Reification API support will be removed in Jena5.
Introduction
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
OntModel
s, 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) 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.)
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 whose elements are ReifiedStatement
s 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 reifyings
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
Statement
s 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)