我对使用 Java 代码的 SPARQL 端点有疑问。
特别是,我有这个 Java 类:
public class example {
public static void main(String[] args) {
String value = "http://dbpedia.org/resource/Fred_Guy";
example exam = example();
QueryExecution qe = exam.query(value);
ResultSet results = ResultSetFactory.copyResults( qe.execSelect() );
}
public QueryExecution query(String stringa){
ParameterizedSparqlString qs = new ParameterizedSparqlString( "" +
"prefix dbpediaont: <http://dbpedia.org/ontology/>\n" +
"prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n" +
"\n" +
"select ?resource where {\n" +
"?mat rdf:type ?resource\n" +
"filter strstarts(str(?resource), dbpediaont:)\n" +
"}" );
Resource risorsa = ResourceFactory.createResource(stringa);
qs.setParam( "mat", risorsa );
QueryExecution exec = QueryExecutionFactory.sparqlService( "http://dbpedia.org/sparql", qs.asQuery() );
ResultSet results = ResultSetFactory.copyResults( exec.execSelect() );
while ( results.hasNext() ) {
System.out.println( results.next().get( "resource" ));
}
// A simpler way of printing the results.
ResultSetFormatter.out( results );
return exec;
}
}
我想检索其谓词“RDF:type”的资源“ http://dbpedia.org/resource/Fred_Guy ”的对象。但我有这个我不明白的错误:
Exception in thread "main" HttpException: 500
at com.hp.hpl.jena.sparql.engine.http.HttpQuery.execGet(HttpQuery.java:340)
at com.hp.hpl.jena.sparql.engine.http.HttpQuery.exec(HttpQuery.java:276)
at com.hp.hpl.jena.sparql.engine.http.QueryEngineHTTP.execSelect(QueryEngineHTTP.java:345)
at MyPackage.example.main(example.java:19)
为什么我会收到此错误?
我正在尝试执行此查询
不写 strstarts 我得到这个错误:
Virtuoso 37000 Error SP031: SPARQL compiler: No one quad map pattern is suitable for GRAPH <http://dbpedia.org> { "http://dbpedia.org/resource/Fred_Guy" <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> ?resource } triple at line 7
SPARQL query:
define sql:big-data-const 0
#output-format:text/html
define sql:signal-void-variables 1 define input:default-graph-uri <http://dbpedia.org> prefix dbpediaont: <http://dbpedia.org/ontology/>
prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
select ?resource where {
"http://dbpedia.org/resource/Fred_Guy" rdf:type ?resource
}
我在这里做错了什么?
我尝试在 Virtuoso 中编写此代码:
prefix dbpediaont: <http://dbpedia.org/ontology/>
prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
select ?resource where {
dbpedia:Fred_Guy rdf:type ?resource
}
我如何用 Jena 代码编写它?