How to generate new EMF model refactorings using Henshin transformations

This manual presents the specification of an EMF model refactoring in EMF Refactor using EMF model transformations formulated in henshin. More precisely, we demonstrate the model refactoring Move EAttribute for Ecore models. Please note, that EMF Refactor can be used for refactorings of any models whose meta model is based on EMF Ecore.

Let's take a look to the following Ecore diagram presenting a first model concerning EMF model refactorings in an early stage of the EMF Refactor development process. A ModelRefactoring has a name and conforms to a MetaModel that is specified by name, namespace prefix, and namespace URI. Furthermore, it has a label that should be shown as an Entry in the ContextMenu of an arbitrary ModelElement. A ModelElement belongs to a Model that is specified by a name and stored in a file with a specific name. Furthermore, a Model conforms to a MetaModel and each ModelElement is typed over a specific MetaModelType belonging to the corresponding MetaModel. Besides the afore mentioned attributes, each ModelRefactoring is related to a MetaModelType representing the type of the contextual element the refactoring can be applied on.

Fig. a_1

During software design it became questionable whether attribute label of class ModelRefactoring could be better placed in class Entry. So, model refactoring Move EAttribute is the next task to be performed. Since EMF Refactor can be used on arbitrary EMF based models the generation of a specific refactoring is mainly triggered from within the EMF instance editor. The next figure shows the example model from above using this tree-based editor.

Fig. a_2

EMF model refactoring Move EAttribute can be specified in the following way: First, it has to be checked whether the contextual EAttribute is not marked as ID of the containing class, and whether this class has at least one referenced class. If these (initial) checks pass the user has to put in the name of the class the attribute has to be moved to. Then, it has to be checked whether the containing class has a referenced class with the specified name, and whether this class does not already owns an attribute with the same name as the contextual attribute. If these (final) checks pass the contextual attribute can finally be moved to the specified class.

Before triggering the code generation process in EMF Refactor we specify the corresponding EMF model transformations using henshin. Henshin is a new approach for inplace transformations of EMF models and uses pattern-based rules which can be structured into nested transformation units with well-defined operational semantics.

Each part of a EMF model refactoring (initial check, final check, and the proper model transformation) has to be specified by a henshin transformation unit named mainUnit to be executed by EMF Refactor. Then, this transformation unit can reference the corresponding henshin rules. Besides refactoring specific parameters, each main unit must have a parameter named selectedEObject representing the contextual model element the refactoring should be applied on.

The following figure shows the henshin rule move specifying the movement of the contextual EAttribute from its containing EClass to a referenced EClass. This rule is contained in a henshin SequentialUnit named mainUnit to be executed. Refactoring Move EAttribute (i.e. the corresponding main unit) has one parameter, eClassName, representing the name of the class the attribute has to be moved to. The value of this parameter is passed to rule parameter referencedEClass and the value of parameter selectedEObject is passed to rule parameter selectedEAttribute.

Fig. h_1

This rule uses the abstract syntax of EMF Ecore. It specifies the selected EAttribute (in the upper left corner) that is contained in an EClass (in the lower left corner). This class also has an EReference to another EClass (in the upper right corner) with the specified name given by parameter referencedEClass. The containment relationship between the containing class and the contextual attribute has to be removed (represented by tags <<delete>>) whereas a new one between the referenced class and the contextual attribute has to be created. All other elements remain unchanged (represented by tags <<preserve>>).

In EMF Refactor, initial and final precondition checks can also be specified using henshin transformations. Here, each conflicting situation is defined by a rule pattern using the abstract syntax of the underlying modeling language. These rules must be included in a henshin unit following the same conventions as the execution unit (see above). Furthermore, parameters in the main checking unit must be equally named to the corresponding ones in the main execution unit (in our case selectedEObject and eClassName, respectively). The following figure shows both henshin rules specifying the initial checks of refactoring Move EAttribute.

Fig. h_2

Rule check_id checks whether the selected attribute is marked as ID of the containing class. Rule check_references checks whether the containing class has no referenced classes. The absence of a referenced class is modeled using tags <<forbid>>. These rules are contained in a henshin IndependentUnit to be executed. If rule check_id can be applied, EMF Refactor uses its description value to provide a detailed error message (see following figure).

Fig. h_3

The following figure shows the corresponding description (respectively error message) of rule check_references.

Fig. h_4

As mentioned above, there are two final conditions that have to be checked. First, there must be a class with the user specified name that is referenced by the containing class of the contextual attribute. The rule pattern for the absence of such a class is shown in the following figure. Again, the value of unit parameter eClassName is passed to rule parameter referencedEClass for this purpose.

Fig. h_5

The following figure shows the corresponding description (respectively error message) of rule check_input_eClassName.

Fig. h_6

The second (and last) final precondition that has to be checked is specified by rule check_existing_EAttribute as shown in the following figure. Besides the already known parameters selectedEAttribute and referencedEClass, this rule has another parameter, eAttributeName. When selecting the contextual attribute, this parameter is set to the attribute's name. Then, the rule checks whether the referenced class (with the user given name) already owns an attribute respectively reference with the same name as the contextual attribute using parameter eAttributeName.

Fig. h_7

The following figure shows the corresponding description (respectively error message) of rule check_existing_EAttribute.

Fig. h_8

After constructing the necessary henshin transformations we can start the code generation process of EMF Refactor. This refactoring specification process can be triggered from within the context menu of a certain model element in the tree-based EMF instance editor. The next figure shows the context menu of an arbitrary EAttribute representing the contextual type of our example EMF model refactoring Move EAttribute. Here, we select entry Specify EMF Model Refactoring using Henshin.

Fig. h_9

In the first page of the upcoming refactoring generation dialog three refactoring specifics have to be given (see following figure). First, you have to type in the name of the new refactoring. This name also serves as id of the new refactoring. Then, the text of the label has to be specified concerning the context menu entry when triggering the refactoring application. Finally, an Eclipse plug-in project has to be selected in which the corresponding refactoring Java code should be generated to. Further specifics concerning the contextual model element type are set automatically.

Fig. h_10

In the second page of the refactoring generation dialog the henshin transformation files that specify the three parts of the refactoring has to be selected. These files must be available in a folder named transformation in the plug-in project that has been selected in the previous dialog page. The following figure shows the selection for our example refactoring Move EAttribute.

Fig. h_11

The third page of the refactoring generation dialog specifies the parameters of the corresponding model refactoring. It shows the parameters of the main transformation unit of the corresponding execution henshin file selected in the previous dialog page (except for the contextual parameter selectedEObject). For refactoring Move EAttribute we select parameter eClassName as shown in the following figure.

Fig. h_12

After finishing the refactoring generation dialog, EMF Refactor adds some additional information to the selected Eclipse plug-in project. First, EMF Refactor adds additionally required plug-in dependencies like shown in the following figure.

Fig. h_13

To register the new EMF model refactoring the selected Eclipse plug-in project has to serve a specific extension point, org.eclipse.emf.refactor.common.Refactoring, defined by EMF Refactor. Besides the given refactoring specifics id, namespaceUri and menulabel additional references to two Java classes are needed. The following figure shows the generated extension point serving for our example refactoring Move EAttribute (using henshin).

Fig. h_14

EMF Refactor generates altogether six refactoring specific Java classes as shown in the following figure. These classes are needed by the application module of EMF Refactor to execute the specified refactoring. Furthermore, a specific package is created containing the generated Java classes.

Fig. h_15

Now, the newly specified refactoring Move EAttribute can be applied, either by deploying the Eclipse plug-in project or by starting the Eclipse runtime environment.

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