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| <h1 class="title">Reification HowTo</h1> |
| |
| <h2 id="introduction">Introduction</h2> |
| <p>This document describes the Jena API support for reification. |
| it. As always, consult the Javadoc for interface |
| details.</p> |
| <p>Reification in RDF and Jena is the ability to treat a <code>Statement</code> |
| as a <code>Resource</code>, and hence to make assertions <em>about</em> that |
| statement. A statement may be reified as many different resources, |
| allowing different manifestations (“statings”) of that statement to |
| be treated differently if required.</p> |
| <p>RDF represents a reified statement as four statements with |
| particular RDF properties and objects: the statement <code>(S, P, O)</code>, |
| reified by resource <code>R</code>, is represented by:</p> |
| <ul> |
| <li><code>R rdf:type rdf:Statement</code></li> |
| <li><code>R rdf:subject S</code></li> |
| <li><code>R rdf:predicate P</code></li> |
| <li><code>R rdf:object O</code></li> |
| </ul> |
| <p>We shall call these four such statements a <em>reification quad</em> and |
| the components <em>quadlets</em>. Users of reification in Jena may, by |
| default, simply manipulate reified statements as these quads. |
| However, just as for <code>Bag</code>, <code>Seq</code>, <code>Alt</code> and <code>RDF lists</code> in |
| ordinary models, or ontology classes and individuals in |
| <code>OntModel</code>s, Jena has additional support for manipulating reified |
| statements.</p> |
| <p>The interface <code>ReifiedStatement</code> is used to represent a reified |
| statement as a Jena <code>Resource</code> that has direct access to the |
| statement it reifies. The method</p> |
| <ul> |
| <li><code>ReifiedStatement::getStatement()</code></li> |
| </ul> |
| <p>returns the <code>Statement</code> that the resource is reifying. All the |
| other <code>Resource</code> methods, of course, may be applied to a |
| <code>ReifiedStatement</code>.</p> |
| <h2 id="converting-resources-to-reified-statements">Converting resources to reified statements</h2> |
| <p>If a resource <code>R</code> is associated with a reified statement, but might |
| not itself be a <code>ReifiedStatement</code> object, the conversion method |
| <code>RDFNode::as(Class)</code> can be used to find (or create) a |
| <code>ReifiedStatement</code>:</p> |
| <ul> |
| <li><code>(ReifiedStatement) R.as(ReifiedStatement.class)</code></li> |
| </ul> |
| <p>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.</p> |
| <p>If there is no such associated reified statement, a |
| <code>CannotReifyException</code> is thrown. To find out in advance if the |
| conversion is possible, use the predicate |
| <code>RDFNode::canAs(ReifiedStatement.class)</code>. (Jena only counts as “an |
| associated reified statement” a resource with exactly one |
| <code>rdf:subject</code>, <code>rdf:predicate</code>, and <code>rdf:object</code> which has |
| <code>rdf:type rdf:Statement</code>. It can of course have <em>other</em> |
| properties.)</p> |
| <h2 id="testing-statements-for-reification">Testing statements for reification</h2> |
| <p>You may wish to know if some <code>Statement</code> is reified. The methods |
| <code>Statement::isReified()</code> and <code>Model::isReified(Statement)</code> return |
| true if (and only if) the statement has been reified in the model. |
| Note that the <code>Statement</code> method tests to see if the statement is |
| reified in its own model, and the model method tests to see if the |
| <code>Statement</code> is reified in <em>that</em> model; there is no test to see if |
| a <code>Statement</code> is reified in any other models.</p> |
| <h2 id="listing-reified-statements">Listing reified statements</h2> |
| <p>Just as <code>listStatements</code> 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 <code>RSIterator</code> object, which is an |
| iterator each of whose elements are <code>ReifiedStatement</code>s and for |
| which the convenience method <code>nextRS()</code> will deliver a |
| suitably-cast reified statement.</p> |
| <ul> |
| <li><code>Statement::listReifiedStatements()</code> - all the reifications of |
| this statement in its model.</li> |
| <li><code>Model::listReifiedStatements()</code> - all the reified statements |
| in this model.</li> |
| <li><code>Model::listReifiedStatements(Statement s)</code> - all the reified |
| statements reifying <code>s</code> in this model.</li> |
| </ul> |
| <h2 id="creating-reified-statements-directly">Creating reified statements directly</h2> |
| <p>You do not have to create reified statements by asserting the |
| quad into a <code>Model</code>; they can be created directly from their |
| <code>Statement</code>s using one of the methods:</p> |
| <ul> |
| <li><code>Statement::createReifiedStatement()</code></li> |
| <li><code>Statement::createReifiedStatement(String)</code></li> |
| <li><code>Model::createReifiedStatement(Statement)</code></li> |
| <li><code>Model::createReifiedStatement(String,Statement)</code></li> |
| </ul> |
| <p>Each of these returns a <code>ReifiedStatement</code> who’s <code>getStatement()</code> |
| method delivers the original statement (actually, a <code>.equals()</code> |
| statement; it may not be the identical statement object). If the creation |
| method passed in a (non-null) <code>String</code>, the <code>ReifiedStatement</code> is a |
| named resource and that string is its URI. Otherwise it is a |
| newly-minted bnode. The methods on <code>Statement</code> create a reified |
| statement in that statements model; those on <code>Model</code> create a |
| reified statement in that model.</p> |
| <p>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 <code>AlreadyReifiedException</code>.</p> |
| <p>The additional method <code>Model::getAnyReifiedStatement(Statement)</code> |
| returns some reification of the supplied <code>Statement</code>; an existing |
| one if possible, otherwise a fresh one (reified by a fresh bnode).</p> |
| <h2 id="removing-reified-statements">Removing reified statements</h2> |
| <p>There are two methods which remove all the reifications of a |
| <code>Statement</code> in some <code>Model</code>:</p> |
| <ul> |
| <li><code>Statement::removeReification()</code></li> |
| <li><code>Model::removeAllReifications(Statement)</code></li> |
| </ul> |
| <p>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</p> |
| <ul> |
| <li><code>Model::removeReification(ReifiedStatement)</code></li> |
| </ul> |
| <h2 id="reification-styles">Reification styles</h2> |
| <p>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.</p> |
| <p>The old documentation is <a href="reification_previous.html">still available</a>.</p> |
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