Jena – A Semantic Web Framework for Java

Introduction Semantic Web technologies Jena

Jena – A Semantic Web Framework for Java

Aleksander Pohl

Katedra Informatyki, Akademia Górniczo-Hutnicza

2. luty 2009

Aleksander Pohl KI AGH


Agenda

Introduction

Semantic Web technologies

Jena



What is the Semantic Web?

The Semantic Web is an:
... extension of current web in which
◮

... information is given well deﬁned meaning
◮

... better enabling people and computers to work in
◮
cooperations.
The Semantic Web, Tim Barners-Lee, James Hendler and Ora
Lassil



Motivation

providing information about Web resources and the
◮
systems that use them
allowing data to be processed outside the particular
◮
environment in which it was created
combining data from several applications to arrive at new
◮
information



Core Semantic Web technologies

RDF – Resource Description Framework
◮

RDFS – RDF Schema (vocabulary)
◮

OWL – Web Ontology Language
◮

SPARQL – Query Language for RDF
◮



RDF – Resource Description Framework

based on XML and URIs
◮

directed, labeled graph format for representing
◮
information in the web
formal semantics allows well founded deductions in RDF
◮
data
extensible vocabulary
◮



RDF – graph data model

any expression in RDF is a collection of triples
◮

every triple consists of a subject, a predicate and an object
◮

the arc direction is signiﬁcant – it always points toward the
◮
object
assertion of an RDF triple says that some relationship,
◮
indicated by the predicate, holds between the things
denoted by subject and object of the triple
meaning of an RDF graph is the conjunction (logical AND)
◮
of the statements corresponding to all the triples it contains


RDF – subject, predicate & object

nodes of an RDF graph are its subjects and objects
◮

a node may be:
◮
URI with optional fragment identiﬁer, eg.
◮

http://www.example.org/stafﬁd/1234#xyz
literal, eg. quot;Georgequot;
◮

blank
◮

the subject of a triple might be only URI reference or
◮
a blank node
the predicate of the triple must be URI reference
◮



RDF – example

<rdf:Description rdf:about=quot;http://www.ivan-herman.netquot;>
<foaf:name>Ivan</foaf:name>
<abc:myCalendar rdf:resource=quot;http://.../myCalendarquot;/>
<foaf:surname>Herman</foaf:surname>
</rdf:Description>



RDFS – RDF Vocabulary Description Language
RDF itself provides no means for defining
◮
application-specific classes and properties
They are described in RDFS – RDF Vocabulary
◮
Description Language (aka RDF Schema)
RDF Schema provides a type system for RDF, similar to
◮
type systems in programming languages:
allows resources to be defined as instances of one or more
◮

classes
allows classes to be organized in a hierarchical fashion
◮

URIref prefix
◮
http://www.w3.org/2000/01/rdf-schema#,
conventionally associated with the QName prefix rdfs:



RDFS – basic notions (1)
Classes are described using the RDFS resources:
◮
rdfs:Class
◮

rdfs:Resource
◮

and the properties:
◮
rdf:type
◮

rdfs:subClassOf
◮

Example:
◮
ex:MotorVehicle rdf:type rdfs:Class.
◮

Java: class MotorVehicle {· · · }
exthings:myCar rdf:type ex:MotorVehicle.
◮

Java: myCar = new MotorVehicle();
ex:Van rdfs:subClassOf ex:MotorVehicle.
◮

Java: class Van extends MotorVehicle {· · · }



OWL – Web Ontology Language
Intended to be used when the information needs to be
◮
processed by applications (not merely presented to
humans)
Provides additional vocabulary along with a formal
◮
semantics
Sublanguages:
◮
Lite – classiﬁcation hierarchy and simple constraints
◮

(extension of restricted RDF)
DL (Description Logic) – maximum expressiveness while
◮

retaining computational completeness (extension of
restricted RDF)
Full – maximum expressiveness and the syntactic freedom
◮

of RDF with no computational guarantees (real extension of
RDF)


OWL Lite additions

(In)Equality: equivalentClass,
◮
equivalentProperty, sameAs, differentFrom, ...
Property characteristics: inverseOf,
◮
TransitiveProperty, SymmetricProperty, ...
Restricted Cardinality: minCardinality,
◮
maxCardinality, cardinality
Annotation Properties: rdfs:label, rdfs:comment,
◮
rdfs:seeAlso, rdfs:isDefinedBy,
AnnotationProperty, OntologyProperty



Semantic Web – ontologies overview



SPARQL – overview

SPARQL is a query language for RDF
◮

It is similar to SQL in terms of purpose and syntax, but
◮
different in terms of application (relational data vs. graph
based data)
Human-friendly syntax
◮

Data integration and aggregation – multiple sources
◮
supported by default
Allows to make yes/no questions
◮



SPARQL Example
Data:
◮
<http://example.org/book/book1>
<http://purl.org/dc/elements/1.1/title>
”SPARQL Tutorial” .
Query:
◮
SELECT ?title WHERE {
<http://example.org/book/book1>
<http://purl.org/dc/elements/1.1/title>
?title . }
Result:
◮

title
”SPARQL Tutorial”



Jena2 – overview

Developed by HP Laboratories
◮

Open-source Java implementation of core Semantic Web
◮
technologies:
RDF graph manipulation API
◮

RDFS and OWL reasoning API
◮

Includes the de facto reference RDF/XML parser
◮

RDF/XML, N3 and N-triple Input/Output
◮

SPQRQL query engine
◮

rule-based inference engine
◮

In-memory or persistent storage
◮



RDF API example
// some definitions
1
static String personURI = quot;http://somewhere/JohnSmithquot;;
2
static String fullName = quot;John Smithquot;;
3
4
// create an empty Model
5
Model model = ModelFactory.createDefaultModel();
6
7
// create the resource
8
Resource johnSmith = model.createResource(personURI);
9
10
// add the property
11
johnSmith.addProperty(VCARD.FN, fullName);
12



SPARQL API example
Model model = ... ;
1
String queryString = quot; .... quot; ;
2
Query query = QueryFactory.create(queryString) ;
3
QueryExecution qexec = QueryExecutionFactory.create(query, model) ;
4
try {
5
ResultSet results = qexec.execSelect() ;
6
for ( ; results.hasNext() ; )
7
{
8
QuerySolution soln = results.nextSolution() ;
9
// Get a result variable by name.
10
RDFNode x = soln.get(quot;varNamequot;) ;
11
// Get a result variable - must be a resource
12
Resource r = soln.getResource(quot;VarRquot;) ;
13
// Get a result variable - must be a literal
14
Literal l = soln.getLiteral(quot;VarLquot;) ;
15
}
16
} finally { qexec.close() ; }
17



Jena comparison



Bibliography (1)

Brickley, Dan; Guha, Ramanathan (2004). RDF Vocabulary
◮
Description Language 1.0: RDF Schema,
http://www.w3.org/TR/rdf-schema/
Carroll, Jeremy J.; Dickinson, Ian; Dollin, Chris; Reynolds,
◮
Dave; Seaborne, Andy; Wilkinson, Kevin (2003). Jena:
Implementing the Semantic Web Recommendations, HP
Laboratories, Bristol, technical report HPL-2003-146.
Herman, Ivan Questions (and Answers) on the semantic
◮
Web, http://www.w3.org/People/Ivan/-
CorePresentations/SW_QA



Bibliography (2)

Klyne, Graham; Carroll, Jeremy J. (2004). Resource
◮
Description Framework (RDF): Concepts and Abstract
Syntax http://www.w3.org/TR/rdf-concepts/
McBride, Brian (2002), The Semantic Web, HP
◮
Laboratories, Bristol, Euroweb 2002 invited talk.
McGuinness, Deborah; van Harmelen, Frank (2004). OWL
◮
Web Ontology Language Overview,
http://www.w3.org/TR/owl-features/


Jena – A Semantic Web Framework for Java

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