This document provides an overview and tutorial on querying DBpedia using the Jena framework. It introduces Jena and its capabilities for working with RDF data, describes how to set up a development environment in Netbeans or Eclipse, and provides examples of querying DBpedia's SPARQL endpoint to retrieve information about people and locations. Other APIs for working with RDF in languages like PHP, Python, and C are also briefly mentioned.

1. Tutorial: Querying DBpedia
Web Technology
2ID60
14 November 2013
Dr. Katrien Verbert
Dr. ir. Natasha Stash
Dr. George Fletcher

2. Overview
•
•
•
•
•
Introduction to Jena
Setting up the environment
Querying DBpedia
Other APIs
PHP example

3. Jena
• Jena is a Java framework for the creation of applications
for the Semantic Web
• Provides interfaces and classes for the creation and
manipulation of RDF repositories

4. RDF concepts

5. Capabilities of Jena
•
•
•
•
RDF API
Reading and writing in RDF/XML, N-Triples
In-memory and persistent storage
SPARQL query engine

6. RDF concepts
• The Jena RDF API contains classes and interfaces for every
important aspect of the RDF specification
• They can be used in order to construct RDF graphs from
scratch, or edit existent graphs
• These classes/interfaces reside in the
com.hp.hpl.jena.rdf.model package
• In Jena, the Model interface is used to represent RDF
graphs
• Through Model, statements can be obtained/ created/
removed etc

7. RDF concepts
// Create an empty model
Model model = ModelFactory.createDefaultModel();
String ns = new String("http://www.example.com/example#");
// Create two Resources
Resource john = model.createResource(ns + "John");
Resource jane = model.createResource(ns + "Jane");
// Create the 'hasBrother' Property declaration
Property hasBrother = model.createProperty(ns, "hasBrother");
// Associate jane to john through 'hasBrother'
jane.addProperty(hasBrother, john);
// Create the 'hasSister' Property declaration
Property hasSister = model.createProperty(ns, "hasSister");
// Associate john and jane through 'hasSister' with a Statement
Statement sisterStmt = model.createStatement(john, hasSister, jane);
model.add(sisterStmt);

8. SPARQL query processing
• Jena uses the ARQ engine for the processing of
SPARQL queries
• The ARQ API classes are found in com.hp.hpl.jena.query
• Basic classes in ARQ:
• Query: Represents a single SPARQL query.
• Dataset: The knowledge base on which queries are executed
(Equivalent to RDF Models)
• QueryFactory: Can be used to generate Query objects from
SPARQL strings
• QueryExecution: Provides methods for the execution of queries
• ResultSet: Contains the results obtained from an executed query
• QuerySolution: Represents a row of query results.
• If there are many answers to a query, a ResultSet is returned after
the query is executed. The ResultSet contains many QuerySolutions

9. SPARQL query processing
// Prepare query string
String queryString =
"PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>n" +
"PREFIX : <http://www.example.com/onto1#>n" +
"SELECT ?married ?spouse WHERE {" +
"?married rdf:type :MarriedPerson.n" +
"?married :hasSpouse ?spouse." +
"}";
// Use the ontology model to create a Dataset object
// Note: If no reasoner has been attached to the model, no results
// will be returned (MarriedPerson has no asserted instances)
Dataset dataset = DatasetFactory.create(ontModel);
// Parse query string and create Query object
Query q = QueryFactory.create(queryString);
// Execute query and obtain result set
QueryExecution qexec = QueryExecutionFactory.create(q, dataset);
ResultSet resultSet = qexec.execSelect();

10. SPARQL query processing
// Print results
while(resultSet.hasNext()) {
// Each row contains two fields: ‘married’ and ‘spouse’,
// as defined in the query string
QuerySolution row = (QuerySolution)resultSet.next();
RDFNode nextMarried = row.get("married");
System.out.print(nextMarried.toString());
System.out.print(" is married to ");
RDFNode nextSpouse = row.get("spouse");
System.out.println(nextSpouse.toString());
}

11. ARQ Application API
http://jena.apache.org/documentation/query/app_api.html

12. Overview
•
•
•
•
Introduction to Jena
Setting up the environment
Querying Dbpedia
Other APIs

13. Setting up the environment
Download Netbeans Java EE version:
https://netbeans.org/downloads/

14. Downloading Jena
http://jena.apache.org

15. Download binary distribution
http://www.apache.org/dist/jena/

16. Getting started with Jena in Netbeans
Create a new Java project

17. Create a Java project

18. Add Jena libraries to class path

19. Add Jena libraries to class path

20. Add all jars in lib folder of Jena distribution

21. Add all jars in lib folder

22. Using Jena with Eclipse
• http://www.iandickinson.me.uk/articles/jena-eclipsehelloworld/

23. Tutorials
http://jena.apache.org/getting_started/

24. Overview
•
•
•
•
Introduction to Jena
Setting up the environment
Querying Dbpedia
Other APIs

25. QueryFactory
• has various create() methods to read a textual query
• these create() methods
• return a Query object,
• which encapsulates a parsed query.

26. QueryExecutionFactory
Create a QueryExecution that will access a SPARQL
service over HTTP
QueryExecutionFactory.sparqlService(String service,
Query query)

27. Querying Dbpedia
SPARQL endpoint
http://dbpedia.org/sparql

28. Example
String service = "http://dbpedia.org/sparql";
String query = "ASK { }";
QueryExecution qe = QueryExecutionFactory.sparqlService(service,
query);

29. Test connection
import com.hp.hpl.jena.query.QueryExecution;
import com.hp.hpl.jena.query.QueryExecutionFactory;
import com.hp.hpl.jena.sparql.engine.http.QueryExceptionHTTP;
public class QueryTest {
public static void main(String[] args) {
String service = "http://dbpedia.org/sparql";
String query = "ASK { }";
QueryExecution qe = QueryExecutionFactory.sparqlService(service, query);
try {
if (qe.execAsk()) {
System.out.println(service + " is UP");
} // end if
} catch (QueryExceptionHTTP e) {
System.out.println(service + " is DOWN");
} finally {
qe.close();
}
}
}

30. Example queries
http://wiki.dbpedia.org/OnlineAccess#h28-5

31. Example query: people who were born in Eindhoven
String service="http://dbpedia.org/sparql";
String query="PREFIX dbo:<http://dbpedia.org/ontology/>"
+ "PREFIX : <http://dbpedia.org/resource/>"
+ "select ?person where {?person dbo:birthPlace :Eindhoven.}";
QueryExecution qe=QueryExecutionFactory.sparqlService(service, query);
ResultSet rs=qe.execSelect();
while (rs.hasNext()){
QuerySolution s=rs.nextSolution();
System.out.println(s.getResource("?person").toString());
}
03/28/11

32. Processing results
QuerySolution soln = results.nextSolution() ;
RDFNode x = soln.get("varName") ; // Get a result variable by name.
Resource r = soln.getResource("VarR") ; // Get a result variable - must be a resource
Literal l = soln.getLiteral("VarL") ; // Get a result variable - must be a literal

33. Example
String service="http://dbpedia.org/sparql";
String query="PREFIX dbo:<http://dbpedia.org/ontology/>"
+ "PREFIX : <http://dbpedia.org/resource/>"
+ "PREFIX foaf:<http://xmlns.com/foaf/0.1/>"
+ "select ?person ?name where {?person dbo:birthPlace :Eindhoven."
+ "?person foaf:name ?name}";
QueryExecution qe=QueryExecutionFactory.sparqlService(service, query);
ResultSet rs=qe.execSelect();
while (rs.hasNext()){
QuerySolution s=rs.nextSolution();
Resource r=s.getResource("?person");
Literal name=s.getLiteral("?name");
System.out.println(s.getResource("?person").toString());
System.out.println(s.getLiteral("?name").getString());
}
03/28/11

34. Example query: people who were born in Berlin
before 1900
PREFIX dbo: http://dbpedia.org/ontology/
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX foaf: <http://xmlns.com/foaf/0.1/>
PREFIX : http://dbpedia.org/resource/
SELECT ?name ?birth ?death ?person WHERE {
?person dbo:birthPlace :Berlin .
?person dbo:birthDate ?birth .
?person foaf:name ?name .
?person dbo:deathDate ?death .
FILTER (?birth < "1900-01-01"^^xsd:date) .
}
ORDER BY ?name

35. Other APIs
PHP:
RAP
–
RDF
h+p://www.seasr.org/wp-­‐content/plugins/meandre/rdfapi-­‐php/doc/
Python:
RDFLib
h+p://www.rdflib.net/
C:
Redland
h+p://librdf.org/

36. Installing PHP
Mac OS:
https://netbeans.org/kb/docs/php/configure-phpenvironment-mac-os.html
Windows:
https://netbeans.org/kb/docs/php/configure-phpenvironment-windows.html

37. Create new PHP project

38. Install RAP
• Download at:
http://wifo5-03.informatik.uni-mannheim.de/bizer/rdfapi/
• Unpack the zip file.
• Include RDF API into your scripts:
• define("RDFAPI_INCLUDE_DIR", "C:/Apache/htdocs/rdf_api/
api/");
• include(RDFAPI_INCLUDE_DIR . "RDFAPI.php");
• Change the constant RDFAPI_INCLUDE_DIR to the
directory in which you have unpacked the zip file.

39. PHP RAP: example

40. k.verbert@tue.nl
n.v.stash@tue.nl
g.l.fletcher@tue.nl
03/28/11

41. Sources
• Konstantinos Tzonas. The Jena RDF Framework.