There is a library of classes to access a remote Joseki server in the package
com.hp.hpl.jena.joseki. There are classes to perform
queries and
fetch RDF data objects, both
at the protocol level and within the RDQL query framework of
Jena, and classes
to perform the non-query operations like "ping", "options" as well as modify remote
RDF graphs by adding and removing RDF graphs from remote models. For operations to succeed, the
remote server must allow such operations on the target model.
There are also libraries for performing queries for Python and Perl.
See Python/ and Perl/.
Query languages provided in the Joseki distribution are:
See the "Query Languages" page for full details about these query languages.
The protocol used for access remote RDF is a simple use of HTTP GET which returns the smallest subgraph that matches the query (i.e. it gives the same results as the query on the original graph). Other operations use HTTP POST.
There is also a client library of Python, which issues queries to a Joseki server and uses the RDFLib toolkit for client-side handling of the results.
Access to the fetch functionality on RDF data objects is provided by the class HttpFetch. The "fetch" query language specifies the target model and the resource. A "fetch" request returns all the RDF associated with the resource known to the model. The exact model of RDF statements is determined by the server configuration.
HttpFetch fetch = new HttpFetch(modelURI, resourceURI) ; Model obj = fetch.exec() ;
Because fetching data objects is a query, requests use HTTP GET with all the usual caching and bookmarking.
The primary class is QueryEngineHTTP, which implements the standard Jena QueryExecution
interface for variable bindings. Remote queries are executed in the same manner
as local queries, except that a different query execution object is used.
The following code fragments show using the client API in Java. See the Jena
javadoc for more detailed documentation
on the general query classes and the Joseki javadoc for
QueryEngineHTTP.
Query q = new Query(queryString) ;
QueryExecution qe = new QueryEngineHTTP(q, modelURI) ;
QueryResults results = qe.exec() ;
for ( Iterator iter = results ; iter.hasNext() ; )
{
ResultBinding rbind = (ResultBinding)iter.next() ;
// Assuming the query has a variable called "x" ...
Object obj = rbind.get("x") ;
// obj will be a Jena object: resource, property or RDFNode.
System.out.println("x = "+obj.toString() ;
}
results.close() ;
The class HttpQuery provides a wrapper to HTTP GET and is used
by HttpFetch and QueryEngineHTTP.
See the javadoc for the package
com.hp.hpl.jena.joseki.