Singleton Design Pattern

Definition:
Singleton is a part of Gang of Four design pattern and it is categorized under creational design patterns.

In this article we are going to take a deeper look into the usage of the Singleton pattern. It is one of the most simple design pattern in terms of the modelling but on the other hand this is one of the most controversial pattern in terms of complexity of usage.

In Java the Singleton pattern will ensure that there is only one instance of a class is created in the Java Virtual Machine. It is used to provide global point of access to the object. In terms of practical use Singleton patterns are used in logging, caches, thread pools, configuration settings, device driver objects. Design pattern is often used in conjunction with Factory design pattern. This pattern is also used in Service Locator JEE pattern.

Structure:

 

Singleton Class Diagram

• Static member : This contains the instance of the singleton class.
• Private constructor : This will prevent anybody else to instantiate the Singleton class.
• Static public method : This provides the global point of access to the Singleton object and returns the instance to the client calling class.

Implementation Example: Lazy initialization

Let us look into a singleton implementation example in Java. The below code uses Lazy initialization process.



Singleton Pattern Code Explanation
When this class will be called from the client side using SingletonExample.getSingletonInstance().printSingleton(); then at the first time only an instance will be created. During second time onwards for all subsequent calls we will be referring to the same object and the getSingletonInstance() method returns the same instance of the SingletonExample class which was created during the first time. You can test this by adding a print statement the following code as:



If we now call Singleton class from the client class as:



The output of the above call is:

Implementation Example: Double check locking

The above code works absolutely fine in a single threaded environment and processes the result faster because of lazy initialization. However the above code might create some abrupt behaviour in the results in a multithreaded environment as in this situation multiple threads can possibly create multiple instance of the same SingletonExample class if they try to access the getSingletonInstance() method at the same time. Imagine a practical scenario where we have to create a log file and update it or while using a shared resource like Printer. To prevent this we must use some locking mechanism so that the second thread cannot use this getInstance() method until the first thread has completed the process.

 

 

Singleton in heap
In Figure 3 we show how multiple threads access the singleton instance. Since the singleton instance is a static class variable in the stored in the PermGen space of the heap. This applies to getSingletonInstance() instance method as well since it is static too. In the multithreading environment to prevent each thread to create another instance of singleton object and thus creating concurrency issue we will need to use locking mechanism. This can be achieved by synchronized keyword. By using this synchronized keyword we prevent Thread2 or Thread3 to access the singleton instance while Thread1 inside the method getSingletonInstance().
From code perspective it means:

 

So this means that every time the getSingletonInstance() is called it gives us an additional overhead . To prevent this expensive operation we will use double checked locking so that the synchronization happens only during the first call and we limit this expensive operation to happen only once. It is only required for:

singletonInstance = new SingletonExample();

Code example: 

 

In the above code snippet imagine that multiple threads comes concurrently and tries to create the new instance. In such situation the may be three or more threads are waiting on the synchronized block to get access. Since we have used synchronized only one thread will be given access. All the remaining threads which were waiting on the synchronized block will be given access when first thread exits this block. However when the remaining concurrent thread enters the synchronized block they are prevented to enter further due to the double check : null check. Since the first thread has already created an instance no other thread will enter this loop.
All the remaining threads that were not lucky to enter the synchronized block along with the first thread will be blocked at the first null check. This mechanism is called double checked locking and it provides significant performance benefit and also it is cost effective solution.

Implementation Example: Volatile Keyword

We can also use the volatile keyword to the instance variable declaration.

private volatile static SingletonExample singletonInstance;

The volatile keyword helps as concurrency control tool in a multithreaded environment and provides the latest update in a most accurate manner.However please note that double check locking might not work before Java 5. In such situation we can use early loading mechanism. If we remember about the original sample code we had used lazy loading. In case of early loading we will instantiate the SingletonExample class at the start and this will be referred to the private static instance field. 


In this approach the singleton object is created before it is needed. The JVM takes care of the static variable initialization and ensures that the process is thread safe and that the instance is created before the threads tries to access it. In case of Lazy loading the singletonInstance is created when the client class calls the getSingleInstance() whereas in case of the early loading the singletonInstance is create when the class is loaded in the memory.

Implementation Example: Using enum

Implementing Singleton in Java 5 or above version using Enum:
Enum is thread safe and implementation of Singleton through Enum ensures that your singleton will have only one instance even in a multithreaded environment. Let us see a simple implementation: