我需要将从查询中获取的Jena com.hp.hpl.jena.query.ResultSet
流式传输到转换为RDF输出格式的远程端点。
我知道Jena为此提供ResultSetFormatter.toModel
设施,但是我有以下限制:
Model
并用ResultSet
填充它,但是在我迭代时流出三元组来控制内存消耗。我见过StreamRDF
界面,但我不清楚如何有效地使用它。
在这种情况下,什么是正确的方法?
答案 0 :(得分:0)
我实施的解决方案对我有用,所以我将其作为答案发布。
下面的代码段应该足够提供信息,因为它涵盖了问题中的要求。值得注意的是,我创建了两个类,一个ResultSetTripleIterator
和一个QuerySolutionToTripleAdapter
接口。第一个是负责向作者传输三元组,而第二个是从每个QuerySolution
构建一个三元组的迭代器。
public class ResultSetTripleIterator implements Iterator<Triple> {
private ResultSet rs;
private QuerySolutionToTripleAdapter ad;
private Iterator<Triple> it = null;
public ResultSetTripleIterator(ResultSet resultSet, QuerySolutionToTripleAdapter adapter) {
this.rs = resultSet;
this.ad = adapter;
}
@Override
public boolean hasNext() {
if(it != null && it.hasNext()){
return true;
}
it = null;
return rs.hasNext();
}
@Override
public Triple next() {
if(it == null){
it = ad.adapt(rs.next());
}
return it.next();
}
}
public interface QuerySolutionToTripleAdapter {
public Iterator<Triple> adapt(QuerySolution qs);
}
以下是应用程序示例:
// Can be any OutputStream
OutputStream os = new ByteArrayOutputStream();
StreamRDF stream = StreamRDFWriter.getWriterStream(os, Lang.TRIG);
QueryExecution qe = QueryExecutionFactory.sparqlService(
"http://data.open.ac.uk/sparql", "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> SELECT ?A ?B ?C WHERE {?A a ?B . ?A rdf:type ?C} LIMIT 100");
Iterator<Triple> iter = new ResultSetTripleIterator(qe.execSelect(), new QuerySolutionToTripleAdapter() {
Integer rowIndex = 0;
@Override
public Iterator<Triple> adapt(QuerySolution qs) {
rowIndex++;
String ns = "http://www.example.org/test/row#";
String pns = "http://www.example.org/test/col#";
Resource subject = ResourceFactory.createResource(ns + Integer.toString(rowIndex));
Property property;
List<Triple> list = new ArrayList<Triple>();
Iterator<String> cn = qs.varNames();
while (cn.hasNext()) {
String c = cn.next();
property = ResourceFactory.createProperty(pns + c);
list.add(new Triple(subject.asNode(), property.asNode(), qs.get(c).asNode()));
}
return list.iterator();
}
});
stream.start();
StreamOps.sendTriplesToStream(iter, stream);
stream.finish();
但是,Jena似乎不支持某些用于流式传输的RDF序列化,即XML和JSON格式,例如导致org.apache.jena.riot.RiotException: No serialization for language Lang:rdf/null
。
修改强>
感谢Jena用户邮件列表中的反馈,可以使用Jena的实用程序类来操作迭代器和执行转换来压缩上面的代码。
该片段可以按如下方式重写:
OutputStream os = new ByteArrayOutputStream();
StreamRDF stream = StreamRDFWriter.getWriterStream(os, Lang.RDFTHRIFT);
QueryExecution qe = QueryExecutionFactory.sparqlService(
"http://data.open.ac.uk/sparql", "PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> SELECT ?A ?B ?C WHERE {?A a ?B . ?A rdf:type ?C} LIMIT 100");
Transform<QuerySolution, Iterator<Triple>> m = new Transform<QuerySolution, Iterator<Triple>>() {
Integer rowIndex = 0;
@Override
public Iterator<Triple> convert(QuerySolution qs) {
rowIndex++;
String ns = "http://www.example.org/test/row#";
String pns = "http://www.example.org/test/col#";
Resource subject = ResourceFactory.createResource(ns + Integer.toString(rowIndex));
Property property;
List<Triple> list = new ArrayList<Triple>();
Iterator<String> cn = qs.varNames();
while (cn.hasNext()) {
String c = cn.next();
property = ResourceFactory.createProperty(pns + c);
list.add(new Triple(subject.asNode(), property.asNode(), qs.get(c).asNode()));
}
return list.iterator();
}
};
Iterator<Triple> iter = WrappedIterator.createIteratorIterator( Iter.map( qe.execSelect(), m ));
stream.start();
StreamOps.sendTriplesToStream(iter, stream);
stream.finish();