This paper presents a systematic approach supporting the translation of UML use case diagrams, describing the functional requirements of a system, into a Maude formal specification. The proposed approach also considers the static and dynamic features of object-oriented systems. The formal and object-oriented language Maude, based on rewriting logic, supports formal specification and programming of concurrent systems. The major motivations of this work are: (1) translating the functional requirements of an object-oriented system, specified using UML use case diagrams, into a Maude specification, (2) translating its static and dynamic aspects, described using UML class, communication and state-transitions diagrams respectively, into a Maude specification, and (3) integrating the formal verification of the consistency of the models, since the analysis phase. A case study is presented to illustrate our approach.
UML (Unified Modeling Language) is a language for specifying, visualizing and constructing the artifacts of software systems [OMG05]. UML has become a standard for object-oriented modeling. Its graphical notation makes it eas y to underst an d, in particular during the first phases of the development process. However, the fact that UML lacks formal semantics can lead to serious problems [Led01]. This weakness can lead, in particular, to inconsistencies within the developed models. R equirements analysis is a primordial step of the development process. The quality of the models produced at this phase is extremely important for the remaining phases of the development process. Their formal validation allows avoiding many problems that may affect the quality of the development as well as its cost [Dan07].
In this context, the use case diagrams and their related UML models play an important role. The use case diagram allows describing the functional requirements of an object-oriented system and represents an interesting communication tool between developers and users. However, like the other UML diagrams, it offers only a graphical and semi-formal description of requirements. A more formal approach, in the description of the use cases, will reduce confusion and misunderstanding risks between developers and users [She03]. As mentioned in [Sno01, Bru98], an appropriate combination of object-oriented techniques and formal methods may make practically the software development more rigorous. To achieve this purpose, one of the possible approaches consists in deriving formal specifications (Maude in our case) from UML models and analyzing them formally. An object based development process begins [Jac93, Rum94, Gli00], by building the use case diagram and the class diagram of the application domain. Communication and state-transitions diagrams are also added to these models in the following steps of the analysis process to describe, respectively, the collective and individual behaviour of objects. Each use case is described by at least one communication diagram where exchanged messages between objects appear as methods in the class diagram. The behaviour of objects of the same class is described by one state-transitions diagram. The collaborative behaviour of objects, described by UML communication diagrams, shows how objects (a group of objects) interact to realize the use cases (operations of use cases). These different models allow describing, in a complementary manner, several facets of the use cases. They represent the first models of an object-oriented iterative development. It is important to ensure, at this level, their consistency.
This paper presents a systematic approach supporting the translation of UML use case diagrams into a Maude formal specification. The proposed approach also considers, the static (UML class diagram) and dynamic (UML communication and state-transitions diagrams) features of the system. The formal and object-oriented language Maude, based on rewriting logic, offers formal and sound basis for the specification and programming of concurrent systems. Furthermore, a Maude specification constitutes a coherent and executable representation of the specifications described using UML artefacts. The principal motivations of this work are: (1) translating the functional requirements of an object-oriented system, specified using UML use case diagrams, into a Maude specification, (2) translating its static and dynamic aspects, described by UML class, communication and state-transitions diagrams respectively, into a Maude specification, and (3) integrating the formal verification of the consistency of the models, since the analysis phase. A case study is presented to illustrate our approach.
The remainder of the paper is organized as follows. In section 2, we give a brief overview of related works. Section 3 briefly presents the UML diagrams we use. In section 4 we give an overview of rewriting logic and Maude. The proposed translation process is presented in section 5. Section 6 illustrates the translation process using a concrete case study. Finally, we give a conclusion and future work directions in section 7.
2 RELATED WORK
In [Mor99], the authors have proposed a translation process of the use case model into Object-Z. They have formalized use cases using temporal logic to define invariants in the Object-Z classes schemes. However, in their work, they only consider use case and sequence diagrams. The structural aspects of the system have not been covered. Furthermore, the adopted approach does not allow the formal validation of the considered models. The developed framework also does not take into account concurrency aspects such as an object that receives a same message from several senders simultaneously.
With the objective of using jointly UML and B in a rigorous, unified and practically development process, Ledang et al. [Led01] have proposed an approach for translating the class, communication and use case diagrams into a B specification. In this approach, the UML descriptions are analyzed through the generated B specification. However, this approach does not consider explicitely the concurrential aspects of the described system. Furthermore, the B notation is not object-oriented and consequently the abstract specification generated form an object modeling may be different from which we would write directly [Tat01].
In [Fer07], the authors have proposed an approach for translating the UML 2.0 use case and sequence diagrams into a CPN formal description. The objective of this work is to develop a tool allowing software engineers to use jointly UML and CPNs. Although the CPN description is formal, it remains graphical and needs to be supported by a tool for a formal verification. The structural aspects are not covered in this approach.
We present in this paper, a generic approach supporting the generation of a Maude specification describing the functional requirements of an object-oriented system. The approach takes into account jointly the UML models mentioned in section 1, and also considers the concurrential aspects of the system. Furthermore, the use of UML and Maude is motived by the desire to use them jointly in a practically, unified and rigorous development approach. Our choice of Maude is mainly motivated by its powerful description of object-oriented concurrent systems. It offers, indeed, a powerful formal framework for the description of intra and inter objects concurrency. Furthermore, Maude is supported by a tool, which allows validating the UML diagrams through their Maude description. This tool also incorporates a model-checker which allows analysing and verifying formally the system's properties.
3 UML DIAGRAMS
UML class diagrams express the static structure of a system, in terms of classes and relationships between classes. Classes are essentially organized through aggregation, inheritance or association relationships [Mul00, OMG05].
UML state diagrams [Mul00] describe, using finite state machines, the life cycle of objects. Different types of events are defined by UML. We will focus only on the events of the "Call" type.
UML communication diagrams [Boo98, Mul00] describe how a set of objects collaborate to accomplish a specific task (for example an operation of a use case). They emphasize the dynamic interactions between those objects (message exchanges) as well as their synchronization. The concept of synchronization between messages is accomplished using the "/" symbol. A synchronization point is used to note the necessity of the completion of a particular message before the execution of another can begin, for example.
Use case diagram
UML use case diagrams describe, in the form of action and reaction, the system's behaviour from the user's point of view. They allow defining the system's limits and the relationships between the system and the environment [Mul00]. The use case diagrams represent use cases, actors and the relationships between the use cases and the actors.
4 REWRITING LOGIC AND MAUDE
Rewriting logic, having a sound and complete semantic, was introduced by Meseguer [Mes92]. It allows describing concurrent systems [Mes03, McC03, Eke02, Cla05]. This logic unifies all the formal models that express concurrency [Mes92]. The rewriting rules are based on the general form of R: [t] → [t'] if C, which indicates that, according to rule R, term t becomes or is transformed into t' if a certain condition C is verified. This rule is of the conditional form. There also exist unconditional rules where the conditional term C is not present.
Figure 1. Example of a portion of a Maude program.
The example shown in Figure 1 gives the definition of three types: Configuration, Object and Msg (those two last being subtypes of Configuration). In the case where there is no floating messages or live objects, the global configuration of the system is empty. The construction of a new configuration, in terms of other configurations, is done with the operation given on line 7. This operation satisfies the structural laws of associability and commutability and possesses a neutral element called null.
Maude is a specification and programming language based on rewriting logic [Mes92, Cla01, Cla05, McC03]. Three types of modules are defined in Maude. Functional modules allow defining data types and their functions. System modules allow defining the dynamic behaviour of a system. This type of module augments the functional modules by introducing rewriting rules. Finally, object-oriented modules, which can be reduced to system modules, offer a more appropriate syntax to describe the basic entities of the object paradigm. Maude environment has an incorporated model checker. However, model checking is out of the scope of this paper, but will be addressed in a future work.
5 TRANSLATION PROCESS
Figure 2 illustrates the translation process supported by our approach. The methodology of our approach needs, in a first time, defining the system's functional requirements using a use case diagram. For realizing use cases, we associate to each use case one or several communication diagrams, representing different possible scenarios. The messages exchanged between objects carry on methods defined in the class diagram.
Figure 2: Methodology of the approach.
The individual behaviours of objects involved in the collaboration, to realize an operation of a use case, are described by state-transitions diagrams. The considered diagrams go through a first step of an inter-models validation in order to verify the system consistency. For example, each message sent to a destination object in the communication diagram must exist in the state diagram of the object and it is accessible. In the same way, each use case in the use case diagram must be realized by at least one communication diagram.
The proposed translation process consists of generating a Maude formal description from use case, class communication and state diagrams analysis (Figure 2). During this process, several modules are generated. Figure 3 illustrates those modules. Please note that the modules in bold are object-oriented modules, while all others are functional modules.
The functional module METHOD (Figure 4) contains all the types used to describe a method. Types Parameter and ParameterList are generic. They describe the type of parameters a method uses. Furthermore, ResultType and Void describe the type of the result returned by the method. ResultType is generic, and Void is a particular case of ResultType. The operation (_,_) is a constructor for the parameter list of a function.
Figure 4. The functional module METHOD.
A functional module is associated to each state diagram for which the name is the concatenation of the class' name and the string 'STATEVALUES '. This module describes the state values a class can take according to its state diagram. The functional module IDENTIFICATION is generated to describe the identification mechanism of the objects of the communication diagram. For each class of the class diagram, we associate an object-oriented module bearing the same name as the class, while adopting a generic form for the classes (Figure 5).
In the case where one of such a class is in relation to other classes in the class diagram, the module associated to it must import all the other modules associated to those classes. The class is declared in a module with a state attribute called State and for which its type is declared in the corresponding functional module. In the case of an aggregation class, an identification list of all the aggregated classes must also be present.
Figure 5. Generic form adopted for the classes.
The methods of a class are also defined in the corresponding module using the following form (Figure 6):
Figure 6. Form adopted for the methods.
We define an object-oriented module MESSAGE in which are defined the forms of the messages exchanged between objects as well as the form of the synchronization message (see Figure 7).
Figure 7. Form adopted for the messages.
Each message exchanged between two objects of the communication diagram is translated in the form of a ComingMsg shown in Figure 7. With this message, we specify two things. On the first hand, we identify the destination object (Receiver) and, on the other hand, the result type of the operation to be executed. In fact, each sending of a message in the communication diagram corresponds to a Call Event, launching a transition in the state diagram of the destination object.
To implement the concept of Synchronization Point of the messages sent within a communication diagram, a new message called IsAccomplished is introduced (see Figure 7). The rewriting rule that implements transition corresponding to the sending of a message on which depend other messages must generate a number of IsAccomplished messages equal to the number of messages to be sent. This message is also used in the case where the sending of an asynchronous message depends on the sending of another message. Furthermore, to each use case is associated an object-oriented module Use-Casei bearing the same name as the corresponding use case. In each module Use-Casei are defined the rewriting rules describing the different interaction scenarios between the objects defined in the different communication diagrams, instances of the use case. A module describing a use case can import (optional importation) another describing a use case which is linked to it. Once generated, the modules Use-Casei are imported in the object-oriented module SYSTEM REQUIEREMENTS representing the principal module (see Figure 8). This module describes, in fact, the system's dynamic behaviour from the user's point of view.
Figure 8. The principal module SYSTEM-REQUIERMENTS.
6 CASE STUDY: THE ELEVATOR
This section illustrates the application of the proposed approach on a concrete example taken from [Mul00]. This example was simplified for the present study. It carries on the elevator working. Figure 9 shows the class diagram of that system. The functional requirements are described by the use case diagram of figure 10. The use case TransportByElevator is realized by the communication diagram of figure 11. It consists of the procedure done by a user at a given moment to use the elevator after it was started properly. Figure 12 shows respectively the state diagrams for classes Door, SignalLight, Cabin, and Elevator.
Figure 11. Communication diagram.
Figure 12. State diagrams for classes Door (a), SignalLight (b), Cabin (c) and Elevator (d).
Application of the translation process
By applying the translation process, we obtain the modules described in what follows. We have four functional modules: ELEVATOR-STATEVALUES, CABIN-STATEVALUES, SIGNALLIGHT-STATEVALUES and DOOR-STATEVALUES. These modules contain respectively the states of the different classes: Elevator, Cabin, SignalLight and Door. For reasons of space limitation, only the code for one of them is given, namely ELEVATOR-STATEVALUES (see Figure 13).
Figure 13. Module ELEVATOR-STATEVALUES.
A module IDENTIFICATION (see Figure 14) imports the predefined CONFIGURATION module. This module contains the definition of types Eoid, Coid, Doid, Soid which describe the identification mechanism of the objects E, C, P and S, instance of classes Elevator, Cabin, Door and SignalLight respectively.
Figure 14. Module IDENTIFICATION.
Figure 15. Module CABIN.
We have four object-oriented modules: ELEVATOR, CABIN, SIGNALLIGHT and DOOR. In each module, a class is defined with a State attribute describing the current state of the object, and a list of composing objects in case of aggregate classes, as well as the different methods of the class. The code for only one of those modules is presented here, namely CABIN (see Figure 15).
Three object-oriented modules implementing the use cases are generated by our approach; CALL-ELEVATOR, TRANSPORTBYElEVATOR and PREPARE-ELEVATOR. We give in what follows the code of the module TRANSPORTBYELEVATOR (sww Figure 16).
Figure 16. Module TRANSPORTBYELEVATOR.
This module imports, on the first hand, the modules CALL-ELEVATOR and PREPARE-ELEVATOR, and on the other hand the modules IDENTIFICATION and MESSAGE. Furthermore, it extends modules ELEVATOR, CABIN, DOOR and SIGNALLIGHT. Figure 16 presents two rewriting rules. The rewriting rule 'E1' describes the reception of message Initialize by object E. After its execution, the rule generates a message IsAccomplished that will be used to allow asynchronous message Start to be sent. The execution of the second rule, namely 'E2', needs, aside from the IsAccomplished message generated by the first rule, the arrival of message Start. Such a rule generates two IsAccomplished messages, to allow two other messages, namely SelectFutureFloor and ExternalCall, to be sent (see Figure 11).
The object-oriented module SYSTEM-REQUIEREMENTS (Figure 17) constitutes the principal module generated by our approach. It imports modules TRANSPORTBYElEVATOR, CALL-ELEVATOR and PREPARE-ELEVATOR.
Figure 17: Module SYSTEM-REQUIEREMENTS.
Validation of the generated description
To illustrate the validation of the generated description, two essential cases are presented: the case where the elevator receives an external call, after it was initialized and started by the maintenance technician, and the case where the elevator receives a message for selecting the next floor. For the first case, the initial configuration is given by figure 18:
Figure 18. First initial configuration.
This configuration represents, an object E, instance of the class Elevator in starting state (Started ), an object C of the class CABIN in waiting state (Waiting ), an object D of the class DOOR in closed state (Closed ) and an object S of the class SIGNALLIGHT in extinguished state (Off ). It also shows the arrival of an external call accomplished by the user from the floor number 4. The unlimited rewriting of this configuration returns the result given in figure 19.
Figure 19. Result of the unlimited rewriting of the first initial configuration.
This result shows that, the Elevator is in its Started state, the Door is closed, the SignalLight is lit, but the Cabin is always in a waiting state because the user has not selected yet its destination.
For the second case, we extend the configuration of figure 19. We relaunch the rewriting process from the result of the rewriting of the first configuration while adding to it the arrival of a message for selecting the next floor SelectFuturFloor (figure 20).
Figure 20. Second initial configuration.
The unlimited rewriting of the second configuration returns the result given by the figure 21. This configuration is similar to the one of figure 19 except that the Cabin is in moving.
Figure 21. Result of the unlimited rewriting of the second initial configuration.
Figure 22 shows part of the code developed in the Maude language, namely the principal module SYSTEM-REQUIERMENTS of our framework which describes the system's functional requirements illustrated by the use case diagram of figure 10. This module imports the others modules (CALL-ELAVATOR, PREPAR-ELEVATOR and TRANSPORT-BY-ELEVATOR) implementing the different scenarios of the different use cases of figure 10.
The following figure also illustrates, the launching of the unlimited rewriting process of the configurations of figures 18 and 20, and the limited rewriting (after two rewriting steps) of the first initial configuration of figure 18.
Figure 22. Part of the developed code.
Figure 23. Results of the rewriting of the different configurations.
The results of the rewriting of the different configurations are illustrated by figure 23. This figure shows, the results of the unlimited rewriting of the two first configurations. These results are like to those presented in figures 19 and 21. The figure shows also the result of the limited rewriting of the first configuration. This last result represents an intermediate configuration illustrating: the elevator in starting, the cabin in waiting, the door is open, the signallight is extinguished and the arrival of a message indicating that the user is ready to accede to the cabin.
7 CONCLUSION AND FUTURE WORK
In this paper, we proposed a generic approach that allows translating functional requirements described by UML use case diagrams into a Maude formal specification. This thematic has been addressed in several papers published in the literature. However, the structural and/or concurrential aspects of the systems have not been covered in most of these papers.
The proposed approach takes into account the system's structural and dynamic (individual and collective) features. Furthermore, concurrential aspects have also been considered. Maude is very appropriate for describing object-oriented concurrent systems. The Maude language is supported by a tool, which allowed us to validate the generated code by simulation.
Maude offers a model checker in its environment, which uses Linear Temporal Logic (LTL) to verify properties among the developed models. As future work, we plan on extending the formal framework we developed to analize and verify system's functional requirements though their Maude descriptions using the model checker incorporated in Maude environment. This model checker uses on-the-fly techniques to manage the state-space problem from which model checking techniques suffer. Linear Time Logic is used to define desirable or non desirable properties that are to be checked in the system under development.
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About the authors
Farid Mokhati, Mourad Badri: "Generating Maude Specifications From UML Use Case Diagrams", in Journal of Object Technology, vol. 8, no. 2, March-April 2009, pp. 119-136 http://www.jot.fm/issues/issue_2009_03/article2/