java - Swing GUI 未显示错误但未运行

Question

我有一个简单的登录表单，可以编译并且不会显示任何编译或构建错误，但当我尝试运行它时不会显示界面。我已经检查并设置了“setVisible(true)”，但仍然没有运气。我不太确定问题是什么，但我们将不胜感激。我正在使用 Netbeans IDE。这是源代码。

package client.system;

import java.awt.HeadlessException;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.swing.JOptionPane;


public class LoginForm extends javax.swing.JPanel {
    Connection con = null;
    PreparedStatement pst = null;
    ResultSet rs = null;

    /**
     * Creates new form LoginForm
     */
    public LoginForm() {
        initComponents();
    }

    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        username = new javax.swing.JTextField();
        jLabel3 = new javax.swing.JLabel();
        password = new javax.swing.JPasswordField();
        login = new javax.swing.JButton();

        jLabel1.setFont(new java.awt.Font("Tahoma", 1, 20)); // NOI18N
        jLabel1.setText("Log In");

        jLabel2.setLabelFor(username);
        jLabel2.setText("Username:");

        username.setText("Username");

        jLabel3.setLabelFor(password);
        jLabel3.setText("Password:");

        password.setText("Password");

        login.setFont(new java.awt.Font("Tahoma", 0, 20)); // NOI18N
        login.setText("Login");
        login.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                loginActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addGap(50, 50, 50)
                .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jLabel1, javax.swing.GroupLayout.Alignment.TRAILING, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING, false)
                            .addComponent(password, javax.swing.GroupLayout.DEFAULT_SIZE, 283, Short.MAX_VALUE)
                            .addComponent(jLabel2, javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(username))
                        .addContainerGap())
                    .addGroup(jPanel1Layout.createSequentialGroup()
                        .addGroup(jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(login, javax.swing.GroupLayout.PREFERRED_SIZE, 283, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jLabel3))
                        .addGap(0, 43, Short.MAX_VALUE))))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jLabel1)
                .addGap(18, 18, 18)
                .addComponent(jLabel2)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(username, javax.swing.GroupLayout.PREFERRED_SIZE, 34, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jLabel3)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(password, javax.swing.GroupLayout.PREFERRED_SIZE, 34, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addComponent(login, javax.swing.GroupLayout.PREFERRED_SIZE, 40, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(70, Short.MAX_VALUE))
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addComponent(jPanel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
        );
    }// </editor-fold>                        

    private void loginActionPerformed(java.awt.event.ActionEvent evt) {                                      
        // TODO add your handling code here:
        String sql="select * from login where username=? and password=?";
        try{
            con = DriverManager.getConnection("jdbc:mysql://localhost/testsql","root","mysql");
            pst = con.prepareStatement(sql);
            pst.setString(1, username.getText()); 
            pst.setString(2,password.getText());
            rs = pst.executeQuery();
            if(rs.next()){
                JOptionPane.showMessageDialog(null,"username and password matched");
            }else{
                JOptionPane.showMessageDialog(null, "Incorrect Username or Password");
            }
        }catch(SQLException | HeadlessException ex){
        JOptionPane.showMessageDialog(null,ex);
        }
    }                                     
    public static void main(String args[]){
        java.awt.EventQueue.invokeLater(new Runnable(){
            public void run(){
                new LoginForm().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JButton login;
    private javax.swing.JPasswordField password;
    private javax.swing.JTextField username;
    // End of variables declaration                   
}

Answer 1

如果表单/JPanel 是使用 NetBeans 生成的

创建一个新的 JFrame 表单。将 JPanel 表单从项目窗口拖到新创建的 JFrame 表单上。您必须这样做，因为 JPanel 需要 JFrame 根父级在屏幕上可见。它没有任何窗口逻辑。

如果您没有使用 NetBeans 创建表单/JPanel

创建一个新的 JFrame 并将表单/JPanel 作为子项添加到其中。这是一个非常简单的示例，说明您如何实现这一目标:

public class ExampleFrame extends JFrame {

    public ExampleFrame() {
        MyPanel panel = new MyPanel();
        getContentPane().add(panel);

        setTitle("My Cool Custom Panel");
        setSize(300, 200);
        setLocationRelativeTo(null);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public static void main(String[] args) {
        new ExampleFrame().setVisible(true);
    }
}