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From i..@apache.org
Subject svn commit: r1150938 [2/2] - /incubator/jena/site/trunk/content/jena/documentation/ontology/ontology.mdtext
Date Mon, 25 Jul 2011 22:20:19 GMT

Added: incubator/jena/site/trunk/content/jena/documentation/ontology/ontology.mdtext
URL: http://svn.apache.org/viewvc/incubator/jena/site/trunk/content/jena/documentation/ontology/ontology.mdtext?rev=1150938&view=auto
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--- incubator/jena/site/trunk/content/jena/documentation/ontology/ontology.mdtext (added)
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+# Jena Ontology API
+
+In this section I will give a general introduction to the the Jena2
+ontology API. I'll also describe a range of common user tasks. I
+won't go into all of the many details of the API here: you should
+expect to refer to the [Jena2 Javadoc](../javadoc/index.html) to
+get full details of the capabilities of the API. Feedback, (via the
+[jena-dev](http://tech.groups.yahoo.com/group/jena-dev) support
+list) is welcomed.
+
+## Prerequisites
+
+I will assume that you have a basic familiarity with RDF and with
+Jena. There are many other Jena
+[help documents](../documentation.html) you can read for background
+on these topics. There is also a
+[Jena programming tutorial](../tutorial/index.html) you can follow
+through.
+
+Jena is a programming toolkit, using the Java programming language.
+While there are a few command-line tools to help you perform some
+key tasks using Jena, mostly you use Jena by writing Java programs.
+The examples in this document will be primarily code samples.
+
+I also won't be explaining the OWL or RDFS ontology languages in
+much detail in this document. You should expect to refer to
+supporting documentation for details on those languages, for
+example the [W3C OWL document index](http://www.w3.org/2004/OWL/).
+
+**Note:** At the time of writing, work on OWL version 1.1 is still
+underway. No decision has yet been made about when Jena will
+support the new OWL 1.1 features. I will only use OWL 1.0 features
+in this document.
+
+## Overview
+
+The section of the manual is broken into a number of sections. You
+do not need to read them in sequence, though later sections may
+refer to concepts and techniques introduced in earlier sections.
+The sections are:
+
+1.  Overview
+2.  [General concepts](#generalConcepts)
+3.  [Running example: the ESWC ontology](#eswcExample)
+4.  [Creating ontology models](#creatingModels)
+5.  [Compound ontology documents and imports processing](#documentHandling)
+6.  [The generic ontology type: OntResource](#ontResource)
+7.  [Ontology classes and basic class expressions](#classesBasic)
+8.  [Ontology properties](#properties)
+9.  [More complex class expressions](#classesComplex)
+10. [Instances or individuals](#instances)
+11. [Ontology meta-data](#metadata)
+12. [Ontology inference: overview](#inferenceIntro)
+13. [Working with DAML+OIL ontologies](#legacyDAML)
+14. [Working with persistent ontologies](#persistence)
+15. [Experimental ontology tools](#ontTools)
+16. [Common ontology application problems and sample programs](common-problems.html)
+
+### Further assistance
+
+I hope that this document will be sufficient to help most readers
+to get started using the Jena ontology API. For further support,
+please post questions to the Jena support list:
+[jena-dev@yahoogroups](http://tech.groups.yahoo.com/group/jena-dev).
+You can also report bugs directly into the
+[Jena bug tracker](https://sourceforge.net/tracker/?atid=430288&group_id=40417&func=browse)
+on [SourceForge](http://sourceforge.net/projects/jena).
+
+*Please note that we ask that you use the support list or the bug-tracker to communicate with the Jena team, rather than send email to the team members directly. This helps us manage Jena support more effectively, and facilitates contributions from other Jena community members.*
+
+## General concepts
+
+In a widely-quoted definition, an ontology is
+
+> "… a specification of a conceptualization"
+> [[Gruber, T.](http://www-ksl.stanford.edu/kst/what-is-an-ontology.html)
+> 1993]
+
+Let's unpack that brief characterisation a bit. It means that an
+ontology allows a programmer to specify, in an open, meaningful,
+way the concepts and relationships that collectively characterise
+some domain. Examples might be the concepts of red and white wine,
+grape varieties, vintage years, wineries and so forth that
+characterise the domain of 'wine', and relationships such as
+'wineries produce wines', 'wines have a year of production'. This
+*wine ontology* might be developed initially for a particular
+application, such as a stock-control system at a wine warehouse. As
+such, it may be considered similar to a well-defined database
+schema. The advantage to an ontology is that it is an explicit,
+first-class description. So having been developed for one purpose,
+it can be published and reused for other purposes. For example, a
+given winery may use the wine ontology to link its production
+schedule to the stock system at the wine warehouse. Alternatively,
+a wine recommendation program may use the wine ontology, and a
+description (ontology) of different dishes to recommend wines for a
+given menu.
+
+There are many ways of writing down an ontology, and a variety of
+opinions as to what kinds of definition should go in one. In
+practice, the contents of an ontology are largely driven by the
+kinds of application it will be used to support. In Jena, we do not
+take a particular view on the minimal or necessary components of an
+ontology. Rather, we try to support a variety of common techniques.
+In this manual I try to explain what is – and to some extent what
+isn't – possible using Jena's ontology support.
+
+Since Jena is fundamentally an RDF platform, Jena's ontology
+support is limited to ontology formalisms built on top of RDF.
+Specifically this means [RDFS](http://en.wikipedia.org/wiki/RDFS),
+the varieties of
+[OWL](http://en.wikipedia.org/wiki/Web_Ontology_Language), and the
+now-obsolete [DAML+OIL](http://en.wikipedia.org/wiki/DAML+OIL). I
+will provide a very brief introduction to these languages here.
+Please refer to the extensive on-line documentation for these
+formalisms for complete and authoritative detail.
+
+### RDFS
+
+RDFS is the weakest ontology language supported by Jena. RDFS
+allows the ontologist to build a simple hierarchy of concepts, and
+a hierarchy of properties. Consider the following trivial
+characterisation (with apologies to biology-trained readers!):
+
+![image of simple class hierarchy](../images/Simple-hierarchy.png)
+
+Table 1: A simple concept hierarchy
+
+Using RDFS, I can say that my ontology has five *classes*, and that
+`Plant` is a *sub-class of* `Organism` and so on. So every animal
+is also an organism. A good way to think of these classes is as
+describing sets of *individuals*: organism is intended to describe
+a set of living things, some of which are animals (i.e. a sub-set
+of the set of organisms is the set of animals), and some animals
+are fish (a subset of the set of all animals is the set of all
+fish).
+
+To describe the attributes of these classes, I can associate
+*properties* with the classes. For example, animals have sensory
+organs (noses, eyes, etc.). A general property of an animal might
+be `senseOrgan`, to denote any given sensory organs a particular
+animal has. In general, fish have eyes, so a fish might have a
+`eyes` property to refer to a description of the particular eye
+structure of some species. Since eyes are a type of sensory organ,
+we can capture this relationship between these properties by saying
+that `eye` is a sub-property-of `senseOrgan`. Thus if a given fish
+has two eyes, it also has two sense organs. (It may have more, but
+we know that it must have two).
+
+I can describe this simple hierarchy with RDFS. In general, the
+class hierarchy is a graph rather than a tree (i.e. not like Java
+class inheritance). The
+[slime mold](http://en.wikipedia.org/wiki/Slime_mold) is popularly,
+though perhaps not accurately, thought of as an organism that has
+characteristics of both plants and animals. I might model a slime
+mold in my ontology as a class that has both plant and animal
+classes among its super-classes. RDFS is too weak a language to
+express that a thing cannot be both a plant and an animal (which is
+perhaps lucky for the slime molds). In RDFS, I can only name the
+classes, I cannot construct expressions to describe interesting
+classes. However, for many applications it is sufficient to state
+the basic vocabulary, and RDFS is perfectly well suited to this.
+
+Note also that I can both describe classes, in general terms, and I
+can describe particular *instances* of those classes. So there may
+be a particular individual Fred who is a Fish (i.e. has
+`rdf:type Fish`), and who has two eyes. His companion Freda, a
+[Mexican Tetra](http://en.wikipedia.org/wiki/Mexican_tetra), or
+blind cave fish, has no eyes. One use of an ontology is to allow us
+to fill-in missing information about individuals. Thus, though it
+is not stated directly, we can deduce that Fred is also an Animal
+and an Organism. Assume that there was no `rdf:type` asserting that
+Freda is a Fish. We may still infer Freda's `rdf:type` since Freda
+has [lateral lines](http://en.wikipedia.org/wiki/Lateral_line) as
+sense organs, and these only occur in fish. In RDFS, we state that
+the *domain* of the `lateralLines` property is the `Fish` class, so
+an RDFS reasoner can infer that Freda must be a fish.
+
+### OWL
+
+In general, OWL allows me to say everything that RDFS allows, and
+much more besides. A key part of OWL is the ability to describe
+classes in more interesting and complex ways. For example, in OWL
+we can say that Plant and Animal are *disjoint classes*: no
+individual can be both a plant and an animal (which would have the
+unfortunate consequence of making `SlimeMold` an empty class).
+`SaltwaterFish` might be the *intersection* of `Fish` and the class
+`SeaDwellers` (which also includes, for example, cetaceans and sea
+plants).
+
+Suppose I have a property `covering`, intended to represent the
+scales of a fish or the fur of a mammal. I can now refine the
+mammal class to be 'animals that have a covering that is hair',
+using a *property restriction* to express the condition that
+property `covering` has a value from the class `Hair`. Similarly
+`TropicalFish` might be the intersection of the class of `Fish` and
+the class of things that have `TropicalOcean` as their habitat.
+
+Finally (for this brief overview), I can say more about properties
+in OWL. In RDFS, properties can be related via a property
+hierarchy. OWL extends this by allowing properties to be denoted as
+*transitive*, *symmetric* or *functional*, and allow one property
+to be declared to be the *inverse* of another. OWL also makes a
+distinction between properties that have data-values (known as
+*literals* in RDF terminology) as their range, or other
+individuals. Respectively these are *datatype properties* and
+*object properties*. A consequence of the RDF lineage of OWL is
+that OWL ontologies cannot make statements about literal values. I
+cannot say in RDF that 7 has the property of being a prime number.
+I can, of course, say that the class of primes includes 7, but that
+doesn't require a number to be the subject of an RDF statement. In
+OWL, this distinction is important since only object properties can
+be transitive or symmetric.
+
+The OWL language is sub-divided into three syntax classes:
+*OWL Lite*, *OWL DL* and *OWL Full*. OWL DL does not permit some
+constructions allowed in OWL Full, and OWL Lite has all the
+constraints of OWL DL plus some more. The intent for OWL Lite and
+OWL DL is to make the task of reasoning with expressions in that
+subset more tractable. Specifically, OWL DL is intended to be able
+to be processed efficiently by a
+[*description logic*](http://en.wikipedia.org/wiki/Description_logic)
+reasoner. OWL Lite is intended to be amenable to processing by a
+variety of reasonably simple inference algorithms, though experts
+in the field have challenged how successfully this has been
+achieved.
+
+While the OWL standards documents note that OWL builds on top of
+the (revised) RDF specifications, it is possible to treat OWL as a
+separate language in its own right, and not something that is built
+on an RDF foundation. This view uses RDF as a serialisation syntax;
+the RDF-centric view treats RDF triples as the core of the OWL
+formalism. While both views are valid, in Jena we take the
+RDF-centric view.
+
+### DAML+OIL
+
+DAML+OIL is very similar to OWL Full. This is not surprising, since
+the W3C's Web Ontology Working Group, who designed OWL, took
+DAML+OIL as their starting point. Some constructions in OWL have
+been added to the capabilities of DAML+OIL, and one or two have
+been removed. For the purposes of this brief overview, however, the
+expressiveness of DAML+OIL is comparable to that of OWL.
+
+**Note:** DAML+OIL is effectively now obsolete. While you can still
+find some DAML ontologies on the web, you should work with OWL or
+RDFS to maximise the longevity of your models.
+
+### Ontology languages and the Jena Ontology API
+
+As I outlined above, there are various different ontology languages
+available for representing ontology information on the semantic
+web. They range from the most expressive, OWL Full, through to the
+weakest, RDFS. Through the Ontology API, Jena aims to provide a
+consistent programming interface for ontology application
+development, independent of which ontology language you are using
+in your programs.
+
+The Jena Ontology API is language-neutral: the Java class names do
+not mention the underlying language. For example, the `OntClass`
+Java class can represent an OWL class, RDFS class, or DAML class.
+To represent the differences between the various representations,
+each of the ontology languages has a *profile*, which lists the
+permitted constructs and the names of the classes and properties.
+Thus in the DAML profile, the URI for object property is
+`daml:ObjectProperty` (short for
+`http://www.daml.org/2001/03/daml+oil#ObjectProperty`), in the OWL
+profile is it `owl:ObjectProperty` (short for
+`http://www.w3.org/2002/07/owl#ObjectProperty`) and in the RDFS
+profile it is `null` since RDFS does not define object properties.
+
+The profile is bound to an *ontology model*, which is an extended
+version of Jena's
+[`Model`](../javadoc/com/hp/hpl/jena/rdf/model/Model.html) class.
+The base `Model` allows access to the statements in a collection of
+RDF data.
+[`OntModel`](../javadoc/com/hp/hpl/jena/ontology/OntModel.html)
+extends this by adding support for the kinds of objects expected to
+be in an ontology: classes (in a class hierarchy), properties (in a
+property hierarchy) and individuals. When you're working with an
+ontology in Jena, all of the state information remains encoded as
+RDF triples (which Jena calls `Statement`s) stored in the RDF
+model. The ontology API
+*doesn't change the RDF representation of ontologies*. What it does
+do is add a set of convenience classes and methods that make it
+easier for you to write programs that manipulate the RDF
+statements.
+
+The predicate names defined in the ontology language correspond to
+the accessor methods on the Java classes in the API. For example,
+an `OntClass` has a method to list its super-classes, which
+corresponds to the values of the `subClassOf` property in the RDF
+representation. This point is worth re-emphasising: no information
+is stored in the `OntClass` object itself. When you call the
+OntClass `listSuperClasses()` method, Jena will retrieve the
+information from the underlying RDF statements. Similarly adding a
+subclass to an `OntClass` asserts an additional RDF statement into
+the model.
+
+### Ontologies and reasoning
+
+One of the main reasons for building an ontology-based application
+is to use a reasoner to derive additional truths about the concepts
+you are modelling. We saw a simple instance of this above: the
+assertion "Fred is a Fish" *entails* the deduction "Fred is an
+Animal". There are many different styles of automated reasoner, and
+very many different reasoning algorithms. Jena includes support for
+a variety of reasoners through the
+[inference API](../inference/index.html). A common feature of Jena
+reasoners is that they create a new RDF model which appears to
+contain the triples that are derived from reasoning, as well as the
+triples that were asserted in the base model. This extended model
+is, nevertheless, still conforms to the contract for Jena models.
+So it can be used wherever a base model can be used. The ontology
+API exploits this feature: the convenience methods the ontology API
+provides can query an extended inference model in just the same way
+as a plain RDF model. In fact, this is such a common pattern that
+we provide simple recipes for constructing ontology models whose
+language, storage model and reasoning engine can all be simply
+specified when an `OntModel` is created. I'll show examples later.
+
+Figure 2 shows one way of visualising this:
+
+![image of layering of graphs in model](../images/Ont-model-layers.png)
+
+Figure 2: the statements seen by the OntModel
+
+`Graph` is an internal Jena interface that supports the composition
+of sets of RDF triples. The asserted statements, which may have
+been read in from an ontology document, are held in the base graph.
+The reasoner, or inference engine, can use the contents of the base
+graph and the semantic rules of the language to show a more
+complete set of statements. This is also presented via a `Graph`
+interface, so the model works only with the outermost interface.
+This regularity allows us to very easily build ontology models with
+or without a reasoner. It also means that the base graph can be an
+in-memory store, a database-backed persistent store, or some other
+storage structure altogether (e.g. an LDAP directory) again without
+affecting the operation of the ontology model.
+
+### RDF-level polymorphism and Java
+
+Deciding which Java abstract class to use to represent a given RDF
+resource can be surprisingly subtle. Consider the following RDF
+sample:
+
+    <owl:Class rdf:ID="DigitalCamera">
+    </owl:Class>
+
+This declares that the resource with the relative URI
+`#DigitalCamera` is an OWL ontology class. It suggests that it
+would be appropriate to model that declaration in Java with an
+instance of an `OntClass`. Now suppose I add a triple to the RDF
+model to augment the class declaration with some more information:
+
+    <owl:Class rdf:ID="DigitalCamera">
+      <rdf:type owl:Restriction />
+    </owl:Class>
+
+Now I am saying that `#DigitalCamera` is an OWL Restriction.
+Restriction is a subclass of `owl:Class`, so this is a perfectly
+consistent operation. The problem I have is that Java does not
+allow me to dynamically change the Java class of the object
+representing this resource. The resource has not changed: it still
+has URI `#DigitalCamera`. But the appropriate Java class Jena might
+choose to model it has changed from `OntClass` to `Restriction`.
+Conversely, if I subsequently remove the `rdf:type owl:Restriction`
+from the model, using the `Restriction` Java class is no longer
+appropriate.
+
+Even worse, OWL Full allows me to state the following (rather
+counter-intuitive) construction:
+
+    <owl:Class rdf:ID="DigitalCamera">
+      <rdf:type owl:ObjectProperty />
+    </owl:Class>
+
+That is, `#DigitalCamera` is both a class *and* a property. While
+this may not be a very useful operation, it illustrates a basic
+point: we cannot rely on a consistent or unique mapping between an
+RDF resource and the appropriate Java abstraction.
+
+Jena accepts this basic characteristic of polymorphism at the RDF
+level by considering that the Java abstraction (`OntClass`,
+`Restriction`, `DatatypeProperty`, etc.) is just a view or *facet*
+of the resource. That is, there is a one-to-many mapping from a
+resource to the facets that the resource can present. If the
+resource is typed as an `owl:Class`, it can present the `OntClass`
+facet; given other types, it can present other facets. Jena
+provides the `.as()` method to efficiently map from an RDF object
+to one of its allowable facets. Given a RDF object (i.e. an
+instance of `com.hp.hpl.jena.rdf.model.RDFNode` or one of its
+sub-types), I can get a facet by invoking `as()` with an argument
+that denotes the facet required. Specifically, the facet is
+identified by the Java class object of the desired facet. For
+example, to get the `OntClass` facet of a resource, I can do:
+
+    Resource r = myModel.getResource( myNS + "DigitalCamera" );
+    OntClass cls = (OntClass) r.as( OntClass.class );
+
+This pattern allows me to defer decisions about the correct Java
+abstraction to use until run-time. The choice can depend on the
+properties of the resource itself. If a given RDFNode will not
+support the conversion to a given facet, it will raise a
+`ConversionException`. I can test if `.as()` will succeed for a
+given facet with `canAs()`. This RDF-level polymorphism is used
+extensively in the Jena ontology API to allow maximum flexibility
+in handling ontology data.
+
+## Running example: the ESWC ontology
+
+To illustrate the principles of using the ontology API, I will use
+examples drawn from Tom Heath's
+[ESWC ontology](http://www.schemaweb.info/schema/SchemaDetails.aspx?id=282).
+This ontology presents a simple model for describing the concepts
+and activities associated with a typical academic conference. A
+copy of the ontology serialized in RDF/XML is included with the
+Jena download, see:
+[[`eswc-2006-09-21.rdf`](../../src-examples/data/eswc-2006-09-21.rdf)]
+(note that you may need to view the page source in some browsers to
+see the XML code).
+
+A subset of the classes and properties from the ontology are shown
+in Figure 3:
+
+![Image of the example class hierarchy](../images/eswc-classes.png)
+
+Figure 3: Classes and properties from ESWC ontology
+
+I will use elements from this ontology to illustrate the ontology
+API throughout the rest of this document.
+
+## Creating ontology models
+
+An ontology model is an extension of the Jena RDF model that
+provides extra capabilities for handling ontologies. Ontology
+models are created through the Jena
+[`ModelFactory`](../javadoc/com/hp/hpl/jena/rdf/model/ModelFactory.html).
+The simplest way to create an ontology model is as follows:
+
+    OntModel m = ModelFactory.createOntologyModel();
+
+This will create an ontology model with the *default* settings,
+which are set for maximum compatibility with the previous version
+of Jena. These defaults are:
+
+-   OWL-Full language
+-   in-memory storage
+-   RDFS inference, which principally produces entailments from the
+    sub-class and sub-property hierarchies.
+
+In many applications, such as driving a GUI, RDFS inference is too
+strong (for example, every class is inferred to be a sub-class of
+`owl:Thing`. In other applications, stronger reasoning is needed.
+In general, to create an `OntModel` with a particular reasoner or
+language profile, you should pass a model specification to the
+`createOntologyModel` call. For example, an OWL model that performs
+no reasoning at all can be created with:
+
+    OntModel m = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM );
+
+To create an ontology model for a particular language, but leaving
+all of the other values as defaults, you should pass the URI of the
+ontology language to the model factory. The URI strings for the
+various language profiles are:
+
+Ontology language
+URI
+RDFS
+`http://www.w3.org/2000/01/rdf-schema#`
+DAML+OIL
+`http://www.daml.org/2001/03/daml+oil#`
+OWL Full
+`http://www.w3.org/2002/07/owl#`
+OWL DL
+`http://www.w3.org/TR/owl-features/#term_OWLDL`
+OWL Lite
+`http://www.w3.org/TR/owl-features/#term_OWLLite`
+These URI's are used to look-up the language profile from the
+[`ProfileRegistry`](../javadoc/com/hp/hpl/jena/ontology/ProfileRegistry.html).
+Fortunately, the profile registry contains declared constants so
+that you do not have to remember these URI's. Please note that the
+URI's denoting OWL Lite and OWL DL are not officially sanctioned by
+the OWL standard.
+
+To create an ontology model for handling DAML ontologies, use
+either of:
+
+    OntModel m = ModelFactory.createOntologyModel( OntModelSpec.DAML_MEM );
+    OntModel m = ModelFactory.createOntologyModel( ProfileRegistry.DAML_LANG );
+
+Beyond these basic choices, the complexities of configuring an
+ontology model are wrapped up in a recipe object called
+`<a href="../javadoc/com/hp/hpl/jena/ontology/OntModelSpec.html">OntModelSpec</a>`.
+This specification allows complete control over the configuration
+choices for the ontology model, including the language profile in
+use, the reasoner, and the means of handling compound documents. A
+number of common recipes are pre-declared as constants in
+`OntModelSpec`, and listed below.
+
+OntModelSpec
+Language profile
+Storage model
+Reasoner
+OWL\_MEM
+OWL full
+in-memory
+none
+OWL\_MEM\_TRANS\_INF
+OWL full
+in-memory
+transitive class-hierarchy inference
+OWL\_MEM\_RULE\_INF
+OWL full
+in-memory
+rule-based reasoner with OWL rules
+OWL\_MEM\_MICRO\_RULE\_INF
+OWL full
+in-memory
+optimised rule-based reasoner with OWL rules
+OWL\_MEM\_MINI\_RULE\_INF
+OWL full
+in-memory
+rule-based reasoner with subset of OWL rules
+OWL\_DL\_MEM
+OWL DL
+in-memory
+none
+OWL\_DL\_MEM\_RDFS\_INF
+OWL DL
+in-memory
+rule reasoner with RDFS-level entailment-rules
+OWL\_DL\_MEM\_TRANS\_INF
+OWL DL
+in-memory
+transitive class-hierarchy inference
+OWL\_DL\_MEM\_RULE\_INF
+OWL DL
+in-memory
+rule-based reasoner with OWL rules
+OWL\_LITE\_MEM
+OWL Lite
+in-memory
+none
+OWL\_LITE\_MEM\_TRANS\_INF
+OWL Lite
+in-memory
+transitive class-hierarchy inference
+OWL\_LITE\_MEM\_RDFS\_INF
+OWL Lite
+in-memory
+rule reasoner with RDFS-level entailment-rules
+OWL\_LITE\_MEM\_RULES\_INF
+OWL Lite
+in-memory
+rule-based reasoner with OWL rules
+DAML\_MEM
+DAML+OIL
+in-memory
+none
+DAML\_MEM\_TRANS\_INF
+DAML+OIL
+in-memory
+transitive class-hierarchy inference
+DAML\_MEM\_RDFS\_INF
+DAML+OIL
+in-memory
+rule reasoner with RDFS-level entailment-rules
+DAML\_MEM\_RULE\_INF
+DAML+OIL
+in-memory
+rule-based reasoner with DAML rules
+RDFS\_MEM
+RDFS
+in-memory
+none
+RDFS\_MEM\_TRANS\_INF
+RDFS
+in-memory
+transitive class-hierarchy inference
+RDFS\_MEM\_RDFS\_INF
+RDFS
+in-memory
+rule reasoner with RDFS-level entailment-rules
+For details of reasoner capabilities, please see the
+[inference documentation](../inference/index.html) and the Javadoc
+for
+[OntModelSpec](../javadoc/com/hp/hpl/jena/ontology/OntModelSpec.html).
+See also further discussion [below](#inferenceIntro).
+
+**Note:** it is primarily the choice of reasoner, rather than the
+choice of language profile, that determines which entailments are
+seen by the ontology model. However, using an OWL reasoner with
+DAML source data will result in few additional entailments being
+seen by the ontology model.
+
+To create a model with a given specification, you should invoke the
+`ModelFactory` as follows:
+
+    OntModel m = ModelFactory.createOntologyModel( <model spec> );
+
+for example:
+
+    OntModel m = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM_MICRO_RULE_INF );
+
+To create a custom model specification, you can create a new one
+from its constructor, and call the various setter methods to set
+the appropriate values. More often, you want a variation on an
+existing recipe. In this case, you copy an existing specification
+and then update the copy as necessary:
+
+    OntModelSpec s = new OntModelSpec( OntModelSpec.OWL_MEM );
+    s.setDocumentManager( myDocMgr );
+    OntModel m = ModelFactory.createOntologyModel( s );
+
+## Compound ontology documents and imports processing
+
+The OWL and DAML ontology languages include some facilities for
+creating modular ontologies that can be re-used in a similar manner
+to software modules. In particular, one ontology can *import*
+another. Jena helps ontology developers to work with modular
+ontologies by automatically handling the imports statements in
+ontology models.
+
+The key idea is that the base model of an ontology model is
+actually a collection of models, one per imported model. That means
+we have to modify figure 2 a bit. Figure 4 shows how the ontology
+model builds a collection of import models:
+
+![Diagram of compound document for imports](../images/Ont-model-layers-import.png)
+
+Figure 4: ontology model compound document structure for imports
+
+I will use the term *document* to describe an ontology serialized
+in some transport syntax, such as RDF/XML or N3. This terminology
+isn't used by the OWL or RDFS standards, but it is a convenient way
+to refer to the written artifacts. However, from a broad view of
+the interlinked semantic web, a document view imposes artificial
+boundaries between regions of the global web of data.
+
+We will load an ontology document into an ontology model in the
+same way as a normal Jena model, using the `read` method. There are
+several variants on read, that handle differences in the source of
+the document (to be read from a resolvable URL or directly from an
+input stream or reader), the base URI that will resolve any
+relative URI's in the source document, and the serialisation
+language. In summary, these variants are:
+
+    read( String url )
+    read( Reader reader, String base )
+    read( InputStream reader, String base )
+    read( String url, String lang )
+    read( Reader reader, String base, String Lang )
+    read( InputStream reader, String base, String Lang )
+
+You can use any of these methods to load an ontology document. Note
+that we advise that you avoid the the `read()` variants that accept
+a `java.io.Reader` argument when loading XML documents containing
+internationalised character sets, since the handling of character
+encoding by the Reader and by XML parsers is not compatible.
+
+By default, when an ontology model reads an ontology document, it
+will also locate and load the document's imports. An OWL or DAML
+document may contain an individual of class `Ontology`, which
+contains meta-data about that document itself. For example:
+
+    <owl:Ontology rdf:about="">
+      <dc:creator rdf:value="Ian Dickinson" />
+      <owl:imports rdf:resource="http://jena.hpl.hp.com/example-ont" />
+    </owl:Ontology>
+
+The construct `rdf:about=""` is a relative URI that will resolve to
+the document's base URI: in other words it's a shorthand way of
+referring to the document itself. The `owl:imports` line states
+that this ontology is constructed using classes, properties and
+individuals from the referenced ontology. When `OntModel` reads
+this document, it will notice the `owl:imports` line and attempt to
+load the imported ontology into a sub-model of the ontology model
+being built. The definitions from both the base ontology and all of
+the imports will be visible to the reasoner.
+
+Each imported ontology document is held in a separate graph
+structure. This is important: I want to keep the original source
+ontology separate from the imports. When I write the model out
+again, normally only the base model is written (the alternative is
+that all you see is a confusing union of everything). And when I
+update the model, only the base model changes.
+
+Imports are processed recursively, so if my base document imports
+ontology A, and A imports B, I will end up with the structure shown
+in Figure 4. Note that the imports have been flattened out. A cycle
+check is used to prevent the document handler getting stuck if, for
+example, A imports B which imports A!
+
+### The ontology document manager
+
+Each ontology model has an associated *document manager* that
+assists with the processing and handling of documents-related
+concerns. For convenience, there is one global document manager
+that is used by default by ontology models. You can get a reference
+to this shared instance through `OntDocumentManager.getInstance()`.
+In many cases, it will be sufficient to simply change the settings
+on the global document manager to suit your application's needs.
+However, for more fine-grain control, you can create separate
+document managers, and pass them to the ontology model when it is
+created through the model factory. To do this, create an ontology
+specification object (see above), and set the document manager. For
+example:
+
+    OntDocumentManager mgr = new OntDocumentManager();
+    // set the mgr's properties now
+     … some code …
+    // now use it
+    OntModelSpec s = new OntModelSpec( OntModelSpec.RDFS_MEM );
+    s.setDocumentManager( mgr );
+    OntModel m = ModelFactory.createOntologyModel( s );
+
+Note that the model retains a reference to the document manager it
+was created with. Thus if you change a document manager's
+properties, it will affect models that have previously been
+constructed with that document manager.
+
+### Document manager policy
+
+Since the document manager has a large number of configurable
+options, there are two ways in which you can customise it to your
+application requirements. Firstly, you can set the individual
+parameters of the document manager by Java code. Alternatively,
+when a given document manager is created it can load values for the
+various parameters from a policy file, expressed in RDF. The
+document manager has a list of URL's which it will search for a
+policy document. It will stop at the first entry on the list that
+resolves to a retrievable document. The default search path for the
+policy is: `file:./etc/ont-policy.rdf;file:ont-policy.rdf`. You can
+find the default policy, which can serve as a template for defining
+your own policies, in the `etc/` directory under the Jena download
+directory.
+
+You can set the general properties of the document manager in the
+policy as follows:
+
+    <DocumentManagerPolicy>
+      <!-- policy for controlling the document manager's behaviour -->
+      <processImports rdf:datatype="&xsd;boolean">true</processImports>
+      <cacheModels rdf:datatype="&xsd;boolean">true</cacheModels>
+    </DocumentManagerPolicy>
+
+You can find the simple schema that declares the various properties
+that you can use in such an ontology document policy in the
+vocabularies directory of the Jena download. It's called
+`ont-manager.rdf`. To change the search path that the document
+manager will use to initialise itself, you can either pass the new
+search path as a string when creating a new document manager
+object, or call the method `setMetadataSearchPath()`.
+
+### The ModelMaker: creating storage on demand
+
+In order for the document manager to build the union of the
+imported documents (which we may refer to as the
+*imports closure*), there must be some means of creating new graphs
+to store the imported ontologies. Loading a new import means that
+there needs to be a new graph added Jena defines a *model maker* as
+a simple interface that allows different kinds of model storage (in
+memory, file-backed, in a persistent database, etc.) to be created
+on demand. For the database case, this may include passing the
+database user-name and password and other connection parameters.
+New model makers can be created with the
+`<a href="../javadoc/com/hp/hpl/jena/rdf/model/ModelFactory.html">ModelFactory</a>`.
+
+There are two cases in which we may want to create storage for
+models on-demand. The first is when creating the `OntModel` for the
+first time, some variants of `createOntologyModel` will allocate
+space for the *base model* (instead of, for example, being handed a
+base model to use as one of the method arguments). The second case
+when storage must be allocated is when adding an imported document
+to the union of imports. These cases often require different
+policies, so the `OntModelSpec` contains two model maker
+parameters: the *base model maker* and *imports model maker*,
+available via `getBaseModelMaker()` and `getImportsModelMaker()`
+methods respectively.
+
+The default specifications in OntModelSpec that begin MEM\_ use an
+in-memory model maker for the both the base model and the imported
+documents.
+
+**Implementation note**: internally to Jena, we use `Graph` as a
+primary data structure. However, your code will almost always refer
+to models, not graphs. What's happening is that a `Model` is a
+wrapper around the `Graph`, which balances a rich, convenient
+programming interface (`Model`) with a simple, manageable internal
+data structure (`Graph`). Hence some potential confusion in that
+Figure 4, above, refers to a structure containing graphs, but we
+use a
+`<a       href="../javadoc/com/hp/hpl/jena/rdf/model/ModelMaker.html"     >ModelMaker</a>`
+to generate new stores. The document manager extracts the
+appropriate graph from the containing model. Except in cases where
+you are extending Jena's internal structures, you should think of
+`Model` as the container of RDF and ontology data.
+
+### Controlling imports processing
+
+Normally, loading imports during the `read()` call automatic. To
+`read()` an ontology without building the imports closure, call the
+method `setProcessImports( false )` on the document manager object
+before calling `read()`. Alternatively, you can set the
+`processImports` property in the policy file. You can also be more
+selective, and ignore only certain URI's when loading the imported
+documents. To selectively skip certain named imports, call the
+method `addIgnoreImport( String uri )` on the document manager
+object, or set the `ignoreImport` property in the policy.
+
+### Managing file references
+
+An advantage of working with ontologies is that we can reuse work
+done by other ontologists, by importing their published ontologies
+into our own. Sometimes, however, this means that there is an
+Internet firewall between the ontology-based application and the
+source of an imported ontology. Even if it's possible to traverse
+the firewall through an HTTP proxy, retrieving files from an HTTP
+connection may impose unacceptable delays when starting an
+application. In the worst case, we may find that ontology on which
+our application depends is temporarily or permanently unavailable
+from the original published URL. To alleviate these commonly
+experienced problems, you can use the ontology document manager to
+set up a a local indirection, so that an attempt to import a
+document from a given published URL means that a local copy of the
+document is retrieved instead. This may be a file copy, or simply a
+pointer to a local mirror web site.
+
+To specify this local redirection in the policy file, use the
+following declarations:
+
+    <OntologySpec>
+      <publicURI rdf:resource="… the public URI to map from…"    />
+      <altURL rdf:resource="… the local URL to map to …" />
+      <!-- optional ontology language term -->
+      <language rdf:resource="… encoding used …" />
+      <!-- optional prefix to associate with the public URL -->
+      <prefix rdf:datatype="&xsd;string">a prefix</prefix>
+    </OntologySpec>
+
+For example:
+
+    <OntologySpec>
+      <!-- local version of the RDFS vocabulary -->
+      <publicURI rdf:resource="http://www.w3.org/2000/01/rdf-schema"    />
+      <altURL rdf:resource="file:vocabularies/rdf-schema.rdf" />
+    </OntologySpec>
+
+This specifies that an attempt to load the RDFS vocabulary from
+`http://www.w3.org/2000/01/rdf-schema` will transparently cause
+`file:vocabularies/rdf-schema.rdf` to be fetched instead. You can
+specify any number of such re-directions in the policy file, or you
+can add them to the document manager object directly by calling the
+various setter methods (see the Javadoc for details). As a
+side-effect, this mechanism also means that ontologies may be named
+with any legal URI (not necessarily resolvable) - so long as the
+`altURL` is itself resolvable.
+
+Note that the `OntDocumentManager` is an application of Jena's
+[File Manager](../how-to/filemanager.html). See the notes on
+FileManager for details of additional options.
+
+In the following example, I programmatically declare that the ESWC
+ontology is replicated locally on disk, and then I load it using
+the public URL. Assume that the constant `JENA` has been
+initialised to the directory in which Jena was installed.
+
+    OntModel m = ModelFactory.createOntologyModel();
+      OntDocumentManager dm = m.getDocumentManager();
+      dm.addAltEntry( "http://www.eswc2006.org/technologies/ontology",
+                      "file:" + JENA + "src-examples/data/eswc-2006-09-21.rdf"    );
+      m.read( "http://www.eswc2006.org/technologies/ontology" );
+
+### Specifying prefixes
+
+A model keeps a table of URI prefixes that can be used to render
+URI's in the shortened prefix:name form, which is useful in
+displaying URI's in a readable way in user interfaces, and is
+essential in producing legal XML names that denote arbitrary URI's.
+The ontology model's table of prefixes can be initialized from a
+table kept by the document manager, which contains the standard
+prefixes plus any that are declared by in the policy file (or added
+to subsequently by method calls). To prevent the model's prefix
+table from being initialized in this way, use the property
+`useDeclaredNsPrefixes` in the policy file (with value `false`), or
+call the method `setUseDeclaredPrefixes` on the ontology object.
+
+### Caching of imported models
+
+You can use the document manager to assist with loading ontology
+documents through its cache. Suppose two ontologies, A and B both
+import ontology C. We would like not to have to read C twice when
+loading A and then B. The document manager supports this use case
+by optionally caching C's model, indexed by URI. When A tries to
+import C, there is no cached copy, so a new model is created for C,
+the contents of C's URL read in to the model, then the C model is
+used in the compound document for A. Subsequently, when ontology B
+is loading imports, the document manager checks in its cache and
+finds an existing copy of C. This will be used in preference to
+reading a fresh copy of C from C's source URL, saving both time and
+storage space.
+
+Caching of import models is switched on by default. To turn it off,
+use the policy property `cacheModels`, or call the method
+`setCacheModels( boolean caching )` with `caching = false`. The
+document manager's current model cache can be cleared at any time
+by calling `clearCache()`.
+
+## The generic ontology type: OntResource
+
+All of the classes in the ontology API that represent ontology
+values have
+`<a href="../javadoc/com/hp/hpl/jena/ontology/OntResource.html">OntResource</a>`
+as a common super-class. This makes `OntResource` a good place to
+put shared functionality for all such classes, and makes a handy
+common return value for general methods. The Java interface
+`OntResource` extends Jena's RDF
+`<a href="../javadoc/com/hp/hpl/jena/rdf/model/Resource.html">Resource</a>`
+interface, so any general method that accepts a resource or an
+`<a href="../javadoc/com/hp/hpl/jena/rdf/model/RDFNode.html">RDFNode</a>`
+will also accept an `OntResource`, and consequently, any other
+ontology value.
+
+Some of the common attributes of ontology resources that are
+expressed through methods on OntResource are shown below:
+
+Attribute
+Meaning
+versionInfo
+A string documenting the version or history of this resource
+comment
+A general comment associated with this value
+label
+A human-readable label
+seeAlso
+Another web location to consult for more information about this
+resource
+isDefinedBy
+A specialisation of seeAlso that is intended to supply a definition
+of this resource
+sameAs
+Denotes another resource that this resource is equivalent to
+differentFrom
+Denotes another resource that is distinct from this resource (by
+definition)
+For each of these properties, there is a standard pattern of
+available methods:
+
+Method
+Effect
+add<property\>
+Add an additional value for the given property
+set<property\>
+Remove any existing values for the property, then add the given
+value
+list<property\>
+Return an iterator ranging over the values of the property
+get<property\>
+Return the value for the given property, if the resource has one.
+If not, return null. If it has more than one value, an arbitrary
+selection is made.
+has<property\>
+Return true if there is at least one value for the given property.
+Depending on the name of the property, this is sometimes
+is<property\>
+remove<property\>
+Removes a given value from the values of the property on this
+resource. Has no effect if the resource does not have that value.
+For example: `addSameAs( Resource r )`, or
+`isSameAs( Resource r )`. For full details of the individual
+methods, please consult the Javadoc.
+
+`OntResource` defines some other general utility methods. For
+example, to find out how many values a resource has for a given
+property, you can call `getCardinality( Property p )`. To delete
+the resource from the ontology altogether, you can call `remove()`.
+The effect of this is to remove every statement that mentions this
+resource as a subject or object of a statement.
+
+To get or set the value of a given property, use
+`getPropertyValue( Property p )` or
+`setPropertyValue( Property p, RDFNode value )`. Continuing the
+naming pattern, the values of a named property can be listed (with
+`listPropertyValues`), removed (with `removeProperty`) or added
+(with `addProperty`).
+
+Finally, `OntResource` provides methods for listing, getting and
+setting the `rdf:type` of a resource, which denotes a class to
+which the resource belongs (remember that in RDF and OWL, a
+resource can belong to many classes at once). The `rdf:type`
+property is one for which many entailment rules are defined in the
+semantic models of the various ontology languages. Therefore, the
+values that `listRDFTypes()` returns is more than usually dependent
+on the actual reasoner bound to the ontology model. For example,
+suppose I have class A, class B which is a subclass of A, and
+resource x whose asserted `rdf:type` is B. With no reasoner,
+listing x's RDF types will return only B. If the reasoner is able
+to calculate the closure of the subclass hierarchy (and most can),
+X's RDF types would also include A. A complete OWL reasoner would
+also infer that x has `rdf:type` `owl:Thing` and `rdf:Resource`.
+
+For some tasks, getting a complete list of the RDF types of a
+resource is exactly what is needed. For other tasks, this is not
+the case. If you are developing an ontology editor, for example,
+you may want to distinguish in its display between inferred and
+asserted types. In the above example, only `x rdf:type B` is
+asserted, everything else is inferred. One way to make this
+distinction is to make use of the base model (see Figure 4).
+Getting the resource from the base model and listing the type
+properties there would return only the asserted values. For
+example:
+
+    // create the base model
+    String SOURCE = "http://www.eswc2006.org/technologies/ontology";
+    String NS = SOURCE + "#";
+    OntModel base = ModelFactory.createOntologyModel( OWL_MEM );
+    base.read( SOURCE, "RDF/XML" );
+    
+    // create the reasoning model using the base
+    OntModel inf = ModelFactory.createOntologyModel( OWL_MEM_MICRO_RULE_INF, base );
+    
+    // create a dummy paper for this example
+    OntClass paper = base.getOntClass( NS + "Paper" );
+    Individual p1 = base.createIndividual( NS + "paper1", paper );
+    
+    // list the asserted types
+    for (Iterator i = p1.listRDFTypes(); i.hasNext(); ) {
+        System.out.println( p1.getURI() + " is asserted in class " + i.next() );
+    }
+    
+    // list the inferred types
+    p1 = inf.getIndividual( NS + "paper1" );
+    for (Iterator i = p1.listRDFTypes(); i.hasNext(); ) {
+        System.out.println( p1.getURI() + " is inferred to be in class " + i.next() );
+    }
+
+For other user interface or presentation tasks, we may want
+something between the complete list of types and the base list of
+only the asserted values. Consider the class hierarchy in figure 5
+(i):
+
+![Diagram showing direct relationships](../images/Direct-hierarchy.png)
+
+Figure 5: asserted and inferred relationships
+
+Figure 5 (i) shows a base model, containing a class hierarchy and
+an instance x. Figure 5 (ii) shows the full set of relationships
+that might be inferred from this base model. In Figure 5 (iii), we
+see only the *direct* or maximally specific relationships. For
+example, in 5 (iii) x does not have `rdf:type A`, since this is an
+relationship that is covered by the facts that x has `rdf:type D`,
+and D is a subclass of A. Notice also that the `rdf:type B` link is
+also removed from the direct graph, for a similar reason. Thus the
+direct graph hides relationships from both the inferred and
+asserted graphs. When displaying instance x in a GUI, particularly
+in a tree view of some kind, the direct graph is often the most
+useful as it contains the useful information in the most compact
+form.
+
+To list the RDF types of a resource, use:
+
+    listRDFTypes()                 // assumes not-direct
+    listRDFTypes( boolean direct ) // if direct=true, show only direct relationships
+
+Related methods allow the `rdf:type` to be tested, set and
+returned.
+
+## Ontology classes and basic class expressions
+
+Classes are the basic building blocks of an ontology. A simple
+class is represented in Jena by an
+[OntClass](../javadoc/com/hp/hpl/jena/ontology/OntClass.html)
+object. As I [mentioned above](#rdfPolymorphism), an ontology class
+is a facet of an RDF resource. One way, therefore, to get an
+ontology class is to convert an RDF resource. Assume that `m` is a
+suitable defined `OntModel`, into which the ESWC ontology has
+already been read, and that `NS` is a variable denoting the
+ontology namespace:
+
+    Resource r = m.getResource( NS + "Paper" );
+    OntClass paper = (OntClass) r.as( OntClass.class );
+
+This can be shortened by calling `getOntClass()` on the ontology
+model:
+
+    OntClass paper = m.getOntClass( NS + "Paper" );
+
+The `getOntClass` method will retrieve the resource with the given
+URI, and attempt to obtain the `OntClass` facet. If either of these
+operations fail, `getOntClass()` will return null. Compare this
+with the `createClass` method, which will reuse an existing
+resource if possible, or create a new class resource if not:
+
+    OntClass paper     = m.createClass( NS + "Paper" );
+    OntClass bestPaper = m.createClass( NS + "BestPaper" );
+
+You can use the create class method to create an anonymous class –
+a class description with no associated URI. Anonymous classes are
+often used when building more complex ontologies in OWL or DAML.
+They are less useful in RDFS.
+
+    OntClass anonClass = m.createClass();
+
+Once you have the ontology class object, you can begin processing
+it through the methods defined on `OntClass`. The attributes of a
+class are handled in a similar way to the attributes of
+OntResource, above, with a collection of methods to set, add, get,
+test, list and remove values. Properties of classes that are
+handled in this way are:
+
+Attribute
+Meaning
+subClass
+A subclass of this class, i.e. those classes that are declared
+`subClassOf` this class.
+superClass
+A super-class of this class, i.e. a class that this class is a
+`subClassOf`.
+equivalentClass
+A class that represents the same concept as this class. This is not
+just having the same class extension: the class 'British Prime
+Minister in 2003' contains the same individual as the class 'the
+husband of Cherie Blair', but they represent different concepts.
+disjointWith
+Denotes a class with which this class has no instances in common.
+Thus, in our example ontology, we can print a list the subclasses
+of an Artefact as follows:
+
+    OntClass artefact = m.getOntClass( NS + "Artefact" );
+    for (Iterator i = artefact.listSubClasses(); i.hasNext(); ) {
+      OntClass c = (OntClass) i.next();
+      System.out.println( c.getURI() );
+    }
+
+Note that, under RDFS and OWL semantics, each class is a sub-class
+of itself (in other words, `rdfs:subClassOf` is reflexive). While
+this is true, under the semantics, Jena users have reported finding
+it unhelpful. Therefore, the `listSubClasses` and
+`listSuperClasses` methods remove the reflexive from the list of
+results returned by the iterator. However, if you use the plain
+`Model` API to query for `rdfs:subClassOf` triples, assuming that a
+reasoner is in use, the reflexive triple will appear in the
+deductions model.
+
+Given an `OntClass` object, you can create or remove members of the
+class extension – individuals that are instances of the class –
+using the following methods:
+
+Method
+Meaning
+listInstances()  
+listInstances( boolean direct )
+Returns an iterator over those instances that include this class
+among their `rdf:type` values. The `direct` flag can be used to
+select individuals that are direct members of the class, rather
+than indirectly through the class hierarchy. Thus if `p1` has
+`rdf:type :Paper`, it will appear in the iterator returned by
+`listInstances` on `:Artefact`, but not in the iterator returned by
+`listInstances(false)` on `:Artefact`.
+createIndividual()  
+createIndividual( String uri )
+Adds a resource to the model, whose asserted `rdf:type` is this
+ontology class. If no URI is given, the individual is an anonymous
+resource.
+dropIndividual( Resource individual )
+Removes the association between the given individual and this
+ontology class. Effectively, this removes the `rdf:type` link
+between this class and the resource. Note that this is not the same
+as removing the individual altogether, unless the only thing that
+is known about the resource is that it is a member of the class. To
+delete an `OntResource`, including classes and individuals, use the
+`remove()` method.
+To test whether a class is a root of the class hierarchy in this
+model (i.e. it has no known super-classes), call
+`isHierarchyRoot()`.
+
+The domain of a property is intended to allow entailments about the
+class of an individual, given that it appears as a statement
+subject. It is not a constraint that can be used to validate a
+document, in the way that XML schema can do. Nevertheless, many
+developers find it convenient to use the domain of a property to
+document the design intent that the property only applies to known
+instances of the domain class. Given this observation, it can be a
+useful debugging or display aide to show the properties that have
+this class among their domain classes. The method
+`listDeclaredProperties()` attempts to identify the properties that
+are intended to apply to instances of this class. Using
+`listDeclaredProperties` is explained in detail in the
+[RDF frames how-to](../how-to/rdf-frames.html).
+
+## Ontology properties
+
+In an ontology, a *property* denotes the name of a relationship
+between resources, or between a resource and a data value. It
+corresponds to a predicate in logic representations. One
+interesting aspect of languages like RDFS and OWL is that
+properties are not defined as aspects of some enclosing class, but
+are first-class objects in their own right. This means that
+ontologies and ontology-applications can store, retrieve and make
+assertions about properties directly. Consequently, Jena has a set
+of Java classes that allow you to conveniently manipulate the
+properties represented in an ontology model.
+
+A property in an ontology model is an extension of the core Jena
+API class
+`<a href="../javadoc/com/hp/hpl/jena/rdf/model/Property.html">Property</a>`,
+and allows access to the additional information that can be
+asserted about properties in an ontology language. The common API
+super-class for representing ontology properties in Java is
+`<a href="../javadoc/com/hp/hpl/jena/ontology/OntProperty.html">OntProperty</a>`.
+Again, using the pattern of add, set, get, list, has, and remove
+methods, we can access the following attributes of an
+`OntProperty`:
+
+Attribute
+Meaning
+subProperty
+A sub property of this property; i.e. a property which is declared
+to be a `subPropertyOf` this property. If p is a sub property of q,
+and we know that `A p B` is true, we can infer that `A q B` is also
+true.
+superProperty
+A super property of this property, i.e. a property that this
+property is a `subPropertyOf`
+domain
+Denotes the class or classes that form the domain of this property.
+Multiple domain values are interpreted as a conjunction. The domain
+denotes the class of value the property maps from.
+range
+Denotes the class or classes that form the range of this property.
+Multiple range values are interpreted as a conjunction. The range
+denotes the class of values the property maps to.
+equivalentProperty
+Denotes a property that is the same as this property.
+inverse
+Denotes a property that is the inverse of this property. Thus if q
+is the inverse of p, and we know that `A q B`, then we can infer
+that `B p A`.
+In the example ontology, the property `hasProgramme` has a domain
+of `OrganizedEvent`, a range of `Programme` and the human-readable
+label "has programme". I can reconstruct this definition in an
+empty ontology model as follows:
+
+    OntModel m = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM );
+    OntClass programme = m.createClass( NS + "Programme" );
+    OntClass orgEvent = m.createClass( NS + "OrganizedEvent" );
+    
+    ObjectProperty hasProgramme = m.createObjectProperty( NS + "hasProgramme" );
+    
+    hasProgramme.addDomain( orgEvent );
+    body.addRange( programme );
+    body.addLabel( "has programme", "en" );
+
+As a further example, I can alternatively add information to an
+existing ontology. Let's add a super-property `hasDeadline` to
+generalise the separate properties denoting the submission
+deadline, notification deadline and camera-ready deadline:
+
+    OntModel m = ModelFactory.createOntologyModel( OntModelSpec.OWL_MEM );
+    m.read( "http://www.eswc2006.org/technologies/ontology" );
+    
+    DatatypeProperty subDeadline = m.getDatatypeProperty( NS + "hasSubmissionDeadline" );
+    DatatypeProperty notifyDeadline = m.getDatatypeProperty( NS + "hasNotificationDeadline" );
+    DatatypeProperty cameraDeadline = m.getDatatypeProperty( NS + "hasCameraReadyDeadline" );
+    
+    DatatypeProperty deadline = m.createDatatypeProperty( NS + "deadline" );
+    deadline.addDomain( m.getOntClass( NS + "Call" ) );
+    deadline.addRange( XSD.dateTime );
+    
+    deadline.addSubProperty( subDeadline );
+    deadline.addSubProperty( notifyDeadline );
+    deadline.addSubProperty( cameraDeadline );
+
+Note that, although I called the `addSubProperty` method on the
+object representing the new super-property, the serialized form of
+the ontology will contain `rdfs:subPropertyOf` axioms on each of
+the sub-property resources, since this is what the language
+defines. Jena will, in general, try to allow symmetric access to
+sub-properties and sub-classes from either direction.
+
+### Object and Datatype properties
+
+OWL and DAML+OIL refine the basic property type from RDF into two
+sub-types: *object properties* and *datatype properties* (for more
+details see
+[[OWL Reference](http://www.w3.org/TR/owl-ref/#Property)]). The
+difference between them is that an object property can have only
+individuals in its range, while a datatype property has concrete
+data literals (only) in its range. Some OWL reasoners are able to
+exploit the differences between object and datatype properties to
+perform more efficient reasoning over ontologies. OWL also adds an
+*annotation property*, which is defined to have no semantic
+entailments, and so is useful when annotating ontology documents,
+for example.
+
+In Jena, the Java interfaces
+`<a href="../javadoc/com/hp/hpl/jena/ontology/ObjectProperty.html">ObjectProperty</a>`,
+`<a href="../javadoc/com/hp/hpl/jena/ontology/DatatypeProperty.html">DatatypeProperty</a>`
+and
+`<a href="../javadoc/com/hp/hpl/jena/ontology/AnnotationProperty.html">AnnotationProperty</a>`
+are sub-types of `OntProperty`. However, they do not have any
+behaviours (methods) particular to themselves. Their existence
+allows the more complex sub-types of ObjectProperty – transitive
+properties and so forth – to be kept separate in the class
+hierarchy. However, when you create an object property or datatype
+property in a model, it will have the effect of asserting different
+`rdf:type` statements into the underlying triple store.
+
+### Functional properties
+
+OWL permits object and datatype properties to be *functional* –
+that is, for a given individual in the domain, the range value will
+always be the same. In particular, if `father` is a functional
+property, and individual `:bryn` has `father :ijd` and
+`father :ian`, a reasoner is entitled to conclude that `:ijd` and
+`:ian` denote the same individual. A functional property is
+equivalent to stating that the property has a maximum cardinality
+of one.
+
+Being a functional property is represented through the
+`<a href="../javadoc/com/hp/hpl/jena/ontology/FunctionalProperty.html">FunctionalProperty</a>`
+facet of an ontology property object. If a property is declared
+functional (test using the `isFunctional()` method), then the
+method `asFunctionalProperty()` conveniently returns the functional
+facet. A non-functional property can be made functional through the
+`convertToFunctionalProperty()` method. When you are creating a
+property object, you also have the option of passing a Boolean
+parameter to the `createObjectProperty()` method on `OntModel`.
+
+### Other property types
+
+There are several additional sub-types of ObjectProperty that
+represent additional capabilities of ontology properties. A
+`<a href="../javadoc/com/hp/hpl/jena/ontology/TransitiveProperty.html">TransitiveProperty</a>`
+means that if p is transitive, and we know `:a p :b` and also
+`b p :c`, we can infer that `:a p :c`. A
+`<a href="../javadoc/com/hp/hpl/jena/ontology/SymmetricProperty.html">SymmetricProperty</a>`
+means that if p is symmetric, and we know `:a p :b`, we can infer
+`:b p :a`. An
+`<a href="../javadoc/com/hp/hpl/jena/ontology/InverseFunctionalProperty.html">InverseFunctionalProperty</a>`
+means that for any given range element, the domain value is unique.
+(Note that in DAML+OIL, the terminology for inverse functional
+property is *unambiguous property*).
+
+Given that all properties are `RDFNode` objects, and therefore
+support the `as()` method, you can use `as()` to change from an
+object property facet to a transitive property facet. To make this
+more straightforward, the `OntProperty` Java class has a number of
+methods that support directly switching to the corresponding facet
+view:
+
+    public TransitiveProperty asTransitiveProperty();
+    public FunctionalProperty asFunctionalProperty();
+    public SymmetricProperty asSymmetricPropery();
+    public InverseFunctionalProperty asInverseFunctionalProperty();
+
+These methods all assume that the underlying model will support
+this change in perspective. If not, the operation will fail with a
+`ConversionException`. For example, if a given property `p` is not
+already a transitive property, then invoking
+`p.asTransitiveProperty()` will throw a conversion exception. The
+following methods will, if necessary, add additional information
+(i.e. the additional `rdf:type` statement) to allow the conversion
+to an alternative facet to succeed.
+
+    public TransitiveProperty convertToTransitiveProperty();
+    public FunctionalProperty convertToFunctionalProperty();
+    public SymmetricProperty convertToSymmetricPropery();
+    public InverseFunctionalProperty convertToInverseFunctionalProperty();
+
+Finally, methods beginning `is…` (e.g. `isTransitiveProperty`)
+allow you to test whether a given property would support a given
+sub-type facet.
+
+## More complex class expressions
+
+I introduced the handling of basic, named classes above. These are
+the only kind of class descriptions available in RDFS. In OWL,
+however, there are a number of additional types of class
+expression, which allow richer and more expressive descriptions of
+concepts. There are two main categories of additional class
+expression: *restrictions* and *Boolean expressions*. Let's examine
+each in turn.
+
+### Restriction class expressions
+
+A
+[restriction](../javadoc/com/hp/hpl/jena/ontology/Restriction.html)
+defines a class by reference to one of the properties of the
+individuals that comprise the members of the class, and then
+placing some constraint on that property. For example, in a simple
+view of animal taxonomy, we might say that mammals are covered in
+fur, and birds in feathers. Thus the property `hasCovering` is in
+one case restricted to have the value `fur`, in the other to have
+the value `feathers`. This is a *has value restriction*. Six
+restriction types are currently defined by OWL:
+
+Restriction type
+Meaning
+has value
+The restricted property has exactly the given value.
+all values from
+All values of the restricted property, if it has any, are members
+of the given class.
+some values from
+The property has at least one value which is a member of the given
+class.
+cardinality
+The property has exactly *n* values, for some positive integer n.
+min cardinality
+The property has at least *n* values, for some positive integer n.
+max cardinality
+The property has at most *n* values, for some positive integer n.
+Note that in DAML+OIL terminology, an all-values-from restriction
+is a *toClass* restriction, while a some-values-from restriction is
+a *hasClass* restriction. Note also that, at present, the Jena
+ontology API has only limited support for DAML's qualified
+cardinality restrictions (i.e. `cardinalityQ`, `minCardinalityQ`
+and `maxCardinalityQ`). Qualified cardinality restrictions are
+encapsulated in the interfaces `CardinalityQRestriction`,
+`MinCardinalityQRestriction` and `CardinalityQRestriction`.
+`OntModel` also provides methods for creating and accessing
+qualified cardinality restrictions. Since they are not part of the
+OWL 1.0 language definition, qualified cardinality restrictions are
+not supported in OWL ontologies. Qualified cardinality restrictions
+may be added to the forthcoming OWL 1.1 update, at which point it
+is likely that the Jena ontology API will be updated. Note however,
+that we are not currently making any commitment on the timescale
+for such an update.
+
+Jena provides a number of ways of creating restrictions, or
+retrieving them from a model. Firstly, you can retrieve a general
+restriction from the model by its URI, if known.
+
+    // get restriction with a given URI
+    Restriction r = m.getRestriction( NS + "theName" );
+
+You can create a new restriction created by nominating the property
+that the restriction applies to:
+
+    // anonymous restriction on property p
+    OntProperty p = m.createOntProperty( NS + "p" );
+    Restriction anonR = m.createRestriction( p );
+
+Since a restriction is typically not assigned a URI in an ontology,
+retrieving an existing restriction by name may not be possible.
+However, you can list all of the restrictions in a model and search
+for the one you want:
+
+    Iterator i = m.listRestrictions();
+    while (i.hasNext()) {
+        Restriction r = (Restriction) i.next();
+        if (isTheOne( r )) {
+            // handle the restriction
+        }
+    }
+
+A common case is that you want the restrictions on some property
+`p`. In this case, from an object denoting `p` you can list the
+restrictions that mention that property:
+
+    OntProperty p = m.getProperty( NS + "p" );
+    Iterator i = p.listReferringRestrictions();
+    while (i.hasNext()) {
+        Restriction r = (Restriction) i.next();
+        // now handle the restriction …
+    }
+
+A general restriction can be converted to a specific type of
+restriction via `as…` methods (if the information is already in the
+model), or, if the information is not in the model, via
+`convertTo…` methods. For example, to convert the example
+restriction `r` from the example above to an all values from
+restriction, you can do the following:
+
+    OntClass c = m.createClass( NS + "SomeClass" );
+    AllValuesFromRestriction avf = r.convertToAllValuesFromRestriction( c );
+
+To create a particular restriction *ab initio*, you can use the
+creation methods defined on `OntModel`. For example:
+
+    OntClass c = m.createClass( NS + "SomeClass" );
+    ObjectProperty p = m.createObjectProperty( NS + "p" );
+    
+    // null denotes the URI in an anonymous restriction
+    AllValuesFromRestriction avf = m.createAllValuesFromRestriction( null, p, c );
+
+Assuming that the above code fragment was using a model `m` that
+was created with the OWL language profile, it creates a instance of
+an OWL restriction that would have the following definition in
+RDF/XML:
+
+    <owl:Restriction>
+      <owl:onProperty rdf:resource="#p"/>
+      <owl:allValuesFrom rdf:resource="#SomeClass"/>
+    </owl:Restriction>
+
+Once you have a particular restriction object, there are methods
+following the standard add, get, set and test naming pattern to
+access the aspects of the restriction. For example, in a camera
+ontology, we might find this definition of a class describing
+Large-Format cameras:
+
+    <owl:Class rdf:ID="Large-Format">
+      <rdfs:subClassOf rdf:resource="#Camera"/>
+      <rdfs:subClassOf>
+        <owl:Restriction>
+          <owl:onProperty rdf:resource="#body"/>
+          <owl:allValuesFrom rdf:resource="#BodyWithNonAdjustableShutterSpeed"/>
+       </owl:Restriction>
+      </rdfs:subClassOf>
+    </owl:Class>
+
+Here's one way to access the components of the all values from
+restriction. Assume m contains a suitable camera ontology:
+
+    OntClass largeFormat = m.getOntClass( camNS + "Large-Format" );
+    for (Iterator i = LargeFormat.listSuperClasses( true ); i.hasNext(); ) {
+      OntClass c = (OntClass) i.next();
+    
+      if (c.isRestriction()) {
+        Restriction r = c.asRestriction();
+    
+        if (r.isAllValuesFromRestriction()) {
+          AllValuesFromRestriction av = r.asAllValuesFromRestriction();
+          System.out.println( "AllValuesFrom class " +
+                              av.getAllValuesFrom().getURI() +
+                              " on property " + av.getOnProperty().getURI() );
+        }
+      }
+    }
+
+### Boolean class expressions
+
+Most programmers are familiar with the use of Boolean operators to
+construct propositional expressions: conjunction (and), disjunction
+(or) and negation (not). OWL provides a means for construction
+expressions describing classes with analogous operators, by
+considering class descriptions in terms of the set of individuals
+that comprise the members of the class.
+
+Suppose we wish to say that an instance x has `rdf:type` A **and**
+`rdf:type` B. This means that x is both a member of the set of
+individuals in A, and in the set of individuals in B. Thus, x lies
+in the *intersection* of classes A and B. If, on the other hand, A
+is either has `rdf:type` A **or** B, then x must lie in the *union*
+of A and B. Finally, to say that x does **not** have `rdf:type` A,
+it must lie in the *complement* of A. These operations, union,
+intersection and complement are the Boolean operators for
+constructing class expressions. While complement takes only a
+single argument, union and intersection must necessarily take more
+than one argument. Before continuing with constructing and using
+Boolean class expressions, I digress briefly to discuss lists.
+
+### List expressions
+
+RDF originally had three container types: `Seq`, `Alt` and `Bag`.
+While useful, these are all open forms: it is not possible to say
+that a given container has a fixed number of values. The DAML+OIL
+standard introduced a fourth container type - lists - in order to
+have a closed collection. Lists have subsequently been added to the
+core RDF specification, and are used extensively in OWL. A list
+follows the well-known *cons cell* pattern from Lisp, Prolog and
+other list-handling languages. Each cell of a list is either the
+end-of-list terminator (`nil` in Lisp), or is a pair consisting of
+a value and a pointer to the cell that is the first cell on the
+tail of the list. In RDF lists, the end-of-list is marked by a
+resource with name `rdf:nil`, while each list cell is an anonymous
+resource with two properties, one denoting the tail and the other
+the value. Fortunately, this complexity is hidden by some simple
+syntax:
+
+    <p rdf:parseType="collection">
+      <A />
+      <B />
+    </p>
+
+According to the RDF specification, this list of two elements has
+the following expansion as RDF triples:
+
+    <p>
+      <rdf:first><A /></rdf:first>
+      <rdf:rest>
+        <rdf:first><B /></rdf:first>
+        <rdf:rest rdf:resource="http://www.w3.org/1999/02/22-rdf-syntax-ns#nil"/>
+      </rdf:rest>
+    </p>
+
+Given this construction, a well formed list (one with exactly one
+`rdf:first` and `rdf:rest` per cons cell) has a precisely
+determined set of members. Incidentally, the same list in N3/Turtle
+is even more compact:
+
+    :example
+        :p ( :A :B ).
+
+Although lists are defined in the generic RDF model in Jena, they
+are extensively used by the ontology API so I mention them here.
+Full details of the methods defined on the
+[`RDFList`](../javadoc/com/hp/hpl/jena/rdf/model/RDFList.html)
+class are available in the Jena Javadoc.
+
+A number of means of constructing lists is defined in
+[`Model`](../javadoc/com/hp/hpl/jena/rdf/model/Model.html), as
+variants on `createList`. For example, you can construct a list of
+three classes as follows:
+
+    OntModel m = ModelFactory.createOntModel();
+    OntClass c0 = m.createClass( NS + "c0" );
+    OntClass c1 = m.createClass( NS + "c1" );
+    OntClass c2 = m.createClass( NS + "c2" );
+    
+    RDFList cs = m.createList( new RDFNode[] {c0, c1, c2} );
+
+Alternatively, you can build a list one element at at time:
+
+    OntModel m = ModelFactory.createOntModel();
+    RDFList cs = m.createList(); // Cs is empty
+    cs = cs.cons( m.createClass( NS + "c0" ) );
+    cs = cs.cons( m.createClass( NS + "c1" ) );
+    cs = cs.cons( m.createClass( NS + "c2" ) );
+
+Note that these two approaches end with the classes in the lists in
+opposite orders, since the `cons` operation adds a new list cell to
+the front of the list. Thus the second list will run c2 to c0. In
+the ontology operations we are discussing here, the order of values
+in the list is not considered significant.
+
+Once the list has been constructed or obtained from the model (a
+resource which is a cell in a list sequence will accept
+`.as( RDFList.class )`),
+[`RDFList`](../javadoc/com/hp/hpl/jena/rdf/model/RDFList.html)
+methods may be used to access members of the list, iterate over the
+list, and so forth. For example:
+
+    System.out.println( "List has " + myRDFList.size() + " members:" );
+    for (Iterator i = myRDFList.iterator(); i.hasNext(); ) {
+      System.out.println( i.next() );
+    }
+
+### Intersection, union and complement class expressions
+
+Given Jena's ability to construct lists, building intersection and
+union class expressions is straightforward. The `create` methods on
+OntModel allow you to construct an intersection or union directly.
+Alternatively, given an existing OntClass, you can use the
+`convertTo…` methods to construct facet representing the more
+specialised expressions. For example, we can define the class of UK
+industry-related conferences as the intersection of conferences
+with a UK location and conferences with an industrial track. Here's
+the XML declaration:
+
+    <owl:Class rdf:ID="UKIndustrialConference">
+      <owl:intersectionOf rdf:parseType="Collection">
+        <owl:Restriction>
+          <owl:onProperty rdf:resource="#hasLocation"/>
+          <owl:hasValue rdf:resource="#united_kingdom"/>
+        </owl:Restriction>
+      </owl:intersectionOf>
+        <owl:Restriction>
+          <owl:onProperty rdf:resource="#hasPart"/>
+          <owl:someValuesFrom rdf:resource="#IndustryTrack"/>
+        </owl:Restriction>
+      </owl:intersectionOf>
+    </owl:Class>
+
+Or, more compactly in N3/Turtle:
+
+    :UKIndustrialConference a owl:Class
+        ; owl:intersectionOf (
+           [a owl:Restriction
+            ; owl:onProperty :hasLocation
+            ; owl:hasValue :united_kingdom]
+           [a owl:Restriction
+            ; owl:onProperty :hasPart
+            ; owl:someValuesFrom :IndustryTrack]
+          )
+
+Here is code to create this class declaration using Jena, assuming
+that `m` is a model into which the ESWC ontology has been read:
+
+    // get the class references
+    OntClass place = m.getOntClass( NS + "Place" );
+    OntClass indTrack = m.getOntClass( NS + "IndustryTrack" );
+    
+    // get the property references
+    ObjectProperty hasPart = m.getObjectProperty( NS + "hasPart" );
+    ObjectProperty hasLoc = m.getObjectProperty( NS + "hasLocation" );
+    
+    // create the UK instance
+    Individual uk = place.createIndividual( NS + "united_kingdom" );
+    
+    // now the anonymous restrictions
+    HasValueRestriction ukLocation =
+        m.createHasValueRestriction( null, hasLoc, uk );
+    SomeValuesFromRestriction hasIndTrack =
+        m.createHasValueRestriction( null, hasPart, indTrack );
+    
+    // finally create the intersection class
+    IntersectionClass ukIndustrialConf =
+        m.createIntersectionClass( NS + "UKIndustrialConference",
+                                   m.createList( new RDFNode[] {ukLocation, hasIndTrack} ) );
+
+Given the similarity between union and intersection class
+expressions, rather than separate methods to set the components of
+the expression, Jena defines a common super-class
+`<a href="../javadoc/com/hp/hpl/jena/ontology/BooleanClassDescription.html">BooleanClassDescription</a>`.
+This class provides access to the *operands* to the expression. In
+the above example, the operands are the two restrictions. The
+`BooleanClassDescription` class allows you to set the operands
+*en masse* by supplying a list, or to be added or deleted one at a
+time.
+
+Complement class expressions are very similar. The principal
+difference is that they take only a single class as operand, and
+therefore do not accept a list of operands.
+
+### Enumerated classes
+
+The final type class expression allows by OWL is the enumerated
+class. Recall that a class is a set of individuals. Often, we want
+to define the members of the *implicitly*: for example, "the class
+of UK conferences". Sometimes it is convenient to define a class
+*explicitly*, by stating the individuals the class contains. An
+*[enumerated class](../javadoc/com/hp/hpl/jena/ontology/EnumeratedClass.html)*
+is exactly the class whose members are the given individuals. For
+example, we know that the class of PrimaryColours contains exactly
+red, green and blue, and no others.
+
+In Jena, an enumerated class is created in a similar way to other
+classes. The set of values that comprise the enumeration is
+described by an RDFList. For example, here's a class defining the
+countries that comprise the United Kingdom:
+
+    <owl:Class rdf:ID="UKCountries">
+      <owl:oneOf rdf:parseType="Collection">
+        <eswc:Place rdf:about="#england"/>
+        <eswc:Place rdf:about="#scotland"/>
+        <eswc:Place rdf:about="#wales"/>
+        <eswc:Place rdf:about="#northern_ireland"/>
+      </owl:oneOf>
+    </owl:Class>
+
+To list the contents of this enumeration, we could do the
+following:
+
+    OntClass place = m.getOntClass( NS + "Place" );
+    
+    EnumeratedClass ukCountries =
+        m.createEnumeratedClass( NS + "UKCountries", null );
+    ukCountries.addOneOf( place.createIndividual( NS + "england" ) );
+    ukCountries.addOneOf( place.createIndividual( NS + "scotland" ) );
+    ukCountries.addOneOf( place.createIndividual( NS + "wales" ) );
+    ukCountries.addOneOf( place.createIndividual( NS + "northern_ireland" ) );
+    
+    for (Iterator i = UKCountries.listOneOf(); i.hasNext(); ) {
+      Resource r = (Resource) i.next();
+      System.out.println( r.getURI() );
+    }
+
+### Listing classes
+
+In many applications, you will need to inspect the set of classes
+in an ontology. The `list…` methods on `OntModel` provide a variety
+of means of listing types of class. The methods available include:
+
+    public ExtendedIterator listClasses();
+    public ExtendedIterator listEnumeratedClasses();
+    public ExtendedIterator listUnionClasses();
+    public ExtendedIterator listComplementClasses();
+    public ExtendedIterator listIntersectionClasses();
+    public ExtendedIterator listRestrictions();
+    public ExtendedIterator listNamedClasses();
+    public ExtendedIterator listHierarchyRootClasses();
+
+The last two methods deserve special mention. In OWL, class
+expressions are typically not named, but are denoted by anonymous
+resources (aka *bNodes*). In many applications, such as displaying
+an ontology in a user interface, we want to pick out the named
+classes only, ignoring those denoted by bNodes. This is what
+`listNamedClasses()` does. The method `listHierarchyRootClasses()`
+identifies the classes that are uppermost in the class hierarchy
+contained in the given model. These are the classes that have no
+super-classes. The iteration returned by
+`listHierarchyRootClasses()` **may** contain anonymous classes. To
+get a list of named hierarchy root classes, i.e. the named classes
+that lie closest to the top of the hierarchy (alternatively: the
+shallowest fringe of the hierarchy consisting solely of named
+classes), use the
+[OntTools](../javadoc/com/hp/hpl/jena/ontology/OntTools.html)
+method `namedHierarchyRoots()`.
+
+You should also note that it is important to close the iterators
+returned from the `list…` methods, particularly when the underlying
+store is a database. This is necessary so that any state (e.g. the
+database connection resources) can be released. Closing happens
+automatically when the `hasNext()` method on the iterator returns
+false. If your code does not iterate all the way to the end of the
+iterator, you should call the `close()` method explicitly. Note
+also that the values returned by these iterators will depend on the
+asserted data and the reasoner being used. For example, if the
+model contains a `Restriction`, that restriction will only be
+returned by the listClasses() iterator if the model is bound to a
+reasoner that can infer that any restriction is also be a class,
+since `Restriction` is a subClassOf `Class`. This difference can be
+exploited by the programmer: to list classes and restrictions
+separately, perform the `listClasses()` and `listRestictions()`
+methods on the base model only, or on a model with no reasoner
+attached.
+
+## Instances or individuals
+
+In OWL Full (and in DAML+OIL) any value can be an individual – and
+thus the subject of triples in the RDF graph other than ontology
+declarations. In OWL Lite and DL, the language terms and the
+instance data that the application is working with are kept
+separate, by definition of the language. Jena therefore supports a
+simple notion of an
+[`Individual`](../javadoc/com/hp/hpl/jena/ontology/Individual.html),
+which is essentially an alias for `Resource`. While `Individual`s
+are largely synonymous with `Resource`s, they do provide an
+programming interface that is consistent with the other Java
+classes in the ontology API.
+
+There are two ways to create individuals. Both requires the class
+to which the individual will initially belong:
+
+    OntClass c = m.createClass( NS + "SomeClass" );
+    
+    // first way: use a call on OntModel
+    Individual ind0 = m.createIndividual( NS + "ind0", c );
+    
+    // second way: use a call on OntClass
+    Individual ind1 = c.createIndividual( NS + "ind1" );
+
+The only real difference between these approaches is that the
+second way will create the individual in the same model that the
+class is attached to (see the `getModel()` method). In both of the
+above examples the individual is named, but this is not necessary.
+The method `OntModel.createIndividual( Resource cls )` creates an
+anonymous individual belonging to the given class. Note that the
+type of the class parameter is only `Resource`. You are not
+required to use `as()` to present a `Resource` to an `OntClass`
+before calling this method, though of course an `OntClass` is a
+`Resource` so using an `OntClass` will work perfectly well.
+
+`Individual` provides a set of methods for testing and manipulating
+the ontology classes to which an individual belongs. This is a
+convenience: OWL and RDFS denote class membership through the
+`rdf:type` property, and methods for manipulating and testing
+`rdf:type` are defined on `OntResource`. You may use either
+approach interchangeably.
+
+## Ontology meta-data
+
+In OWL and DAML+OIL, but not RDFS, meta-data about the ontology
+itself is encoded as properties on an individual of class
+`owl:Ontology` or `daml:Ontology` as appropriate. By convention,
+the URI of this individual is the URI of the ontology document
+itself. In the XML serialisation, this is typically shown as:
+
+    <owl:Ontology rdf:about="">
+    </owl:Ontology>
+
+Note that the construct `rdf:about=""` does *not* indicate a
+resource with no URI; it is in fact a shorthand way of referencing
+the *base URI* of the document containing the ontology. The base
+URI may be stated in the document through an `xml:base` declaration
+in the XML preamble. The base URI can also be specified when
+reading the document via Jena's Model API (see the `read()` methods
+on [`OntModel`](../javadoc/com/hp/hpl/jena/ontology/OntModel.html)
+for reference).
+
+You can attach various meta-data statements to this object to
+indicate attributes of the ontology as a whole. The Java object
+`Ontology` represents this special instance, and uses the standard
+add, set, get, list, test and delete pattern to provide access to
+the following attributes:
+
+Attribute
+Meaning
+backwardCompatibleWith
+Names a prior version of this ontology that this version is
+compatible with.
+incompatibleWith
+Names a prior version of this ontology that this version is not
+compatible with
+priorVersion
+Names a prior version of this ontology.
+imports
+Names an ontology whose definitions this ontology includes
+In addition to these attributes, the Ontology element typically
+contains common meta-data properties, such as comment, label and
+version information.
+
+In the Jena API, the ontology's metadata properties can be accessed
+through the
+[`Ontology`](../javadoc/com/hp/hpl/jena/ontology/Ontology.html)
+interface. Suppose I wish to know the list of URI's that the
+ontology imports. First I must obtain the resource representing the
+ontology itself:
+
+    String base = …; // the base URI of the ontology
+    OntModel m = …;  // the model containing the ontology statements
+    Ontology ont = m.getOntology( base );
+    
+    // now list the ontology imports
+    for (Iterator i = ont.listImports(); i.hasNext(); ) {
+        System.out.println( "Ontology " + base + " imports " + i.next() );
+    }
+
+If the base URI of the ontology is not known, you can list all
+resources of `rdf:type` `Ontology` in a given model by
+`OntModel.listOntologies()`. If there is only one of these, it
+should be safe to assume that it is *the* Ontology resource for the
+ontology. However, you should note that if more than one ontology
+document has been read in to the model (for example by including
+the imports of a document), there may well be more than one
+`Ontology` resource in the model. In this case, you may find it
+useful to list the ontology resources in just the base model:
+
+    OntModel m = … // the model, including imports
+    OntModel mBase = ModelFactory.createOntologyModel(
+                          OntModelSpec.OWL_MEM, m.getBaseModel() );
+    
+    for (Iterator i = mBase.listOntologies(); i.hasNext(); ) {
+        Ontology ont = (Ontology) i.next();
+        // m's base model has ont as an import ...
+    }
+
+A common practice is also to use the Ontology element to attach
+Dublin Core metadata to the ontology document. Jena provides a copy
+of the Dublin Core vocabulary, in `com.hp.hpl.jena.vocabulary.DC`.
+To attach a statement saying that the ontology was authored by John
+Smith, we can say:
+
+    Ontology ont = m.getOntology( baseURI );
+    ont.addProperty( DC.creator, "John Smith" );
+
+It is also possible to programatically add imports and other
+meta-data to a model, for example:
+
+    String base = …; // the base URI of the ontology
+    OntModel m = …;
+    
+    Ontology ont = m.createOntology( base );
+    ont.addImport( m.createResource( "http://example.com/import1" ) );
+    ont.addImport( m.createResource( "http://example.com/import2" ) );
+
+Note that under default conditions, simply adding (or removing) an
+`owl:imports` statement to a model will not cause the corresponding
+document to be imported (or removed). However, by calling

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