Object Constraint Language

Object Constraint Language Design by Contract and Queries Dan Massey Y&L Consulting

OCL Topics • Object Constraint Language (OCL) • OCL for Design by Contract • OCL for Queries • Another OCL Example

Object Constraint Language • OCL 2.0 • Product of the Object Management Group (OMG) • Add-on to the Unified Modeling Language (UML™) 2.0 specification • UML combined with OCL forms a solid base for Model Driven Architecture (MDA®)

Model Driven Architecture • Work with Platform Independent Model (PIM) • Transform PIM into Platform Specific Model (PSM) • Transform PSM into implementation

Model Constraints • Originally viewed as a way to constrain or to restrict values in a model • In addition OCL supports: • Query expressions • Derived values • Conditions • Business rules

Reasons to use OCL • Express concepts that are not supported by UML diagrams • Make models more precise • Support transformation tools and code generation (MDA)

Approach • We’re going to explore OCL by example to give you ideas as to where it might apply in your work. • At the end I’ll point you toward sources of more complete information.

Design by Contract • Why do we use contracts? • We know the rules. We know what happens if we break them. • Design by Contract does the same for software. • Provide a formal specification of behavior.

More Design by Contract • winningBid():Bid is a signature, not a specification. It’s like giving someone a phone without providing the format for a correct phone number. • Are there any conditions that must be met for a call to winningBid() to succeed?

And… Design by Contract • Specify preconditions, postconditions, and invariants. Defensive programming? • We’ll cover each of these in turn. • Bertrand Meyer is the father of DbC • http://www2.inf.ethz.ch/personal/meyer/

Auction Domain • A partial auction domain to get us started. Includes “Auction,” “Bid,” and “Money.” • We have the structure, but what are the rules?

OCL Expression Context • Every OCL expression takes place in a context: • contextAuction • contextAuction::winningBid():Bid

Preconditions • Preconditions are the conditions that must be true prior to successfully invoking the method in question. • Often preconditions that fail result in exceptions.

Example Preconditions • contextAuction::placeBid(bidder:Id,bid:Money) • pre:notbidder.oclIsUndefined() • pre:notbid.oclIsUndefined() • pre:bid.currency= • self.bids->first().amount.currency • pre:bid>self.currentBid().money

Simple Preconditions • contextAuction::placeBid(bidder:Id,bid:Money) • pre:notbidder.oclIsUndefined() • pre:notbid.oclIsUndefined() • This operation requires a bidder identifier and a bid.

Implementing Preconditions • … • publicvoid placeBid(Id bidder, • Money bid) { • if (bidder == null) { • thrownew NullPointerException( • “bidder may not be null”); • } • …

More Involved Preconditions • contextAuction::placeBid(bidder:Id,bid:Money) • pre:bid.currency= • self.bids->first().amount.currency • pre:bid>self.currentBid().money • The bid must be in the same currency as the starting bid. The bid must be higher than the current high bid.

Implementing Preconditions • … • if (bid.getAmount().getCurrency() != • ((Bid) bids.first()).getAmount() • .getCurrency()) { • thrownew IllegalArgumentException( • “currencies must match”); • } • …

Assertion Temptation • // Don’t do this. Anyone who runs your app • // can “turn off” assertions. • … • assert bidder != null : • “bidder may not be null”; • assert bid.getAmount().getCurrency() == • ((Bid) bids.first()).getAmount() • .getCurrency() : • “currencies must match”; • …

OCL Comments • contextAuction::placeBid(bidder:Id,bid:Money) • -- currencies must match • pre:bid.currency= • self.bids->first().amount.currency • prealwaysTrue:1=1 • -- and /**/ are valid comment forms in OCL

More Preconditions • Preconditions are not limited to checking parameters. They can specify any condition that must be true prior to executing the operation. • What are some other possible preconditions?

More Preconditions • contextAuction::winningBid():Bid • -- auction must end to have a winner • pre:TimeStamp::now()>=self.expires() • contextBid::>(bid:Bid):Boolean • -- _gt is our name for “>” • pre:notbid.oclIsUndefined()

Invariants • Invariants are conditions that are always true. • They are always true. Think conservation of energy, not “the color of the sky is blue.”

Invariants • If an invariant is violated something is wrong with the implementation of your system. • What are some invariants in our domain?

Example Invariants • contextAuction • inv:self.expires()>self.start • contextBid • inv:self.amount.amount>0.0 • /* Note this could be specified in • the Auction context, but don’t. */

Postconditions • Postconditions are the conditions that your operation guarantees will be true when it has completed. • The contract: You meet the preconditions and I’ll meet the postconditions.

PostConditions • contextAuction::placeBid(bidder:Id, • bid:Money) • post: self.currentBid().amount=bid • Notice that the postcondition assumes serial or single-threaded execution. This postcondition isn’t necessarily enforceable in an unprotected multi-threaded context.

PostConditions • contextAuction::placeBid(bidder:Id, • bid:Money) • post:self.currentBid()> • self.currentBid@pre() • Postconditions can reach back in time. • @pre references the value before the operation. • Another way to state a precondition…

Implementing PostConditions • … • publicvoid placeBid(Id bidder, • Money bid) { • … • Bid preCurrentBid = this.currentBid(); • … • if (this.currentBid() <= preCurrentBid) { • …

OCL Contracts • Add more formal specifications to your models. • Specify what will happen, without the how. • Start at component boundaries. • You can start out with simple contracts.

OCL as Query Language • You can think of OCL as equivalent to SQL when it comes to querying object models. • Borland’s ECO framework uses OCL for querying (as well as constraints, derived values, etc.) • Other query alternatives include OQL and Xquery

Test Results Mini Domain • Below is a simple domain for describing test results from test sessions at a competition for teams of logicians. • This is our domain for examining OCL queries.

Example Test Data Set • Here’s some example test result data from a logician competition.

Derived Values • contextTestResult::compositeScore:Integer • derive:(2*logicScore)+illogicScore

OCL Collection Operations • contextTestSession::numberOfResults:Integer • derive:results->size()

OCL Collection Choices • Set: no duplicates, not ordered,result of navigating on association • OrderedSet: not sorted, just ordered • Bag: may contain duplicates, not ordered, • usually the result of navigating multiple associations • Sequence: an ordered Bag

OCL Collections Classification

OCL select() Query • contextTestSession::numberOfScoresOverTarget( target:Integer):Integer • body:results->select( compositeScore>target)->size()

Translating OCL Queries • Let’s look at what a transformation tool might do with one of our queries in SQL. • SELECT COUNT(*) FROM test_resultsWHERE composite_score > :target;

Returning a Collection • contextTestSession::testResultsForTeam( testTeam:Team):Set(TestResult) • body:results->select(team=testTeam)

Building Up Queries • contextTestSession::averageScoreForTeam( testTeam:Team):Real • body:testResultsForTeam(testTeam).compositeScore • ->sum()/testResultsForTeam(testTeam)->size()

Which Logicians Won? • contextTestSession::bestResults():Set(TestResult) • body:results->select(compositeScore= • results->sortedBy(compositeScore) ->last().compositeScore)

OCL Resources • The Object Constraint Language, 2edJos Warmer and Anneke Kleppe • Klasse Objecten’s OCL pagehttp://www.klasse.nl/ocl/ • OCL 2.0 Specification at OMGhttp://www.omg.org/cgi-bin/doc?ptc/2003-10-14

Questions and Answers

Object Constraint Language