Question

I have written following code for extratcting URIs from a web page with content type application/rdf-xml for Linked Data application.

public static void test(String url) {
    try {
        Model read = ModelFactory.createDefaultModel().read(url);
        System.out.println("to go");
        StmtIterator si;
        si = read.listStatements();
        System.out.println("to go");
        while(si.hasNext()) {
            Statement s=si.nextStatement();
            Resource r=s.getSubject();
            Property p=s.getPredicate();
            RDFNode o=s.getObject();
            System.out.println(r.getURI());
            System.out.println(p.getURI());
            System.out.println(o.asResource().getURI());
        }
    }
    catch(JenaException | NoSuchElementException c) {}
}

But for the input

<?xml version="1.0"?>
<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:dc="http://purl.org/dc/elements/1.1/" xmlns:ex="http://example.org/stuff/1.0/">
    <rdf:Description rdf:about="http://www.w3.org/TR/rdf-syntax-grammar"
        dc:title="RDF/XML Syntax Specification (Revised)">
        <ex:editor>
            <rdf:Description ex:fullName="Dave Beckett">
                <ex:homePage rdf:resource="http://purl.org/net/dajobe/" />
            </rdf:Description>
        </ex:editor>
    </rdf:Description>
</rdf:RDF>

The output is :

Subject URI is http://www.w3.org/TR/rdf-syntax-grammar
Predicate  URI is http://example.org/stuff/1.0/editor
Object URI is null
Subject URI is http://www.w3.org/TR/rdf-syntax-grammar
Predicate  URI is http://purl.org/dc/elements/1.1/title
Website is read

I require in the output all the URIs present on that page to build a web crawler for RDF pages.
I require all following links in output:

       http://www.w3.org/TR/rdf-syntax-grammar
       http://example.org/stuff/1.0/editor
       http://purl.org/net/dajobe
       http://example.org/stuff/1.0/fullName
       http://www.w3.org/TR/rdf-syntax-grammar
       http://purl.org/dc/elements/1.1/title

Answer 1

Minor mistake: you mean application/rdf+xml (note the plus).

Anyway, your problem is very simple:

catch(JenaException | NoSuchElementException c) {}

Bad! You’re missing the error thrown here, and the output is being truncated:

System.out.println(o.asResource().getURI());

o isn’t always a resource, and this will break on the triple

<http://www.w3.org/TR/rdf-syntax-grammar> dc:title "RDF/XML Syntax ..."

so you need to guard against that:

if (o.isResource()) System.out.println(o.asResource().getURI());

or even more specific:

if (o.isURIResource()) System.out.println(o.asResource().getURI());

which will skip the null output you see for ex:editor.

Now write one thousand times I will not swallow exceptions 🙂