我对Java EE编程非常陌生，并且应该做一些练习（以便很快完成学业）。
我的问题可能很简单，但是却找不到任何答案。
在一个练习中，我应该能够登录到java / jsp / mysql类型的服务。
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。传递表单时，应转到
ServletLogin.java以启动登录顺序。相反，我是从Glassfish得到的：
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，我们的辅导老师留下了一个注释，即整个项目应包括一个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告诉我我已经安装了，但是显然它没有安装或者不起作用。
这是
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();
}
}
我是否缺少某些东西，例如进口商品？
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，并且结果相同。我现在需要的只是能够查看代码是否有效，但是NullPointerException阻止了我这样做。通过Netbeans 8.0.2上的项目属性->库添加
mysql-connector-java-5.1.23-bin.jar似乎也无法解决。将MySQL注册到GlassFish，仍然适用于
NullPointerException。 最佳答案
转到服务标签
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。
关于java - 将MySQL驱动程序库添加到NetBeans项目，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/31013289/