jsp连接数据库，tomcat服务器启动后第一次打开正常，再刷新就报空指针异常。

jsp页面：<%@ page language="java" contentType="text/html; charset=utf-8"%>
<%@ page import="dao.TDao" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Insert title here</title>
</head>
<body>
<%
TDao t=new TDao();
out.print(t.getLastDate());
%>
</body>
</html>

DBHelper.java

package tools;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

public class DBHelper {
private static String driver="com.mysql.jdbc.Driver";
private static String username="root";
private static String password="root";
private static String url="jdbc:mysql://localhost:3306/datamanagement";
private static Connection conn=null;
static {
  try {
    Class.forName(driver);
  } catch (ClassNotFoundException e) {
   // TODO Auto-generated catch block
   e.printStackTrace();
  }
}
public static Connection getConnection() {

  if(conn==null) {
   try {
    conn=DriverManager.getConnection(url,username,password);
   } catch (SQLException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
   }
   return conn;
  }else {
   return null;
  }
}

public static void main(String[] args) {
  Connection conn=DBHelper.getConnection();
  if(conn==null) {
   System.out.println("连接失败");
  }else {
   System.out.println("连接成功!!!");
  }
}
}

TDao.java

package dao;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;

import tools.DBHelper;

public class TDao {

public String getLastDate() {
  String lastDate=null;
  PreparedStatement stmt=null;
  ResultSet rs=null;
    try {
     Connection conn=DBHelper.getConnection();
     String sql="select riqi from daily_report order by riqi desc";
     stmt=conn.prepareStatement(sql);
     rs=stmt.executeQuery();
     rs.next();
     lastDate=rs.getString("riqi");
    }catch(Exception e) {
     e.printStackTrace();
    }finally {
     try {
      rs.close();
      stmt.close();
     }catch(Exception e) {
      e.printStackTrace();
     }
    }
  return lastDate;
}

public static void main(String[] args) {
  TDao t=new TDao();
  System.out.println(t.getLastDate());
}
}

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