将MySQL驱动程序库添加到NetBeans项目中

我对Java EE编程还很陌生，我应该做一些练习（很快完成我的学业）。我的问题可能很简单，但我还没有找到任何答案。在一个练习中，我应该能够登录到java/jsp/mysql类型的服务。

<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>

是的，相当标准的登录。传递表单时，它应该转到ServletLogin.java以启动登录序列。相反，我从Glassfish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注释，整个项目应该包括一个MySQL驱动程序库，以避免NullPointerException。

我的问题是，如何使用Netbeans做到这一点？Netbeans告诉我已经安装了，但显然它要么没有，要么不起作用。

这里是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();
    }

}

我是不是错过了什么，比如进口还是？我已经安装了mysql-connector-java-5.1.23-bin.jar，并将其设置在多个不同的位置，结果相同。我现在所需要的只是能够查看代码是否有效，但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似乎也不能解决问题。已将MySQL注册到GlassFish，仍然使用NullPointerException。

转到"服务"选项卡
选择数据库
右键单击并选择"注册MYSQL驱动程序"
右键单击项目中的库，然后从添加库中选择MYSQL

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