Problem with using SERVICE for Federated Query

Hi,

I'm using JENA TDB Joseki in my work. I need to query multiple datasets and I know that the SPARQL 1.1 allow it, but when I submit any query nothing is returned. Someone can help me please.

I read all FAQs about federated query and none is the same problem.

This is my code:

/ * To change this template, choose Tools | Templates * and open the template in the editor. / package src;

import com.hp.hpl.jena.query.Query; import com.hp.hpl.jena.query.QueryExecution; import com.hp.hpl.jena.query.QueryExecutionFactory; import com.hp.hpl.jena.query.QueryFactory; import com.hp.hpl.jena.query.ResultSet; import com.hp.hpl.jena.query.ResultSetFormatter;

/ * @author nusa / public class FederatedQuery {

    final static String esquemaMediacao = "http://127.0.0.1:2020/esquemamediacao";

    public static void main(String[] args) {

          String queryString =  
                    "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> "  
                    "PREFIX akt: <http://www.aktors.org/ontology/portal#> "  
                    "SELECT * WHERE {  "  
                    "SERVICE <http://127.0.0.1:2020/dblp> {?s ?p ?o}" 
                    "}" 
                    "LIMIT 10";

            //executando a consulta em cada um dos SPARQL endpoints:

    Query query = QueryFactory.create(queryString);  // exception happens here
    QueryExecution qexec = QueryExecutionFactory.sparqlService(esquemaMediacao,query);

try {
    ResultSet rs = qexec.execSelect();
    if ( rs.hasNext() ) {
        // show the result, more can be done here
        System.out.println(ResultSetFormatter.asText(rs));
    }
} 
catch(Exception e) { 
    System.out.println(e.getMessage());
}
finally {
    qexec.close();
}
System.out.println("\nall done.");

} }