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 height
slot 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</age>
</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.declaredProperties();
OntClass.declaredProperties( boolean direct );
OntClass.hasDeclaredProperty( OntProperty prop, boolean direct );
OntProperty.declaringClasses();
OntProperty.declaringClasses( boolean direct );
All of the above API methods return a Java Stream
.
Where
direct
is used to restrict the associations to only the local
associations.
See also
Figure 5: asserted and inferred relationships.
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:
String ns = "http://ex.com#";
OntModel m0 = OntModelFactory.createModel(OntSpecification.OWL2_DL_MEM);
OntClass a0 = m0.createOntClass( ns + "A" );
OntClass b0 = m0.createOntClass( ns + "B" );
OntObjectProperty p = m0.createObjectProperty(ns + "p");
OntObjectProperty q = m0.createObjectProperty(ns + "q");
p.addSuperProperty(q);
q.addDomain(b0);
a0.addSubClass(b0);
OntModel m1 = OntModelFactory.createModel(m0.getBaseGraph(), OntSpecification.OWL2_DL_MEM_RULES_INF);
OntClass b1 = m1.getOntClass( ns + "B" );
Stream<OntProperty> s0 = b0.declaredProperties(true);
Stream<OntProperty> s1 = b0.declaredProperties(true);
Stream s1
will return p
and q
, while s0
will return only
q
.