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I have an UML diagram on which I want to mark some classes as singletons (because they are). How do I do that? Do I invent a new stereotype or do I just add a comment or are there some existing means to do that?
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To specify that a class is a singleton, you can write a constraint in between braces: { number of instances = 1 }. This constraint should be put in a constraints compartment in the class rectangle. The UML 2.5 specification, §7.6.4 defines the notation for constraints in general and §9.2.4 specifies how to show the constraints of a classifier:
In a UML diagram, such a class can be marked with a ‘1’ in the upper right corner of the name compartment. See Figure 16.17. Figure 16.17 Showing a singleton.
Classic Singleton creates an instance only if there is no instance created so far; otherwise, it will return the instance that is already created. Let’s take a look at the below code. Here, in the __new__ method, we will check whether an instance is created or not. If created, it will return the instance; otherwise, it will create a new instance.
Let’s check what happens when we subclass a singleton class. Here, you can see that SingletonChild has the same instance of SingletonClass and also shares the same state. But there are scenarios, where we need a different instance, but should share the same state. This state sharing can be achieved using Borg singleton.
"What will most clearly represent the concept to your reader."
There is a reasonable example of the basic appearance over on Wikipedia. It shows a private constructor and a public getInstance() method that returns an object of type Singleton. I'd also find a stereotype to be a perfectly reasonable way to make it explicit without requiring your reader to parse the details of the class object.
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Craig Larman's "Applying UML and Patterns" book (3rd edition) gives the following notation for a sequence diagram:
Note that it implies that before calling doA the :Register object manages to get access to the singleton via the standard way, e.g., getInstance(). This makes the diagram more agile.
For a class diagram:
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