Singleton why use

What is singleton class in Android? The Singleton Pattern is a software design pattern that guarantees a class has one instance only and a global point of access to it is provided by that class. This Singleton class may be responsible for instantiating itself, or you can delegate the object creation to a factory class. Can we extend Singleton class? All you need to extend a singleton class is a constructor with protected or package-default in the singleton class.

If there are only private constructors you simply won't be able to extend it. If there are public constructors then it's not a singleton class.

How does Singleton class work? A singleton is a class that allows only a single instance of itself to be created and gives access to that created instance. It contains static variables that can accommodate unique and private instances of itself. It is used in scenarios when a user wants to restrict instantiation of a class to only one object.

Why Singleton is not thread safe? In the above code, getInstance method is not thread-safe. Multiple threads can access it at the same time and for the first few threads when the instance variable is not initialized, multiple threads can enters the if loop and create multiple instances and break our singleton implementation.

Why use a singleton instead of static methods? Intention of a singleton pattern is to ensure that a single instance of a class is instantiated. Then I decided to write an article about that: Why and how we should use the singleton pattern over a static class? OK, I agree that creating singleton class or static class, doesn't matter, is not a good practice. However, in practical, it can be unavoidable in some points. So, which one we should prefer?

While the article title already answers this question, I will explain details of this decision in this article. While this article uses C as the programming language, the principles can be applied in any object oriented language. You can get the source code from my GitHub samples repository. Then you are lucky and you are doing a good thing, you can use the singleton lifetime. NET Core allows you to register a class with the singleton lifetime.

Assuming that you've a MyCache class and you want to register it with the singleton lifetime. Write this inside the ConfigureServices method of your Startup class and that's all:. Whenever you need to the MyCache service, just inject it like any other service.

However, there can be some reasons to manually implement the singleton pattern even if you use the dependency injection:. There are two main reasons that you should prefer singleton pattern over a static class. I will introduce briefly, then I will explain them in the next sections by details.

I created a solution that implements two caching services, one with the singleton pattern, the other one is a static class. I also created unit tests for both of the services:. The rest of the article will be based on this solution. It would be better that you have a GetOrThrowException method that throws exception if given key is not present:.

If SingletonCache is in a library that you are using, you can't change it source code. But you can inherit and override a method if you like. A singleton allows access to a single created instance - that instance or rather, a reference to that instance can be passed as a parameter to other methods, and treated as a normal object.

A static class allows only static methods. If you only need static methods from your singleton, then just use a static class to expose these static methods - There is no reason to implement your own singleton. However, if you need to be able to use your singleton as if it were a regular instance of a class, then read on. The single instance of the class will be distributed from logic internal to our singleton implementation.

It must ensure that one, and only one, instance is ever created. Now that we know vaguely what a singleton is, lets look at how we might implement one. Here, our Logger class has a private constructor. This restricts outside sources from creating new instances of the class. The Instance property on the Logger class has a getter which holds the logic for how the entire class is referenced. First, we check to see if instance , a private field of type Logger, is null.

The next time that the getter is called, the private field, instance , will no longer be null. In the code above, evenMoreInteresting has complete control over the control flow, because it gets a concrete implementation of all required collaborators and decides what to do with them.

If you change some existing code to dependency injection, especially constructor injection, you will notice that some of your constructors have lots and lots of parameters. Take that a chance for refactoring: This class is clearly doing too much! Changing code is much riskier than adding new code.

Every responsibility of a class is also a reason to change. Also, if your class has multiple responsibilities, you have to test all of them.

And you probably cannot test them in isolation. Also, abstractions should never depend on details. Singletons often implement some low-level detail. Imagine a vending machine: You select a product, pay the price, get the product. When the implementation of this high-level workflow has any dependencies to low-level details, like how the payment works, you might have to change code dealing with this high-level workflow when you add some new payment method like NFC payment.

Often, multiple classes depend on the concrete implementation of a singleton. This increases coupling in your system: When you want to change the behavior of the singleton, you probably have to check and change all the callers. You always have common coupling between all classes that use the singleton : They share a global variable - the singleton!

In a system with low cohesion, it is hard to even find out where to make a change: It can be any one of 10 different modules. Or it could be all of them. If you really want only one instance of a certain class, implement inversion of control for your dependencies. If you are already using a dependency injection container like Guice or Spring , just make sure all your singleton objects are managed by that container.

All those containers have a way to declare objects as singleton.