Inferring datatypes when declaring literals in Sesame

Question

I've just started a project where I'm using the Sesame libraries to do some SemWebbery. I was looking for a good way to declare a method that takes two URIs and an Object, returning a Statement with the object declared to be either a Resource or a properly-typed Literal:

public Statement makeStatement(URI subj, URI pred, Object obj) { ... }

The tricky part, of course, is figuring out whether that 'obj' is a valid URI or a valid type of Literal. It seems that the most straightforward approach is to do something like:

if (XMLDatatypeUtil.isValidBoolean(obj)) { 
   do something fancy with XMLDatatypeUtil.qnameToURI(obj);
}

However, that's a lot of typing. Does Sesame have something akin to Jena's TypeMapper class? Or maybe a method call that does some behind-the-scenes magic?

Accepted Answer

3

I think what you want is the ValueFactory:

 if ( <insert check to see that your object is in fact URI> ) {
    result = valueFactory.createURI(obj); 
 }
 else { // obj is a literal of some sort
    // create a datatyped literal for the suplied object.
    result = valueFactory.createLiteral(obj); 
 }

createLiteral is overloaded to accept several kinds of input object. See the javadoc for a list of all supported types.

link

answered 17 Dec '12, 23:53

Jeen Broekstra ♦
8.5k●3●12
accept rate: 32%

edited 17 Dec '12, 23:53

1

Valuefactory.createLiteral doesn't take an Object argument, so your code doesn't compile. If you do it this way you need to cast the obj to one of the overloaded types first, which requires a whole lot of additional "else if (obj instanceof ...)" code blocks

(18 Dec '12, 09:09) Gerrit V

1

@Gerrit V, good point, hadn't thought of that. Feature request logged. https://openrdf.atlassian.net/browse/SES-1671 . Should be trivial.

(18 Dec '12, 14:13) Jeen Broekstra ♦

Appreciate the feature request! For now, though, we have enough control over literal values coming into the system (ie nobody's passing a java.io.File object into that position) that I can just call result= ValueFactory.createLiteral(obj.toString()).

(21 Dec '12, 08:48) Ryan Kohl

Answer 2

Starting from Sesame-2.7.0 you can also solve the issue with the new DatatypeHandler interface, either extending the default XMLSchemaDatatypeHandler or replacing it with your own.

To implement the replacement you need to use ParserConfig to tell the Parser's and RepositoryConnection's what to use:

ParserConfig parserConfig = new ParserConfig();
parserConfig.set(BasicParserSettings.DATATYPE_HANDLERS, Arrays.asList(customDatatypeHandlers...))
// Normalisation is off by default
parserConfig.set(BasicParserSettings.NORMALIZE_DATATYPE_VALUES, true);
repositoryConnection.setParserConfig(parserConfig);
// or:
rdfParser.setParserConfig(parserConfig);

Inside of DatatypeHandler.normalizeLiteral you can create the Literal using any method you want, including not using the given ValueFactory if necessary.

By the way, Jeen's idea for ValueFactory.createLiteral(Object) was migrated to a new helper method as Literals.createLiteral(ValueFactory, Object) method in the final 2.7.0 release. Having a ValueFactory.createLiteral(Object) method may have hampered our ability to add to the interface in future. The Literals method will resort to using obj.toString if the datatype is unknown, but you can also using Literals.createLiteralOrFail if you wish to fail instead of using obj.toString as a backup.

Inferring datatypes when declaring literals in Sesame

Follow this question

Upcoming Events

Related questions