In Design Pattern article, we have seen different types of design patterns. In this article, we will understand one of the simplest Creational Design Pattern i.e. Singleton Design Pattern.

What is the purpose of Singleton?

The purpose of the singleton class is to control object creation, limiting the number of objects to only one. The singleton allows only one entry point to create the new instance of the class.

Since there is only one Singleton instance, any instance fields of a Singleton will instantiate only once per class, just like static fields. Singletons are often useful where we have to control the resources, such as database connections or sockets.

Singleton design pattern restricts the instantiation of a class and ensures that only one instance of the class exists in the java virtual machine.

Approaches for writing Singleton Class

It seems to be a very simple design pattern but when it comes to implementation, it comes with a lot of implementation concerns.

Eager Initialization

What are the essential points that you should consider while writing Singleton class?

1. We want to take control over creation of the instances, so we will provide only private constructor. (Also read  What is the use of private constructor in java?)
2. If nobody can explicitly create object of our class (as there is no public constructor), then who will create the instance? Of course, we need to create one instance. So we will create one instance and store it in private static variable named as instance.
3. To access this private static variable, we will provide one public static accessor method getInstance().

Java Program Implementation

class EagerInitializedSingleton {

	// Private static instance is created at the time of class loading
	private static EagerInitializedSingleton instance = new EagerInitializedSingleton();

	// Constructor made private so that no new instance can be created
	private EagerInitializedSingleton() {

	}

	// Static method to return the instance to the client
	public static EagerInitializedSingleton getInstance() {
		return instance;
	}
}

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Why this approach is known as Eager Initialization?

If you notice, initialization of the instance is done at the time of class loading. It will not wait for client to call getInstance() method.  In other words, it will eagerly initialize the object.

What if client will never call getInstance() method?

instance will still get created and will remain in the Heap Memory which is waste of memory. This is the disadvantage of the eager initialization. This problem will be resolved using Lazy Initialization Approach.

What if some exception occurs while creating an object i.e. inside constructor?

We can write try catch block inside constructor.  Another way to solve this problem is by using static block Initialization approach.

Static block initialization

Static block initialization implementation is similar to eager initialization, except that instance of class is created in the static block that provides option for exception handling. Below is the implementation:

public class StaticBlockSingleton {

	private static StaticBlockSingleton instance;

	private StaticBlockSingleton() {

	}

	static {
		try {
			instance = new StaticBlockSingleton();
		} catch (Exception e) {
			throw new RuntimeException("Exception occured in creating instance");
		}
	}

	public static StaticBlockSingleton getInstance() {
		return instance;
	}
}

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Lazy Initialization

In practice, one should avoid using Eager Initialization and static initializer approach as in most of the cases the Singleton design pattern is used for creating heavy resources e.g. SessionFactory. It is preferred to initialize instance when clients asked for it i.e. when client calls getInstance() method. How do we achieve this in code?

Lets tweak the implementation a bit to convert eager initialization code to lazy initialization. We will create new instance when client will call getInstance() method. Also we will check null condition before creating new instance to ensure that it has not already created.

public class LazzyInitializedSingleton {

	private static LazzyInitializedSingleton instance;

	private LazzyInitializedSingleton() {

	}

	// Lazy initialization is done when client first time request for the instance
	public static LazzyInitializedSingleton getInstance() {
		if (instance == null) {
			instance = new LazzyInitializedSingleton();
		}
		return instance;
	}
}

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Lazy Initialization implementation will work when the given environment is single threaded. However in multithreaded environment, above code may create more than one instances. For example,

• Assume that two threads T1 and T2 are calling getInstance() method concurrently.
• Thread T1 has checked the null condition on line 11 and entered inside if block as no instance has been created yet.
• T1 is inside the if condition, but it has not created instance yet. At the same time another thread T2 reaches to the null condition on line 11. As instance is still null, T2 also enters the if condition.
• Now both threads will create instance on line 12 which will break the singleton design pattern.

Here is the graphical representation for the same :

Thread Safe Lazy Intialization Singleton Pattern

If you notice the code in First two approaches i.e. Eager Initialization and Static block Initialization, then you  can see that we are just returning the instance variable in getInstance() method. Process of creating instance happens eagerly at the time of class loading. Hence, we can conclude that both of these approaches are thread-safe, as there is no critical section inside getInstance() method. Only drawback is eager initialization.

On the other hand, if we want to initialize object lazily, then we need to handle thread-safety explicitly using synchronized keyword.

Howerver, There are many ways to implement Lazy Initialization Singleton Design Pattern in multithreaded environment like Synchronizing getInstance() method, Double check locking mechanism and Bill Pugh Singleton Implementation Using Inner Class. We have dedicated entire article on thread safe singleteon design pattern as it will require detailed discussion.

Next Article in the series of Singleton Design Pattern :  Singleton Design Pattern using Double Checked Locking