问题
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?
回答1:
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:
回答2:
"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.
回答3:
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:
If a Classifier owns Constraints, a conforming tool may implement a compartment to show the owned Constraints listed within a separate compartment of the owning Classifier’s rectangle. The name of this optional compartment is “constraints.”
Alternatively, you could invent your own stereotype ≪singleton≫.
来源：https://stackoverflow.com/questions/8386785/how-do-i-mark-a-class-as-a-singleton-in-uml