The application API is in the package com.hp.hpl.jena.query
.
Other packages contain various parts of the system (execution engine, parsers, testing etc). Most applications will only need to use the main package. Only applications wishing to programmatically build queries or modify the behaviour of the query engine need to use the others packages directly.
The package com.hp.hpl.jena.query
is the main application
package.
Query
- a class that represents the application query. It is
a container for all the details of the query. Objects of class
Query are normally created by calling one of the methods of
QueryFactory
methods which provide access to the various parsers.QueryExecution
- represents one execution of a query.QueryExecutionFactory
- a place to get QueryExecution
instancesDatasetFactory
- a place to make datasets, including making a
DataSource
(an updatable Dataset
)QuerySolution
- A single solution to the queryResultSet
- All the QuerySolutions. An iterator.ResultSetFormatter
- turn a ResultSet into various forms;
into text, into an RDF graph (Model, in Jena terminology) or as
plain XMLThe basic steps in making a SELECT query are outlined in the
example below. A query is created from a string using the
QueryFactory
. The query and model or RDF dataset to be queried
are then passed to QueryExecutionFactory
to produce an instance
of a query execution. Result are handled in a loop and finally the
query execution is closed.
import com.hp.hpl.jena.query.* ; Model model = ... ; String queryString = " .... " ; Query query = QueryFactory.create(queryString) ; QueryExecution qexec = QueryExecutionFactory.create(query, model) ; try { ResultSet results = qexec.execSelect() ; for ( ; results.hasNext() ; ) { QuerySolution soln = results.nextSolution() ; RDFNode x = soln.get("varName") ; // Get a result variable by name. Resource r = soln.getResource("VarR") ; // Get a result variable - must be a resource Literal l = soln.getLiteral("VarL") ; // Get a result variable - must be a literal } } finally { qexec.close() ; }
It is important to cleanly close the query execution when finished. System resources connected to persistent storage may need to be released.
A ResultSetARQ supports the Java iterator interface so the following is also a way to process the results if preferred:
Iterator<QuerySolution> results = qexec.execSelect() ; for ( ; results.hasNext() ; ) { QuerySolution soln = results.next() ; . . . }
The step of creating a query and then a query execution can be reduced to one step in some common cases:
import com.hp.hpl.jena.query.* ; Model model = ... ; String queryString = " .... " ; QueryExecution qexec = QueryExecutionFactory.create(queryString, model) ; try { ResultSet results = qexec.execSelect() ; . . . } finally { qexec.close() ; }
Instead of a loop to deal with each row in the result set, the application can call an operation of the ResultSetFormatter. This is what the command line applications do.
Example: processing results to produce a simple text presentation:
ResultSetFormatter fmt = new ResultSetFormatter(results, query) ; fmt.printAll(System.out) ;
or simply:
ResultSetFormatter.out(System.out, results, query) ;
The results are objects from the Jena RDF API and API calls, which do not modify the model, can be mixed with query results processing:
for ( ; results.hasNext() ; ) { // Access variables: soln.get("x") ; RDFNode n = soln.get("x") ; // "x" is a variable in the query // If you need to test the thing returned if ( n.isLiteral() ) ((Literal)n).getLexicalForm() ; if ( n.isResource() ) { Resource r = (Resource)n ; if ( ! r.isAnon() ) { ... r.getURI() ... } } }
Updates to the model must be carried out after the query execution has finished. Typically, this involves collecting results of interest in a local datastructure and looping over that structure after the query execution has finished and been closed.
CONSTRUCT
queries return a single RDF graph. As usual, the query
execution should be closed after use.
Query query = QueryFactory.create(queryString) ; QueryExecution qexec = QueryExecutionFactory.create(query, model) ; Model resultModel = qexec.execConstruct() ; qexec.close() ;
DESCRIBE
queries return a single RDF graph.
Different handlers for the
DESCRIBE
operation can be loaded by added by the application.
Query query = QueryFactory.create(queryString) ; QueryExecution qexec = QueryExecutionFactory.create(query, model) ; Model resultModel = qexec.execDescribe() ; qexec.close() ;
The operation Query.execAsk() returns a boolean value indicating whether the query pattern matched the graph or dataset or not.
Query query = QueryFactory.create(queryString) ; QueryExecution qexec = QueryExecutionFactory.create(query, model) ; boolean result = qexec.execAsk() ; qexec.close() ;
The ResultSetFormatter
class has methods to write out the
SPARQL Query Results XML Format.
See ResultSetFormatter.outputAsXML method.
The examples above are all queries on a single model. A SPARQL
query is made on a dataset, which is a default graph and zero or
more named graphs. Datasets can be constructed using the
DatasetFactory
:
String dftGraphURI = "file:default-graph.ttl" ; List namedGraphURIs = new ArrayList() ; namedGraphURIs.add("file:named-1.ttl") ; namedGraphURIs.add("file:named-2.ttl") ; Query query = QueryFactory.create(queryString) ; Dataset dataset = DatasetFactory.create(dftGraphURI, namedGraphURIs) ; QueryExecution qExec = QueryExecutionFactory.create(query, dataset) ; try { ... } finally { qExec.close() ; }
Already existing models can also be used: A DataSource
is an
updatable dataset:
DataSource dataSource = DatsetFactory.create() ; dataSource.setDefaultModel(model) ; dataSource.addNamedModel("http://example/named-1", modelX) ; dataSource.addNamedModel("http://example/named-2", modelY) ; QueryExecution qExec = QueryExecutionFactory.create(query, dataSource) ;