Jena - How to know whether a specific resource is in the model?

Question

I am trying to discover whether I had a specific resource in the model. For that I am using:

model.getResource("example")

Checking the doc, this method behaves exactly as createResource. Then, even if it is not in the model, I will get a new resource.

How can I check I have the resource avoiding its creation when it's not?

Thanks in advance!

Answer 1

In Jena, Resource objects by themselves are not in the model. The model only contains triples - Statement objects containing a subject, predicate and object (usually abbreviated SPO). Any one of S, P or O can be a resource (noting that a Property is a sub-type of Resource in Jena and in the RDF standard). So you need to refine your question from "does this model contain this resource" to either:

• does model M contain resource R as a subject?

• does model M contain resource R as a subject, predicate or object?

This can be achieved as:

Resource r = ... ;
Model m = ... ;

// does m contain r as a subject?
if (m.contains( r, null, (RDFNode) null )) {
  ..
}

// does m contain r as s, p or o?
if (m.containsResource( r )) {
  ..
}

Incidentally, in your code sample you have

model.getResource("example")

This returns a Resource object corresponding to the given URI, but does not side-effect the triples in the model. This is the reason that Model has both getResource and createResource - get is potentially slightly more efficient since it re-uses resource objects, but the semantics are essentially identical. However, the argument you pass to getResource or createResource should be a URI. You are borrowing trouble from the future if you start using tokens like "example" in place of full URI's, so I would advise stopping this bad habit before you get comfortable with it!