Java Singleton Design Pattern

Here you will learn about java singleton design pattern with example.

What is Singleton Design Pattern?

Defining a class that can have single instance and provides a global access point to get the instance.
This design pattern is mainly used in multi-threaded and database applications.
It saves memory because only one instance is created and used again and again.

There are various ways to implement singleton design pattern. Here I am discussing two popular ways, Early Instantiation and Lazy Instantiation.

A singleton class must have following three things.

Private Static Variable: It contains the instance of the class.

Private Constructor: It prevents further instantiation of class.

Public Static Method: It is a global access point to get the instance of class from outer world.

Early Instantiation

In this implementation, the instance is created at the time of class loading. Below example shows how to implement singleton design pattern using early instantiation.

SingletonDesginPattern.java

package com;

public class SingletonDesignPattern {
	//instance will be created at the time of class loading
	private static SingletonDesignPattern obj = new SingletonDesignPattern();
	
	//private constructor
	private SingletonDesignPattern(){}
	
	//global point of access to the instance
	public static SingletonDesignPattern getInstance(){
		return obj;
	}
	
	public void printMessage(){
		System.out.println("Java Singleton Design Pattern Using Early Instantiation");
	}
}

Demo.java

package com;

public class Demo {
	public static void main(String...s){
		SingletonDesignPattern obj = SingletonDesignPattern.getInstance();
		obj.printMessage();
	}
}

Output

Java Singleton Design Pattern Using Early Instantiation

Lazy Instantiation

In this implementation, the instance is created at request time. Below example shows how to implement singleton design pattern using lazy instantiation.

SingletonDesginPattern.java

package com;

public class SingletonDesignPattern {
	private static SingletonDesignPattern obj = null;
	
	//private constructor
	private SingletonDesignPattern(){}
	
	//global point of access to the instance
	public static SingletonDesignPattern getInstance(){
	
		//instance will be created when requested
		if(obj == null){
			obj = new SingletonDesignPattern();
		}
		
		return obj;
	}
	
	public void printMessage(){
		System.out.println("Java Singleton Design Pattern Using Lazy Instantiation");
	}
}

Demo.java

package com;

public class Demo {
	public static void main(String...s){
		SingletonDesignPattern obj = SingletonDesignPattern.getInstance();
		obj.printMessage();
	}
}

Output

Java Singleton Design Pattern Using Lazy Instantiation

To make the class thread safe we can create the instance inside synchronized block. It can be done in following way.

//global point of access to the instance
public static SingletonDesignPattern getInstance(){
	//instance will be created when requested
	if(obj == null){
		synchronized(SingletonDesignPattern.class){
			if(obj == null){
				obj = new SingletonDesignPattern();
			}
		}
	}
		
	return obj;
}

Comment below if you found any information incorrect or missing in above tutorial.

Java Singleton Design Pattern