Reification HowTo


This document describes the Jena APi support for reification. it. As always, consult the Javadoc for interface details.

Reification in RDF and Jena is the ability to treat a Statement as a Resource, and hence to make assertions about that statement. A statement may be reified as many different resources, allowing different manifestations ("statings") of that statement to be treated differently if required.

RDF represents a reified statement as four statements with particular RDF properties and objects: the statement (S, P, O), reified by resource R, is represented by:

  • R rdf:type rdf:Statement
  • R rdf:subject S
  • R rdf:predicate P
  • R rdf:object O

We shall call these four such statements a reification quad and the components quadlets. Users of reification in Jena may, by default, simply manipulate reified statements as these quads. However, just as for Bag, Seq, Alt and RDF lists in ordinary models, or ontology classes and individuals in OntModels, Jena has additional support for manipulating reified statements.

The interface ReifiedStatement is used to represent a reified statement as a Jena Resource that has direct access to the statement it reifies. The method

  • ReifiedStatement::getStatement()

returns the Statement that the resource is reifying. All the other Resource methods, of course, may be applied to a ReifiedStatement.

Converting resources to reified statements

If a resource R is associated with a reified statement, but might not itself be a ReifiedStatement object, the conversion method RDFNode::as(Class) can be used to find (or create) a ReifiedStatement: - (ReifiedStatement)

For example, a model that has been read in from an RDF/XML file may have reified statements: knowing the name of the resource allows a ReifiedStatement object to be constructed without knowing the statement itself.

If there is no such associated reified statement, a CannotReifyException is thrown. To find out in advance if the conversion is possible, use the predicate RDFNode::canAs(ReifiedStatement.class). (Jena only counts as "an associated reified statement" a resource with exactly one rdf:subject, rdf:predicate, and rdf:object which has rdf:type rdf:Statement. It can of course have other properties.)

Testing statements for reification

You may wish to know if some Statement is reified. The methods Statement::isReified() and Model::isReified(Statement) return true if (and only if) the statement has been reified in the model. Note that the Statement method tests to see if the statement is reified in its own model, and the model method tests to see if the Statement is reified in that model; there is no test to see if a Statement is reified in any other models.

Listing reified statements

Just as listStatements is used to find the statements present in some model, there are methods for finding the reified statements of a model. Each of them returns a RSIterator object, which is an iterator each of who's elements are ReifiedStatements and for which the convenience method nextRS() will deliver a suitably-cast reified statement. - Statement::listReifiedStatements() - all the reifications of this statement in its model. - Model::listReifiedStatements() - all the reified statements in this model. - Model::listReifiedStatements(Statement s) - all the reified statements reifiying s in this model.

Creating reified statements directly

You do not have to create reified statements by asserting the quad into a Model; they can be created directly from their Statements using one of the methods: - Statement::createReifiedStatement() - Statement::createReifiedStatement(String) - Model::createReifiedStatement(Statement) - Model::createReifiedStatement(String,Statement)

Each of these returns a ReifiedStatement who's getStatement() method delivers the original statement (actually, a .equals() statement; it may not be the identical statement object). If the creation method passed in a (non-null) String, the ReifiedStatement is a named resource and that string is its URI. Otherwise it is a newly-minted bnode. The methods on Statement create a reified statement in that statements model; those on Model create a reified statement in that model.

It is not permitted for two different (non-equals) statements to be reified onto the same resource. An attempt to do so will generate an AlreadyReifiedException.

The additional method Model::getAnyReifiedStatement(Statement) returns some reification of the supplied Statement; an existing one if possible, otherwise a fresh one (reified by a fresh bnode).

Removing reified statements

There are two methods which remove all the reifications of a Statement in some Model: - Statement::removeReification() - Model::removeAllReifications(Statement)

All the reified statements in the model that reify the given statement are removed, whatever their reifying resource. To remove a particular reified statement only, use - Model::removeReification(ReifiedStatement)

Reification styles

Prior to version 2.10.0 of Jena, there were 3 styles of reificiation, "standard", "minimal" and "convenient". As of 2.10.0 and later, only what was prviosuly the "standard" style is supported.

The old documentation is still available.