使用servlet从mysql检索数据时出现空指针异常

import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class MyServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();

        Connection conn = null;
        PreparedStatement stmt = null;
        ResultSet rs = null;

        try {
            // 连接数据库
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/mydatabase", "username", "password");

            // 执行查询
            String sql = "SELECT * FROM mytable";
            stmt = conn.prepareStatement(sql);
            rs = stmt.executeQuery();

            // 处理查询结果
            if (rs.next()) {
                // 获取数据并进行处理
                String data = rs.getString("column_name");
                out.println("Data: " + data);
            } else {
                out.println("No data found.");
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            // 关闭资源
            try {
                if (rs != null) {
                    rs.close();
                }
                if (stmt != null) {
                    stmt.close();
                }
                if (conn != null) {
                    conn.close();
                }
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}