当我运行管理员登录表单时，出现nullpointerexception错误。请解决这个问题。谢谢 [英] When I run admin login form , then nullpointerexception error appear.please solve this my issue.thanks
问题描述
//这是管理员登录表单
//This is Admin Login form
package Admin_Login;
import AdminPanel.AdminSection;
import database.db;
import java.sql.*;
import javax.swing.JOptionPane;
public class AdminLogin extends javax.swing.JFrame
{
Connection conn;
PreparedStatement pst;
ResultSet rs;
AdminSection as;
public AdminLogin()
{
initComponents();
}
private void LoginActionPerformed(java.awt.event.ActionEvent evt) {
String sql="select id,username,password from Users where(username=? and password=?)";
try
{
int count =0;
pst = conn.prepareStatement(sql);
pst.setString(1, username.getText());
pst.setString(2, password.getText());
rs =pst.executeQuery();
while(rs.next())
{
count =count+1;
}
if(count==1)
{
JOptionPane.showMessageDialog(null, "Success Login");
as= new AdminSection();
as.setVisible(true);
this.dispose();
}
else
{
JOptionPane.showMessageDialog(null, "Username and
Password you entered are not valid");
}
}
catch(Exception e)
{
}
finally
{
try
{
pst.close();
}
catch(Exception e)
{
JOptionPane.showMessageDialog(null, e);
}
}
}
/* Create and display the form */
java.awt.EventQueue.invokeLater(new Runnable() {
public void run()
{
new AdminLogin().setVisible(true);
}
});
}
// Variables declaration - do not modify
private javax.swing.JButton Login;
private javax.swing.JLabel jLabel1;
private javax.swing.JLabel jLabel2;
private javax.swing.JLabel jLabel3;
private javax.swing.JPanel jPanel1;
private javax.swing.JTextField password;
private javax.swing.JTextField username;
// End of variables declaration
}
**
//这是数据库类
------------------------
**
**
//This is Database Class
------------------------
**
package database;
import Classes.add_Librarian;
import java.sql.*;
import javax.swing.*;
public class db
{
private Connection conn;
private Statement st;
private ResultSet rs;
public static Connection establishConnection()
{
try
{
Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
Connection conn=DriverManager.getConnection("jdbc:sqlserver://localhost\\sqlexpress:1433; database=Employee Payroll; user=pak; password=123");
if(conn==null)
{
JOptionPane.showMessageDialog(null, "Not Connected");
}
else
{
JOptionPane.showMessageDialog(null, "Connected");
}
return conn;
}catch(Exception ex)
{
JOptionPane.showMessageDialog(null,"Connection not established");
return null;
}
}
public int addlibrarian (add_Librarian lb)
{
int n=0;
try
{
n=st.executeUpdate("insert into Add_Librarian values ('"+lb.getName()+"','"+lb.getFatherName()+"',"
+lb.getID_CardNo()+"','"+lb.getQualification()+"','"+lb.getContactNo()+"','"+lb.getAddress()+"')");
}
catch(Exception e)
{
JOptionPane.showMessageDialog(null,e);
}
return n;
}
public void closeConnection()
{
try
{
conn.close();
}
catch(Exception ex)
{
JOptionPane.showMessageDialog(null,ex);
}
}
}
我尝试过：
//这是管理员登录表格
H，当我运行此AdminLogin表单时，我一次又一次地面对
Java.Lang.NullPointerException错误的错误。请，任何人，解决我的问题.T hanks。
我附加了数据库DB类和AdminLogin表单代码。
--------- -------------------------------------------------- -------------
*
What I have tried:
//This is Admin Login form
H, When I run this AdminLogin form, then I face again and again Error of
Java.Lang.NullPointerException error. Please, Anybody, solve my problem.T hanks.
I have attached code of database DB class and AdminLogin Form code both.
------------------------------------------------------------------------
*
推荐答案
忽略你已经注意到的问题......你有更大的你不是;知道。
永远不要连接字符串来构建SQL命令。它让您对意外或故意的SQL注入攻击持开放态度，这可能会破坏您的整个数据库。总是使用参数化查询。
连接字符串时会导致问题，因为SQL会收到如下命令：
Ignore ing the problem you have noticed for a moment ... you have bigger ones you arent; aware of.
Never concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Always use Parameterized queries instead.
When you concatenate strings, you cause problems because SQL receives commands like:
SELECT * FROM MyTable WHERE StreetAddress = 'Baker's Wood'就SQL而言，用户添加的引号会终止字符串，并且您会遇到问题。但情况可能更糟。如果我来并改为输入：x'; DROP TABLE MyTable; - 然后SQL收到一个非常不同的命令：
The quote the user added terminates the string as far as SQL is concerned and you get problems. But it could be worse. If I come along and type this instead: "x';DROP TABLE MyTable;--" Then SQL receives a very different command:
SELECT * FROM MyTable WHERE StreetAddress = 'x';DROP TABLE MyTable;--'哪个SQL看作三个单独的命令：
Which SQL sees as three separate commands:
SELECT * FROM MyTable WHERE StreetAddress = 'x';完全有效的SELECT
A perfectly valid SELECT
DROP TABLE MyTable;完全有效的删除表格通讯和
A perfectly valid "delete the table" command
--'其他一切都是评论。
所以它确实：选择任何匹配的行，从数据库中删除表，并忽略其他任何内容。
所以总是使用参数化查询！或者准备好经常从备份中恢复数据库。你经常定期备份，不是吗？
我知道你知道如何 - 你的登录代码确实如此 - 所以总是这样做 - 或者你的INSERT代码会咬你，很难。并且您将不知道是谁做了，或者何时或如何修复它...
参数化查询是关于您的登录代码唯一正确的问题...切勿以明文形式存储密码 - 这是一个主要的安全风险。有关如何在此处执行此操作的信息：密码存储：如何做到这一点。 [ ^ ] - 代码在C＃中，但对于有Java经验的人来说应该是非常明显的。
您注意到的问题是什么？我们无法为您解决问题，因为我们无法在您执行的情况下运行您的代码;我们无权访问您的数据库输入。因此，您可以自行修复它。这是我们被问到的最常见问题之一，也是我们最不能回答的问题，但你最有能力回答自己。
让我只是解释错误的含义：您尝试使用变量，属性或方法返回值，但它包含null - 这意味着变量中没有类的实例。
它有点像一个口袋：你的衬衫里有一个口袋，用来握笔。如果你进入口袋并发现那里没有笔，你就不能在一张纸上签名 - 如果你尝试的话，你会得到非常有趣的外观！空口袋给你一个空值（这里没有笔！）所以你不能做任何你检索笔通常做的事情。它为什么空？这就是问题 - 可能是你今天早上离开家时忘了拿起你的笔，或者你昨晚把它拿到昨天的衬衫口袋里时可能会把笔留下来。
我们无法分辨，因为我们不在那里，更重要的是，我们甚至看不到你的衬衫，更不用说口袋里的东西了！
回到计算机，你做了同样的事情，不知何故 - 我们看不到你的代码，更不用说运行它了，找不到包含null的东西。
但是你可以 - 而Visual Studio将在这里帮助你。在调试器中运行您的程序，当它失败时，VS将向您显示它发现问题的行。然后，您可以开始查看它的各个部分，以查看哪个值为null，并开始回顾代码以找出原因。因此，在包含错误行的方法的开头放置一个断点，然后从头再次运行程序。这一次，VS会在错误发生前停止，让你通过查看代码来查看你的价值来检查发生了什么。
但我们做不到那 - 我们没有您的代码，如果我们拥有它，我们不知道如何使用它，我们没有您的数据。所以试试吧 - 看看你能找到多少信息！
And everything else is a comment.
So it does: selects any matching rows, deletes the table from the DB, and ignores anything else.
So ALWAYS use parameterized queries! Or be prepared to restore your DB from backup frequently. You do take backups regularly, don't you?
I know you know how to - your login code does - so always do it - or your INSERT code is going to bite you, hard. And you will have no idea who did it, or when, or how to fix it ...
And parameterised queries are about the only thing your login code gets right ... Never store passwords in clear text - it is a major security risk. There is some information on how to do it here: Password Storage: How to do it.[^] - the code is in C#, but it should be pretty obvious to anyone with Java experience.
And the problem you noticed? We can't fix it for you, because we can't run your code under the same circumstances you do; we don't have any access to your inputs of database. So it will be up to you to fix it. This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.
Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterdays shirt when you took it off last night.
We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!
Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and Visual Studio will help you here. Run your program in the debugger and when it fails, VS will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, VS will stop before the error, and let you examine what is going on by stepping through the code looking at your values.
But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!
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