无法解决 Java Swing 应用程序中的 NullPointerException

Posted 2023-02-19

【中文标题】无法解决 Java Swing 应用程序中的 NullPointerException

【英文标题】：Unable to resolve NullPointerException in java swing application

【发布时间】：2022-01-10 10:57:45

【问题描述】：

我在 Eclipse 中使用 Java Swing 制作了一个简单的登录页面，一旦我运行并输入输入，我就会收到错误 java.lang.NullPointerException

import java.lang.String;
import java.awt.EventQueue;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.EmptyBorder;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JTextField;
import javax.swing.JPasswordField;
import javax.swing.JButton;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

public class trail extends JFrame 
    Connection connection =null;
    private JPanel contentPane;
    private JTextField id;
    private JPasswordField passwordField;

    public static void main(String[] args) 
        String jdbcURL = "jdbc:postgresql://localhost:5432/postgres";
        String username = "postgres";
        String password = ".......";
        try 
            Connection connection = DriverManager.getConnection(jdbcURL, username, password);
            System.out.print("Connected");
            connection.close();
        
        catch(SQLException e) 
            System.out.println("Error in connection");
            e.printStackTrace();
        
        EventQueue.invokeLater(new Runnable() 
            public void run() 
                try 
                    trail frame = new trail();
                    frame.setVisible(true);
                 catch (Exception e) 
                    e.printStackTrace();
                
            
        );
    

    /**
     * Create the frame.
     */
    public trail() 
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setBounds(100, 100, 519, 550);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        setContentPane(contentPane);
        contentPane.setLayout(null);
        JLabel lblNewLabel = new JLabel("username");
        lblNewLabel.setBounds(94, 114, 45, 13);
        contentPane.add(lblNewLabel);

        id = new JTextField();
        id.setBounds(246, 111, 96, 19);
        contentPane.add(id);
        id.setColumns(10);

        JLabel lblNewLabel_1 = new JLabel("password");
        lblNewLabel_1.setBounds(94, 178, 45, 13);
        contentPane.add(lblNewLabel_1);

        passwordField = new JPasswordField();
        passwordField.setBounds(246, 175, 96, 19);
        contentPane.add(passwordField);

        JButton btnlogin = new JButton("login");
        btnlogin.addActionListener(new ActionListener() 
            @SuppressWarnings("deprecation")
            public void actionPerformed(ActionEvent e) 
                try 
                    String query = "select * from users where username = ? and password = ?";
                    PreparedStatement pst=connection.prepareStatement(query);
                    pst.setString(1, id.getText());
                    pst.setString(2, passwordField.getText());
                    ResultSet rs = pst.executeQuery();
                    int count = 0;
                    while(rs.next()) 
                        count = count + 1;
                        if(count == 1)
                        
                            JOptionPane.showMessageDialog(null, "Password and username is correct");
                        
                        else if(count>1)
                        
                            JOptionPane.showMessageDialog(null, "redundancy detected");
                        
                        else
                        
                            JOptionPane.showMessageDialog(null,"Incorrect credentials");
                        
                    
                    rs.close();
                    pst.close();
                
                catch(Exception e1)
                
                    JOptionPane.showMessageDialog(null, e1);
                
            
        );
        btnlogin.setBounds(179, 259, 85, 21);
        contentPane.add(btnlogin);
    

【问题讨论】：

查看图片下导致异常的代码会很有帮助。编辑您的帖子并将代码复制粘贴到其中。在执行查询之前，您需要建立与数据库的连接。

数据库连接建立成功！！！ @DevilsHnd

为什么一打开就关闭那个连接！！！！

我改变了它知道...仍然是相同的 nullpointe 异常错误@DevilsHnd

好吧，错误被底部的catch(e1)捕获了……所以在btnlogin底部@DevilsHnd的actionlistener下方的try块中可能出了问题

【参考方案1】：

你稍后在你使用的 try 块中声明了Connection connection =null;

Connection connection = DriverManager.getConnection(jdbcURL, username, password) 是完全不同的 Connection 对象。你应该使用类似的东西。

 this.connection=DriverManager.getConnection(jdbcURL, username, password)

 

你也可以用不同的方法移动你的代码，比如

 public void  initialize() 
      String jdbcURL = "jdbc:postgresql://localhost:5432/your_db";
      String username = "postgres";
      String password = "password";

  try 
  this.connection = DriverManager.getConnection(jdbcURL, username, password);
  System.out.print("Connected");
  connection.close();      
  
  catch(SQLException e) 
      System.out.println("Error in connection");
      e.printStackTrace();
   

并在构造函数内部调用

 public trail() 
    initialize();
    ....