/**
* Query SPARQL endpoint with a SELECT query
*
* @param qExec QueryExecution encapsulating the query
* @return model retrieved by querying the endpoint
*/
private Model getSelectModel(QueryExecution qExec) {
Model model = ModelFactory.createDefaultModel();
Graph graph = model.getGraph();
ResultSet results = qExec.execSelect();
while (results.hasNext()) {
QuerySolution sol = results.next();
String subject;
String predicate;
RDFNode object;
try {
subject = sol.getResource("s").toString();
predicate = sol.getResource("p").toString();
object = sol.get("o");
} catch (NoSuchElementException e) {
logger.error("SELECT query does not return a (?s ?p ?o) Triple");
continue;
}
Node objNode;
if (object.isLiteral()) {
Literal obj = object.asLiteral();
objNode = NodeFactory.createLiteral(obj.getString(), obj.getDatatype());
} else {
objNode = NodeFactory.createLiteral(object.toString());
}
graph.add(
new Triple(NodeFactory.createURI(subject), NodeFactory.createURI(predicate), objNode));
}
return model;
}
/**
* Returns the string value of the first of the properties in the uriDescriptionList for the given
* resource (as an URI). In case the resource does not have any of the properties mentioned, its
* URI is returned. The value is obtained by querying the endpoint and the endpoint is queried
* repeatedly until it gives a response (value or the lack of it)
*
* <p>It is highly recommended that the list contains properties like labels or titles, with test
* values.
*
* @param uri - the URI for which a label is required
* @return a String value, either a label for the parameter or its value if no label is obtained
* from the endpoint
*/
private String getLabelForUri(String uri) {
String result;
if (uriLabelCache.containsKey(uri)) {
return uriLabelCache.get(uri);
}
for (String prop : uriDescriptionList) {
String innerQuery = "SELECT ?r WHERE {<" + uri + "> <" + prop + "> ?r } LIMIT 1";
try {
Query query = QueryFactory.create(innerQuery);
QueryExecution qExec = QueryExecutionFactory.sparqlService(rdfEndpoint, query);
boolean keepTrying = true;
while (keepTrying) {
keepTrying = false;
try {
ResultSet results = qExec.execSelect();
if (results.hasNext()) {
QuerySolution sol = results.nextSolution();
result = EEASettings.parseForJson(sol.getLiteral("r").getLexicalForm());
if (!result.isEmpty()) {
uriLabelCache.put(uri, result);
return result;
}
}
} catch (Exception e) {
keepTrying = true;
logger.warn("Could not get label for uri {}. Retrying.", uri);
} finally {
qExec.close();
}
}
} catch (QueryParseException qpe) {
logger.error("Exception for query {}. The label cannot be obtained", innerQuery);
}
}
return uri;
}
/**
* Get a set of unique queryObjName returned from a select query
*
* <p>Used to retrieve sets of modified objects used in sync
*
* @param rdfQuery query to execute
* @param queryObjName name of the object returned
* @return set of values for queryObjectName in the rdfQuery result
*/
HashSet<String> executeSyncQuery(String rdfQuery, String queryObjName) {
HashSet<String> rdfUrls = new HashSet<String>();
Query query;
try {
query = QueryFactory.create(rdfQuery);
} catch (QueryParseException qpe) {
logger.warn(
"Could not parse [{}]. Please provide a relevant query. {}",
rdfQuery,
qpe.getLocalizedMessage());
return null;
}
QueryExecution qExec = QueryExecutionFactory.sparqlService(rdfEndpoint, query);
try {
ResultSet results = qExec.execSelect();
while (results.hasNext()) {
QuerySolution sol = results.nextSolution();
try {
String value = sol.getResource(queryObjName).toString();
rdfUrls.add(value);
} catch (NoSuchElementException e) {
logger.error("Encountered a NoSuchElementException: " + e.getLocalizedMessage());
return null;
}
}
} catch (Exception e) {
logger.error(
"Encountered a [{}] while querying the endpoint for sync", e.getLocalizedMessage());
return null;
} finally {
qExec.close();
}
return rdfUrls;
}