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| <h1 class="title">Presenting RDF as frames</h1> |
| |
| <p>The origins of RDF as a representation language include |
| <em>frame languages</em>, in which an object, or frame, was the main unit |
| of structuring data. Frames have <em>slots</em>, for example a <code>Person</code> |
| frame might have an <code>age</code> slot, a <code>height</code>slot etc. RDF, however, |
| has taken a step beyond frame languages by making <code>rdf:Property</code> a |
| first class value, not an element of a frame or resource <em>per se</em>. |
| In RDF, for example, an age property can be defined: |
| <code><rdf:Property rdf:ID="age"></code>, and then applied to any resource, |
| including, but not limited to a <code>Person</code> resource.</p> |
| <p>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 <code>rdfs:domain</code> 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:</p> |
| <pre><code> <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> |
| </code></pre> |
| <p>Whereas many object-oriented or frame-oriented representations |
| would regard it as an error that the <code>age</code> property was not being |
| applied to a <code>Person</code>, RDF-based applications are simply entitled |
| to infer that <code>truck1</code> is a (that is, has <code>rdf:type</code>) <code>Truck</code> as |
| well as a <code>Person</code>. This is unlikely to be the case in any |
| real-world domain, but it is a valid RDF inference.</p> |
| <p>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 <em>C</em>?". 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.</p> |
| <h2 id="basic-principles-the-properties-of-a-class">Basic principles: the properties of a class</h2> |
| <p>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:</p> |
| <pre><code> <rdfs:Class rdf:ID="Person" /> |
| <rdf:Property rdf:ID="age" /> |
| |
| <Person rdf:ID="jane_doe"> |
| <age>23</a> |
| </Person> |
| </code></pre> |
| <p>Now consider that we add to this fragment that:</p> |
| <pre><code> <rdf:Property rdf:about="age"> |
| <rdfs:domain rdf:resource="Person" /> |
| </rdf:Property> |
| </code></pre> |
| <p>This additional information about the domain of the <code>age</code> property |
| does not add any new entailments to the model. Why? Because we |
| already know that <code>jane_doe</code> is a Person. So we can consider <code>age</code> |
| to be one of the properties of <code>Person</code> type resources, because if |
| we use the property as a predicate of that resource, it doesn’t add |
| any new <code>rdf:type</code> information about the resource. Conversely, if |
| we know that some resource has an <code>age</code>, we don’t learn any new |
| information by declaring that it has <code>rdf:type Person</code>. In summary, |
| for the purposes of this HOWTO we define the |
| <em>properties of a class</em> 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.</p> |
| <h2 id="sub-classes-and-more-complex-class-expressions">Sub-classes, and more complex class expressions</h2> |
| <p>Given these basic principles, now consider the following RDF |
| fragment:</p> |
| <pre><code> <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> |
| </code></pre> |
| <p>Is <code>hasSkeleton</code> one of the properties of <code>Animal</code>? Yes, because |
| any resource of <code>rdf:type Animal</code> can have a <code>hasSkeleton</code> property |
| (with value either true or false) without adding type information. |
| Similarly, any resource that is a <code>Mammal</code> also has |
| <code>rdf:type Animal</code> (by the sub-class relation), so <code>hasSkeleton</code> is |
| a property of <code>Mammal</code>. However, <code>hasSkeleton</code> is <em>not</em> a property |
| of <code>LivingThing</code>, since we don’t automatically know that a living |
| thing is an animal - it may be a plant. Stating that a given |
| <code>LivingThing</code> has a <code>hasSkeleton</code> property, even if the value is |
| false, would entail the additional <code>rdf:type</code> statement that the |
| <code>LivingThing</code> is also an <code>Animal</code>.</p> |
| <p>For more complex class expressions in the domain, we look to see |
| what simple domain constraints are entailed. For example, a domain |
| constraint <code>A ∩ B</code> (i.e. “A intersection B”) for property <code>p</code> |
| entails that both <code>p rdfs:domain A</code> and <code>p rdfs:domain B</code> are true. |
| However, the properties of neither <code>A</code> nor <code>B</code> will include <code>p</code>. To |
| see this, suppose we have a resource <code>x</code> that we already know is of |
| type <code>A</code>, and a statement <code>x p y</code>. This entails <code>x rdf:type A</code> |
| which we already know, but also <code>x rdf:type B</code>. So information is |
| added, even if we know that <code>x</code> is an instance <code>A</code>, so <code>p</code> is not a |
| property of <code>A</code>. The symmetrical argument holds for <code>p</code> not being a |
| property of <code>B</code>.</p> |
| <p>However, if the domain of <code>p</code> is <code>A ∪ B</code> (i.e. “A union B”), then |
| both <code>A</code> and <code>B</code> will have <code>p</code> as a property, since an occurrence |
| of, say <code>x p y</code> does not allow us to conclude that either |
| <code>x rdf:type A</code> or <code>x rdf:type B</code>.</p> |
| <h2 id="property-hierarchies">Property hierarchies</h2> |
| <p>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 <code>Animal</code> and <code>Mammal</code> include both <code>hasSkeleton</code> and |
| <code>hasEndoSkeleton</code>:</p> |
| <pre><code> <rdf:Property rdf:ID="hasSkeleton"> |
| <rdfs:domain rdf:resource="Animal" /> |
| </rdf:Property> |
| |
| <rdf:Property rdf:ID="hasEndoSkeleton"> |
| <rdfs:subPropertyOf rdf:resource="#hasSkeleton" /> |
| </rdf:Property> |
| </code></pre> |
| <p>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.</p> |
| <h2 id="global-properties">Global properties</h2> |
| <p>Under the principled definition that we propose here, properties |
| which do not express a domain value are <em>global</em>, 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 <code>rdfs:Resource</code> or <code>owl:Thing</code>, 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.</p> |
| <p>A commonly used idiom in some OWL ontologies is to use |
| <em>Restrictions</em> 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 <code>Person</code> should have a |
| <code>familyName</code> property:</p> |
| <pre><code> <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> |
| </code></pre> |
| <p>This approach shows the intent of the ontology designer that |
| <code>Person</code> instances have <code>familyName</code> properties. We do regard |
| <code>familyName</code> as one of the <em>properties of</em> <code>Person</code>, but only |
| because of the global properties principle. Unless a domain |
| constraint is also specified for <code>familyName</code>, it will appear as |
| one of the properties of classes other than <code>Person</code>. |
| <strong>Note that this is a behaviour change from versions of Jena prior to release 2.2</strong>. |
| 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.</p> |
| <h2 id="the-java-api">The Java API</h2> |
| <p>Support for frame-like views of classes and properties is provided |
| through the <a href="../ontology/index.html">ontology API</a>. The following |
| methods are used to access the properties of a class, and the |
| converse for properties:</p> |
| <pre><code> OntClass.listDeclaredProperties(); |
| OntClass.listDeclaredProperties( boolean direct ); |
| OntClass.hasDeclaredProperty( Property prop, boolean direct ); |
| OntProperty.listDeclaringClasses(); |
| OntProperty.listDeclaringClasses( boolean direct ); |
| </code></pre> |
| <p>All of the above API methods return a Jena |
| <a href="/documentation/javadoc/jena/org/apache/jena/util/iterator/ExtendedIterator.html"><code>ExtendedIterator</code></a>.</p> |
| <p><strong>Note a change from the Jena 2.1 interface:</strong> the optional Boolean |
| parameter on <code>listDeclaredProperties</code> has changed name from <code>all</code> |
| (Jena 2.1 and earlier) to <code>direct</code> (Jena 2.2 and later). The |
| meaning of the parameter has also changed: <code>all</code> was intended to |
| simulate some reasoning steps in the absence of a reasoner, whereas |
| <code>direct</code> is used to restrict the associations to only the local |
| associations. See more on |
| <a href="../ontology/index.html#direct_relationships">direct associations</a>.</p> |
| <p>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:</p> |
| <pre><code> <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(); |
| </code></pre> |
| <p>Iterator <code>i1</code> will return <code>p</code> and <code>q</code>, while <code>i0</code> will return only |
| <code>p</code>.</p> |
| <h2 id="summary-of-changes-from-jena-22-beta-2-and-older">Summary of changes from Jena 2.2-beta-2 and older</h2> |
| <p>For users updating code that uses <code>listDeclaredProperties</code> from |
| versions of Jena prior to 2.2-final, the following changes should |
| be noted:</p> |
| <ul> |
| <li>Global properties |
| <code>listDeclaredProperties</code> will treat properties with no specified |
| domain as global, and regard them as properties of all classes. The |
| use of the <code>direct</code> flag can hide global properties from non-root |
| classes.</li> |
| <li>Restriction properties |
| <code>listDeclaredProperties</code> no longer heuristically returns properties |
| associated with a class via the <code>owl:onProperty</code> predicate of a |
| restriction.</li> |
| <li>Limited simulated inference |
| The old version of <code>listDeclaredProperties</code> attempted to simulate |
| the entailed associations between classes and properties. Users are |
| now advised to attach a reasoner to their models to do this.</li> |
| <li>Change in parameter semantics |
| The old version of <code>listDeclaredProperties(boolean all)</code> 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 <code>listDeclaredProperties(boolean direct)</code>, where |
| calling the method with <code>direct = true</code> will compress the returned |
| results to use only the |
| <a href="../ontology/index.html#direct_relationships">direct</a> |
| associations.</li> |
| </ul> |
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