为什么我无法在jsp中调用这个函数？

我写了这么一个函数用来核对用户名密码是否匹配

public static boolean login(String account,String pass) throws SQLException {
        String sql = "SELECT * FROM USER WHERE USER_NAME='"+account+"' OR EMAIL = '"+account+"'";//user表中有username和email两个值可以充当用户名，两者只要有一个满足即可。
        Connection conn = Conn.getConnection();
        conn.setAutoCommit(false);
        PreparedStatement statement = conn.prepareStatement(sql);
        ResultSet result = statement.executeQuery();
        while(result.next()) {
            String password = result.getString("password");
            if(pass.equals(password))return true;
        }
        return false;
    }

在login.jsp中读取输入的账号密码：

<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Insert title here</title>
</head>
<body>
    <form action="logincheck.jsp" method="post">
        <table>
            <tr><td>用户名</td><td><input name="username" type="text"></td></tr>
            <tr><td>密    码</td><td><input name="pass" type="password"></td></tr>
            <tr><td><input type="submit" value="登录"></td></tr>
        </table>
    </form>
</body>
</html>

在logincheck.jsp中调用login函数进行核对：

<%@page import="com.ahtcm.zcy.controller.dao.UserDao"%>
<%@page import="java.awt.Window"%>
<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Insert title here</title>
</head>
<body>
    <%
    request.setCharacterEncoding("utf-8");
    String user = request.getParameter("username");
    String pass = request.getParameter("pass");
    boolean isloged = UserDao.login(user, pass);
    //out.print(user);out.print(pass);
    if(isloged)out.print("ok");
    %>

</body>
</html>

但是在运行中却报出一下错误信息：

HTTP Status 500 – Internal Server Error
Type Exception Report

Message An exception occurred processing [/logincheck.jsp] at line [15]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.apache.jasper.JasperException: An exception occurred processing [/logincheck.jsp] at line [15]

12:     request.setCharacterEncoding("utf-8");
13:     String user = request.getParameter("username");
14:     String pass = request.getParameter("pass");
15:     boolean isloged = UserDao.login(user, pass);
16:     out.print(user);out.print(pass);
17:     if(isloged)out.print("ok");
18:     %>


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:626)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:515)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Root Cause

java.lang.NullPointerException
    com.ahtcm.zcy.controller.dao.UserDao.login(UserDao.java:18)
    org.apache.jsp.logincheck_jsp._jspService(logincheck_jsp.java:135)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:477)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:386)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:330)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Note The full stack trace of the root cause is available in the server logs.

请教一下大佬到底是哪里出现了问题？

补充：
在发现问题后我也尝试过在java中调用该函数：

package com.ahtcm.zcy;

import java.sql.SQLException;
import com.ahtcm.zcy.controller.dao.UserDao;
import com.ahtcm.zcy.model.User;

public class Test {
    public static void login(User user) throws SQLException {
        if(UserDao.login(user.getUserName(), user.getPass())) {
            System.out.println("ok!");
        }
        else System.out.println("N/A");

        if(UserDao.login(user.getEmail(), user.getPass())) {
            System.out.println("ok!");
        }
        else System.out.println("N/A");
    }



    public static void main(String[] args) throws SQLException {
        User user = new User();
        user.setUserName("testaccount1");
        user.setPass("123456");
        user.setEmail("testaccount1@test.com");
        user.setMob("***********");
        Test.login(user);
    }
}

实际上此时函数可以被正常调用。
我也曾猜测是否是因为在logincheck.jsp中没有正常读取username和pass的值，但是经过测试发现这两个参数值被正常读取：

<body>
    <%
    request.setCharacterEncoding("utf-8");
    String user = request.getParameter("username");
    String pass = request.getParameter("pass");
    //boolean isloged = UserDao.login(user, pass);
    out.print(user);out.print(pass);
    //if(isloged)out.print("ok");
    %>

</body>