将MySQL驱动程序库添加到NetBeans项目

我对Java EE编程还是个新手，我应该做一些关于它的练习（很快就要完成学业了）。我的问题可能很简单，但我还没有找到答案。在一个练习中，我应该能够登录到java/jsp/mysql类型的服务。
Login.htm

<html>
<head>
<title>Login</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<link rel="stylesheet" href="css/style.css" />
</head>

<body>
<h3>Login</h3>
<form name="login" method="post" action="ServletLogin">
<table>
    <tr> 
      <td>Email:</td>
      <td><input name="email" type="text" id="email"></td>
    </tr>
    <tr> 
      <td>Salasana</td>
      <td><input name="password" type="password" id="password"></td>
    </tr>
    <tr> 
      <td colspan="2">
          <input type="submit" name="Submit" value="Log in;">
        </td>
    </tr>
  </table>
</form>
</body>
</html>

非常标准的登录，是的。当传递表单时，它应该转到一个ServletLogin.java来启动登录序列。相反，我从Glassfish那里得到了这个：

HTTP Status 500 - Internal Server Error

type Exception report

messageInternal Server Error

descriptionThe server encountered an internal error that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
note The full stack traces of the exception and its root causes are available in the GlassFish Server Open Source Edition 4.1 logs.

GlassFish Server Open Source Edition 4.1

在ServletLogin.java中，我们的导师留下了一个备注，整个项目应该包括一个MySQL驱动库，以避免NullPointerException。
我的问题是，我如何使用Netbeans做到这一点？Netbeans告诉我我已经安装了，但显然它要么没有安装，要么不工作。
下面是ServletLogin.java代码：

/*
 * Remember to install a MYSQL driver library
 * or the file will alert
 * NullPointerException.
 */

package Servlets; //the package in which the Servlet is part of.

import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@WebServlet(name = "ServletLogin", urlPatterns = {"/ServletLogin"})
public class ServletLogin extends HttpServlet {

    Connection conn = null;

    /**
     * Initializes the servlet.
     */
    @Override
    public void init(ServletConfig config) throws ServletException {
        super.init(config);

        //Connect to database with database-class
        try {
        conn = Classes.SQL.openConnection();
        }
        catch (Exception e) {
           System.out.println("Cannot connect to database " + e);
        }
    }

    /**
     * Destroys the servlet.
     */
    @Override
    public void destroy() {

    //Closing the database connection
         try {
          conn.close();
        } catch ( SQLException se ) {
           System.out.println("Exception " + se);
        }
    }

    //Everything is done in doPost method.
    //This Servlet does not use doGet() or ProcessRequest()

    /** 
     * Handles the HTTP <code>POST</code> method.
     * @param request servlet request
     * @param response servlet response
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {

    PrintWriter out = response.getWriter();

    /*Unless there is a session, one is created.
        It is used to check if the user has logged in.*/

     HttpSession session = request.getSession(true);

        //Reading the parameters from login form.
        String email = request.getParameter("email");
        String password = request.getParameter("password");

        out.print(email);

        //Created login_ok and set it to false by default.
        boolean login_ok = false;

     try {
     //let's create a Statement with which the sql can be run with.
     Statement stmt = conn.createStatement();

     //Run SQL and save it to ResultSet rs.
     ResultSet rs = stmt.executeQuery("SELECT email, password FROM clients");

     //Go trough the results with while loop and next-method, which returns the value true, until we've reached last result.
     while(rs.next())
     {
       //Reading the data
       String email2 = rs.getString("email");
       String salasana2 = rs.getString("password);

       //If user input has been found from database,
       //login_ok becomes true and looping ends.
       if(email.compareTo(email2) == 0 && password.compareTo(password2) == 0)
       {
           login_ok = true;
           break;
       }

     }

     //If login_ok is true, save info about logging to session and guide the user to Clients.jsp
        if( login_ok == true )
        {
            //session is informed about login
            session.setAttribute("login", "ok");
            //Debugging printed to console.
            System.out.println("Login ok");
            //Proceeding to clients
            response.sendRedirect("clients.jsp");
            //Return stops the servlet run.
            return;
        }
      //Login false -> redirected to login page.
        else {
            response.sendRedirect("login.htm");
        }

    }

     catch(SQLException se){

     out.println("Error: " + se);

     }

        out.close();
    }

}

我是不是遗漏了什么东西，比如在导入或？我已经安装了mysql-connector-java-5.1.23-bin.jar，并将其设置到多个不同的位置，结果都是一样的。我现在需要的是能够看到代码是否工作，但是NullPointerException阻止了我这样做。通过项目属性-〉库在Netbeans 8.0.2上添加mysql-connector-java-5.1.23-bin.jar似乎并不能解决这个问题，注册MySQL到GlassFish，仍然是NullPointerException。

将MySQL驱动程序库添加到NetBeans项目

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