Design Pattern - Singleton Pattern

Singleton Design Pattern
From definition to implementation
Mudasir Qazi - mudasirqazi00@gmail.com 116-Dec-14

Contents / Agenda
• Definition and types
• Advantages and usage
• UML – Class diagram
• UML – Sequence diagram
• Singleton in Memory (Memory allocation)
• Implementation – Lazy Singleton (C# and Java)
• Implementation – Early Singleton (C#)
• Thread-Safe Singleton in C#
• Thread-Safe Singleton in Java
• Double-check locking singleton (thread-safe)
• Static Block Implementation (thread-safe)
• Implementation – N-Singleton (C# and Java)

Definition
• Ensures that a class has only one instance, and provide a global point
to access this instance.
• In other words, a class must ensure that only single instance should be
created and single object can be used by all other classes.
• There are two forms of singleton design pattern
1. Early Instantiation: Creation of instance at load time.
2. Lazy Instantiation: Creation of instance when required.
3. N-Singleton: Create specific number of objects.
• It comes under “Creational Design Patterns” category.

Advantages and Usage
• Advantages
• Saves memory because object is not created at each request. Only single
instance is reused again and again.
• In cases when object creation is very costly (time taking), we don’t have to
create new object each time we need it. We just access already created object.
• Usage
• Singleton pattern is mostly used in multi-threaded and database applications.
• It is used in logging, caching, thread pools, configuration settings etc.
• For database connection, because one connection is enough for most
applications and too much connections can make application slow.

Usage Example

UML - Class Diagram
Mudasir Qazi - mudasirqazi00@gmail.com 6
We need
1) Private Static Instance of Class
2) Private Constructor
3) Public Static method with return type of
Class to access that instance.
16-Dec-14

UML - Sequence Diagram

Singleton in Heap
Only one object is created, all
other threads call the same
object without creating new
one.
16-Dec-14

Lazy Instantiation (C# and Java)
This is very common implementation
of Singleton but is not very good in
Multithreaded applications.
Because there is a possibility that
multiple thread can call the method
getInstance on same time due to
Race Condition. If happens so, then it
would create multiple instances.
Means meaning of singleton can will
not be achieved.
16-Dec-14

Lazy Instantiation (C# and Java) - Test

Early / Eager Instantiation (C#)
This is possible in C# because of
‘readonly’ keyword. Java don’t
have ‘readonly’ keyword (but it
can be achieved using final
keyword, see next slide). And
also note that ‘getInstance’ is a
property (getter/setter) for
variable instance, it has no
parenthesis after its name.
Note that, is this not thread
safe. If you are working in multi
threaded system then you
should not use this
implementation.
16-Dec-14

Thread-Safe Singleton in C#
Implementation in this picture will be thread
safe. In any condition there would be one and
only one instance of the Singleton class in
Multithreaded system.
16-Dec-14

Thread-Safe Singleton in Java
Advantages:
1) The instance is not constructed until the class is used.
2) There is no need to synchronize the getInstance() method meaning all threads will see the same instance
and no (expensive) locking is required.
3) The final keyword means that the instance can not be redefined, ensuring that one (and only one) instance
will ever exists.
4) This is thread safe. (This is best implementation for both Multithreaded and single threaded applications)
16-Dec-14

Double Check Locking Implementation (Lazy)
These implementations also comes under
synchronized or thread safe implementations.
Left one is better.
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Static Block Initialization
This is not very common use of singleton. But it also exists so I mentioned it.
16-Dec-14

N-Singleton Implementation (N=3)
MAX = 3,
Means that maximum 3
instances can created.
16-Dec-14

N-Singleton Implementation (N=3) - Output
Output shows that all 3
instances are different.
If you do same test with
Singleton (or give MAX = 0
in N-Singleton) the output
would be
“obj1=obj2=obj3”
16-Dec-14

N-Singleton as Singleton (N=0)
Here N=0 (we give count < MAX
condition so N=0 actually means
that 1 instance will be created. If
we have given <= condition then
we have given N=1) to create
single instance.
16-Dec-14

N-Singleton as Singleton (N=0) - Output
Output proves that only
one instance is created.
16-Dec-14

Design Pattern - Singleton Pattern

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