Executable UML – UML2 The Need for xUML With Acknowledgement to Kennedy-Carter: Moh Ibrahim, Alex Fedorec & Keith Rennolls The University of Greenwich

2 Bonn – NEFIS Why is the UML 1.x… not executable ?  It is ‘computationally’ incomplete  UML describes a system in, broadly, two ways:  by specifying the desired results (use cases, sequence diagrams)  by specifying the software that, when executed, will produce the desired results (classes, associations, operations, state machines)  The latter specification of behaviour is the basis for implementing the system, but is missing key ingredients:  the implementation of operations (methods) are specified only by a “language dependent” text string  the actions associated with state machines (and methods) are specified either by a text string, or by using the “action” part of the UML Action Terminate Action DestroyAction Uninterpreted Action CreateAction CallActionReturnAction SendAction

3 Bonn – NEFIS Action Specifications in UML 1.x  Cannot be attached to methods and therefore  cannot be used to specify operations  Do not cover the full range of information required to model real behaviour  No conditional logic  Do not deal with internal data within an Action Sequence  Cannot describe reading and writing of attributes  Cannot describe manipulation and navigation of associations  Cannot describe parallel behaviour  Not related to the UML type model  Do not have fully defined semantics..... were intended only as a place holder for future UML development

4 Bonn – NEFIS Why is the current UML not executable ?  It is big  UML was conceived as a Universal as well as Unified modelling language  UML covers many development paradigms  Synchronous and asynchronous behaviour  State dependent and stateless behaviour  Mealy state machines and Moore state machines  Flat state models and Harel state charts  Abstract analysis modelling and code specific design modelling  Language specific modelling and abstract modelling  Sometimes the breadth of coverage can lead to ambiguity... Transaction in Progress Transaction Complete entry/ log transaction Transaction Completed/ if is_aborted then return e.g. what happens if you execute a “return” in a transition action stimulated by “call” (synchronous) event ? I have absolutely no idea because that combination of behaviour is undefined!

5 Bonn – NEFIS Actions and the UML  the Action Specification Language, operates at the same level of abstraction as UML… …but embodies the precision to allow models to be executed and consequently supports translation of the models into any language.  The OMG has recognised the need for a full action specification in the UML  In November 1998 the OMG issued a Request for Proposals on Precise Action Semantics for the UML

6 Bonn – NEFIS What is xUML? UML V1.x xUML = Semantically Weak Elements Semantically Weak Elements - Precisely Defined Action Semantics +  xUML is  an executable version of the UML, with…  clearly defined simple model structure  a precise semantics for actions, incorporated into the UML 2 standard  a compliant action specification language  an accompanying process  a proven development process, oriented towards…  executable modelling  large-scale reuse  pattern based design

7 Bonn – NEFIS Why an Executable UML ?  Measurable Deliverables  Models that execute tests correctly  Can be delivered in phases with progressive integration  Use case by use case Admit In Patient User Interface U log incident Patient AdminStatementsResource Alloc 1:dialogue ok hit 2:admit in patient 3:assign bed 4:bed assigned 5:confirm admission 6:display dialogue 8:reject admission 9:display dialogue 1:op selects admit in patient 2: 3:find a suitable bed 4:if bed available then 5: confirm admission 6: display “success” dialogue 7: log admission details 8:else reject admission 9: display “failure” dialogue  Class group by class group  Early Verification  Requirements can be validated before extensive system design and coding This supports iterative and incremental development An executable model can be developed and tested even if it supports only a single use case Groups of classes at the collaboration, subsystem, package or domain level can be modelled and executed

8 Bonn – NEFIS Why an Executable UML ?  Improves the quality of the modelling  Models can be judged not just on subjective criteria, but on whether they exhibit the desired behaviour and thus meet requirements  Focused analyst objective àThe aim is to build models that execute correctly àAvoids “analysis paralysis” àReviews criteria are objective and therefore more useful Aircraft clearedRunway() Runway deallocate() 0..1 is_ allocated_to R Does this identify the correct runway to deallocate ? Does this identify the correct runway to deallocate ? # obtain an instance handle for # the runway we just landed on theRunway = this -> R9.”is_allocated_to” # remove the association unlink this R9 theRunway # tell the runway that aircraft has landed [] = deallocate[] on theRunway

9 Bonn – NEFIS Why an Executable UML ?..... eliminates maintenance problems due to redundant analysis and design models  Supports extensive code generation if desired  Lack of ambiguity means automatic code generation will deliver functionally correct code GenericContainer HashTableLinkedListBoundedArray GenericClass 0..* 1  A solid specification  Supports multiple development teams  Well defined models mean well defined interfaces  Easier Transition to Design and Code  No ambiguity in the models - the models have a single clear interpretation  Design can be specified using abstract patterns

10 Bonn – NEFIS Why an Executable UML ?  and finally..... It’s more fun !  Analysts see the results of their efforts more rapidly and have more confidence that what they are doing is correct  Managers get more confidence that progress is being made Round-trip Software Engineering - Quality Assurance Problem / Requirement Solution / Implementation Can miss the solution to the problem?