Querying DBpedia with SPARQL and Jena

Question

I cannot understand how can I query DBpedia using Jena. In the tutorials like here(Listing 4) model is initialized as follows:

// Open the bloggers RDF graph from the filesystem
InputStream in = new FileInputStream(new File("bloggers.rdf"));

// Create an empty in-memory model and populate it from the graph
Model model = ModelFactory.createMemModelMaker().createModel();
model.read(in,null); // null base URI, since model URIs are absolute
in.close();

Let's say I want to write a query that will list churches in Paris. In SPARQL it will look like (taken from this mailing list message):

PREFIX p: <http://dbpedia.org/property/>
PREFIX dbpedia: <http://dbpedia.org/resource/>
PREFIX category: <http://dbpedia.org/resource/Category:>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX skos: <http://www.w3.org/2004/02/skos/core#>
PREFIX geo: <http://www.georss.org/georss/>

SELECT DISTINCT ?m ?n ?p ?d
WHERE {
 ?m rdfs:label ?n.
 ?m skos:subject ?c.
 ?c skos:broader category:Churches_in_Paris.
 ?m p:abstract ?d.
 ?m geo:point ?p
 FILTER ( lang(?n) = "fr" )
 FILTER ( lang(?d) = "fr" )
 }

How will this query look in Java? Particularly, I'm interested in how the model object is initialized.

Pentagon

Do you want to know how to run SPARQL queries in Jena, or a procedure that performs an equivalent query to that one? – Bingle

Yes. Actually I found an answer which I was looking for but I would look with pleasure to your answer. Thanks. – Pentagon

Answer 1

After browsing tons and tons of pages I found the answer. Perhaps I didn't ask the question clearly enough, but anyway below is the code that worked for me.

String queryString=
"PREFIX p: <http://dbpedia.org/property/>"+
"PREFIX dbpedia: <http://dbpedia.org/resource/>"+
"PREFIX category: <http://dbpedia.org/resource/Category:>"+
"PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>"+
"PREFIX skos: <http://www.w3.org/2004/02/skos/core#>"+
"PREFIX geo: <http://www.georss.org/georss/>"+

"SELECT DISTINCT ?m ?n ?p ?d"+
"WHERE {"+
" ?m rdfs:label ?n."+
" ?m skos:subject ?c."+
" ?c skos:broader category:Churches_in_Paris."+
" ?m p:abstract ?d."+
" ?m geo:point ?p"+
" FILTER ( lang(?n) = "fr" )"+
" FILTER ( lang(?d) = "fr" )"+
" }"

// now creating query object
Query query = QueryFactory.create(queryString);
// initializing queryExecution factory with remote service.
// **this actually was the main problem I couldn't figure out.**
QueryExecution qexec = QueryExecutionFactory.sparqlService("http://dbpedia.org/sparql", query);

//after it goes standard query execution and result processing which can
// be found in almost any Jena/SPARQL tutorial.
try {
    ResultSet results = qexec.execSelect();
    for (; results.hasNext();) {

    // Result processing is done here.
    }
}
finally {
   qexec.close();
}

This answer I found on dbpedia-discussion of www.mail-archive.com page.

Pentagon

The code in this answer isn't quite right, and contains some subtle bugs. String concatenation like "SELECT DISTINCT ?m ?n ?p ?d"+ "WHERE {" leads to "SELECT DISTINCT ... ?dWHERE {` with a variable named dWHERE (and this is still legal, since the WHERE in a SPARQL where is optional; i.e., select ?x { } and select ?x WHERE {} are the same). – Homage

And escapes are needed in the language tags, in " FILTER ( lang(?n) = "fr" )"+. But on top of that, language tags should be compared with langMatches: filter langMatches(lang(?n),'fr'). – Homage