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:StatementR rdf:subject SR rdf:predicate PR rdf:object OWe 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.
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) R.as(ReifiedStatement.class)
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.)
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.
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.
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).
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)
Prior to version 2.10.0 of Jena, there were 3 styles of reification, "standard", "minimal" and "convenient". As of 2.10.0 and later, only what was previously the "standard" style is supported.
The old documentation is still available.