jena.query.ResultSet and jena.query.QuerySolution:

问题:

A have a problem with receiving SPARQL response. A problem is method ((ResultSet) response).hasNext() returns false despite response shouldn't be empty.

Request is:

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
PREFIX owl: <http://www.w3.org/2002/07/owl#> 
PREFIX xsd: <http://www.w3.org/2001/XMLSchema#> 
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> 
PREFIX ontology: <http://www.semanticweb.org/kseniia/ontologies/2013/1/untitled-ontology-12#> SELECT ?x 
WHERE {?x rdfs:subClassOf ontology:Visual}

This works correctly in Protege and returns 3 objects:

Location
Relation
Descriptive

Query was executed in jena such way:

Query query = QueryFactory.create(queryString);
QueryExecution qexec = QueryExecutionFactory.create(query, model);
Object response = qexec.execSelect();
qexec.close();
for ( ; ((ResultSet) response).hasNext(); ) {   // always false
   QuerySolution soln = ((ResultSet) response).nextSolution();
   // etc
}

Maybe I missed something?

回答1:

You are closing the execution with qexec.close, then iterating over the results. Except the results are closed by the qexec.close and no longer available.

Move the qexec.close to after the loop.

Improvement:

Object response ==> ResultSet response