o cl by example lecture

7/28/2019 o Cl by Example Lecture

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Department of Computer Science, Institute for Software and Multimedia Technology

OCL

by Example

.


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In theory, there is no difference

between theory and practice.

But, in practice, there is.

Jan L. A. van de Snepscheut/Yogi Bera

Dr. Birgit Demuth MINE Summer School, Nida, 2009 2


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Main Goals of the Lecture

Bridge the gap between practically used softwarespecifications (UML) and formal languages

, ,

Learn how to specify semantics using OCL

Learn what are interestin OCL use cases

Inform what OCL tools can already be used



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Foundation: Assertions

An assertion is a predicate (i.e., a truefalse statement)

placed in a program to indicate that the developer thinksa e pre ca e s a ways rue a a p ace pe a .

Usa e in

Hoare logic [Hoare 1969] Design by contract [Meyer 1986, Eiffel]

- asser , , , ,)

During the development cycle (debugging) Static assertions at compile time



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Object Constraint

Model-based assertion

[Warmer and Kleppe] define a constraint as follows:

of (part of) an object-oriented model or system.

OCL as specification language for object constraints



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History of OCL

Developed at IBM in 1995 originally as a businessengineering language

Part of the official OMG standard for UML (fromversion 1.1 on)

Used for precisely defining the well-formedness rules(WFRs) for UML and further OMG-related metamodels

.



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OCL (Object Constraint Language)

Extends the Unified Modeling Language (UML)

Formal lan ua e for the definition of constraints andqueries on UML models

Declarative

Side effect free Add precise semantics to visual (UML-) models

Meanwhile generally accepted

Many extensions such as for temporal constraints Core Language of other OMG languages (QVT, PRR)



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Literature

1 Warmer J. Kle e A.: The Ob ect Constraint Lan ua e.Precise Modeling with UML. Addison-Wesley, 1999

[2] Warmer, J., Kleppe, A.: The Object Constraint LanguageSecond Edition.

Getting Your Models Ready For MDA. Addison-Wesley, 2003[3] OMG UML specification,

_ _g.htm#UML

[4] OMG UML 2.0 OCL,. . .

[5] Heinrich Humann: Formal Specification of SoftwareSystems. Course, 2000, Technische Universitt Dresden



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Constraint

Definition

A constraint is a restriction on one or morevalues of (part of) an object-oriented model or

.

A constraint is formulated on the level of classes, butits semantics is applied on the level of objects.

originally formulated in the syntactic context of a UML. .



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Definition

An invariant is a constraint that should be true for anobject during its complete lifetime.

Invariants often represent rules that should hold for thereal-life objects after which the software objects aremodeled.

context

inv []:



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OCL/UML By Example



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-

context Meeting inv: self.end > self.start

Equivalent Formulations

-- "self" always refers to the object identifier from which the

constraint is evaluated.

context Meeting inv startEndConstraint:

self.end > self.start

-- Names can be given to the constraint



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r

operation without stating an algorithm or implementation

Are attached to an operation in a class diagram



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Precondition

Constraint that must be true just prior to the executionof an operation

Syntax

pre []:



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Precondition - Examples

context Meeting::shift(d:Integer)

pre: self.isConfirmed = false


context Meeting::shift(d:Integer)pre: self.isConfirmed = false and d>0



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Definition ons ra n a mus e rue us a er o e execu on

of an operation

Postconditions are the way how the actual effect of anoperation is described in OCL.

Syntax

context :: < arameters>

post []:



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Postcondition - Examples

context Meeting::duration():Integer

post: result = self.end self.start

-- eywor resu re ers o e resu o e opera on

context Meeting::confirm()

post: self.isConfirmed = true



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Postcondition Examples (cont.)


post: start = start@pre +d and end = end@pre + d

--

-- which is to be evaluated in the original state-- before execution of the operation

-- startrefers to the value upon completion of the operation

--



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Postcondition Examples (cont.)

messaging only in postconditions

hasSent (^) operator

context Subject::hasChanged()

post: observer^update(2,4)

/* standard observer attern:

results in true if an update message with arguments 2 and 4

was sent to the observer object during execution of the

o eration hasChan ed

*/



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Buildin OCL Ex ressions 1

Boolean expressions

Standard library ofprimitive types and associated operations

Basic types (Boolean, Integer, Real, String)

Collection

Set Ordered Set (only OCL2)

Bag



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Building OCL Expressions (2)

User defined types (OCLType)

ass ype o e ype :

Classifier in a class diagram (implicitly defined)

Generalisation among classiefiers leads to Supertypes

A class has the following Features: Attributes ( s ta r t )

Class attributes (Dat e: : t oday)

Class operations Association ends (navigation expressions)

Enumeration t e Gender Gender : : mal e



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OCL Type Hierarchy

T

T T T T



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OCL Type Conformance Rules

OCL is a strongly typed language .

The parser has to check the conformance:

Type1 con orms to Type2 i an instance o Type1 can esubstituted at each place where an instance of Type2 isexpected.

General rules:

ac ype con orms o eac o s super ypes.

Type conformance is transitive.


.


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OCL Constraints and Inheritance

Constraints are inherited.

Wherever an instance of a class is expected, one canalways substitute an instance of any of its subclasses.

An invariant for a superclass is inherited by its subclass.

A subclass may strengthen the invariant but cannot weakenit.

A precondition may be weakened but not strengthened ina redefinition of an operation in a subclass.

A postcondition may be strengthened but not weakened ina redefinition of an operation in a subclass.



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Navigation Expressions

from one object in the model to another object.

Navigations are treated as attributes (dot-Notation).

User defined type

(association end with multiplicity at most 1) Collection

(association end with multiplicity > 1)



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av gat on xpress ons - xamp es

User defined type

Navigation from Meet i ng to moder at orresults in typeTeammember

inv: self.moderator.gender = Gender::female



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Navigation Expressions - Examples

Collection

Navigation von Meeting to par t i ci pant s

context Meeting

inv: self->collect(participants)->size()>=2

context Meeting inv: self.participants->size()>=2



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Collection Operations (1)

22 operations with variant meaning depending on the

collection type such as

equals (=) and not equals operation ()

Transformations (asBag(), asSet(), asOrderedSet(),asSequence())

including(object) and excluding(object)

Set {Bag{1, 2, 2}, Bag{2}} Set {1, 2}

Typical set operations, , ,

Operations on ordered collections only (OrderedSet,Sequence) (such as first(), last(), indexOf())



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Collection Operations (2)

Loop operations (Iterators) on all collection types

collect(expr)

exists(expr)

forAll(expr)isUnique(expr)

one(expr)

select(expr)

sortedBy(expr)



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Loo O eration iterate

Collection->iterate( element : Type1;

| }

All other loop operations can be described as a specialcase ofiterate( ) such as in the following simpleexample:

Set {1,2,3}->sum()

Set{1,2,3}->

iterate{i: Integer, sum: Integer=0 | sum + i }



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Further Examples for Collection Operations (1)

A teammeetin has to be or anized for a whole team( forAll()) :

context Teammeeting

inv: participants->forAll(team=self.for)

contextMeeting inv: oclIsTypeOf(Teammeeting)

implies participants->forAll(team=self.for)



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Further Examples for collection operations (2)

Postconditions select :

context Teammember::numMeeting():Integer

post: result=meetings->size()

post:

result=meetin s->select(isConfirmed)->size()



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Flattening of Collections

Automatic flattenin rule for all nested collections

self.participants.meetings

in the context Meeting

sel f . par t i ci pant s delivers a Set(Person)

self. artici ants.meetin s delivers aBag(Set(Person)

Results in a Bag(Person)



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Derivation Rule derive OCL2

context Team::size:Integer

-

Derived association (conf l i ct )

defines a set of meetings that are in conflict with each other

derive: select(m|mself and self.inConflict(m))



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Initial Value (init, OCL2)

Examp es

context Meeting::isConfirmed : Boolean

init: false

init: Set{}

Note that an initial value must be valid only at the objectcreation time!



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Query Operation (body, OCL2)

Operations that do not change the state of the system

Can be used as a query language

ower o

Example

context

Teammember:: etMeetin Titles(): Ba (Strin )body: meetings->collect(title)



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Let Expression (let)

Interesting for complex expressions

Define a local variable noConf l i ct that can be usedinstead of a sub-expression

context Meeting inv:

let noConflict : Boolean =

-.

m.isConfirmed implies not self.inConflict(m))

in isConfirmed implies noConflict



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Defining New Attributes and Operations(def, OCL2)

Adding attributes and query operations to the model

Helpful for the reuse of OCL expressions in severalconstraints

context Meetin

def: noConflict : Boolean =

participants.meetings->forAll(m|mself and

m.isConfirmed implies not

self.inConflict(m))



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Packaging OCL Expressions

package MeetingExample

context Meeting::isConfirmed : Boolean

context Teammember:meetings : Set(Meetings)

init: Set{}

..

endpackage



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No su ort for inconsistenc detection for OCL

Frame Problem

Operations are specified by what they change (in post-conditions), with the implicit assumption that everything

else (the frame) remains unchanged

Limited recursion

allInstances() Problem:

Per son. al l I nst ances( ) allowed

not allowed for infinite types such asI nt eger . al l I nst ances( )



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Building complete models with OCL

Statechart diagram

Activity diagram Com onent dia ram

Use case diagram



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OCL in Statecharts Example (oclInState())

operation on all objects (Typ Ocl Any)

oc n a e s: c a e : oo ean

context Vector::removeElement(d:Data)

pre: oclInState(notEmpty)

post: size@pre = 1 implies oclInState(empty)



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An OCL expression can evaluate to undefined (OclVoid)

where no value is existent in the respective object

Strictness Principle

Whenever a subexpression of an OCL expressionevaluates to undefined then the whole term evaluates toundefined

Exceptions

rue or un e ne = rue

False and undefined = False

False implies undefined = True



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The OclVoid Type

Undefined value is the onl instance Operation for testing if the value of an expression is

undefined

oclIsUndefined(): Boolean

--

-- otherwise false



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Some Tips for Writing OCL Expressions

Constraints should be easy to read and write:

Avoid complex navigation expressions

Choose appropriate context

Avoid allInstances()

Use the collect shorthand

Use association end names (role names) instead ofassociation names in modeling



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Typical Use Cases for OCL

Model Layer Examples

Metamodels: {MOF-, Ecore-based} X {UML, CWM, ODM, SBVR, PRR, DSLs}

M2(Metamodel)

Specification ofWFRs in OMG standards

Definition ofModeling Guidelines for DSLs

Specification ofModel Transformations

M1 (Model) Model Verification ( CASE-Tool)

Execution of model transformations

Specification ofBusiness Rules/Constraints

Specification ofTest Cases

M0Ob ects

Evaluation of Business Rules/Constraints



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Examples for OCL on Metamodel

WFR in UML metamodel

context Classifier inv:

not self.allParents->includes self

-- Generalization cycles are not allowed

context Classifier inv SingleInheritance:

self.generalization->size()


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Some UML/OCL Tools

12 OCL tools/libraries (see OCL Portal)

MagicDraw Enterprise Edition v16.5 Borland Together 2008 (OCL/QVT)

Eclipse MDT/OCL for EMF Based Models

ArgoUML



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Decennial Anniversary of Dresden OCL in 2009



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Dresden OCL2 for Eclipse



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Dresden OCL2 for Eclipse



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XMI Import into Dresden OCL2 for Eclipse

TopCased (EMF UML2 XMI)

MagicDraw (EMF UML2 XMI)

Visual Paradigm (EMF UML2 XMI)

Eclipse UML2 / UML2 Tools (EMF UML2 XMI)



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OCL Support in MagicDraw Enterprise Edition

OCL validation rules(based on Dresden OCL2 Toolkit)

1. Specification on UML metamodel (M2) /Verification on UML models (M1)

2. Specification of Stereotypes (M2) /Verification of UML models (M1)

3. Specification on UML models (M1) /Verification of UML instances (objects)



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Acronyms

OCL

OMG

Object Constraint Language

Object Management Group

PRR

e a- ec ac y

Production Rule Representation

UML

WFR

Unified Modeling Language

Well-Formedness Rule



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