Decode Singleton Pattern

When to use

• Application needs “only one instance” of a class.
• To have complete control over the instance creation.

Intent

Ensure a class has only one instance, and provide a global point of access to it.

Components

• Singleton Class

Structure

Implementation

Create a Singleton class with a static instance and a private constructor. Provide a static method to allow access to this 'only' instance.

1 Create a Singleton Class

package com.gaurav.singleton;

public class Singleton {

  /* the singleton obj */
  private static Singleton instance = null;

  /* private constructor to avoid
     external instantiation of this class */
  private Singleton() {
  }

  /* method to get the singleton obj */
  public static Singleton getInstance() {
    if (instance == null) {
      synchronized (Singleton.class) {
        /* double checked locking */
        if (instance == null) {
          instance = new Singleton();
        }
      }
    }
    return instance;
  }

  public void printObj() {
    System.out.println("Unique Id of the obj: "
    + System.identityHashCode(this));
  }

}

2 Access the singleton instance

class Demo {

  public static void main(String[] args) {
    Singleton obj1 = Singleton.getInstance();
    obj1.printObj();
    Singleton obj2 = Singleton.getInstance();
    obj2.printObj();
  }

}

Output

Unique Id of the obj: 1916222108
Unique Id of the obj: 1916222108

Benefits

• Controlled instantiation
• Supports both EAGER and LAZY initializations
• Singletons can be converted to Multitons (to support limited number of instances identified by keys)

Drawbacks

• Singleton is often seen as 'not-so-good' design as it resembles global variables
• Special handling is required if the Singleton object needs to be deleted
• Singletons that maintain global state may cause issues

Real World Examples

• The Office of the President (there can be only one President at any given time)

Software Examples

• Logger classes
• Window Manager
• Printer Spooler

Java SDK Examples

java.lang.Runtime.getRuntime( )
java.awt.Toolkit.getDefaultToolkit( )
java.awt.Desktop.getDesktop( )
java.util.logging.LogManager.getLogManager( )
java.lang.System#getSecurityManager( )

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