写一段servlet+JDBC完成MVC架构下的增删改查的代码

在MVC（Model-Controller）架构下，Servlet通常作为控制器，负责处理HTTP请求，而JDBC用于数据库操作。下面是一个简单的示例，展示如何使用Servlet和JDBC实现CRUD操作：

// 导入必要的包
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.sql.*;

public class UserController extends HttpServlet {
    
    private static final String DATABASE_URL = "jdbc:mysql://localhost:3306/mydb";
    private static final String USER_TABLE = "users";
    private static final String USER_ID_COLUMN = "id";
    private static final String USER_NAME_COLUMN = "name";

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 获取数据
        int id = Integer.parseInt(request.getParameter("id"));
        String action = request.getParameter("action");
        String userName = request.getParameter("username");

        String sql;
        try (Connection conn = DriverManager.getConnection(DATABASE_URL, "root", "password");) {
            if ("add".equals(action)) {
                sql = "INSERT INTO " + USER_TABLE + "(name) VALUES (?)";
                PreparedStatement pstmt = conn.prepareStatement(sql);
                pstmt.setString(1, userName);
                pstmt.executeUpdate();
                response.sendRedirect("view-users.jsp");
            } else if ("update".equals(action)) {
                sql = "UPDATE " + USER_TABLE + " SET name=? WHERE " + USER_ID_COLUMN + "=?";
                PreparedStatement pstmt = conn.prepareStatement(sql);
                pstmt.setString(1, userName);
                pstmt.setInt(2, id);
                pstmt.executeUpdate();
                response.sendRedirect("view-users.jsp");
            } else if ("delete".equals(action)) {
                sql = "DELETE FROM " + USER_TABLE + " WHERE " + USER_ID_COLUMN + "=?";
                PreparedStatement pstmt = conn.prepareStatement(sql);
                pstmt.setInt(1, id);
                pstmt.executeUpdate();
                response.sendRedirect("view-users.jsp");
            } else {
                throw new IllegalArgumentException("Invalid action: " + action);
            }
        } catch (SQLException e) {
            response.getWriter().println("Error: " + e.getMessage());
        }
    }

    // 其他方法如get请求、异常处理等可根据需要添加

}

在这个例子中，`doPost`方法处理POST请求，并根据传来的`action`参数执行对应的增删改操作。注意，这是一个简化的示例，实际生产环境中需要更全面的错误处理以及安全性措施。