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Serialisation is the process of creating a linear stream of bytes or characters corresponding to an object structure (a graph), so the same structure can be re-constructed by de-serialising the stream. This technique is used both to save the object structure to file and to transmit it across a network.

Introduction

Persistence is the term used for storing (application) state in between sessions, in our case model instances represented by objects structures (graphs), and covers file and database storage, including SQL, object and NoSQL databases. The logic of files and databases are so different that it is easier to treat them separately. A file is essentially a sequence or array of bytes, which often encode characters, and serialisation is the term used for generating this byte (or character) sequence from an object structure. The opposite process, de-serialisation, is the process of reconstructing the object structure from the byte stream.

The Resource

 

EMF uses a(n instance of the) Resource (class) as the container of the object graph and scope of serialisation. The object graph will mainly consist of EObjects, since this is the base superclass of all modelled data, hence we'll use EObjects as a synonym for graph of instances of an Ecore model. Thus, you serialise the EObjects contained in a Resource to an OutputStream (e.g. a FileOutputStream) with its save method, and de-serialise from an InputStream (e.g. a ByteArrayInputStream) into a Resource with its load method, which then will contain the re-constructed EObjects.

The Resource class is abstract, and it's the Resource subclass you use that implements the serialisation logic which determines the format of the byte or character sequence. E.g. the XMIResource class supports a generic format named XML Metadata Interchange (XMI), which is an XML-based model format standardised by the Object Management Group (OMG), so if you want your EObjects serialised as XMI, you use an XMIResource as the container. You can implement your own custom format by making your own Resource subclass, e.g. to support existing legacy formats or a format designed to be easier to read and write for humans (e.g. a DSL).

The (structure of) EObjects within a Resource is often self-contained, but cross-links between Resources, are supported, and this must be handled by the serialisation and de-serialisation mechanism. EMF uses a dual technique based on URIs: 1) a URI is used to identify a Resource , e.g. "file:/Users/hal/resource2.xmi" and 2) a fragment is used to identify an EObject within a Resource. The fragment is computed by the target Resource's getURIFragment(EObject) method and is typically a path-like string like "/orgUnits/0/workers/2". Together this gives URIs like "file:/Users/hal/resource2.xmi#/orgUnits/0/workers/2" where "#" is used as separator according to the URI standard. The fragment is useful on its own, as it can be used to serialise links within a Resource, too.

So, when serialising (saving) a link to a target EObject, a URI to this EObject is created as described above, and the resulting string is used. When de-serialising (loading) a Resource with a cross-link, the URI is split into a base URI and a fragment, and the base URI is used to identify and auto-load the target Resource, before looking up the target EObject by giving the fragment to the Resource's getEObject(String) method.

Det circles in Resource 1 and 2 are instances of subclasses of EObject, where the classes typically are generated from your Ecore model. The arrows from circles in Resource 1 to Resource 2 are cross-links.

From http://www.informit.com/articles/article.aspx?p=1323360&seqNum=5

The ResourceSet

As described above, when loading a Resource, you may need to load other Resources as you encounter cross-links. To avoid loading a Resource more than once, it needs to be stored and reused if its URI is encountered later. A ResourceSet is used as the container for Resources, giving a hierarchy of (at least) three levels: ResourceSet, Resource and EObjects, where each Resource contains part of a large EObject graph, as shown in the figure above. Before loading a Resource, you must add it to a ResourceSet so it 1) can be used for lookup up Resources by URI later and 2) can store the other Resources that are implicitly auto-loaded in the process. The code may look like this:

ResourceSet resourceSet = new ResourceSetImpl();
Resource resource = new XMIResourceImpl(URI.create("file:/Users/hal/resource2.xmi"));
resourceSet.getResources().add(resource);
resource.load(null); // TODO: catch exceptions
// create an instance of the default ResourceSet implementation
// create an instance of the default XMIResource implementation
// add the Resource to the ResourceSet
// load using default loading options
// now the size of resourceSet.getResources() is >= 1

The Resource.Factory

The auto-loading mechanism requires a Resource to (be able to) create other Resources. As described above, the Resource implements the serialisation and de-serialisation logic, so it's crucial that the correct Resource subclass is instantiated. But how does the Resource know which? The actual instantiation is done by a Resource.Factory, and global and local registries of such objects are used to find the appropriate one to use. The global registry is stored in Resource.Factory.Registry.INSTANCE and the local one is stored in the ResourceSet and retrieved by the getResourceFactoryRegistry method. The lookup and instantiation is encapsulated in the ResourceSet's createResource(URI) method, so we only need to write the following code:

ResourceSet resourceSet = new ResourceSetImpl();
Resource resource = resourceSet.createResource(URI.create("file:/Users/hal/resource2.xmi"));
// create an instance of the default ResourceSet implementation
// use it to create the Resource

However, first we need to make sure the registry is initialised with the relevant Resource.Factory implementations, so our custom ones are found (whether generated or hand-written):

ResourceSet resourceSet = new ResourceSetImpl();
Resource.Factory.Registry registry = resourceSet.getResourceFactoryRegistry();
registry.getExtensionToFactoryMap().put("org", new OrgResourceFactoryImpl());
// create an instance of the default ResourceSet implementation
// retrieve the Resource.Factory.Registry
// map the file extension to our Resource.Factory
 

To register our Resource.Factory globally, use Resource.Factory.Registry.INSTANCE instead of retrieving the one in the ResourceSet. Note that if you use genmodel and generate code, and install your EMF project into Eclipse, this is done automatically, using Eclipse's plugin.xml-based extension mechanism. If running standalone, as in an ordinary JUnit test you must include similar code, in the case of JUnit in the setUp or @BeforeClass method.

The XMI format

As mention above, EMF has support for the XMI format, which is based on the hierarchical XML syntax.

<org:OrgModel xmi:version="2.0"
   xmlns:org="platform:/plugin/no.hal.org/model/org.ecore">
   <orgUnits>
      <workers
         name="Hallvard &quot;hal&quot; Trætteberg"
         role="//@roles.0"/>
   </orgUnits>
   <roles
      name="manager"/>
</org:OrgModel>
the root object is of the type org:OrgModel
relates the org prefix to our "org" package's identifying URI
   a contained instance that is orgUnits-related to the outer object, implicitly an OrgUnit
      a contained instance that is workers-related to the outer object, implicitly a Person
        name attribute and value
        role reference and link to object referenced by URI fragment
   
   a contained instance that is roles-related to the outer object, implicitly a Role
      name attribute and value
 

Model instances are graphs in general, but it is easier to think of them as mostly hierarchical, with links across. Hence, a serialisation mechanism will typically need to support the following features: 

  • type information, i.e. references to EClasses in the model, so instances can be created
  • attribute values, corresponding to EAttributes, so attributes of instances can be set or filled
  • links, corresponding to EReferences, similar to attributes where the values are strings that can resolve to objects in the context of the object hierarchy
  • containment, corresponds to EReference with containment flag set

Given these features, it is pretty easy to make a general algorithm for serialising and de-serialising object structures. To serialise, identify the root object(s) and traverse the object hierarchy. For each object output the EClass reference, and for each EStructuralFeature, output the name and values (for EAttributes) or resolvable object references (for EReferences). For containment, use some kind of nesting indicator, like curly braces and/or indentation, combined with the name of the containment EReference's name. To de-serialise, create the objects and set or fill the attributes while parsing, and store the EReference names and object identities. Containment can also handles during parsing, based on the nesting and EReference names. After the object hierarchy has been created, link them together by resolving the object references and setting or filling the EReferences.

 

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