Forewords
EMF provides a runtime and tools that allow you to create ecore object models. The starting point is a model definition. It may be created out of a XML Schema (XSD), annotated java classes, etc. but you will mostly craft one by hand. This document attempts to describe the process involved in the later. It will show you the basic steps to create an ecore model implementation and give you some more advanced hints here and there.
There are a few tutorials available on eclipse.org, the best one (to my eyes's) the one provided by Lars Vogella.
Get started, create an EMF Project
To get started, create an empty EMF project.
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15705-14-5332/449-428/new-empty-emf-project.jpg
You'll get a new project, that's set up to work with the EMF framework.
What's pops to your eyes is that there's a model directory in this project. That where the model definitions, the ecore files, are put (not a must but a standard so far).
Create a Model Definition
Create a new ecore model file in this folder by selecting the folder and triggering the new ecore model wizard.
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15705-14-5333/450-436/new-ecore-file.jpg
Create a Package
We created the empty ecore model so far, so the next step is to create a package for your model. You set its name, Ns Prefix and Ns URI.
The good practice (or at least what most modelers do) is to set them to:
- name: a simple term (not required to be unique)
- Ns prefix: ~shoretened 'java package' name (not required to be unique)
- Ns URI: some real (or bogus) unique URI where the scheme might be found.
example: org.eclipse.emf.cdo.ui.defs
I have a plugin/module in cdo called org.eclipse.emf.cdo.ui.defs
The ecore model for it has the following definitions:
name: defs
Ns prefix: cdo.ui.defs
Ns URI: http://www.eclipse.org/emf/CDO/ui/defs/1.0.0
Model Your Classes
You can now add classes to your package. There are different editors at hand that you may use to edit your ecore file. The most common one is the Sample Ecore Model Editor that is included in the Eclipse modeling edition. Alternatively you may also use text based editors like Emfatic or Xtext. There are also graphical editors around like the Ecore Diagram Editor or the eUML2 Editor.
Ecore files define classes that will be generated in a later step. The generated java classes are not plain POJOs but extend EObject. You may therefore only define elements that EObjects may have. There are Classes, Attributes, References, Operations, etc. A quick look at the EObject class diagram may help you get in touch with the terms used in EMF.
EObjects offer various capabilities that are not (or at least at high cost) offered with plain POJOs. Extending EObjects in your model classes offers you all these capabilities free of charge:
- serialization (XML, XMI, binary, database based persistence, etc.)
- change propagation
- validation
- object query
- etc.
Furthermore the EMF ecosystem holds plenty of frameworks that extend it in various areas. Using EObject based POJOs allows you to take advantage of all these powerful frameworks.
A disadvantage is that EMF's pretty invasive. You get all its power, but you mostly have to stick to EObjects. This usually isn't prroblematic in client (aka Eclipse IDE/RCP or other RCP platforms) projects, but might be a problem in server side projects. You may tell the generator not to extend EObjects, but you'll loose all benefits the powerful EMF runtime offers to you. Another solution emerged lately with Eclipse Texo. Texo generates plain POJOs and offers a runtime that unleasehes most of the emf runtime capabilities.
All Ways lead to Rome
There are usually several ways to get to the desired result (aka generated java code). The best way to find out about them is to get Eclipse Modeling Framework book or trial and error.
A rule of thumb is to have all referenced classes available in your model definition. This is evident for ecore classes. But if your ecore classes use references to plain java types (that are not part of your ecore model in the strict sense) you'll have to declare those java types in the ecore model. In other words, the ecore model needs to know about all types (ecore or plain java) that are part of your model.
example: Use EDataTypes for Java types
Let's say that my modeled class CDOEditorDefs has a method execute() that throws an ExecutionException. I could add that method by hand but as a matter of taste I prefer to declare that method in my model.
My model does not know anything about this exception so far, so there's no way to get the correct signature generated out of the box . I'll therefore have to declare this exception in model. I create a DataType ExecutionException.
create an EDataType:
http://community.jboss.org/servlet/JiveServlet/downloadImage/4911/declare-datatype.png
Give it an instance type name so it won't be generated but is referencable:
http://community.jboss.org/servlet/JiveServlet/downloadImage/4913/declare-executionexception.jpg
Set the execute method (operation) to throw the ExecutionException:
http://community.jboss.org/servlet/JiveServlet/downloadImage/4914/declare-throws-executionexception.jpg
The generated method now throws the given Exception:
http://community.jboss.org/servlet/JiveServlet/downloadImage/4915/generated-throws-executionexception.jpg
example: Use Instance Type Name to model Java Interfaces
A very common problem is to have modeled (ecore-) classes extend Java Interfaces. For instance this could be java.lang.Comparable
The best way to achieve that is to model a class Comparable. Do not model its operation as this is just a mirror of the real java interface in the ecore-world.
Interfaces are modeled as supertypes in ecore. You therefore cannot use EDataTypes here as they cannot be supertypes of ecore classes. Nevertheless you can achieve that in a slight different manner: You model a (real) ecore class, but you set its instance type name: java.lang.Comparable. You can now add the Comparable class to the super type of each ecore-class that shall implement Comparable. The generator will not generate an ecore class that's called Comparable but it will include java.lang.Comparable in the interface that your ecore-classes implement.
Not sure I follow this example and what we're trying to illustrate. Would example code help?
Get prepared to generate code, create a Genmodel
This is mostly straight forward. Select the ecore file and create a genmodel for it. Select your ecore file and start a new EMF Generator Model wizard. The wizard will allow you to create a so called Generator Model that holds all settings which are important to the code generation process.
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15705-14-5335/450-272/new-genmodel.jpg
There are 2 settings that might be of interest to you:
'All' (property group when the package is selected):
- Base Package: the base package all ecore classes get generated to
- Prefix: Prefix that the factory- and package-class get
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15705-14-5337/311-251/edit-genmodel.jpg
example (still using the cdo.ui.defs example):
Base Package: org.eclipse.emf.cdo.ui
Prefix: CDOUIDefs
Package-class gets CDOUIDefsPackage, Factory gets CDOUIDefsFactory, etc. All classes get generated to the package org.eclipse.emf.cdo.ui
Further modifications you might be interested in are the suffixes of the sub-packages (the defaults creates an 'impl' package where it puts all implementation classes). It can be modified by selecting the 'Package Suffixes' and choosing the properties 'Implementation' and 'Interfaces'.
The naming of the implementation- and interface-classes may be changed, too. You find those settings if you select the root-node of the tree in the genmodel-editor and choose the 'Model' property group. You'll find 'Class Name Pattern' and 'Interface Name Pattern' among the available properties. The explanations for the values show up in the statusbar (default is '{0}impl' and '{0}').
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15697-11-4849/genmodel-classname-pattern.png
Once you're done defining your generator model, you simply need to generate the implementation classes. Select the package you want to generate, right click and select the implementation you want to create. You may choose among the models, the editor, the tests.
http://community.jboss.org/servlet/JiveServlet/downloadImage/102-15697-11-4850/genmodel-generate.png
Modify the Generated Classes
Ecore is built to be modified, the basic usage-pattern is to code and generate hand-in-hand. To tell the generator not to override your modifications you need to set the javadoc-annotation to anything different than @generated. Good practice says that you should set it to '@generated NOT'. Good practice also tells you to annotate any manually added method by @ADDED, but its optional though.
There is another handy that allows you to modify and get the generated code. If you want to have your code instead of the generated one, you just annotate accordingly and the generator will preserve your code. If you want the generated code, too, you'd need to create a method that has the original name + a suffix 'Gen'
example:
/**
*
* @generated NOT
*/
public void setName(String name) {
YOUR OWN CODE
}
/**
*
* @generated
*/
public void setNameGen(String name) {
GENERATOR provides the generated code in here
}
After making your modifications, you simply need to re-generate the ecore classes. (How?)
Refactor generated Code and Regenerate
The code generator in EMF's is pretty capable when it's up to merge manual code changes in generated code. It respects @generated tags and preserves your handwritten code in generated classes.
If you rename a class in your ecore file though, it won't be able to detect your change. EMF has no clue that you renamed ecore class definitions. It will not be able to detect your java files because they have the old name. It will generate new artifacts and merging will not occur. You could copy your changes manually, but this is pretty cumbersome, there's a better approach to this use case:
Refactor/rename the generated java classes in a first step. Rename your ecore class definitions in a second step and regenerate afterwards. EMF will detect the preexisting artifacts and merge your manual code changes with the generated content. You will not have to copy your code manually.
