Object Constraint Language
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Definition
The Object Constraint Language is a declarative language for describing rules that apply to UML models developed at IBM and now part of the UML standard. Initially OCL was only a formal specification language extension to UML. OCL may now be used with any Meta-Object Facility OMG metamodel, including UML. The Object Constraint Language is a precise text language that provides constraint and object query expressions on any Meta-Object Facility model or metamodel that cannot otherwise be expressed by diagrammatic notation. OCL is a key component of the new OMG standard recommendation for transforming models, the QVT specification. Any QVT language like ATL is built on top of OCL and a rule execution engine.
Description
OCL is a descendent of Syntropy, a second-generation object-oriented analysis and design method. The OCL 1.4 definition specified a constraint language. In OCL 2.0, the definition has been extended to include general object query language definitions.
OCL language statements are constructed in four parts:
- a context that defines the limited situation in which the statement is valid
- a property that represents some characteristics of the context (e.g., if the context is a class, a property might be an attribute)
- an operation (e.g., arithmetic, set-oriented) that manipulates or qualifies a property, and
- keywords (e.g., if, then, else, and, or, not, implies) that are used to specify conditional expressions.
OCL and UML
OCL supplements UML by providing expressions that have neither the ambiguities of natural language nor the inherent difficulty of using complex mathematics.
OCL and MOF
OCL allows to make any Meta-Object Facility model more precise by associating to its metaelements a number of assertions.
OCL and QVT
Of particular importance to Model Driven Engineering or Model Driven Architecture is the notion of Model transformation. The OMG has defined a specific standard for model transformation called MOF/QVT or in short QVT. Several model transformation languages like GReAT, VIATRA or the ATL are presently available, with different level of compliance with the QVT standard. Many of these languages are buit on top of OCL, which is the main part of the QVT-compliance.
Alternatives to OCL
As a navigation language, XPath may be considered as an altenative to OCL. However XPath works for XML trees while OCL allows to navigate MOF-based models and metamodels. As a model specification language allowing to decorate a model or a metamodel with side-effect free annotations, OCL could be advantageously replaced by languages like Alloy.
External links
- The UML specification (including OCL)
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.