, HttpServletResponse response) {
        String method = request.getMethod();
        if ("GET".equalsIgnoreCase(method)) {
            doGet(request, response);
        } else {
            response.setStatus(405, "Method Not Allowed");
            response.write("仅支持GET请求");
        }
        response.flush();
    }
}

2.4 业务 Servlet 实现类

package http.impl;

public class DemoServlet extends HttpServlet{

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) {
        // 1. 获取路径与GET参数
        String path = request.getPathInfo();
        String name = request.getParameter("name");
        String ua = request.getHeader("User-Agent");

        // 2. 组装HTML响应
        response.write("<h1>模拟GET请求处理成功</h1>");
        response.write("<p>请求路径：" + path + "</p>");
        response.write("<p>GET参数name：" + (name == null ? "无" : name) + "</p>");
        response.write("<p>客户端UA：" + ua + "</p>");
    }
}

2.5 改造SimpleServer.java

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.SocketException;

import http.impl.DemoServlet;
import http.impl.HttpServlet;
import http.impl.HttpServletRequest;
import http.impl.HttpServletResponse;

public class SimpleServer {

    //模拟简易Tomcat
    public static void main(String[] args) {
        int port = 8080;
        try (ServerSocket serverSocket = new ServerSocket(port)) {
            System.out.println("Mock Tomcat 启动，监听端口：" + port);
            while (true) {
                Socket clientSocket = serverSocket.accept();
                // 开启独立线程处理每个客户端HTTP连接
                new Thread(() -> handleClient(clientSocket)).start();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void handleClient(Socket clientSocket) {
        System.out.println("新客户端接入：" + clientSocket.getRemoteSocketAddress());
        BufferedReader in = null;
        PrintWriter out = null;
        try {
            in = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
            out = new PrintWriter(clientSocket.getOutputStream(), true);

            // 1. 把原始IO封装成模拟Request/Response
            HttpServletRequest request = new HttpServletRequest(in);
            HttpServletResponse response = new HttpServletResponse(out);

            // 2. 创建业务Servlet，分发执行doGet
            HttpServlet servlet = new DemoServlet();
            servlet.service(request, response);

        } catch (SocketException e) {
            String msg = e.getMessage();
            if (msg.contains("中止了一个已建立的连接") || msg.contains("Connection aborted")) {
                System.out.println("WARN：客户端强制断开连接");
            } else {
                e.printStackTrace();
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            try {
                if (in != null) in.close();
                if (out != null) out.close();
                if (!clientSocket.isClosed()) {
                    clientSocket.close();
                }
                System.out.println("客户端连接关闭");
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

2.6 测试

1.启动 SimpleServer；
2.浏览器访问地址：http://127.0.0.1:8080/demo?name=zhangsan

"D:\Program Files\Java\jdk-21\bin\java.exe" "-javaagent:D:\Program Files\JetBrains\IntelliJ IDEA 2026.1\lib\idea_rt.jar=55374" -Dfile.encoding=UTF-8 -Dsun.stdout.encoding=UTF-8 -Dsun.stderr.encoding=UTF-8 -classpath D:\project\deepseek\DNTomcat\out\production\DNTomcat SimpleServer
Mock Tomcat 启动，监听端口：8080
新客户端接入：/127.0.0.1:55395
新客户端接入：/127.0.0.1:55396
客户端连接关闭
客户端连接关闭