	CS 635 Advanced Object-Oriented Design & Programming Spring Semester, 2001 Singleton
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Contents of Doc 12, Singleton

References
Design Patterns: Elements of Resuable Object-Oriented Software, Gamma, Helm, Johnson, Vlissides, Addison Wesley, 1995, pp. 127-134

Pattern-Oriented Software Architecture: A System of Patterns (POSA 1), Buschman, Meunier, Rohnert, Sommerlad, Stal, 1996,

The Design Patterns Smalltalk Companion, Alpert, Brown, Woolf, Addision-Wesley, 1998,pp. 91-101


Doc 12, Singleton Slide # 2

Singleton

Intent

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

Motivation

There are times when a class can only have one instance

Applicability

Use the Singleton pattern when

There must be only one instance of a class, and it must be accessible to clients from a well-known access point

When the sole instance should be extensible by subclassing, and clients should be able to use an extended instance without modifying their code


Doc 12, Singleton Slide # 3

Examples of Using a Singleton
Java Security manager All parts of a program must access the same security manager

Once set a security manager cannot be changed in a program

Logging the activity of a server All parts of the server should use the same instance of the logging system

The server should not be able to change the instance of the logging system was it has been set

Null Object
Null object does not have state, so only need one instance

A binary tree with N non-null nodes will contain N+1 NullNodes

Command Processor
If Command Processor is to control all commands:

Can have only one Command Processor
Entire program needs access to Command Processor


Doc 12, Singleton Slide # 4

Implementation

Java

// Only one object of this class can be created
class Singleton
   {
   private static Singleton _instance = null;
   
   private Singleton()   { fill in the blank }
   
   public static Singleton getInstance()
      {
      if (  _instance == null )
          _instance = new Singleton();
      return _instance;
      }
   public void otherOperations() { blank; }
   }
   
class Program
   {
   public void aMethod()
      {
      X = Singleton.getInstance();
      }
   }


Doc 12, Singleton Slide # 5

Java Singletons, Classes, Garbage Collection
Classes can be garbage collected in Java

Only happens when there are

No references to instances of the class
No references to the class

If a singleton's state is modified and its class is garbage collected, its modified state is lost

To avoid having singletons garbage collected:

Disable class garbage collection with -Xnoclassgc flag

Insure singleton or class always has a reference

Store singleton or class in system property


Doc 12, Singleton Slide # 6

Implementation

C++

// Only one object of this class can be created
class Singleton
   {
   private:
      static Singleton* _instance;
      void otherOperations();
   protected:
      Singleton();
   public:
      static Singleton* getInstance();
   }
   
   
Singleton*  Singleton::_instance = 0;
Singleton* Singleton::getInstance()
   {
   if (  _instance == 0 )
       _instance = new Singleton;
   return _instance;
   }


Doc 12, Singleton Slide # 7

Implementation

Smalltalk

Object subclass: #Singleton
   instanceVariableNames: ''
   classVariableNames: 'UniqueInstance '
   poolDictionaries: ''
   category: 'Whitney-Examples'
   
current
   UniqueInstance ifNil: [UniqueInstance := Singleton basicNew].
   ^UniqueInstance
   
new
   self error: 'Use current to get an instance of Class ' , self name


Doc 12, Singleton Slide # 8

Overriding new in Smalltalk
Since can control what new returns one might be tempted to use:

new
   UniqueInstance ifNil: [UniqueInstance := Singleton basicNew].
   ^UniqueInstance

This can be misleading, user might think they are getting different objects when calling new

Do we have two different windows below or not?

| left right |
left := SingleWindow new.
Right := SingleWindow new.
left position: 100@ 100.
right position: 500@100.


Doc 12, Singleton Slide # 9

Naming the Access Method
GOF uses: instance()

POSA 1 uses: getInstance()

Smalltalk uses default and current

Selecting names is one of the more difficult problems in object-oriented analysis. No name is perfect [1]


Doc 12, Singleton Slide # 10

Singletons and Static

If one needs only one instance of a class why not just implement all methods as static?

Classes do not inherit Object's protocol

Hard to modify design if need more that one instance

Builds bad habits in beginners


Doc 12, Singleton Slide # 11

Consequences

Controlled access to sole instance
Reduced name space
Permits subclassing
Permits a variable number of instances
More flexible than class operations
Leads to improper use of globals


Doc 12, Singleton Slide # 12

[1] Fowler pp. 9, Alpert pp. 98

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2001 SDSU & Roger Whitney, 5500 Campanile Drive, San Diego, CA 92182-7700 USA.
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