Regarding SPARQL queries on DBPedia SPARQL end point in a loop - sparql

Question

while((reader=br.readLine())!=null)
{
System.out.println(reader);
type="";
String queryNew="select ?y ?z where {"+reader+" <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?z . }";
Query query=QueryFactory.create(queryNew);
ARQ.getContext().setTrue(ARQ.useSAX);
QueryExecution qexec = QueryExecutionFactory.sparqlService("http://dbpedia.org/sparql", query);
ResultSet results = qexec.execSelect();
while(results.hasNext())
{
QuerySolution soln=results.nextSolution();
type=type+" "+soln.get("?z");
System.out.println(soln.get("?y")+" "+soln.get("?z"));
}
bw.write(reader+" "+type+"\n");
}
From a file I am reading actor names and then trying to get all the rdf:type links for each actor. While the result prints for the first actor, the rest of the execution gets stuck. The 'reader' is nothing but an actor's name which is the dbpedia resource. Can anyone kindly tell me what might have gone wrong in the code?

Answer 1

You don't show your data, but assuming that the file contains simply the names of actors, as you say, then the query you're actually asking is something like:
select ?y ?z where {
"Matt Damon" rdf:type ?z
}
That's never going to return any results, because the subject of an RDF triple is always an RDF resource, not a string literal.
You should change your query to:
prefix dbpprop: <http://dbpedia.org/property/>
prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
select ?actor ?type where {
?actor dbpprop:name "Matt Damon"#en ;
rdf:type ?type
}
in other words "what are the rdf:types of the resource whose name (in English) is Matt Damon?"

Answer 2

As Ian Dickinson pointed out in his answer, you haven't shown us the data, nor what exactly the value of reader is, so it's hard to tell exactly what's going wrong. It does sound like you need to adjust your query to something along the lines of
?actor rdfs:label ?label
where ?label is the name of the actor that you're interested in. If you use string concatenation like you did in
String queryNew = "select ?y ?z where {" + reader + " <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?z . }";
you're opening yourself up to SPARQL injection attacks. E.g., what would happen if reader contains more SPARQL query text? (Also, because rdf:type is so commonly used, SPARQL actually allows you to abbreviate it with a, as in ?actor a dbpedia-owl:Person.) To avoid this sort of issue, Jena provides a ParameterizedSparqlString that will take care of proper escaping. Also note that RDF distinguishes between plain literals, e.g., "Richard Dreyfuss" and language tagged strings, e.g., "Richard Dreyfuss"#en. This means that you might want to include language tags in your query. Finally, note that there might well be more than one thing in DBpedia with a given label. Heres's code using a parameterized SPARQL string that finds resources with the label "Richard Dreyfuss"#en, along with all of their RDF types:
import com.hp.hpl.jena.query.ParameterizedSparqlString;
import com.hp.hpl.jena.query.QueryExecution;
import com.hp.hpl.jena.query.QueryExecutionFactory;
import com.hp.hpl.jena.query.ResultSet;
import com.hp.hpl.jena.query.ResultSetFormatter;
import com.hp.hpl.jena.rdf.model.Literal;
import com.hp.hpl.jena.rdf.model.ResourceFactory;
public class DBpediaQueryExample {
public static void main(String[] args) {
/*
* A typed literal: the name Richard Dreyfuss in English. In SPARQL
* this is written as "Richard Dreyfuss"#en.
*/
final Literal richardDreyfuss = ResourceFactory.createLangLiteral( "Richard Dreyfuss", "en" );
/*
* A parameterized SPARQL string for the query. Using this, along with
* the various setZZZ(...) methods can prevent some SPARQL injection
* attacks by doing proper escaping.
*/
final ParameterizedSparqlString queryString = new ParameterizedSparqlString(
"prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>\n" +
"\n" +
"select ?resource ?type where {\n" +
" ?resource a ?type ;\n" +
" rdfs:label ?label .\n" +
"}" );
/*
* Fill in "Richard Dreyfuss"#en for the ?label.
*/
queryString.setLiteral("label", richardDreyfuss );
/*
* Get a query execution that will run against the DBpedia SPARQL
* endpoint, and will use the parameterized query.
*/
final QueryExecution exec = QueryExecutionFactory.sparqlService(
"http://dbpedia.org/sparql",
queryString.asQuery() );
/*
* Execute the query to produce a result set, and format it nicely.
*/
final ResultSet results = exec.execSelect();
ResultSetFormatter.out( results );
}
}
-------------------------------------------------------------------------------------------------------------------------------------------------
| resource | type |
=================================================================================================================================================
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://www.w3.org/2002/07/owl#Thing> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/ActorsFromLosAngeles,California> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AmericanFilmActors> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AmericanTelevisionActors> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/ontology/Agent> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/ontology/Person> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://umbel.org/umbel/rc/Actor> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://schema.org/Person> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://xmlns.com/foaf/0.1/Person> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AmericanComedians> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Comedian109940146> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/JewishActors> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/PeopleFromBrooklyn> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Adult109605289> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Communicator109610660> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/LivingPeople> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Object100002684> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/PeopleWithBipolarDisorder> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Person100007846> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Whole100003553> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/YagoLegalActor> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Entertainer109616922> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AlternateHistoryWriters> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Actor109765278> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AmericanConscientiousObjectors> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/AmericanHistoricalNovelists> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/ConscientiousObjector109957013> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Dissenter110018021> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Novelist110363573> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Performer110415638> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Writer110794014> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/CausalAgent100007347> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/LivingThing100004258> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Organism100004475> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/JewishComedians> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/JewishPacifists> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/Pacifist110390199> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/PhysicalEntity100001930> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/YagoLegalActorGeo> |
| <http://dbpedia.org/resource/Richard_Dreyfuss> | <http://dbpedia.org/class/yago/PeopleFromQueens> |
| <http://sw.opencyc.org/2008/06/10/concept/en/RichardDreyfuss> | <http://sw.opencyc.org/2008/06/10/concept/en/ActorInMovies> |
| <http://sw.opencyc.org/2008/06/10/concept/en/RichardDreyfuss> | <http://sw.opencyc.org/2008/06/10/concept/en/MaleHuman> |
| <http://sw.opencyc.org/2008/06/10/concept/Mx4rwOREVpwpEbGdrcN5Y29ycA> | <http://sw.opencyc.org/2008/06/10/concept/Mx4rvVjWoZwpEbGdrcN5Y29ycA> |
| <http://sw.opencyc.org/2008/06/10/concept/Mx4rwOREVpwpEbGdrcN5Y29ycA> | <http://sw.opencyc.org/2008/06/10/concept/Mx4rwMRyTJwpEbGdrcN5Y29ycA> |
-------------------------------------------------------------------------------------------------------------------------------------------------