1.Bean層（Model層）?

1. 角色：就像餐廳里的“菜品”。
2. ?功能：是純數據對象（如Person類），封裝屬性和 getter/setter（例如用戶名、密碼）。
3. ?示例：Person類

package com.bean;public class Person {private int id;private String name;private String pwd;public int getId() {return id;}public void setId(int id) {this.id = id;}public String getName() {return name;}public void setName(String name) {this.name = name;}public String getPwd() {return pwd;}public void setPwd(String pwd) {this.pwd = pwd;}
}

2.Dao層（Data Access Object）?

• 角色：后廚的“廚師”，專注做菜（操作數據）。
• ?功能：直接和數據庫對話，執行增刪改查（CRUD）。
• ?示例：UserDAO?類中的?saveUser(User user)?方法，負責把用戶數據存入數據庫。

接口

package com.dao;import com.bean.Person;import java.sql.SQLException;
import java.util.List;public interface PersonDao {public List<Person> queryAll() throws SQLException;public  Boolean selectOne(Person person) throws SQLException;public int save(Person person) throws SQLException;
}

實現接口

package com.dao.impl;import com.bean.Person;
import com.dao.PersonDao;
import com.db.DB;import java.sql.*;
import java.util.ArrayList;
import java.util.List;public class PersonDaoImpl implements PersonDao {// ------------------------- 方法1：查詢所有用戶 -------------------------@Overridepublic List<Person> queryAll() throws SQLException {Connection connection=new DB().getConnection(); //獲取數據庫連接Statement statement= connection.createStatement(); //創建SQL執行器ResultSet resultSet= statement.executeQuery("select * from person"); //執行查詢SQL語句List list=new ArrayList(); //準備空集合裝數據while (resultSet.next()){  //遍歷查詢結果（一行一行讀）Person p=new Person(); //新建數據盒子p.setId(resultSet.getInt(1));  //取第一列（id）放入盒子p.setName(resultSet.getString(2));  //取第二列（name）放入p.setPwd(resultSet.getString(3));  //取第三列（pwd）放入list.add(p); //把盒子加入集合}resultSet.close();statement.close();connection.close();return list;}// ------------------------- 方法2：檢查用戶名是否存在 -------------------------@Overridepublic Boolean selectOne(Person person) throws SQLException {//Connection connection=new DB().getConnection(); getConnection()為靜態方法就可以之間如下:Connection connection= DB.getConnection(); //獲取一個數據庫連接PreparedStatement statement= connection.prepareStatement("select * from person where name=?");statement.setString(1,person.getName()); //設置參數（替換第一個問號）ResultSet resultSet= statement.executeQuery(); //執行查詢if (resultSet.next()){return false;}else {return true;}}// ------------------------- 方法3：保存用戶 -------------------------@Overridepublic int save(Person person) throws SQLException {Connection connection= DB.getConnection();//  預編譯SQL（提高性能，防止注入）PreparedStatement statement= connection.prepareStatement("insert into person(name,pwd) values(?,?)");statement.setString(1,person.getName()); //設置第一個參數（name）statement.setString(2,person.getPwd()); //設置第二個參數（pwd）int i= statement.executeUpdate(); //執行插入操作statement.close();connection.close();return i; //返回影響的行數（1=成功，0=失敗）}
}

3.DB（驅動連接數據庫）

• ?角色：餐廳的“倉庫”，存儲所有食材。
• ?功能：持久化保存數據（如 MySQL、PostgreSQL）

package com.db;import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;public  class  DB {public static Connection getConnection(){Connection connection=null;try {Class.forName("com.mysql.cj.jdbc.Driver");connection= DriverManager.getConnection("jdbc:mysql://localhost:3306/users?serverTimezone=GMT%2B8","root","123456");} catch (ClassNotFoundException e) {e.printStackTrace();} catch (SQLException e) {e.printStackTrace();}return connection;}}

4.Service層

• 角色：餐廳的“經理”，協調復雜流程。
• ?功能：處理業務邏輯（例如注冊時檢查用戶名是否重復 + 加密密碼）。
• ?示例：UserService?的?registerUser()?方法會調用 DAO 的多個操作，并處理事務。

接口

package com.service;import com.bean.Person;import java.sql.SQLException;
import java.util.List;public interface PersonSercie {public List<Person> AllPerson() throws SQLException;public boolean register(Person person) throws SQLException;
}

實現接口

package com.service.impl;import com.bean.Person;
import com.dao.PersonDao;
import com.dao.impl.PersonDaoImpl;
import com.service.PersonSercie;import java.sql.SQLException;
import java.util.List;public class PersonServiceImpl implements PersonSercie {@Overridepublic List<Person> AllPerson() throws SQLException {PersonDao dao=new PersonDaoImpl(); // 1. 創建DAO對象，準備訪問數據庫return dao.queryAll();  // 2. 調用DAO查詢所有用戶}@Overridepublic boolean register(Person person) throws SQLException {PersonDao dao=new PersonDaoImpl();//檢查用戶名是否已存在if(dao.selectOne(person)){  //調用DAO查詢是否存在同名用戶dao.save(person); //無重復 → 保存用戶return true; //返回注冊成功}else {return false; //返回注冊失敗}}
}

5.Servlet（Controller層）?

• 角色：餐廳的“服務員”，接待客人并傳遞需求。
• ?功能：接收 HTTP 請求（如?POST /login），調用 Service 處理，返回響應（跳轉頁面或 JSON）。
• ?示例：LoginServlet?獲取表單參數，交給?UserService?驗證登錄。

刪除

package com.servlet;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.*;@WebServlet(name = "DeleteServlet",urlPatterns = "/del")
public class DeleteServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String id=request.getParameter("id");try {Class.forName("com.mysql.cj.jdbc.Driver");Connection connection= DriverManager.getConnection("jdbc:mysql://localhost:3306/users?serverTimezone=GMT%2B8","root","123456");//Statement statement= connection.createStatement();PreparedStatement statement= connection.prepareStatement("delete  from person where id=?");statement.setInt(1,Integer.parseInt(id));int i=statement.executeUpdate();if(i>0){response.sendRedirect("listall");}else {response.getWriter().println("Error");}statement.close();connection.close();} catch (ClassNotFoundException e) {e.printStackTrace();} catch (SQLException e) {e.printStackTrace();}}
}

查看

package com.servlet;import com.bean.Person;
import com.service.PersonSercie;
import com.service.impl.PersonServiceImpl;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.*;
import java.util.List;@WebServlet(name = "ListAllServlet",urlPatterns = "/listall")
public class ListAllServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {PersonSercie personSercie=new PersonServiceImpl();try {List<Person> list=personSercie.AllPerson();request.setAttribute("persons",list);request.getRequestDispatcher("listall.jsp").forward(request,response);} catch (SQLException e) {e.printStackTrace();}}
}

增加

package com.servlet;import com.bean.Person;
import com.service.PersonSercie;
import com.service.impl.PersonServiceImpl;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.SQLException;@WebServlet(name = "RegServlet",urlPatterns = "/reg")
public class RegServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String name=request.getParameter("uname");String pwd=request.getParameter("upwd");Person person=new Person();person.setName(name);person.setPwd(pwd);//調用模型中注冊業務邏輯實現PersonSercie personSercie = new PersonServiceImpl(); // 創建服務層對象（業務邏輯處理者）try {if(personSercie.register(person)){  // 調用服務層的注冊方法response.sendRedirect("login.jsp");   // 注冊成功：跳轉到登錄頁}else {response.sendRedirect("reg.jsp");// 注冊失敗：返回注冊頁};} catch (SQLException e) {e.printStackTrace();}}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}
}

改1

package com.servlet;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.SQLException;@WebServlet(name = "UpdateDoServlet",urlPatterns = "/updateDo")
public class UpdateDoServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String id=request.getParameter("id");String name=request.getParameter("username");String pwd=request.getParameter("userpwd");try {Class.forName("com.mysql.cj.jdbc.Driver");Connection connection= DriverManager.getConnection("jdbc:mysql://localhost:3306/users?serverTimezone=GMT%2B8","root","123456");//Statement statement= connection.createStatement();PreparedStatement statement= connection.prepareStatement("update person set name=?,pwd=? where  id=?");statement.setString(1,name);statement.setString(2,pwd);statement.setInt(3,Integer.parseInt(id));int i=statement.executeUpdate();if(i>0){response.sendRedirect("listall");}else {response.getWriter().println("Error");}statement.close();connection.close();} catch (ClassNotFoundException e) {e.printStackTrace();} catch (SQLException e) {e.printStackTrace();}}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}
}

改2

package com.servlet;import com.bean.Person;import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.sql.*;@WebServlet(name = "UpdateServlet",urlPatterns = "/update")
public class UpdateServlet extends HttpServlet {protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {}protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {String id=request.getParameter("id");try {Class.forName("com.mysql.cj.jdbc.Driver");Connection connection= DriverManager.getConnection("jdbc:mysql://localhost:3306/users?serverTimezone=GMT%2B8","root","123456");PreparedStatement statement= connection.prepareStatement("select *  from person where id=?");statement.setInt(1,Integer.parseInt(id));ResultSet resultSet=statement.executeQuery();Person p=new Person();if (resultSet.next()){p.setId(resultSet.getInt(1));p.setName(resultSet.getString(2));p.setPwd(resultSet.getString(3));}request.setAttribute("user",p);resultSet.close();statement.close();connection.close();request.getRequestDispatcher("update.jsp").forward(request,response);} catch (ClassNotFoundException e) {e.printStackTrace();} catch (SQLException e) {e.printStackTrace();}}
}

總結

大概流程：就像網購流程：JSP→ 你下單（Servlet收件）→ 調度中心檢查（Service處理）→ 倉庫存取（DAO操作→倉庫DB）→ 最終收到包裹（響應結果）

1. ?用戶填寫表單
   👉 在?reg.jsp?頁面輸入用戶名和密碼，點擊提交。

2. ?Servlet接收請求
   👉?RegServlet?像快遞員一樣收到包裹（HTTP請求），拆開包裹取出數據（request.getParameter）。

3. ?封裝成Bean
   👉 把數據裝進?Person?盒子（person.setName(name)）。

4. ?Service處理業務
   👉 調度員（PersonServiceImpl）開始工作：

   • 問倉庫管理員（DAO）：“這個用戶名有人用嗎？”（selectOne）
   • 如果沒人用，告訴管理員：“存進倉庫！”（save）

5. ?DAO操作數據庫
   👉 管理員打開倉庫（DB連接），執行 SQL 存入數據。

6. ?返回結果給用戶
   👉 Servlet 根據結果，跳轉到登錄頁（成功）或返回注冊頁（失敗）。

至于Servlet中doGet和doPost方法該使用哪個

?