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Presenting RDF as frames

The origins of RDF as a representation language include frame languages, in which an object, or frame, was the main unit of structuring data. Frames have slots, for example a Person frame might have an age slot, a heightslot etc. RDF, however, has taken a step beyond frame languages by making rdf:Property a first class value, not an element of a frame or resource per se. In RDF, for example, an age property can be defined: <rdf:Property rdf:ID="age">, and then applied to any resource, including, but not limited to a Person resource.

While this introduces an extra element of modelling flexibility in RDF, it is often the case that users want to treat some components in their models in a more structured way, similar to the original idea of frames. It is often assumed that rdfs:domain restricts a property to be used only on resources that are in the domain class. For example, a frequently asked question on the Jena support list is why the following is not an error:

  <rdfs:Class rdf:ID="Person" />
  <rdfs:Class rdf:ID="Truck" />
  <rdf:Property rdf:ID="age">
    <rdfs:domain rdf:resource="Person" />
  </rdf:Property>

  <Truck rdf:ID="truck1">
    <age>2</a>
  </Truck>

Whereas many object-oriented or frame-oriented representations would regard it as an error that the age property was not being applied to a Person, RDF-based applications are simply entitled to infer that truck1 is a (that is, has rdf:type) Truck as well as a Person. This is unlikely to be the case in any real-world domain, but it is a valid RDF inference.

A consequence of RDF's design is that it is not really possible to answer the commonly asked question "Which properties can be applied to resources of class C?". Strictly speaking, the RDF answer is "Any property". However, many developers have a legitimate requirement to present a composite view of classes and their associated properties, forming more a more succinct structuring of an ontology or schema. The purpose of this note is to explain the mechanisms built-in to Jena to support a frame-like view of resources, while remaining correct with respect to RDF (and OWL) semantics.

Basic principles: the properties of a class

Since any RDF property can be applied to any RDF resource, we require a definition of the properties of a given class that respects RDF semantics. Consider the following RDF fragment:

  <rdfs:Class rdf:ID="Person" />
  <rdf:Property rdf:ID="age" />

  <Person rdf:ID="jane_doe">
    <age>23</a>
  </Person>

Now consider that we add to this fragment that:

  <rdf:Property rdf:about="age">
    <rdfs:domain rdf:resource="Person" />
  </rdf:Property>

This additional information about the domain of the age property does not add any new entailments to the model. Why? Because we already know that jane_doe is a Person. So we can consider age to be one of the properties of Person type resources, because if we use the property as a predicate of that resource, it doesn't add any new rdf:type information about the resource. Conversely, if we know that some resource has an age, we don't learn any new information by declaring that it has rdf:type Person. In summary, for the purposes of this HOWTO we define the properties of a class as just those properties that don't entail any new type information when applied to resources that are already known to be of that class.

Sub-classes, and more complex class expressions

Given these basic principles, now consider the following RDF fragment:

  <rdfs:Class rdf:ID="LivingThing" />

  <rdfs:Class rdf:ID="Animal">
    <rdfs:subClassOf rdf:resource="#LivingThing">
  </rdfs:Class>

  <rdfs:Class rdf:ID="Mammal">
    <rdfs:subClassOf rdf:resource="#Animal">
  </rdfs:Class>

  <rdf:Property rdf:ID="hasSkeleton">
    <rdfs:domain rdf:resource="Animal" />
  </rdf:Property>

Is hasSkeleton one of the properties of Animal? Yes, because any resource of rdf:type Animal can have a hasSkeleton property (with value either true or false) without adding type information. Similarly, any resource that is a Mammal also has rdf:type Animal (by the sub-class relation), so hasSkeleton is a property of Mammal. However, hasSkeleton is not a property of LivingThing, since we don't automatically know that a living thing is an animal - it may be a plant. Stating that a given LivingThing has a hasSkeleton property, even if the value is false, would entail the additional rdf:type statement that the LivingThing is also an Animal.

For more complex class expressions in the domain, we look to see what simple domain constraints are entailed. For example, a domain constraint A ∩ B (i.e. "A intersection B") for property p entails that both p rdfs:domain A and p rdfs:domain B are true. However, the properties of neither A nor B will include p. To see this, suppose we have a resource x that we already know is of type A, and a statement x p y. This entails x rdf:type A which we already know, but also x rdf:type B. So information is added, even if we know that x is an instance A, so p is not a property of A. The symmetrical argument holds for p not being a property of B.

However, if the domain of p is A ∪ B (i.e. "A union B"), then both A and B will have p as a property, since an occurrence of, say x p y does not allow us to conclude that either x rdf:type A or x rdf:type B.

Property hierarchies

Since sub-properties inherit the domain constraints of their parent property, the properties of a class will include the closure over the sub-property hierarchy. Extending the previous example, the properties of Animal and Mammal include both hasSkeleton and hasEndoSkeleton:

  <rdf:Property rdf:ID="hasSkeleton">
    <rdfs:domain rdf:resource="Animal" />
  </rdf:Property>

  <rdf:Property rdf:ID="hasEndoSkeleton">
    <rdfs:subPropertyOf rdf:resource="#hasSkeleton" />
  </rdf:Property>

In general, there may be many different ways of deducing simple domain constraints from the axioms asserted in the ontology. Whether or not all of these possible deductions are present in any given RDF model depends on the power and completeness of the reasoner bound to that model.

Global properties

Under the principled definition that we propose here, properties which do not express a domain value are global, in the sense that they can apply to any resource. They do not, by definition, entail any new type information about the individuals they are applied to. Put another way, the domain of a property, if unspecified, is either rdfs:Resource or owl:Thing, depending on the ontology language. These are simply the types that all resources have by default. Therefore, every class has all of the global properties as one of the properties of the class.

A commonly used idiom in some OWL ontologies is to use Restrictions to create an association between a class and the properties of instances of that class. For example, the following fragment shows that all instances of Person should have a familyName property:

  <owl:Class rdf:ID="Person">
    <rdfs:subClassOf>
      <owl:Restriction>
        <owl:onProperty rdf:resource="#familyName" />
        <owl:minCardinality rdf:datatype="&xsd;int">1</owl:minCardinality>
      </owl:Restriction>
    </rdfs:subClassOf>
  </owl:Class>

This approach shows the intent of the ontology designer that Person instances have familyName properties. We do regard familyName as one of the properties of Person, but only because of the global properties principle. Unless a domain constraint is also specified for familyName, it will appear as one of the properties of classes other than Person. Note that this is a behaviour change from versions of Jena prior to release 2.2. Prior to this release, Jena used a heuristic method to attempt to associate restriction properties with the classes sub-classing that restriction. Since there were problems with precisely defining the heuristic, and ensuring correct behaviour (especially with inference models), we have dropped the use of this heuristic from Jena 2.2 onwards.

The Java API

Support for frame-like views of classes and properties is provided through the ontology API. The following methods are used to access the properties of a class, and the converse for properties:

  OntClass.listDeclaredProperties();
  OntClass.listDeclaredProperties( boolean direct );
  OntClass.hasDeclaredProperty( Property prop, boolean direct );
  OntProperty.listDeclaringClasses();
  OntProperty.listDeclaringClasses( boolean direct );

All of the above API methods return a Jena ExtendedIterator.

Note a change from the Jena 2.1 interface: the optional Boolean parameter on listDeclaredProperties has changed name from all (Jena 2.1 and earlier) to direct (Jena 2.2 and later). The meaning of the parameter has also changed: all was intended to simulate some reasoning steps in the absence of a reasoner, whereas direct is used to restrict the associations to only the local associations. See more on direct associations.

A further difference from Jena 2.1 is that the models that are constructed without reasoners perform only very limited simulation of the inference closure of the model. Users who wish the declared properties to include entailments will need to construct their models with one of the built-in or external reasoners. The difference is illustrated by the following code fragment:

  <rdfs:Class rdf:ID="A" />
  <rdfs:Property rdf:ID="p">
    <rdfs:domain rdf:resource="#A" />
  </rdfs:Property>
  <rdfs:Property rdf:ID="q">
    <rdfs:subPropertyOf rdf:resource="#p" />
  </rdfs:Property>

  OntModel mNoInf = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM );
  OntClass a0 = mNoInf.getOntClass( NS + "A" );
  Iterator i0 = a0.listDeclaredProperties();

  OntModel mInf = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM_RULE_INF );
  OntClass a1 = mInf.getOntClass( NS + "A" );
  Iterator i1 = a1.listDeclaredProperties();

Iterator i1 will return p and q, while i0 will return only p.

Summary of changes from Jena 2.2-beta-2 and older

For users updating code that uses listDeclaredProperties from versions of Jena prior to 2.2-final, the following changes should be noted:

  • Global properties listDeclaredProperties will treat properties with no specified domain as global, and regard them as properties of all classes. The use of the direct flag can hide global properties from non-root classes.
  • Restriction properties listDeclaredProperties no longer heuristically returns properties associated with a class via the owl:onProperty predicate of a restriction.
  • Limited simulated inference The old version of listDeclaredProperties attempted to simulate the entailed associations between classes and properties. Users are now advised to attach a reasoner to their models to do this.
  • Change in parameter semantics The old version of listDeclaredProperties(boolean all) took one parameter, a Boolean flag to indicate whether additional declared (implied) properties should be listed. Since this is now covered by the use, or otherwise, of a reasoner attached to the model, the new method signature is listDeclaredProperties(boolean direct), where calling the method with direct = true will compress the returned results to use only the direct associations.