如何防止类实例相乘

Question

How do I prevent multiple instances of the Scanner class from running? There is information about threads & locks etc but I cannot get them to work on my application. For example, I click on Pay, then an instance of Scanner appears, then Cancel which creates Pay. Then when I click on Pay, the number of Scanner(s) doubles. Heres a short summary:

@ PlaceOrder.java, i have:

btnPay.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) 
{
    java.awt.EventQueue.invokeLater(new Runnable() 
    {
           public void run() 
           {
                 PlaceOrder.this.setVisible(false);

                 new boundary.ScannerUI().setVisible(true);  
                 PlaceOrder.this.dispose();                                 
           }
    });             

} });

@ Scanner.java, i have:

btnCancel.addActionListener(new ActionListener() 
        {
            public void actionPerformed(ActionEvent evt)
            {
                //ScannerUI.this.setVisible(false);
                //ScannerUI.this.dispose();                     

                new boundary.PlaceOrder().setVisible(false);
                ScannerUI.this.dispose();
            }
        }); 

Answer 1

What you need is called the Singleton-Pattern , which is a quite common and basic design pattern.

Basicly, the singleton pattern provides a static method which instantiates the object (if it is not already instantiated) and returns the (already) created instance.

Singletons should have

• a private constructor,
• a static instance,
• and a static access method, which creates and returns the instance.

In your case the ScannerUI class would look like this,

public class ScannerUI extends JFrame { //extending JFrame is just my assumption
// as the OP didn't provided the class declaration code


    //static instance
    private static volatile ScannerUI instance;

    //private constructor
    private ScannerUI() {
        // Do nothing.
    }

    //static access method, which will be called instead of the constructor
    //UPDATE: added isDisplayable because it is disposable.
    public static synchronized ScannerUI getInstance() {
        if (instance == null || !instance.isDisplayable()) {
            instance = new ScannerUI();
        }
        return instance;
    }
}

UPDATE

To access an singleton object, you need to call its public accessor method and use it like other objects.

ScannerUI scanUI = ScannerUI.getInstance();
scanUI.setVisible(true);
scanUI.dispose();
etc.

UPDATE

You can of course implement the private constructor as your present one, you should just make sure, that the constructor is not visible outside of this class, as this would violate the concept of singletons.

"Do nothing." is just a placeholder, as I don't know what your actual constructor looks like.

Answer 2

If you want to reuse a single instance of ScannerUI , then don't ever call its constructor; instead provide factory method which will create an instance and save it in a static variable. Basic idea:

static ScannerUI instance;

ScannerUI newScanner() {
   return instance == null || instance.isDisposed()? 
      instance = new ScannerUI() : instance;
}