Singleton Pattern in Java

Implementing Singleton Pattern

To implement this design pattern we need to consider the following 4 steps:

public class SingletonObjectDemo {

	// Note that the constructor is private
	private SingletonObjectDemo() {
		// Optional Code
	}
}

Step 2: Create a Method for getting the reference to the Singleton Object

public class SingletonObjectDemo {

	private static SingletonObject singletonObject;
	// Note that the constructor is private
	private SingletonObjectDemo() {
		// Optional Code
	}
	public static SingletonObjectDemo getSingletonObject() {
		if (singletonObject == null) {
			singletonObject = new SingletonObjectDemo();
		}
		return singletonObject;
	}
}

We write a public static getter or access method to get the instance of the Singleton Object at runtime. First time the object is created inside this method as it is null. Subsequent calls to this method returns the same object created as the object is globally declared (private) and the hence the same referenced object is returned.

Step 3: Make the Access method Synchronized to prevent Thread Problems.

public static synchronized SingletonObjectDemo getSingletonObject()

It could happen that the access method may be called twice from 2 different classes at the same time and hence more than one object being created. This could violate the design patter principle. In order to prevent the simultaneous invocation of the getter method by 2 threads or classes simultaneously we add the synchronized keyword to the method declaration

Step 4: Override the Object clone method to prevent cloning

We can still be able to create a copy of the Object by cloning it using the Object’s clone method. This can be done as shown below

SingletonObjectDemo clonedObject = (SingletonObjectDemo) obj.clone();

This again violates the Singleton Design Pattern’s objective. So to deal with this we need to override the Object’s clone method which throws a CloneNotSupportedException exception.

public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}