How to add a SERVICE clause to SelectBuilder using Apache Jena's query builder?

Question

I’m using the Jena query builder from the Jena-extras and I was wondering if there was a way to add a SERVICE clause to a SelectBuilder object.

My code right now looks like this:

SelectBuilder builder = new SelectBuilder()
        .addPrefix( "rdf",  "http://www.w3.org/1999/02/22-rdf-syntax-ns#" )
        .addPrefix("dbo", "http://dbpedia.org/ontology/")
        .addVar("?uri")
        .addWhere("?uri", "rdf:type", "dbo:Company")
        .setLimit(100);

Query query = builder.build();
System.out.println(query);

which outputs this:

PREFIX  dbo:  <http://dbpedia.org/ontology/>
PREFIX  rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

SELECT  ?uri
WHERE
  { ?uri  rdf:type  dbo:Company}
LIMIT   100

I’ve also tried adding the SERVICE clause to a Query object after calling builder.build(), but it seems like it’s only possible through the ARQ API using body.addElement(new ElementService("http://any.domain/sparql", elementBlock)). The problem is that you can’t get the elementBlock from a Query object or SelectBuilder object.

Is there a way to add a SERVICE clause using the query builder or are there known workarounds without resorting to the verbose ARQ API, or should I try and extend the SelectBuilder API myself?

Answer 1

For anyone having the same problem, I asked the devs who gave me a useful hint (thank you, Andy Seaborne), and the solution seems to be as follows:

SelectBuilder builder = new SelectBuilder()
        .addPrefix( "rdf",  "http://www.w3.org/1999/02/22-rdf-syntax-ns#" )
        .addPrefix("dbo", "http://dbpedia.org/ontology/")
        .addVar("?uri")
        .addWhere("?uri", "rdf:type", "dbo:Company")
        .setLimit(100);

Query query = builder.build();
ElementGroup body = new ElementGroup();
body.addElement(new ElementService("http://dbpedia.org/sparql", query.getQueryPattern()));
query.setQueryPattern(body);

System.out.println(query);

This code outputs the following query:

PREFIX  dbo:  <http://dbpedia.org/ontology/>
PREFIX  rdf:  <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

SELECT  ?uri
WHERE
  { SERVICE <http://dbpedia.org/sparql>
      { ?uri  rdf:type  dbo:Company}
  }
LIMIT   100

Too bad that this isn't a part of the query builder's fluent API out of the box, but any savvy Java engineer can write an extension or helper for this.