Prerequisite: Singleton Design pattern | Inroduction
Singleton Method is a type of Creational Design pattern and is one of the simplest design pattern available to us. It is a way to provide one and only one object of a particular type. It involves only one class to create methods and specify the objects.
Singleton Design Pattern can be understood by a very simple example of Database connectivity. When each object creates a unique Database Connection to the Database, it will highly affect the cost and expenses of the project. So, it is always better to make a single connection rather than making extra irrelevant connections which can be easily done by Singleton Design Pattern.
singleton-pattern
Definition: The singleton pattern is a design pattern that restricts the instantiation of a class to one object.
Now let’s have a look at the different implementations of the Singleton Design pattern.
Method1 : Monostate/Borg Singleton Design pattern
Singleton behavior can be implemented by Borg’s pattern but instead of having only one instance of the class there are multiple instances that share the same state. Here we don’t focus on the sharing of the instance identity instead we focus on the sharing state.
# Singleton Borg pattern class Borg: # state shared by each instance __shared_state = dict() # constructor method def __init__(self): self.__dict__ = self.__shared_state self.state = 'GeeksforGeeks' def __str__(self): return self.state # main method if __name__ == "__main__": person1 = Borg() # object of class Borg person2 = Borg() # object of class Borg person3 = Borg() # object of class Borg person1.state = 'DataStructures' # person1 changed the state person2.state = 'Algorithms' # person2 changed the state print(person1) # output --> Algorithms print(person2) # output --> Algorithms person3.state = 'Geeks' # person3 changed the # the shared state print(person1) # output --> Geeks print(person2) # output --> Geeks print(person3) # output --> Geeks |
Output:
Algorithms Algorithms Geeks Geeks Geeks
Singleton-Design-pattern
Double Checked Locking Singleton Design pattern
It is easy to notice that once an object is created, the synchronization of the threading is no longer useful because now object will never be equal to None and any sequence of operations will lead to consistent results.
So, when the object will be equal to None, then only we will acquire the Lock on the getInstance method.
# Double Checked Locking singleton pattern import threading class SingletonDoubleChecked(object): # resources shared by each and every # instance __singleton_lock = threading.Lock() __singleton_instance = None # define the classmethod @classmethod def instance(cls): # check for the singleton instance if not cls.__singleton_instance: with cls.__singleton_lock: if not cls.__singleton_instance: cls.__singleton_instance = cls() # return the singleton instance return cls.__singleton_instance # main method if __name__ == '__main__': # create class X class X(SingletonDoubleChecked): pass # create class Y class Y(SingletonDoubleChecked): pass A1, A2 = X.instance(), X.instance() B1, B2 = Y.instance(), Y.instance() assert A1 is not B1 assert A1 is A2 assert B1 is B2 print('A1 : ', A1) print('A2 : ', A2) print('B1 : ', B1) print('B2 : ', B2) |
Output:
A1 : __main__.X object at 0x02EA2590 A2 : __main__.X object at 0x02EA2590 B1 : __main__.Y object at 0x02EA25B0 B2 : __main__.Y object at 0x02EA25B0
Classic implementation of Singleton Design Pattern
In the classic implementation of the Singleton Design pattern, we simply use the static method for creating the getInstance method which has the ability to return the shared resource. We also make use of so-called Virtual private Constructor to raise the exception against it although which is not much required.
# classic implementation of Singleton Design pattern class Singleton: __shared_instance = 'GeeksforGeeks' @staticmethod def getInstance(): """Static Access Method""" if Singleton.__shared_instance == 'GeeksforGeeks': Singleton() return Singleton.__shared_instance def __init__(self): """virtual private constructor""" if Singleton.__shared_instance != 'GeeksforGeeks': raise Exception ("This class is a singleton class !") else: Singleton.__shared_instance = self # main method if __name__ == "__main__": # create object of Singleton Class obj = Singleton() print(obj) # pick the instance of the class obj = Singleton.getInstance() print(obj) |
Output:
__main__.Singleton object at 0x014FFE90 __main__.Singleton object at 0x014FFE90
Class diagram
Class Diagram of Singleton Design Pattern
singleton-class-diagram
Advantages of using Singleton Method:
- Initializations: Object created by Singleton method is initialized only when it is requested for the first time.
- Access to the object: We got the global access to the instance of the object.
- Count of instances: In singleton method classes can’t have more than one instance
Disadvantages of using Singleton Method:
- Multithread Environment: Its not easy to use the singleton method in multithread environment, because we have to take care that multithread wouldn’t create singleton object several times.
- Single responsibility Principle: As the Singleton method is solving two problems at a single time, it doesn’t follow the single responsibility principle.
- Unit testing process: As they introduce the global state to the application, it makes the unit testing very hard.
Applicability
- Controlling over global variables: In the projects where we specifically need the strong control over the global variables, it is highy recommended to use Singleton Method
- Daily Developers use: Singleton patterns are generally used in providing the logging, caching, thread pools and configuration settings and oftenly used in conjuction with Factory design pattern.
Further read : Singleton method in Java, Singleton Design Pattern Practices with Examples
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