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javasparqlrdfsemantic-webdbpedia

SPARQL Construct query not working on java

I would like to get all resource/object pairs from DBPedia related to Jupiter and construct query that output results in RDF format in java by means of Apache Jena. My construct query works in http://dbpedia.org/sparql service which is as follows:

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX owl: <http://www.w3.org/2002/07/owl#>


 construct {<http://dbpedia.org/resource/Jupiter>  ?o ?r. }  
 {{ <http://dbpedia.org/resource/Jupiter>  ?o ?r }
  UNION
 {?o ?r <http://dbpedia.org/resource/Jupiter>}}

I wrote a java program to write the CONSTRUCT query results in Jupiter.rdf file in RDF format, but it showed exception. My java code is as follows:

import java.io.BufferedWriter;
import java.io.FileWriter;
import java.io.IOException;
import org.apache.jena.query.ParameterizedSparqlString;
import org.apache.jena.query.QueryExecution;
import org.apache.jena.query.QueryExecutionFactory;
import org.apache.jena.query.QuerySolution;
import org.apache.jena.query.ResultSetFactory;
import org.apache.jena.query.ResultSetRewindable;

public class Jupiter {

    public static void main(String[] args) throws IOException {

        ParameterizedSparqlString querystring = new ParameterizedSparqlString(""
                + "construct {<http://dbpedia.org/resource/Jupiter>  ?o ?r }  where {{ <http://dbpedia.org/resource/Jupiter>  ?o ?r } UNION"
                + " {?r ?o <http://dbpedia.org/resource/Jupiter>}}");

        QueryExecution exec = QueryExecutionFactory.sparqlService("http://dbpedia.org/sparql", querystring.asQuery());

        ResultSetRewindable result = ResultSetFactory.makeRewindable(exec.execSelect());

        try (BufferedWriter bw = new BufferedWriter(new FileWriter("Jupiter.rdf"))) 

        {

            while (result.hasNext()) {

                QuerySolution querysolution = result.next();
                bw.write(querysolution.get("o").toString());
                bw.write(querysolution.get("r").toString());

            }
            bw.flush();
            bw.close();

        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

And the exception is thus:

Exception in thread "main" org.apache.jena.sparql.resultset.ResultSetException: Not a string: key: value
    at org.apache.jena.sparql.resultset.JSONInput.stringOrNull(JSONInput.java:230)
    at org.apache.jena.sparql.resultset.JSONInput.parseOneTerm(JSONInput.java:201)
    at org.apache.jena.sparql.resultset.JSONInput.parse(JSONInput.java:172)
    at org.apache.jena.sparql.resultset.JSONInput.process(JSONInput.java:100)
    at org.apache.jena.sparql.resultset.JSONInput.fromJSON(JSONInput.java:63)
    at org.apache.jena.query.ResultSetFactory.fromJSON(ResultSetFactory.java:331)
    at org.apache.jena.sparql.engine.http.QueryEngineHTTP.execResultSetInner(QueryEngineHTTP.java:385)
    at org.apache.jena.sparql.engine.http.QueryEngineHTTP.execSelect(QueryEngineHTTP.java:351)

Could you tell me why this happens,please? Your help is greatly appreciated

Solution

  • You have a SPARQL CONSTRUCT query, not a SELECT query. Thus, you have to call exec.execConstruct() and you'll get a Model object back, which contains triples instead of a ResultSet.