在那个执行事务管理的地方和最后U调用方法的时候报空指针异常，怎么解决

Question

package dao;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.SQLException;import util.DBHelper;import entity.User;public class Userdao { Connection conn = null; PreparedStatement ptmt = null; public int add() { int updateRows = 0; try { conn.setAutoCommit(false);// 开启jdbc事务管理 Connection conn = DBHelper.getConnection(); String sql = "insert into chaoshi_user(userCode,UserName)" + "value(?,?)"; ptmt = conn.prepareStatement(sql); ptmt.setString(1, "chentao"); ptmt.setString(2, "陈涛"); updateRows = ptmt.executeUpdate(); conn.commit();// 开始执行事务 if (updateRows > 0) { System.out.println("add success"); } else { System.out.println("add failure"); } } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); try { conn.rollback(); } catch (SQLException e1) { // TODO Auto-generated catch block e1.printStackTrace(); } } finally { try { if(ptmt!=null){ ptmt.close();} if(conn!=null){ conn.close();} } catch (SQLException e) { // TODO Auto-generated catch block e.printStackTrace(); } } return updateRows; } public static void main(String[] args) { Userdao u = new Userdao();        u.add(); }}