Developing a DSL Abstract Syntax with Eclipse

omain-specific languages (DSLs) and model-driven development (MDD) offer developers powerful new ways to improve productivity, enhance quality, and insulate systems from rapid technological change. The Eclipse Modeling Project allows developers to create DSLs and use MDD techniques with the open source Eclipse platform. In the book Eclipse Modeling Project: A Domain-Specific Language (DSL) Toolkit, Richard C. Gronback illuminates both the principles and techniques software professionals need to master.

In Chapter 3, “Developing a DSL Abstract Syntax,” Gronback?an Eclipse Modeling Project co-leader?walks through the development of a DSL using the Eclipse Modeling Framework (EMF) and supporting components. This article is an excerpt of Chapter 3.

Editor’s Note: This chapter is an excerpt from the book, Eclipse Modeling Project: A Domain-Specific Language Toolkit, authored by Richard Gronback, published by Addison-Wesley Professional as part of the eclipse series: www.informit.com/series/eclipse, Copyright 2009 Pearson Education, Inc. ISBN 0321534077, published March 2009. Safari Books Online subscribers can access the book here: http://safari.informit.com/.

Chapter 3: Developing a DSL Abstract Syntax

In this chapter, we walk through the development of a domain-specific language (DSL) using the Eclipse Modeling Framework (EMF) and supporting components. Specifically, we develop the DSL’s abstract syntax using the Ecore metamodel. But first we cover some basics on what to consider when creating a DSL and the different implementation strategies you might want to employ when using EMF. Next, we provide an overview of EMF, leaving detailed information to the book [38] dedicated to this purpose. We cover some additional components of EMF and Model Development Tools (MDT) that enable you to further refine DSLs, and we develop a series of domain models for use in the sample projects.

Disclaimer: The domain models developed as samples are constructed to illustrate certain features of the associated tooling and, as such, should not necessarily be considered “best practices” in some cases.

3.1 DSL Considerations

Many considerations are involved in creating a DSL. Does a model already exist that is close enough? If so, can an existing model be extended, or is it fixed? Does the model need to be based on a standard? Does the DSL lend itself to graphical display and editing? Does the DSL require a textual syntax and editor? Will a product line be built on the DSL, and perhaps others? Is the Ecore metamodel expressive enough to suit your needs for a DSL? How can you model dynamic behavior?

Best Practice: Leverage existing models, when appropriate. XML Schema Definition (XSD) and EMF are very popular technologies, and EMF can import just about any XSD, so search for existing domain models before you reinvent the wheel. Also consider publishing your domain model if you think that others might find it useful, if only as part of your application’s API to aid in integration.

A key consideration is the amount of flexibility you need or will tolerate in the DSL. As you can see in the examples, sometimes a change in the domain model makes your transformation definitions much easier to write. Also, frameworks such as GMF have certain limitations?or, rather, were designed with particular use cases in mind. Your particular style of modeling might not lend itself well to graphical representation, but a few changes might allow mapping to diagram elements much easier. For example, certain mappings in Query/View/ Transformation (QVT) and template expressions can be facilitated by adding derived features or methods to the domain model. Complex queries using Object Constraint Language (OCL) (and, therefore, useful ones in QVT and Xtend) can be added to the domain model with code generated for their implementation at runtime. Having a feature available in the model will greatly simplify transformations and templates that access them.

Tip: Don’t be afraid of modifying your domain model to make working with templates, transformations, and diagram definitions easier. Unless you’re using a model that cannot be altered, the Toolsmith will appreciate being able to make certain design decisions in the domain model to suit the tooling, instead of having to create workarounds or write custom code to use the tooling with a domain model.

This is not to say that you should let the tooling influence your DSL to an extent you are not comfortable with. The question is, how do you maintain a satisfactory level of “purity” in your DSL when considering the additional complexity associated with developing and maintaining the other Model-Driven Software Development (MDSD) artifacts? In general, the more complex the metamodel (DSL) is, the more complex the transformation definitions, templates, and diagram definitions are.

A set of conventions and best practices for the definition of DSLs, transformations, and templates likely will arise, as it has for Java and other popular programming languages. With conventions and best practices comes tooling to support refactorings, static analysis, and cleanup. At this stage of the Modeling project’s evolution, operations are still quite manual and even error prone. As an open source project that forms the basis for commercial products, Eclipse eventually will see more advanced features pushed down into it, thereby improving the Toolsmith experience.

3.2 Eclipse Modeling Framework

From the project description, EMF is “a modeling framework and code generation facility for building tools and other applications based on a structured data model.” This pretty much sums it up, but there’s a lot to know about EMF. I highly recommend that you first read, or at least have available, the book on EMF [38] to better understand its use in the context of this book. Alternatively, reading through the online documentation and tutorials on EMF should make its use in this book easy to follow. In other words, the examples in this book only scratch the surface of what is possible using EMF.

You can create models using EMF in many ways. You can use the provided editor (a tree with properties view) or import a set of annotated Java classes. An Ecore diagram is available from the EMFT project. If you have the XSD component installed, you can import an XSD file. If you have the Unified Modeling Language (UML) version 2 (UML2) component installed, you can import a UML2 model. If you have Graphical Modeling Framework (GMF) installed, you can use its example Ecore diagram editor. If you download and install Emfatic [42], you can work in a textual syntax and synchronize with your Ecore model. In the future, you will be able to design your own concrete textual syntax for Ecore, or any model, using the Textual Modeling Framework (TMF) project.

Regardless of the method you choose for importing or working with your domain model, you will find an .ecore file in your workspace?that is, unless you take a purely programmatic approach. If you open this file in a text editor, you will see that it is an XML Metadata Interchange (XMI) serialization of your Ecore-based model. By default, EMF enables you to edit models in a basic (generated) tree editor with a Properties view. You can easily generate a similar editor for your own model.

Before getting into more detail, let’s take a look at the Ecore metamodel.

3.2.1 Ecore Metamodel

The EMF book describes the Ecore metamodel in detail, but here you find a simplified diagram for reference (Figure 3-1), along with some discussion of the more relevant aspects used as we develop our DSL abstract syntax. It’s a fairly simple model, which is part of its strength. In most cases, you can compensate for the lack of features in Ecore by using some of the more advanced modeling techniques, which are discussed in the EMF book. A longstanding topic of debate among EMF users is the lack of an EAssociation class, but we don’t get into that here.

Figure 3-1. Ecore model

Annotations

Sometimes it’s important to add information to a model element for documentation purposes, or to provide parameters to be considered during transformation or generation. EAnnotations provide these for all model elements in EMF. An EAnnotation has a Source field, which serves as a key, and a list of References. An EAnnotation may have zero or more Details Entry children, which have Key and Value properties. This simple capability of annotating models is quite flexible and turns out to be useful for many purposes, including XSD support.

Another particularly useful application of annotations is to declare OCL constraints, method bodies, and derived feature implementation, as discussed in Section 3.2.4, “Applying OCL.”

3.2.2 Runtime Features

The EMF runtime includes facilities for working with instances of your models. No strict dependencies exist on the Eclipse platform for the runtime and generated model and edit code, so these bundles can be used outside of the Eclipse workbench. As bundles, they can be deployed in any Equinox OSGi container, even within a server environment.

The generated code for your model has a dependency on the underlying EMF runtime components. A significant benefit is gained from the generated Application Programming Interface (API) and model implementation working with the provided runtime features. An efficient observer pattern implementation is provided to alert listeners to model change events. A generated reflective API provides an efficient means of working with models dynamically. In fact, EMF can be used in a purely dynamic fashion, requiring neither an .ecore model nor code generation. Finally, it’s possible to have static registration of a dynamic package, but that’s an advanced use case left to the EMF documentation.

When working with model instances, changes can be recorded in a change model that provides a reverse delta and allows for transaction support. A validation framework provides for invariant and constraint support with batch processing. The Model Transaction and Validation Framework components provide enhanced transaction and validation support, respectively.

Figure 3-2. EMF-generated editor

For persistence of models, the EMF runtime provides a default XML serialization. The persistence layer is flexible, allowing for XMI, Universally Unique Identifiers (UUIDs), and even a zip option. A resource set consists of one or more resources, making it possible to persist objects in multiple files, including cross-containment references. Proxy resolution and demand loading improve performance when working with large models across resources. Additionally, use of EMF Technology (EMFT) components Teneo and CDO allow for the persistence of models to a relational database.

The generated editor for EMF models includes a multipage editor and properties view. Drag-and-drop support is provided, as is copy/paste support. A number of menu actions are available in the generated editor, including validation invocation and dynamic instance model creation. Each generated editor comes with a default creation wizard. Figure 3-2 shows an example of the editor, including a context menu showing options to create new elements, cut, copy, paste, delete, validate, and so on.

3.2.3 Code Generation

From an *.ecore (Ecore) model, you need to produce a generator model and supply additional information required for code generation. An EMF generator model has a *.genmodel file extension and is essentially a decorator model for a corresponding *.ecore model. This generator model is fed to Java Emitter Templates (JETs) that are used to write Java and other files. JET is the Java Server Pages (JSP)-like technology used by default when generating text from Ecore models. This book does not cover it in detail, but a tutorial is available online [51] if you want to know more.

You can customize the existing generation output using custom templates. Furthermore, you can extract constraint, pre-/post-condition, and body implementations from OCL annotations for use in generation and invocation at runtime. This is not a native EMF capability, but you can add it using the MDT OCL component. You will use this technique in the context of the sample projects.

When regenerating code, the JMerge component is used to prevent overwriting user modifications. Generated Java code is annotated with @generated javadoc style tags to identify it and distinguish it from user code. Removing the tag or adding NOT after the tag ensures that JMerge will not overwrite the modified code. Typically, using @generated NOT is preferred because it allows the Toolsmith to identify code that was generated and modified, as opposed to newly added code. Note that not all code benefits from merging. Specifically, plugin.xml and MANIFEST.MF files need to be deleted before an update can occur.

3.2.4 Applying OCL

Many opportunities arise for using OCL in EMF models. Class constraints, method bodies, and derived feature implementations can all be provided using MDT OCL and EMF dynamic templates. The approach of using OCL and custom templates in this book comes from an Eclipse Corner article [44] and has been modified only slightly to conform to a similar approach taken to leverage OCL added to models in QVT, as discussed in Section 6.5.6, “Leveraging OCL in EMF Models.” The templates are generic and can easily be added to any project that needs to provide OCL-based implementations in its generated model code. It is also worth noting that GMF uses OCL extensively in its models, employing an EMF Validator to maintain the integrity of its models.

To add an OCL expression to a model element, we begin by adding a normal EAnnotation. For the Source property, enter http://www.eclipse.org/2007/OCL. This URI allows our custom templates and QVT engine to recognize this annotation as OCL, where it can expect to find Details Entry children of type constraint, derive, or body. Note that the original article [44] used http://www.eclipse.org/ocl/examples/OCL as the URI.

Depending on the context, add the appropriate Key (EMF constraint key, derive, or body) to a child Details Entry of the EAnnotation and specify the OCL in the Value property. For invariant constraints, the OCL annotations complement the normal EMF constraint annotations by providing implementation for the validation framework to enforce constraints.

Tip: To test your OCL, it’s helpful to use the Interactive OCL Console with a dynamic instance of your model, as discussed in Section 1.5.4, “Object Constraint Language.” Be sure to select the proper model element for the expression, as well as the proper metalevel in the console.

To invoke the provided OCL at runtime, you must use custom JET templates for your domain model. The generated code retrieves the OCL statement from the model element and invokes it, evaluating the result. An alternative to this is to generate a Java implementation of the OCL during generation and avoid invoking the OCL interpreter at runtime.

The referenced article covers the details of the custom templates, so they are not covered here. Also, the templates are included in the book’s sample projects and are touched upon during the development of the sample projects. For now, we take a look at just the derived feature implementation, both before and after using the OCL with a custom template approach. First, consider the default generated code for a derived reference?in this case, the rootTopics reference from the MapImpl class in our mindmap example.

/** *  *  * @generated */public EList getRootTopics() {  // TODO: implement this method to return the 'Root Topics'   // reference list  // Ensure that you remove @generated or mark it @generated NOT  // The list is expected to implement   // org.eclipse.emf.ecore.util.InternalEList and   // org.eclipse.emf.ecore.EStructuralFeature.Setting  // so it's likely that an appropriate subclass of      // org.eclipse.emf.ecore.util.EcoreEList should be used.	throw new UnsupportedOperationException();}

Let’s add the following OCL statement to the derived feature using the previous convention. Here we see the annotation within the mindmap.ecore model in its native XMI serialization. Note that this OCL statement could be simplified by using the parent eOpposite relationship on our Topic’s subtopics reference, which was added to facilitate the diagram definition of Section 4.3.5, “Subtopic Figure.”

      

Before regeneration, we need to make some changes in the genmodel. To allow the OCL plug-in to be added to our generated manifest dependencies, we need to add OCL_ECORE=org.eclipse.ocl.ecore to the Model Plug-in Variables property of the genmodel root. Also, we need to set the Dynamic Templates property to true and enter the templates path (such as /org.eclipse.dsl.mindmap/templates/domain) to the Template Directory property. After we generate, we can see the following implementation in our MapImpl class.

private static OCLExpression rootTopicsDeriveOCL;private static final String OCL_ANNOTATION_SOURCE =   "http://www.eclipse.org/2007/OCL";/** *  *  * @generated */public EList getRootTopics() {  EStructuralFeature eFeature =       MindmapPackage.Literals.MAP__ROOT_TOPICS;  if (rootTopicsDeriveOCL == null) {     Helper helper = OCL_ENV.createOCLHelper();    helper.setAttributeContext(MindmapPackage.Literals.MAP, eFeature);    EAnnotation ocl = eFeature.getEAnnotation(OCL_ANNOTATION_SOURCE);    String derive = (String) ocl.getDetails().get("derive");    try {      rootTopicsDeriveOCL = helper.createQuery(derive);    } catch (ParserException e) {      throw new UnsupportedOperationException(e.getLocalizedMessage());    }  }  Query query =     OCL_ENV.createQuery(rootTopicsDeriveOCL);  @SuppressWarnings("unchecked")  Collection result = (Collection) query.evaluate(this);    return new EcoreEList.UnmodifiableEList(this, eFeature,     result.size(), result.toArray());}

The generated code checks to see if the OCLExpression for this derivation has been created already; if not, it initializes it by retrieving the statement from the Eannotation and its detail with key derive. Then the expression is evaluated and the list of Topic elements is returned.

As mentioned in the article, some improvements could be made to this approach, but it illustrates the usefulness of adding OCL statements to your EMF models. It’s not hard to imagine how a significant portion of an application could be generated from a model adorned with OCL for invariant constraints, method bodies, and derived features. In GMF, we can see how OCL is used to augment the diagram-mapping model to provide for constraints, feature initialization, audit definition, and model metric definition.

Best Practice: Adding constraints and validation is essential in model-driven software development. Although you can place validation code within QVT, Xpand templates, and so on, it’s most useful to ensure that your model instance is well formed when created, or before moving to a model transformation.

3.2.5 Dynamic Instances

A powerful feature of EMF, and one that is useful to a Toolsmith developing a new DSL, is the capability to create dynamic instances of a model. The reflective framework of EMF is leveraged to allow instantiation of a model element without generating code beforehand. This can be done from within the default Ecore editor by selecting an element and choosing the Create Dynamic Instance context menu item. The instance is stored in an XMI file within the development workspace, so the generation or launch of plug-ins is not required to test a model or, more importantly, to test Xpand templates and QVT transformations under development. This is one important distinction when comparing JET to Xpand. Dynamic instances are used in the context of our sample projects.

Best Practice: Use dynamic instance models for development as much as possible. Xpand templates, QVT transformations, and the OCL console can all work with dynamic instance models and avoid making Toolsmiths generate code and invoke a runtime instance to test their work. GMF diagrams still require code generation to develop effectively, although generated diagrams are capable of working with dynamic instances.
Figure 3-3. Mindmap dynamic instance model

Figure 3-3 is an example of a dynamic instance model for our mindmap domain model, along with the Properties view. It’s similar in functionality to the generated EMF editor, although it requires the metamodel to be loaded and reflected upon, as you can see from the loaded mindmap.ecore resource file.

Tip: You can view any Ecore model using the Sample Reflective Ecore Model Editor, so there’s little need to generate the EMF .editor plug-in. This applies to XMI dynamic instances, such as GMF-based diagrams files where both the domain and notation models are persisted in a single file. Simply right-click the file and select Open With ?> Other ?> Internal Editors ?> Sample Reflective Ecore Model Editor.

3.3 Developing the Mindmap Domain Model

We develop a simple mindmap DSL and use it throughout the book to provide an example of how to use components of the Modeling project as a DSL Toolkit. This model forms the base of our fictitious Requirements Elicitation Project (REP).

This is the beginning of those sections in the book that you can follow in a tutorial fashion. Solution projects are available to save time, although you should be able to begin from scratch and produce these solutions on your own. It’s up to you to follow along or simply review the solutions as we proceed.

Figure 3-4 is a diagram of the basic mindmap DSL we create in this section. Not much explanation should be required here because you can easily see that a Map class serves as the container for Topics and Relationships, which both extend from MapElement. The following sections provide details on setting up a DSL project and creating this model, along with the other DSL artifacts associated with the project.

Figure 3-4. Mindmap domain model

3.3.1 Project Setup

Before getting started defining our mindmap domain model, we need a new project. Although EMF and GMF provide their own project wizards, we use the DSL Project Wizard provided by the Amalgam project to hold our DSL artifacts. You can create an equivalent project by starting with a plug-in project and adding the required dependencies, natures, and builders. The DSL project is also a plug-in project, as we’ll eventually want to deploy the project to facilitate revisioning and extension. Furthermore, Xpand and workflow files currently need to be located in source paths to be developed, so we need a Java project anyway. In the future, this should not be required.

For our mindmap project, switch to the DSL perspective and use File ?> New ?> DSL Project to create a new project named org.eclipse.dsl.mindmap. The wizard creates a set of folders: /model, /diagrams, /templates, /transformations, and /workflows. Not all of these folders are required for each DSL project, but we use them for our mindmap. The wizard also adds natures and builders for QVT and Xpand/Model Workflow Engine (MWE).

3.3.2 Creating the Mindmap Domain Model

As mentioned earlier, creating an Ecore model involves many starting points. If we had an existing XML Schema for our domain, we could import it and EMF would take care of serializing documents conforming to the schema. If we used the UML2 project and associated the UML2 Tools class diagram to model our domain, we could import it to create an EMF model. We begin using “classic” EMF to create our mindmap DSL from scratch.

Typically, we’d begin with File ?> New ?> Other ?> Eclipse Modeling Framework ?> Ecore Model (Ctrl+3 ?> Ecore Model). However, the DSL Toolkit from Amalgam provides some wizard redefinitions to facilitate DSL development and defines capability definitions to hide the original UI contributions from various Modeling projects. To create our model, we select the /model folder and use the File ?> New ?> Domain Model (Ctrl+3 ?> Domain Model) wizard, which is really just the GMF Ecore diagram wizard. Name the model and diagram files mindmap.ecore and mindmap.ecore_diagram, respectively. Optionally, you can use the Ecore Tools component, available from the EMFT project. It provides some capabilities beyond those that the GMF example editor provides.

Before we begin to model the domain, we need to set some defaults in our mindmap Ecore model. First, right-click on the blank diagram surface and select Show Properties View. This shows the properties for the root package in our new Ecore model. Each Ecore model has a single root package, under which we can create a number of additional subpackages. In our case, we set the properties accordingly: name and Ns Prefix should be set to mindmap; Ns URI should be set to http://www.eclipse.org/2008/mindmap.

Using Figure 3-4, model the mindmap domain using the palette and Properties view. It’s a straightforward model to implement, with only a couple noteworthy items: First, the MapElement class is abstract; second, the rootTopics relationship is derived, transient, and volatile. We implement this derived reference using OCL in Section 3.3.5, “Adding OCL.”

The diagram surface has many features to explore, as discussed in Section 10.1, “Overview.” You should note a few things, however, when using the Ecore diagram to create the mindmap domain model:

  • Aggregation links create a reference with the Containment property set to true, in contrast with Association links, which are noncontainment references.
  • Setting the upper bound property of a link to –1 creates a many relationship and causes the link to be displayed with the familiar 0..* notation.
  • References with eOpposites are shown in Figure 3-4 as a single connection, whereas the Ecore diagram shows two separate links.

3.3.3 Creating the Mindmap Generator Model

With our mindmap.ecore model complete, we can validate it and create a generator model. To validate it, open the model in the Sample Ecore Model Editor and right-click on the root package. Select Validate and confirm that no errors exist. If there are errors, correct them and continue. We look into adding validation for our mindmap model later, which leverages a similar validation framework provided for all Ecore models.

To create mindmap.genmodel, right-click the mindmap.ecore file in Explorer view and select New ?> Other ?> Domain-Specific Language ?> Domain Generator Model (Ctrl+3 ?> Domain Gen). Note that the original EMF wizard is found in New ?> Other ?> Eclipse Modeling Framework ?> EMF Model (Ctrl+3 ?> EMF Model). We started by selecting our mindmap.ecore model, so the wizard defaults to the same folder and provides the name we want. It also recognizes that we are importing an Ecore model, but we have to load it ourselves, curiously. We have only one root package, so we can finish the wizard and take a look at the generator model properties.

EMF generator models include several properties to consider. For our mindmap, we need to change only a couple from their default settings. In the root, change the Compliance Level from 1.4 to 5.0 (if it’s not already set to 5.0) and change the Model Directory to be /org.eclipse.mindmap/src. (Note that this changes the edit, editor, and tests properties as well.) We need to manually change the Model Plug-in ID property to org.eclipse.mindmap, however. In the properties for the Mindmap root package, we need to set the Base Package property to org.eclipse to match our new plug-in namespace.

This gets us started, so we can move on to code generation. Later, we return to our mindmap model and add constraints, validation, and other enhancements.

3.3.4 Generate and Run

The last thing to do is generate our mindmap plug-ins and code. Technically, we don’t need to generate code at this time because we plan to leverage dynamic instances as long as we can in the development of our DSLs. However, for those new to EMF, it’s worthwhile to continue with generation at this point to see how things work. This is accomplished by right-clicking the root of the mindmap.genmodel in the editor tree and selecting Generate All. This generates our model code, edit code, editor code, and test skeletons, each in their own plug-in projects. We don’t need the generated editor code because a diagram provides our primary means of working with mindmap instance models. For now, we can continue by running the generated editor to verify our model and configuration.

To run our plug-ins and test the functionality of our editor, we need to be in the Plug-in Development Environment perspective to gain access to the appropriate Run option. Select Run ?> Open Run Dialog (Ctrl+3 ?> Run C) and create a new Eclipse Application run configuration named requirements in a directory named runtime-requirements. Figure 3-5 is an image of this dialog. Figure 3-6 shows the Arguments page with some arguments for launching on Mac OS X. We use this launch configuration throughout our development of the sample projects, hence the general requirements name.


Figure 3-5. Requirements launch configuration
 
Figure 3-6. Requirements launch configuration arguments

Tip: If you get tired of adding arguments to your launch configurations each time you create one, navigate in the Preferences dialog to Plug-In Development ?> Target Platform ?> Launching Arguments and enter them in the field provided. These values will be copied into any new launch configuration you create.

Run this configuration to launch a new instance of Eclipse with the new plug-ins installed. We could trim the plug-in list to launch only those plug-ins we need for our application. This makes launching faster and keeps us aware of our underlying plug-in dependencies. In Chapter 8, “DSL Packaging and Deployment,” we fine-tune our launch settings before creating our product configuration.

In the runtime workbench, create a new project and follow New ?> Example EMF Model Creation Wizards ?> Mindmap Model, giving it whatever name you want and selecting Map as the Model Object. The default EMF-generated editor appears upon finish, ready for you to create new Topic and Relationship instances within the map.

You again need to open the Properties view to set model element properties and establish subtopics and relationship links. Notice that validation is available for our model instances and enforces the basic structural features defined in our model. For example, we declared 1 for the upper and lower bounds on source and target references of our Relationship class. Creating a new Relationship instance in our model and invoking the Validate menu option brings up a dialog that points out that these required features were not set. As we enhance our model further, EMF and the Validation Framework will provide additional validation, as used by GMF for diagram validation.

3.3.5 Adding OCL

As you should recall, we added a derived, transient, volatile rootTopics reference in our Map class. Section 3.2.4, “Applying OCL,” described the basics of adding OCL and using dynamic templates to generate implementations for invariant constraints, method bodies, and derived features. The example in that section covered the rootTopics implementation using OCL and used a set of dynamic templates that we use in this context as well. At this time, rename the default templates folder to be a templates-domain source folder in the mindmap project, and copy the templates provided in the solution into this folder. We’ll have additional templates later for deployment, so we can separate them into different root folders. Each DSL project that uses OCL to refine its domain model will reuse this set of templates. Then return to Section 3.2.4 and configure the mindmap.ecore model to use OCL to implement the rootTopics feature.

We can leverage OCL in our model in additional places to provide an implementation and avoid having to modify our generated code. Let’s begin by adding a method that returns the full set of subtopics for a given Topic.

Finding All Subtopics

Currently, our model has a subtopics reference on each Topic, along with a method, allSubtopics(), that is intended to return a list of all of a Topic‘s subtopics?that is, its subtopics, all of their subtopics, and so on. All methods declared in an Ecore model require an implementation to be provided, so we turn to OCL, where the implementation of this method is trivial, thanks to the nonstandard closure iterator in MDT OCL:

self->closure(subtopics)

We need to add an Eannotation to the method in our model with Source equal to http://www.eclipse.org/2007/OCL. A child Details Entry is added to the annotation with the previous expression as its Value property and with a Key value of body. When we regenerate our model code, we can see that our implementation is provided:

/** * The parsed OCL expression for the body of the  * '{@link #allSubtopics All Subtopics}' operation. *  *  * @see #allSubtopics * @generated */private static OCLExpression allSubtopicsBodyOCL;private static final String OCL_ANNOTATION_SOURCE =   "http://www.eclipse.org/2007/OCL";/** *  *  * @generated */public EList allSubtopics() {  if (allSubtopicsBodyOCL == null) {    EOperation eOperation =       MindmapPackage.Literals.TOPIC.getEOperations().get(0);    OCL.Helper helper = OCL_ENV.createOCLHelper();    helper.setOperationContext(MindmapPackage.Literals.TOPIC,       eOperation);    EAnnotation ocl = eOperation.getEAnnotation(OCL_ANNOTATION_SOURCE);    String body = ocl.getDetails().get("body");    try {      allSubtopicsBodyOCL = helper.createQuery(body);    } catch (ParserException e) {      throw new UnsupportedOperationException(e.getLocalizedMessage());    }  }  Query query =     OCL_ENV.createQuery(allSubtopicsBodyOCL);  @SuppressWarnings("unchecked")  Collection result = (Collection) query.evaluate(this);  return new BasicEList.UnmodifiableEList(result.size(),     result.toArray());}

Again, here we see the boilerplate OCL code that configures an OCLExpression if it’s the first invocation, and then it invokes the expression obtained from the annotation. We leave the mindmap model at this point and move on to develop the second domain model in our product line.

3.4 Developing the Requirements Domain Model

In a similar fashion to our mindmap model, we create a new org.eclipse.dsl.requirements DSL project here to hold our requirements model. This forms the base of our fictitious Requirements Management Project (RMP). We create the new requirements.ecore model using the Domain Model Wizard and GMF Ecore diagram, and we complete it to match the diagram and description of Figure 3-7.

Basically, a model contains a collection of RequirementGroups, which contain a number of children groups and Requirements. Requirements have child references and contain Version and optional Comment elements. A number of enumerations for Priority, State, Type, and Resolution are also in the model. A Requirement can also have a number of dependent requirements, which become the basis for our dependency diagram. Note that the author attributes are simple strings. We could create a Team model and reference these elements to assign to our requirements and comments. We also could have a separate Discussion model to use here and in our mindmap, as a topic might have an associated discussion thread. Many possibilities exist, but for the purposes of our sample application, we keep it simple.

Figure 3-7. Requirements domain model

3.4.1 Requirements Generator Model

We create a requirements.genmodel in the usual manner, using the new Domain Generator Model (Ctrl+3 ?> Domain Gen) wizard and selecting our requirements.ecore model as the input. We’ll make some adjustments to this genmodel and to the generated Edit code because we intend to use the generated EMF editor as part of our solution.

For the display string of a requirement in the editor selection tree, we want to have it be id (major.minor.service):title, where major, minor, and service are from the contained Version element. We’ll be using the Properties view to edit the details of the requirement, so we’ll have plenty of horizontal space to use in the editor, allowing even longer requirement titles to fit. Another option is to navigate using the Project Explorer view, but this is narrow and does not allow for much information display. Furthermore, we’ll have a second tab in the editor to display a requirements dependency diagram, which will also require a bit of editor space. To accomplish the task, we’ll select the requirement element in the genmodel and change its Edit Label Feature to be our id:Estring attribute. Unfortunately, we cannot set two attributes to display for the label, as we can for GMF diagrams. This means we have to modify the generated code.

Before generation, we need to check the other properties and make changes accordingly. As with the mindmap and other models, we want to generate our model, edit, and editor code to their own projects, so we can change the Model Plug-in ID and Model Directory properties to be org.eclipse.requirements.model. We generate the three plug-ins and open the org.eclipse.requirements.provider.RequirementItemProvider class from our Edit plug-in. Modify the getText() method as seen next. Note that if we wanted to preserve the generated method to allow the label feature of the generator model to have an effect, we could use the getTextGen() method approach, as described in the EMF documentation.

/** * This returns the label text for the adapted class. * Modified to show id (major.minor.service) : title *  * @generated NOT */@Overridepublic String getText(Object object) {  StringBuilder sb = new StringBuilder();  sb.append(((Requirement)object).getId());  sb.append(" (");  Version version = ((Requirement)object).getVersion();    if (version != null) {      sb.append(((Requirement)object).getVersion().getMajor());      sb.append(".");      sb.append(((Requirement)object).getVersion().getMinor());      sb.append(".");      sb.append(((Requirement)object).getVersion().getService());    } else {      sb.append("0.0.0");    }    sb.append(") : ");    sb.append(((Requirement)object).getTitle());    String label = sb.toString();    return label == null || label.length() == 0 ?      getString("_UI_Requirement_type") : label;}

We’ve eliminated the redundant Requirement prefix from our label because we’re using a custom icon to distinguish Requirements from RequirementGroups, Comments, and so on. For our RequirementGroup element, we can similarly modify the getText() method to display only the name attribute; we can modify the Comment element to display created, author, and subject.

3.5 Developing the Scenario Domain Model

Because we’re basing the notation for our Scenario diagram on the Business Process Modeling Notation (BPMN) standard, we could simply use its description of the underlying domain model and semantics to develop our DSL. A better approach would have been to find an XSD for BPMN and simply import it into EMF. Unfortunately, no such schema is published with the specification?even worse, a new specification from the OMG, the Business Process Definition Metamodel (BPDM), is slated to provide a domain model for BPMN2. Also unfortunate is that this specification has no serialized format that we can use and is overly complicated for our Scenario DSL. This leaves us to create our own scenario model.

Figure 3-8. Scenario domain model

In a new org.eclipse.dsl.scenario project, we can create our scenario.ecore model as the base of our fictitious Requirements Scenario Project (RSP) project. Figure 3-8 is the model to create using our Ecore diagram.

Elements of a scenario model are maintained in the Process class, which itself extends Element. A Connection maintains target and source references for Elements that are connected in Sequence or Message flows. An Association also connects elements. Elements come in a variety of types, including Tasks, Events, DataObjects, and Gateways. These elements all map in a straightforward manner to notation elements because the model is inherently graphical in nature. The model is actually similar to the description provided in the BPMN specification, although it is a subset.

3.6 Developing the Business Domain Model

Plenty of options exist for developing the domain model that will form the base of our fictitious Business Domain Modeling (BDM) project. We want something less complicated than the Unified Modeling Language (UML) model for structural class modeling, but something expressive enough to generate code either directly or through an intermediate model. Also, the model should constrain the user to the supported methodology and domain of business models. For the purposes of this book, the four archetypes described for business domain modeling in Java Modeling in Color with UML [46] seem like a good option. Figure 3-9 is a partial image of the Domain-Neutral Component (DNC) model, created with the editor we develop in Section 4.6, “Developing the Color Modeling Diagram.” Of course, a black-and-white rendering of a color modeling diagram in the printed form of this book is not very compelling.

Figure 3-9. Color archetypes

Basically, a set of archetypes is used to model moment-interval, role, party, place, thing, and description elements and their relationships. The relationships and several described patterns of interaction are provided in the book, which we want to facilitate in our modeling environment. First, however, we need an underlying model (a DSL).

This DSL is strongly rooted in a general object-oriented DSL, so we begin with just that. Figure 3-10 is an oocore.ecore model that we extend to add our archetypes and other DNC elements. Why begin with a general object-oriented DSL? Well, we might decide to use this model as the basis for another DSL in the future. Why not simply extend Ecore itself, you might ask? Well, it contains elements that we really don’t need or want, leaving us with all those E-prefixed elements and their properties. Besides, it’s straightforward to develop our own object-oriented DSL. We can use the Java EMF Model (JEM) as a transformation target, giving us a chance to see what a model that extends Ecore is like.

Adventurous types can create a new org.eclipse.dsl.oocore DSL project and create the oocore.ecore model, as we have done previously. Complete the model using Figure 3-10 as a reference. Otherwise, simply copy the oocore.ecore domain model from the solutions into your project. Finally, create a new org.eclipse.dsl.dnc DSL project to hold our dnc.ecore model that will extend our core model.

Figure 3-10. Object-oriented core domain model

With our base model complete, we can create our dnc.ecore model. To reference our oocore.ecore model in our newly created dnc.ecore model, we need to use the EMF Load Resource context menu in the default EMF editor. Fortunately, the dialog that appears now contains options to Browse Registered Packages, Browse File System, and Browse Workspace. At one time, you needed to enter platform:/ URIs into the field to load a registered package. In our case, the oocore.ecore model is easily found in our workspace.

In creating our DNC model (Figure 3-11), several options exist, as always. You’ve seen that using an enumeration to define the type is one solution, as was used in the Mindmap domain model’s Relationship class and Type enum. This approach has some drawbacks, including the loss of polymorphic behavior in our templates and transformation scripts. To illustrate the differences, let’s go ahead and create an Archetype abstract class that extends our oocore::Class class. Each of our archetypes will extend the Archetype class. We also add an Association class that extends oocore::Reference and add a property to signify aggregation. Although it is not a true Association class in the UML sense, it aids us in developing our diagram and hiding some complexities of the underlying model to the Practitioner. As we develop the diagram and other DSL artifacts, we’ll revisit this model and refine it as necessary, potentially pulling up some functionality into the domain model to aid in our color modeling and model transformations.

Figure 3-11. Domain-neutral component domain model

3.7 Summary

This chapter explored the capabilities of EMF as the means of describing our DSL abstract syntax. Although we leave the details of EMF to its own book, we covered enough to get started developing our sample projects. The benefits of leveraging a common underlying metamodel and generation capabilities should become clear as we continue to develop the DSL projects.

At this point, we have starter domain models for our fictitious ERP, plus a reference to a fifth oocore DSL. We now move on to describing how to create graphical concrete syntax for those we want to provide diagrams for, understanding that we will most likely revisit and enhance the EMF models we created in this chapter.

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