The Singleton Pattern

The Singleton Pattern
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Some applications require exactly one instance of a class:
- Windowing systems with one event queue
- Word processors with one menu bar (for all documents)
Unfortunately, constructors can be called repeatedly:
- In one method
- In one class
- Across multiple classes

Requirements of a Singleton

Create and return an instance of itself if one doesn't exist
Return the existing instance if one does exist

One Implementation of the Singleton Pattern

An Example

A FileViewer

javaexamples/singleton/v1/FileViewer.java

(Fragment: skeleton)


/**
 * A Frame that is used to view the contents of a file
 *
 * This class  uses the Singleton pattern 
 * to ensure that there is only one
 * instance at any point in time.
 *
 * @author  Prof. David Bernstein, James Madison University
 * @version 1.0
 *
 */
public class FileViewer
{

    private static boolean      exists = false;
    private static FileViewer   instance;

    /**
     * Construct a new FileViewer
     *
     * Note: This method is private and used only internally
     */
    private FileViewer()
    {


        exists = true;
    }

}

An Example (cont.)

A FileViewer

javaexamples/singleton/v1/FileViewer.java

(Fragment: createInstance)

    /**
     * Construct a new FileViewer (if one doesn't exist)
     * or return the existing instance
     */
    public static FileViewer createInstance()
    {
       if (!exists) instance = new FileViewer(); // exists will become true
       
       return instance;

    }

An Example (cont.)

Using a FileViewer

javaexamples/singleton/v1/FileChooser.java

(Fragment: valueChanged)

    /**
     * Handle valueChanged events 
     * (required by ListSelectionListener)
     *
     * @param lse   The ListSelectionEvent to handle
     */
    public void valueChanged(ListSelectionEvent lse)
    {
        FileViewer   fv;
        String       fn;

        fn = (String)list.getSelectedValue();

        fv = FileViewer.createInstance();
        fv.load(fn);
    }

Thread Safety

A Potential Problem:
- If the createInstance() method might be called by multiple threads, problems can arise
Resolutions:
- Make createInstance() synchronized
- Use eager initialization (i.e., instantiate instance when it is declared)

An Example (cont.)

Using Eager Instantiation

javaexamples/singleton/v2/FileViewer.java

(Fragment: eagerInstantiation)

    private static FileViewer   instance = new FileViewer();

javaexamples/singleton/v2/FileViewer.java

(Fragment: createInstance)

    /**
     * Return the singleton instance
     */
    public static FileViewer createInstance()
    {
        return instance;
    }