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!

Deice

maybe this link Interface Model is useful. – Blotch

After researching a little bit I have found the next way. I don't know if this is really the best way to achieve it, but works: Resource toSearch = ResourceFactory.createResource("example"); if(!model.containsResource(toSearch))...; – Deice

If it is solved then post an answer and accept it, so the next person with the same issue will find the answer, you also get a badge! :) – Frequency

@Boaz.Jan Thank you very much for your suggestion, but the system does not allow me to post now. I have to wait 8 hours. I'll do afterwards! :) – Deice

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!

December

Thank you very much. That's definitely the solution. About the bad habit, don't worry, in my development I'm using real URIs. I just used "example" because it didn't matter this time. – Deice

Checking the solution: there are several contains which fit with r, null, null. I would rather suggest: model.contains(resource, null, (RDFNode)null) – Deice

Good catch. Yes, it's a pain that the method signature for contains() is liberal enough to require a cast on the object argument, but it's a design decision that was made a long time ago in Jena's early history and would be too disruptive to change now. – December

This answer needs an update. getResource now behaves same as createResource: jena.apache.org/documentation/javadoc/jena/org/apache/jena/rdf/… – Horoscope

Answer 2

After researching a little bit I have found the next way. I don't know if this is really the best way to achieve it, but works:

Resource toSearch = ResourceFactory.createResource("example");
if(!model.containsResource(toSearch))...;

Deice

This is a right answer but I have to wait 2 days to mark it as right – Deice