有和没有线程的并发

Question

我有一个需要登录用户的应用程序。数据存储在Derby数据库中。下面的表格根据用户名和密码字段查询它，并设置一个用户会话，该会话应该填充用户数据。

但是，即使db正在对用户进行身份验证，会话也会返回空值。我怎么能把一个system.out.println放在这个类的主要方法中，它会根据db查询返回会话数据，而不是立即执行代码并返回null？

注意：数据库工作正常。我可以根据sql语句中的用户名和密码字段获得结果。

public class LoginForm{ 

    private static JTextField userName; 
    private static JTextField password; 
    private static JButton submit; 
    private static int attempts; 
    private static JFrame main; 

    private Dimension dim = Toolkit.getDefaultToolkit().getScreenSize(); 

    final int MAX_ATTEMPTS = 5; 

    private UserSession session; 

    public LoginForm(){ 

     Handler handle = new Handler();        //inner class 
     LoginFormFocusListener fl = new LoginFormFocusListener(); //inner class 

     main = new JFrame(); 

     main.setUndecorated(true); 
     main.setBounds((dim.width/2) - (500/2),(dim.height/2) - (150/2),500, 75); 
     main.setVisible(true); 
     main.setAlwaysOnTop(true); 
     main.setResizable(false); 
     main.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

     userName = new JTextField(10); 
     password = new JTextField(10); 
     main.setLayout(new GridLayout(0,1)); 

     JPanel panel = new JPanel(); 
     main.add(panel); 

     panel.add(new JLabel("Username: ")); 
     panel.add(userName); 
     panel.add(new JLabel("Password: ")); 
     panel.add(password); 
     panel.setBorder(BorderFactory.createTitledBorder(BorderFactory.createBevelBorder(4), "Please Login")); 

     submit = new JButton("Submit"); 
     panel.add(submit); 

     if(attempts > 0){ 
      panel.add(new JLabel("" + (MAX_ATTEMPTS - attempts) + " attempts remaining...")); 
     } 

     main.addWindowFocusListener(fl); 
     submit.addActionListener(handle); 
    } 



    public UserSession getSession(){ 
     return this.session; 
    } 

    /** 
    * creates the session that's returned to main/POSsystem via getSession 
    * @author Matt 
    * 
    */ 
    private class Handler implements ActionListener 
    { 
     @Override 
     public void actionPerformed(ActionEvent e) { 
      String user = userName.getText(); 
      String pass = password.getText(); 

      session = new UserSession(); 
      if(session.authenticate(user, pass)){ 
       System.out.println("User has been authenticated"); 
       JOptionPane.showMessageDialog(null,"Login Successful! ","",JOptionPane.PLAIN_MESSAGE); 
      }else{ 
       System.out.println("Login Failed!"); 
       JOptionPane.showMessageDialog(null,"Login Failed!","", JOptionPane.WARNING_MESSAGE); 
       attempts++; 
       if(attempts < MAX_ATTEMPTS){ 
        new LoginForm(); 
       }else{ 
        JOptionPane.showMessageDialog(null, "Max attempts reached. " + 
                 "Please Contact the administrator of this system. ", 
                 "User Locked", 
                 JOptionPane.ERROR_MESSAGE); 
       } 
      } 
     } 
    } 

    /** 
    * Inner Class 
    * custom focus events for the login form 
    * the user may click away from this pop-up to close it. 
    * @author Matt 
    * 
    */ 
    public class LoginFormFocusListener implements WindowFocusListener{ 

     @Override 
     public void windowGainedFocus(WindowEvent wEvt) {} 

     @Override 
     public void windowLostFocus(WindowEvent wEvt) { 
      ((JFrame) wEvt.getSource()).dispose(); 
     } 
    }  

    //test 
    public static void main(String args[]){ 

     SwingUtilities.invokeLater(new Runnable(){ 
       public void run(){ 
       LoginForm lf = new LoginForm(); 
       System.out.println("Session: " + lf.getSession()); <---NULL!!! 
       } 
      }); 


    } 

} 

Answer 1

如果你真的想要得到的输出到控制台，那么不要把System.out.println代码放在​​main方法中，你应该把它作为actionPerformed方法的最后一行。

如果你真的，真的，真的想在main方法的代码，那么你应该创建实施，允许取回所创建的LoginForm的

这样Runnable接口的类：

final class InitThread implements Runnable { 
LoginForm lf; 

public LoginForm getLfForSystemOut() { 
    while (lf == null) { 
     try { 
      Thread.sleep(500); 
     } catch (final InterruptedException e) { 
      return null; 
     } 
    } 
    synchronized (lf) { 
     try { 
      lf.wait(); 
      return lf; 
     } catch (final InterruptedException e) { 
      e.printStackTrace(); 
     } 
    } 
    return null; 
} 

@Override 
public void run() { 
    lf = new LoginForm(); 
} 
} 

然后在主方法改变为这样：

public static void main(final String args[]) { 

    final InitThread init = new InitThread(); 
    SwingUtilities.invokeLater(init); 

    System.out.println("Session: " + init.getLfForSystemOut().getSession()); 

} 

，最后在actionPerformed方法在该块结束：

synchronized (LoginForm.this) { 
    LoginForm.this.notifyAll(); 
} 

Answer 2

您遇到的问题是因为您要在用户有机会填写表单并按提交按钮之前获取会话。

当提交按钮被按下时，处理程序将被异步调用，在创建窗体后直接调用main中的println。您必须等到会话不再为空或在处理程序中使用经过身份验证的会话。