Jul 7

Understanding Singleton Design Pattern in Java

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Singleton Pattern

The Singleton Pattern is another creational design pattern that ensures a class has only one instance and provides a global point of access to that instance. It's commonly used when exactly one object is needed to coordinate actions across a system.

If you don't have knowledge of Creational Design Patterns, you can read this blog first: Creational Design Pattern
Key Characteristics of the Singleton Pattern

  • Single Instance: The Singleton Pattern ensures that only one instance of the class is created, no matter how many times the class is accessed. This unique instance is shared across the entire application.

  • Global Access Point: The Singleton Pattern provides a global access point to the instance, allowing other classes to access and use the singleton instance from anywhere in the application.

Think of a Singleton as a single shared resource, like a central government. There can only be one central government at a time in a country, and it coordinates all national-level activities. Similarly, in software, a Singleton manages a shared resource or provides coordination.

How to achieve Singleton Pattern in the code

  • Private Constructor: The constructor of the singleton class is private to prevent any other class from instantiating it. This ensures that the only way to create the instance is through the static method provided by the singleton class.

  • StaticMethod: The singleton class provides a static method (often named getInstance() or getSingletonInstance()) that returns the single instance of the class. This method checks whether the instance already exists; if not, it creates and returns it.

  • Static Variable: A static variable holds the single instance of the class. This variable is initialized the first time the static method is called and holds the reference to the single instance.
If you see in above code, we need to handle several complexities to ensure the Singleton property is preserved under various conditions such as multithreading, serialization, cloning and reflection. However, using an enum simplifies the implementation and inherently addresses these concerns.

That's why Enum Is a Better Way for Singleton in Java

  • Enum instances are inherently thread-safe, and the JVM ensures that the instance is created only once, even in a multithreaded environment.

  • Enums are resistant to reflection attacks. It is not possible to use reflection to create another instance of an enum type.

  • The Java serialization mechanism guarantees that enum values are always serialized as a single instance, maintaining the singleton property without additional code.
In traditional Singleton implementations, various conditions such as multithreading, cloning, reflection, and serialization need to be explicitly managed to preserve the Singleton property.

This often results in complex and error-prone code. In contrast, using an enum for Singleton simplifies the implementation by providing inherent thread safety, reflection resistance, and proper serialization handling out of the box. This makes the enum approach a cleaner and more robust solution for implementing Singletons in Java.
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