GB/T 28174.3-2011Unified modeling language(UML) - Part 3: Object constraint language(OCL) (English PDF)

3.1 The role of OCL

UML diagrams (such as class diagrams) are usually not fine enough to provide all aspects related to the specification. Which is missing in the description model

Add constraints on objects. These constraints are often described in natural language. And practice shows that doing so often causes ambiguity. To write

There is no ambiguity of the constraints, has developed several new "formal language". The traditional form of language, the disadvantage is only suitable for a considerable number

It is difficult to use the background staff, the general business or the system modeler.

OCL is to fill this gap and developed. It is a formal language that retains the easy-to-read and easy-to-write features. It's already in IBM's

The insurance division was developed as a business modeling language and was rooted in the Syntropy method.

OCL is a purely regular language, thus ensuring that OCL expressions do not have side effects. When evaluating an OCL expression,

Just return a value. It can not change anything in the model. This means that the state of the system is never evaluated by the OCL expression

And change, even if the OCL expression can be used to specify a state change (for example, in the postcondition).

OCL is not a programming language, so it is not possible to write program logic or flow control in OCL. Within OCL, you can not enable processes or

Activate non-query operation. OCL is a modeling language, so OCL expressions can not be executed directly by definition.

OCL is a typed language, where each OCL expression has a type. OCL expression to be well, it should be

The type of conformity rule of the language. For example, you can not compare Integer with String. Each category defined within the UML model (Classifier), all represent a unique OCL type. In addition, OCL also contains a set of supplemental predefined types (this is in Chapter 7)

"OCL standard library").

As a protocol language, all implementation problems are beyond the scope of OCL and can not be expressed in OCL.

The evaluation of OCL expressions is instantaneous. That is to say, the state of each object in the model can not be changed during the evaluation.

3.1.1 OCL use

OCL can be used for several different purposes.

a) as a query language;

b) specify the invariants of classes and types in class models;

c) specify the type of Stereotype (Stereotype) invariant;