Singleton Design Pattern

1. Introduction

Let's try and understand Singleton design pattern.

Note: We will be using Spring Boot for all code examples, there is no reason for it, it's just the way we code :)

By Definition, singleton class is one which will have exactly one instance of the class at any given point of time in the given container ( JVM or so).

2. Implementation

Let's try to implement the class.

2.1 Eager Implementation

2.2 LazyLoad Implementation

Let's delay the class creation until accessed :

However, this class has one flaw, can you spot?

It is not thread safe.

2.3 LazyLoad Threadsafe

Let's make class (instance creation) thread safe:

2.4 LazyLoad Threadsafe - improved version

2.5 LazyLoad Using Helper class

2.6 LazyLoad Using Enum

2.6 Singleton & Serializable 

The code is availble in github, enjoy design-patterning :)