How many singletons

Create unittests on the class you want to change : using a mock of the interface introduced in 1 and the setter introduced in 2 you can now write unittests on the class.

Inject the singleton instead of getting it statically : in the class that depends on your singleton, have a way to inject that singleton via the interface created in 1. You can use constructor injection, method-based setters Whatever works for you. This is the first time you actually touch the class you are refactoring! The tests introduced in 3 will help you refactor without breaking anything You can avoid step 2 if you want to do it 'all in', but this approach is the most pure in the sense that you avoid changing any classes that do not have tests to check that nothing breaks.

Improve this answer. Add a comment. Kilian Foth Kilian Foth k 43 43 gold badges silver badges bronze badges. Most often when I see a singleton being implemented there isn't a need for having exactly one instance. For example it doesn't matter how many DAOs I have as long as they all do participate in the same transaction. That seems to be what many people don't realize when they implement - needing one instance is different than having at most one instance.

As someone who has suffered many hundreds of hours of hair-pulling frustration when trying to test code with singletons in it, I can state categorically that it's hard to overstate just how bad singletons are in almost every context. Even your "making sure only one instance of a specific class exists" comment misses the point. If I have two tests that run in parallel, I might need two different instances of the class simultaneously. It's depressing that, in , such bad advice still gets so many up votes.

The problem with singletons, just like any other programming concept, is not that they are just flat out bad, it's over using them that's bad. There is nothing wrong with having a small handful of them as long ad they are properly maintained.

JasonCrosby, it matters not whether you have 1 or 1, singletons in your code. What matters is whether any of them have state and thus hinder ease of testing. That is what makes them "flat out bad" anti-patterns. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. Make the class of the single instance responsible for access and "initialization on first use".

The single instance is a private static attribute. The accessor function is a public static method. The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It is named after the singleton set, which is defined to be a set containing one element. The office of the President of the United States is a Singleton.

The United States Constitution specifies the means by which a president is elected, limits the term of office, and defines the order of succession. As a result, there can be at most one active president at any given time. So whenever that method is called, the same object is always returned. The government is an excellent example of the Singleton pattern. A country can have only one official government.

The Singleton class declares the static method getInstance that returns the same instance of its own class. Calling the getInstance method should be the only way of getting the Singleton object. In this example, the database connection class acts as a Singleton. This method caches the first created object and returns it in all subsequent calls. Use the Singleton pattern when a class in your program should have just a single instance available to all clients; for example, a single database object shared by different parts of the program.

The Singleton pattern disables all other means of creating objects of a class except for the special creation method. This method either creates a new object or returns an existing one if it has already been created.

Use the Singleton pattern when you need stricter control over global variables. Nothing, except for the Singleton class itself, can replace the cached instance. Note that you can always adjust this limitation and allow creating any number of Singleton instances. The only piece of code that needs changing is the body of the getInstance method. It should create a new object on its first call and put it into the static field. The method should always return that instance on all subsequent calls.

Make the constructor of the class private. The static method of the class will still be able to call the constructor, but not the other objects. A Facade class can often be transformed into a Singleton since a single facade object is sufficient in most cases.

Flyweight would resemble Singleton if you somehow managed to reduce all shared states of the objects to just one flyweight object. But there are two fundamental differences between these patterns:.