A strength of working with ontologies is that we can reuse work done by other ontologists by importing their published ontologies. Often, however, this means that the ontology is only retrievable by traversing an Internet firewall. Even if this is possible, by using an HTTP proxy, it may impose unacceptable delay in loading an application. In the worst case, an ontology on which our application depends is temporarily or permanently unavailable from the original published URL. To assist with this common problem, the ontology document manager allows a local indirection, so that an attempt to import a document from a given published URL means that a local copy of the document is retrieved instead. This may be a file copy or simply duplicated on an internal mirror web site.

To specify this local redirection in the policy file, we use the following declarations:

<OntologySpec>
  <!-- local version of the RDFS vocabulary -->
  <publicURI rdf:resource="http://www.w3.org/2000/01/rdf-schema"    />
  <altURL rdf:resource="file:vocabularies/rdf-schema.rdf" />
  <language rdf:resource="http://www.w3.org/2000/01/rdf-schema" />
  <prefix rdf:datatype="&xsd;string">rdfs</prefix>
</OntologySpec>This specifies that an attempt to load the RDFS vocabulary from http://www.w3.org/2000/01/rdf-schema will transparently cause file:vocabularies/rdf-schema.rdf to be fetched instead. Any number of such re-directions may be specified in the policy file, or they can be added to the document manager object directly by calling the various 'add' methods (see the Javadoc for details). As a side-effect, this mechanism also means that ontologies can be named with any legal URI (not necessarily resolvable) - so long as the altURL is itself resolvable.

Example
In the following example, we programmatically declare that the camera ontology is copied locally, and then load it using the public URL. Assume that the constant JENA has been initialized to the directory in which Jena is installed.

OntModel m = ModelFactory.createOntologyModel();
OntDocumentManager dm = m.getDocumentManager();
dm.addAltEntry( "http://www.xfront.com/owl/ontologies/camera/",
                "file:" + JENA + "src-examples/data/camera.owl"    );
m.read( "http://www.xfront.com/owl/ontologies/camera/" );