Question

我在接收SPARQL回复时遇到问题。问题是....当我使用以下源代码时，rs.hasNext()总是返回false，即使响应不应为空。

SPARQL查询：

PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#> 
SELECT ?s ?sl <http://dbpedia.org/property/name> ?o ?ol
FROM <http://en.dbpedia.org/20120601/>
WHERE {
    ?s rdfs:label ?sl .
    ?s <http://dbpedia.org/property/name> ?o .
    ?o rdfs:label ?ol .
FILTER (   lang(?sl)= "en"  &&  lang(?ol)= "en"  ) }
LIMIT 100

此查询在DBPedia Endpoint中正常运行。

此查询是使用Jena执行的，如下所示。

Soucre Code：

...

            QueryEngineHTTP qexec = new QueryEngineHTTP("http://dbpedia.org/sparql", query));
            // "query" is as above
            qexec.addDefaultGraph(http://en.dbpedia.org/20120601/);
            List<QuerySolution> resultSetList = new ArrayList<QuerySolution>();

            ResultSet rs = qexec.execSelect();

            while (rs.hasNext())
                resultSetList.add(rs.next());

            if (!resultSetList.isEmpty()) {
                if (query.contains("?o rdfs:label ?ol")) {
                    func...1
                    func...2
                }
                else {
                    func...3
                    func...4
                }
            }
            else {
                qexec.close();
                break;
            }

...

SPARQL Endpoint没有问题，因为我可以从SPARQL Query获得结果。但是，我无法使用Jena获得任何结果。正如我所提到的，rs.hasNext()总是返回false，即使回复不应该是empty。什么是rs.hasNext()的解决方案不起作用，并且无法将内容添加到resultSetList？

我认为使用Jena的方式很好......有什么问题吗？

Answer 1

您没有提供完整的和最小示例，因此很难确切地看到代码中出现了什么问题。你的查询甚至不用Jena解析，所以我不确定为什么你不是简单地得到一个错误，而不是一个空的结果集。以下是使用查询的最小示例：

import com.hp.hpl.jena.query.QueryExecutionFactory;
import com.hp.hpl.jena.query.ResultSet;

public class DBpediaExample {
    public static void main(String[] args) {
        String query = "" +
                "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n" +
                "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>\n" +
                "SELECT ?s ?sl <http://dbpedia.org/property/name> ?o ?ol\n" +
                "FROM <http://en.dbpedia.org/20120601/>\n" +
                "WHERE {\n" +
                "  ?s rdfs:label ?sl .\n" +
                "  ?s <http://dbpedia.org/property/name> ?o .\n" +
                "  ?o rdfs:label ?ol .\n" +
                "  FILTER (   lang(?sl)= \"en\"  &&  lang(?ol)= \"en\"  )\n" +
                "}\n" +
                "LIMIT 100";
        ResultSet rs = QueryExecutionFactory.sparqlService( "http://dbpedia.org/sparql", query ).execSelect();

        while ( rs.hasNext() ) {
            System.out.println( rs.next() );
        }
    }
}

Exception in thread "main" com.hp.hpl.jena.query.QueryParseException: Encountered " <IRIref> "<http://dbpedia.org/property/name> "" at line 3, column 15.
Was expecting one of:
    <VAR1> ...
    <VAR2> ...
    "from" ...
    "where" ...
    "(" ...
    "{" ...

    at com.hp.hpl.jena.sparql.lang.ParserSPARQL11.perform(ParserSPARQL11.java:102)
    at com.hp.hpl.jena.sparql.lang.ParserSPARQL11.parse$(ParserSPARQL11.java:53)
    at com.hp.hpl.jena.sparql.lang.SPARQLParser.parse(SPARQLParser.java:37)
    at com.hp.hpl.jena.query.QueryFactory.parse(QueryFactory.java:139)
    at com.hp.hpl.jena.query.QueryFactory.create(QueryFactory.java:79)
    at com.hp.hpl.jena.query.QueryFactory.create(QueryFactory.java:52)
    at com.hp.hpl.jena.query.QueryFactory.create(QueryFactory.java:40)
    at com.hp.hpl.jena.query.QueryExecutionFactory.sparqlService(QueryExecutionFactory.java:358)
    at com.hp.hpl.jena.query.QueryExecutionFactory.sparqlService(QueryExecutionFactory.java:345)
    at DBpediaExample.main(DBpediaExample.java:19)

即使Virtuoso接受，您也不能在投影变量中使用URI引用。这不是合法的SPARQL，您可以使用sparql.org's query validator进行检查。

我不确定您在查询中使用FROM的原因，但如果您实际将IRI放入公共端点的“默认数据集名称（图形IRI）”字段，则不要得到任何结果。也许您想要查询标准数据集？

作为一个额外的问题，您应该使用langMatches来比较语言标记。

如果你解决了这些问题，你最终会得到以下查询和代码，这会显示出很多结果。

import com.hp.hpl.jena.query.QueryExecutionFactory;
import com.hp.hpl.jena.query.ResultSet;

public class DBpediaExample {
    public static void main(String[] args) {
        String query = "" +
                "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>\n" +
                "PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>\n" +
                "SELECT ?s ?sl (<http://dbpedia.org/property/name> as ?p) ?o ?ol\n" +
                "WHERE {\n" +
                "  ?s rdfs:label ?sl .\n" +
                "  ?s <http://dbpedia.org/property/name> ?o .\n" +
                "  ?o rdfs:label ?ol .\n" +
                "  FILTER ( langMatches(lang(?sl),'en') && langMatches(lang(?ol),'en') )\n" +
                "}\n" +
                "LIMIT 100";
        ResultSet rs = QueryExecutionFactory.sparqlService( "http://dbpedia.org/sparql", query ).execSelect();

        while ( rs.hasNext() ) {
            System.out.println( rs.next() );
        }
    }
}

( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1990–91 NOFV-Oberliga"@en ) ( ?s = <http://dbpedia.org/resource/1990%E2%80%9391_NOFV-Oberliga> ) ( ?o = <http://dbpedia.org/resource/Jens_Dowe> ) ( ?ol = "Jens Dowe"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1. FC Heidenheim"@en ) ( ?s = <http://dbpedia.org/resource/1._FC_Heidenheim> ) ( ?o = <http://dbpedia.org/resource/Kevin_Kraus> ) ( ?ol = "Kevin Kraus"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1905–06 FC Barcelona season"@en ) ( ?s = <http://dbpedia.org/resource/1905%E2%80%9306_FC_Barcelona_season> ) ( ?o = <http://dbpedia.org/resource/Carles_Comamala> ) ( ?ol = "Carles Comamala"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1910–11 FC Barcelona season"@en ) ( ?s = <http://dbpedia.org/resource/1910%E2%80%9311_FC_Barcelona_season> ) ( ?o = <http://dbpedia.org/resource/Carles_Comamala> ) ( ?ol = "Carles Comamala"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1910–11 FC Barcelona season"@en ) ( ?s = <http://dbpedia.org/resource/1910%E2%80%9311_FC_Barcelona_season> ) ( ?o = <http://dbpedia.org/resource/Francisco_Bru> ) ( ?ol = "Francisco Bru"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1. FC Heidenheim"@en ) ( ?s = <http://dbpedia.org/resource/1._FC_Heidenheim> ) ( ?o = <http://dbpedia.org/resource/Michael_Thurk> ) ( ?ol = "Michael Thurk"@en )
( ?p = <http://dbpedia.org/property/name> ) ( ?sl = "1990–91 NOFV-Oberliga"@en ) ( ?s = <http://dbpedia.org/resource/1990%E2%80%9391_NOFV-Oberliga> ) ( ?o = <http://dbpedia.org/resource/Hilmar_Weilandt> ) ( ?ol = "Hilmar Weilandt"@en )
…

jena.query.ResultSet和jena.query.QuerySolution：hasNext（）在SPARQL请求后返回false

1 个答案: