Chapter 4 Creational Patterns: Singleton

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Chapter 4 - Creational Patterns Singleton
CIS 476/566 Software Architecture and Design
Patterns

• Dr. Brahim Medjahed
• brahim_at_umich.edu

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Design Pattern Classification
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What are Creational Patterns?

• Abstract the instantiation process
• Make system independent of how objects are
  created, composed, and represented
• Give information in
• What gets created?
• Who created it?
• How it gets created?
• When?

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Object and Class Creational Patterns

• Two categories
• Object creational patterns
• Focus on the delegation of the instantiation to
  another object (e.g., Singleton, Abstract
  Factory)
• Class creational patterns
• Focus on the use of inheritance to decide the
  object to be instantiated (e.g., Factory Method)

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Relationship Between Creational Patterns

• Competitors
• Abstract Factory vs. Prototype
• Complementary
• Prototype uses Singleton

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Singleton Pattern

• Intent
• Ensure a class only has one instance, and provide
  a global point of access to it
• Motivation
• Many printers but only one spooler
• Global variable makes an object accessible but
  does not prevent many instances
• Make the class itself responsible for keeping
  track of its sole instance

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Using Constructors

• How do you create an object?
• Obj new MyObject()
• If you repeatedly call this how many instance
  of this object created?
• Public constructor As many calls to new

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How the Singleton Pattern Works?

• Idea Have a special method for instantiating the
  desired object
• When the method is called
• Check whether the object has already been
  instantiated
• If it has, the method returns a reference to the
  object
• Use protected or private constructor to insure
  this is the only way to instantiate an object

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Using a Private Constructor

• Public MyClass
• private MyClass()
• What does private constructor mean?
• It can be only instantiated within itself.
• Well how do you get access to instance of the
  class?
• Not sure!!! Lets see

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Using Private Constructor? (contd)

• Public MyClass
• private MyClass()
• public static MyClass getInstance()
• return new MyClass()
• Can we do obj MyClassgetInstance()

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Uniqueness of an Instance

• Now can we make it such that only one instance of
  MyClass is instantiated?
• Lets call this Singleton now.
• Public class MyClass
• private static MyClass theInstance
• private MyClass()
• public static MyClass getInstance()
• if (theInstance null)
• theInstance new MyClass()
• return theInstance

Uniqueness of an Instance
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An Example

• See handout given in the class