OCL - An ADL case study

OCL(Object Constraint Language)

OCL is a textual language to define constraints OCL can specify queries, constraints and query operations OCL enables to add extra information to a UML model OCL is a standard extension to UML for,

• Writing queries to access model elements and their values
• Stating constraints on model elements
• Defining query operations

OCL expressions have no side effects because,

• OCL can’t change the value of a model element
• OCL can’t define an operation other than a query operation
• OCL can’t be used to specify business rules dynamically

Features of OCL,

• can store OCL expressions in files with  UML model
• can attach OCL expressions to UML model elements as notes

Uses of OCL

• OCL allows suitably enabled modelling tool to reason about UML models
• OCL allows suitably equipped modelling tools to generate code based on OCL expressions
• OCL allows us to be more precise in our modelling

Negatives of OCL:

• Complex because syntaxes are irregular, have lots of “shortcut” forms
• few modellers and even fewer programmers know OCL
• The level of precision that OCL offers is high, mostly not used

OCL expression syntax

package<packagePath>

     context<contextualINstanceName><modelElement>

<expressionType><expressionName>:

<expressionBody>

<expressionType><expressionName>:

….

endpackage

OCL keyword are represented in bold, optional elements are shown in underlined italics. The angle brackets (<….>) indicate a placeholder that we must replace with the appropriate thing.

OCL expression is divided into3 parts:

• The package context(optional)
• The expression context(mandatory)
• One or more expressions

Package context and pathnames

The package context defines the namespace for the OCL expression. It has the following rules,

• If we don’t specify a package context the namespace for the expression defaults to the whole model
• If we attach an OCL expression directly to a model element the namespace for the expression defaults to the owing package of the element.

E.g.: we could specify a package context as

packageBankAccountExample::SimpleAccounts

……………..

endpackage

The OCL syntax for path name is:
Package1::package2::…packageN::ElementName

The expression context

• The expression context indicates the UML model element to which the OCL expression is attached.
• The expression context defines a contextual instance that has an optional name and a mandatory type. The type of contextual instance depends on expression context. i.e;- If the expression context is a classifier the contextual instance is always an instance of that classifier and if an expression is an operation or an attribute the contextual instance is generally an instance of that classifier that owns the attribute or the operation.

Types of OCL expressions
There are 8 different types in 2 categories

• Those specifying constraints (inv:, pre:, and post:)
• Those specifying attributes, operation bodies, and local variables (init:, body:, def:, let, and derive:)

We can give the expressionName to the operations that constrain (inv:, pre:, and post:). This allows us to refer to them by name, perhaps to link them to use case specifications.

Good OCL style ensures that

• All constraints are named
• Choose descriptive name that summarizes the semantics of the constraint
• Ensure that the constraint names are  unique  within the model
• Use lowerCamelCase for constraint names.

Expressions that specify constraints

Invariant	Precondition	Postcondition
Syntax-inv:	Syntax-pre:	Syntax-post:
Applies to-classifier and it should be met by all instances of the classifier	Applies to operation and it should be true before execution starts	Applies to behavioural features and operations and must be true after the execution

Expressions that specif attribute, operation and local variables

Query operation	Initial value	Define
Syntax-body:	Syntax-init:	Syntax-def:
Applies to query operation	Applies to attributes and association end	Applies to classifier

Let	Derived value
Syntax-let	Syntax-derive:
Applies to OCL expression	Applies to attribute