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| <h1 class="title">An Introduction to RDF and the Jena RDF API</h1> |
| |
| |
| <main class="d-flex flex-xl-row flex-column"> |
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| <aside class="text-muted align-self-start mb-3 p-0 d-xl-none d-block"> |
| <h2 class="h6 sticky-top m-0 p-2 bg-body-tertiary">On this page</h2> |
| <nav id="TableOfContents"> |
| <ul> |
| <li><a href="#preface">Preface</a></li> |
| <li><a href="#ch-Introduction">Introduction</a></li> |
| <li><a href="#ch-Statements">Statements</a></li> |
| <li><a href="#ch-Writing-RDF">Writing RDF</a></li> |
| <li><a href="#ch-Reading-RDF">Reading RDF</a></li> |
| <li><a href="#ch-Prefixes">Controlling Prefixes</a> |
| <ul> |
| <li><a href="#explicit-prefix-definitions">Explicit prefix definitions</a></li> |
| </ul> |
| </li> |
| <li><a href="#ch-Jena-RDF-Packages">Jena RDF Packages</a></li> |
| <li><a href="#ch-Navigating-a-Model">Navigating a Model</a></li> |
| <li><a href="#ch-Querying-a-Model">Querying a Model</a></li> |
| <li><a href="#ch-Operations-on-Models">Operations on Models</a></li> |
| <li><a href="#ch-Containers">Containers</a></li> |
| <li><a href="#ch-More-about-Literals-and-Datatypes">More about Literals and Datatypes</a></li> |
| <li><a href="#ch-Glossary">Glossary</a></li> |
| <li><a href="#footnotes">Footnotes</a></li> |
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| <article class="flex-column me-lg-4"> |
| <h2 id="preface">Preface</h2> |
| <p>This is a tutorial introduction to both W3C’s Resource Description Framework |
| (RDF) and Jena, a Java API for RDF. It is written for the programmer who is |
| unfamiliar with RDF and who learns best by prototyping, or, for other |
| reasons, wishes to move quickly to implementation. Some familiarity |
| with both XML and Java is assumed.</p> |
| <p>Implementing too quickly, without first understanding the RDF data model, |
| leads to frustration and disappointment. Yet studying the data model |
| alone is dry stuff and often leads to tortuous metaphysical conundrums. It |
| is better to approach understanding both the data model and how to use it in |
| parallel. Learn a bit of the data model and try it out. Then |
| learn a bit more and try that out. Then the theory informs the practice |
| and the practice the theory. The data model is quite simple, so this |
| approach does not take long.</p> |
| <p>RDF has an XML syntax and many who are familiar with XML will think of RDF |
| in terms of that syntax. This is a mistake. RDF should be |
| understood in terms of its data model. RDF data can be represented in |
| XML, but understanding the syntax is secondary to understanding the data |
| model.</p> |
| <p>An implementation of the Jena API, including the working source code for |
| all the examples used in this tutorial can be downloaded from |
| <a href="//jena.apache.org/download/index.cgi"><code>jena.apache.org/download/index.cgi</code></a>.</p> |
| <h2 id="ch-Introduction">Introduction</h2> |
| <p>The Resource Description Framework (RDF) is a standard (technically a W3C |
| Recommendation) for describing resources. What is a resource? That is |
| rather a deep question and the precise definition is still the subject of |
| debate. For our purposes we can think of it as anything we can identify. |
| You are a resource, as is your home page, this tutorial, the number one and |
| the great white whale in Moby Dick.</p> |
| <p>Our examples in this tutorial will be about people. They use an <a |
| href="https://www.w3.org/TR/vcard-rdf/">RDF representation of VCARDS</a>. RDF |
| is best thought of in the form of node and arc diagrams. A simple vcard |
| might look like this in RDF:</p> |
| <p align="center"> |
| <img border="0" src="figures/fig1.png" alt="figure 1" width="240" height="180"></p> |
| <p>The <a href="#glos-Resource"><i>resource</i></a>, John Smith, is shown as |
| an ellipse and is identified by a Uniform Resource Identifier (URI)<font |
| size="1"><sup><a href="#fn-01">1</a></sup></font>, in this case |
| "http://.../JohnSmith". If you try to access that resource using your |
| browser, you are unlikely to be successful; April the first jokes not |
| withstanding, you would be rather surprised if your browser were able to |
| deliver John Smith to your desk top. If you are unfamiliar with URI's, |
| think of them simply as rather strange looking names.</p> |
| <p>Resources have <a href="#glos-Property"><i>properties</i></a>. In these |
| examples we are interested in the sort of properties that would appear on |
| John Smith's business card. Figure 1 shows only one property, John |
| Smith's full name. A property is represented by an arc, labeled with the |
| name of a property. The name of a property is also a URI, but as URI's are |
| rather long and cumbersome, the diagram shows it in XML qname form. The |
| part before the ':' is called a namespace prefix and represents a |
| namespace. The part after the ':' is called a local name and represents |
| a name in that namespace. Properties are usually represented in this |
| qname form when written as RDF XML and it is a convenient shorthand for |
| representing them in diagrams and in text. Strictly, however, |
| properties are identified by a URI. The nsprefix:localname form is a |
| shorthand for the URI of the namespace concatenated with the localname. |
| There is no requirement that the URI of a property resolve to anything when |
| accessed by a browser.</p> |
| <p>Each property has a value. In this case the value is a <a |
| href="#glos-Literal"><i>literal</i></a>, which for now we can think of as a |
| strings of characters<font size="1"><sup><a href="#fn-02">2</a></sup></font>. |
| Literals are shown in rectangles.</p> |
| <p>Jena is a Java API which can be used to create and manipulate RDF graphs |
| like this one. Jena has object classes to represent graphs, resources, |
| properties and literals. The interfaces representing resources, |
| properties and literals are called Resource, Property and Literal |
| respectively. In Jena, a graph is called a model and is represented by the Model |
| interface.</p> |
| <p>The code to create this graph, or model, is simple:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// some definitions |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span><span style="color:#a2f;font-weight:bold">static</span> String personURI <span style="color:#666">=</span> <span style="color:#b44">"http://somewhere/JohnSmith"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">static</span> String fullName <span style="color:#666">=</span> <span style="color:#b44">"John Smith"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create an empty Model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Model model <span style="color:#666">=</span> ModelFactory<span style="color:#666">.</span><span style="color:#b44">createDefaultModel</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create the resource |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource johnSmith <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span>personURI<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// add the property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>johnSmith<span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">,</span> fullName<span style="color:#666">);</span> |
| </span></span></code></pre></div><p>It begins with some constant definitions and then creates an empty Model |
| or model, using the <code>ModelFactory</code> method <code>createDefaultModel()</code> |
| to create a memory-based model. Jena contains other implementations |
| of the Model interface, e.g one which uses a relational database: these |
| types of Model are also available from ModelFactory.</p> |
| <p>The John Smith resource is then created and a property added to it. |
| The property is provided by a "constant" class VCARD which holds objects |
| representing all the definitions in the VCARD schema. Jena provides constant |
| classes for other well known schemas, such as RDF and RDF schema themselves, |
| Dublin Core and OWL.</p> |
| <p>The working code for this example can be found in the /src-examples directory of |
| the Jena distribution as |
| <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial01.java">tutorial |
| 1</a>. |
| As an exercise, take this code and modify it to create a simple VCARD for |
| yourself.</p> |
| <p>The code to create the resource and add the property, can be more |
| compactly written in a cascading style:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Resource johnSmith <span style="color:#666">=</span> |
| </span></span><span style="display:flex;"><span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span>personURI<span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">,</span> fullName<span style="color:#666">);</span> |
| </span></span></code></pre></div><p>Now let's add some more detail to the vcard, exploring some more features |
| of RDF and Jena.</p> |
| <p>In the first example, the property value was a literal. RDF |
| properties can also take other resources as their value. Using a common RDF |
| technique, this example shows how to represent the different parts of John |
| Smith's name:</p> |
| <p align="center"> |
| <img border="0" src="figures/fig2.png" alt="figure 2" width="360" height="240"></p> |
| <p>Here we have added a new property, vcard:N, to represent the structure of |
| John Smith's name. There are several things of interest about this |
| Model. Note that the vcard:N property takes a resource as its value. |
| Note also that the ellipse representing the compound name has no URI. |
| It is known as an <i><a href="#glos-blank node">blank Node</a>.</i></p> |
| <p>The Jena code to construct this example, is again very simple. First |
| some declarations and the creation of the empty model.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// some definitions |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>String personURI <span style="color:#666">=</span> <span style="color:#b44">"http://somewhere/JohnSmith"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span>String givenName <span style="color:#666">=</span> <span style="color:#b44">"John"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span>String familyName <span style="color:#666">=</span> <span style="color:#b44">"Smith"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span>String fullName <span style="color:#666">=</span> givenName <span style="color:#666">+</span> <span style="color:#b44">" "</span> <span style="color:#666">+</span> familyName<span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create an empty Model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Model model <span style="color:#666">=</span> ModelFactory<span style="color:#666">.</span><span style="color:#b44">createDefaultModel</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create the resource |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// and add the properties cascading style |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource johnSmith |
| </span></span><span style="display:flex;"><span> <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span>personURI<span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">,</span> fullName<span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">N</span><span style="color:#666">,</span> |
| </span></span><span style="display:flex;"><span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">()</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">Given</span><span style="color:#666">,</span> givenName<span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">Family</span><span style="color:#666">,</span> familyName<span style="color:#666">));</span> |
| </span></span></code></pre></div><p>The working code for this example can be found as <a |
| href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial02.java">tutorial 2</a> in the /src-examples directory |
| of the Jena distribution.</p> |
| <h2 id="ch-Statements">Statements</h2> |
| <p>Each arc in an RDF Model is called a <i><a |
| href="#glos-Statement">statement</a></i>. Each statement asserts a fact |
| about a resource. A statement has three parts:</p> |
| <ul> |
| <li>the <i><a href="#glos-Subject"> subject</a></i> is the resource from |
| which the arc leaves</li> |
| <li>the <i><a href="#glos-Predicate"> predicate</a></i> is the property |
| that labels the arc</li> |
| <li>the <i><a href="#glos-Object"> object</a></i> is the resource or |
| literal pointed to by the arc</li> |
| </ul> |
| <p>A statement is sometimes called a <a href="#glos-Triple">triple</a>, |
| because of its three parts.</p> |
| <p>An RDF Model is represented as a <i>set</i> of statements. Each call of |
| <code>addProperty</code> in tutorial2 added another statement to the Model. |
| (Because a Model is set of statements, adding a duplicate of a statement has no |
| effect.) The Jena model interface defines a <code>listStatements()</code> |
| method which returns an <code>StmtIterator</code>, a subtype of Java's |
| <code>Iterator</code> over all the statements in a Model. |
| <code>StmtIterator</code> has a method <code>nextStatement()</code> |
| which returns the next statement from the iterator (the same one that |
| <code>next()</code> would deliver, already cast to <code>Statement</code>). |
| The <code>Statement</code> interface provides accessor |
| methods to the subject, predicate and object of a statement.</p> |
| <p>Now we will use that interface to extend tutorial2 to list all the |
| statements created and print them out. The complete code for this can be |
| found in <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial03.java">tutorial 3</a>.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// list the statements in the Model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>StmtIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listStatements</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// print out the predicate, subject and object of each statement |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span><span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> Statement stmt <span style="color:#666">=</span> iter<span style="color:#666">.</span><span style="color:#b44">nextStatement</span><span style="color:#666">();</span> <span style="color:#080;font-style:italic">// get next statement |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span> Resource subject <span style="color:#666">=</span> stmt<span style="color:#666">.</span><span style="color:#b44">getSubject</span><span style="color:#666">();</span> <span style="color:#080;font-style:italic">// get the subject |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span> Property predicate <span style="color:#666">=</span> stmt<span style="color:#666">.</span><span style="color:#b44">getPredicate</span><span style="color:#666">();</span> <span style="color:#080;font-style:italic">// get the predicate |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span> RDFNode object <span style="color:#666">=</span> stmt<span style="color:#666">.</span><span style="color:#b44">getObject</span><span style="color:#666">();</span> <span style="color:#080;font-style:italic">// get the object |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">print</span><span style="color:#666">(</span>subject<span style="color:#666">.</span><span style="color:#b44">toString</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">print</span><span style="color:#666">(</span><span style="color:#b44">" "</span> <span style="color:#666">+</span> predicate<span style="color:#666">.</span><span style="color:#b44">toString</span><span style="color:#666">()</span> <span style="color:#666">+</span> <span style="color:#b44">" "</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">if</span> <span style="color:#666">(</span>object <span style="color:#a2f;font-weight:bold">instanceof</span> Resource<span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">print</span><span style="color:#666">(</span>object<span style="color:#666">.</span><span style="color:#b44">toString</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> <span style="color:#a2f;font-weight:bold">else</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#080;font-style:italic">// object is a literal |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">print</span><span style="color:#666">(</span><span style="color:#b44">" \""</span> <span style="color:#666">+</span> object<span style="color:#666">.</span><span style="color:#b44">toString</span><span style="color:#666">()</span> <span style="color:#666">+</span> <span style="color:#b44">"\""</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">" ."</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>Since the object of a statement can be either a resource or a literal, the |
| <code>getObject()</code> method returns an |
| object typed as <code>RDFNode</code>, which is a |
| common superclass of both <code>Resource</code> |
| and <code>Literal</code>. The underlying object |
| is of the appropriate type, so the code uses <code>instanceof</code> to |
| determine which and |
| processes it accordingly.</p> |
| <p>When run, this program should produce output resembling:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-turtle" data-lang="turtle"><span style="display:flex;"><span><span style="">http</span>:<span style="">//somewhere/JohnSmith</span><span style="color:#bbb"> </span><span style="">http</span>:<span style="">//www</span>.<span style="">w</span><span style="color:#666">3</span>.<span style="">org/</span><span style="color:#666">2001</span><span style="">/vcard-rdf/</span><span style="color:#666">3.0</span><span style="color:#080;font-style:italic">#N 413f6415-c3b0-4259-b74d-4bd6e757eb60 .</span><span style="color:#bbb"> |
| </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span><span style="color:#666">413</span><span style="">f</span><span style="color:#666">6415</span><span style="">-c</span><span style="color:#666">3</span><span style="">b</span><span style="color:#666">0-4259</span><span style="">-b</span><span style="color:#666">74</span><span style="">d</span><span style="color:#666">-4</span><span style="">bd</span><span style="color:#666">6e757</span><span style="">eb</span><span style="color:#666">60</span><span style="color:#bbb"> </span><span style="">http</span>:<span style="">//www</span>.<span style="">w</span><span style="color:#666">3</span>.<span style="">org/</span><span style="color:#666">2001</span><span style="">/vcard-rdf/</span><span style="color:#666">3.0</span><span style="color:#080;font-style:italic">#Family "Smith" .</span><span style="color:#bbb"> |
| </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span><span style="color:#666">413</span><span style="">f</span><span style="color:#666">6415</span><span style="">-c</span><span style="color:#666">3</span><span style="">b</span><span style="color:#666">0-4259</span><span style="">-b</span><span style="color:#666">74</span><span style="">d</span><span style="color:#666">-4</span><span style="">bd</span><span style="color:#666">6e757</span><span style="">eb</span><span style="color:#666">60</span><span style="color:#bbb"> </span><span style="">http</span>:<span style="">//www</span>.<span style="">w</span><span style="color:#666">3</span>.<span style="">org/</span><span style="color:#666">2001</span><span style="">/vcard-rdf/</span><span style="color:#666">3.0</span><span style="color:#080;font-style:italic">#Given "John" .</span><span style="color:#bbb"> |
| </span></span></span><span style="display:flex;"><span><span style="color:#bbb"></span><span style="">http</span>:<span style="">//somewhere/JohnSmith</span><span style="color:#bbb"> </span><span style="">http</span>:<span style="">//www</span>.<span style="">w</span><span style="color:#666">3</span>.<span style="">org/</span><span style="color:#666">2001</span><span style="">/vcard-rdf/</span><span style="color:#666">3.0</span><span style="color:#080;font-style:italic">#FN "John Smith" .</span><span style="color:#bbb"> |
| </span></span></span></code></pre></div><p></p> |
| <p>Now you know why it is clearer to draw Models. If you look carefully, you |
| will see that each line consists of three fields representing the subject, |
| predicate and object of each statement. There are four arcs in the Model, so |
| there are four statements. The "14df86:ecc3dee17b:-7fff" is an internal |
| identifier generated by Jena. It is not a URI and should not be confused |
| with one. It is simply an internal label used by the Jena implementation.</p> |
| <p>The W3C <a href="https://www.w3.org/2001/sw/RDFCore/">RDFCore Working |
| Group</a> have defined a similar simple notation called <a |
| href="https://www.w3.org/TR/rdf-testcases/#ntriples">N-Triples</a>. The name |
| means "triple notation". We will see in the next section that Jena has an |
| N-Triples writer built in.</p> |
| <h2 id="ch-Writing-RDF">Writing RDF</h2> |
| <p>Jena has methods for reading and writing RDF as XML. These can be |
| used to save an RDF model to a file and later read it back in again.</p> |
| <p>Tutorial 3 created a model and wrote it out in triple form. <a |
| href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial04.java">Tutorial 4</a> modifies tutorial 3 to write the |
| model in RDF XML form to the standard output stream. The code again, is |
| very simple: <code>model.write</code> can take an <code>OutputStream</code> |
| argument.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// now write the model in XML form to a file |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The output should look something like this:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:vcard=</span><span style="color:#b44">'http://www.w3.org/2001/vcard-rdf/3.0#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">'http://somewhere/JohnSmith'</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>John Smith<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A0"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A0"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>John<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Smith<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>The RDF specifications specify how to represent RDF as XML. The RDF |
| XML syntax is quite complex. The reader is referred to the <a |
| href="https://www.w3.org/TR/rdf-primer/">primer</a> being developed by the |
| RDFCore WG for a more detailed introduction. However, let's take a quick look |
| at how to interpret the above.</p> |
| <p>RDF is usually embedded in an <rdf:RDF> element. The element is |
| optional if there are other ways of knowing that some XML is RDF, but it is |
| usually present. The RDF element defines the two namespaces used in the |
| document. There is then an <rdf:Description> element which describes |
| the resource whose URI is "http://somewhere/JohnSmith". If the rdf:about |
| attribute was missing, this element would represent a blank node.</p> |
| <p>The <vcard:FN> element describes a property of the resource. The |
| property name is the "FN" in the vcard namespace. RDF converts this to a URI |
| reference by concatenating the URI reference for the namespace prefix and |
| "FN", the local name part of the name. This gives a URI reference of |
| "http://www.w3.org/2001/vcard-rdf/3.0#FN". The value of the property |
| is the literal "John Smith".</p> |
| <p>The <vcard:N> element is a resource. In this case the resource is |
| represented by a relative URI reference. RDF converts this to an absolute |
| URI reference by concatenating it with the base URI of the current |
| document.</p> |
| <p>There is an error in this RDF XML; it does not exactly represent the Model |
| we created. The blank node in the Model has been given a URI reference. It |
| is no longer blank. The RDF/XML syntax is not capable of representing all |
| RDF Models; for example it cannot represent a blank node which is the object |
| of two statements. The 'dumb' writer we used to write this RDF/XML makes no |
| attempt to write correctly the subset of Models which can be written |
| correctly. It gives a URI to each blank node, making it no longer blank.</p> |
| <p>Jena has an extensible interface which allows new writers for different |
| serialization languages for RDF to be easily plugged in. The above call |
| invoked the standard 'dumb' writer. Jena also includes a more sophisticated |
| RDF/XML writer which can be invoked by using |
| <code>RDFDataMgr.write</code> function call:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// now write the model in a pretty form |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>RDFDataMgr<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">,</span> model<span style="color:#666">,</span> Lang<span style="color:#666">.</span><span style="color:#b44">RDFXML</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>This writer, the so called PrettyWriter, takes advantage of features of |
| the RDF/XML abbreviated syntax to write a Model more compactly. It is also |
| able to preserve blank nodes where that is possible. It is however, not |
| suitable for writing very large Models, as its performance is unlikely to be |
| acceptable. To write large files and preserve blank nodes, write in |
| N-Triples format:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// now write the model in N-TRIPLES form |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>RDFDataMgr<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">,</span> model<span style="color:#666">,</span> Lang<span style="color:#666">.</span><span style="color:#b44">NTRIPLES</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>This will produce output similar to that of tutorial 3 which conforms to |
| the N-Triples specification.</p> |
| <h2 id="ch-Reading-RDF">Reading RDF</h2> |
| <p><a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial05.java">Tutorial 5</a> demonstrates reading the |
| statements recorded in RDF XML form into a model. With this tutorial, |
| we have provided <a href="sparql_data/vc-db-1.rdf">a small database of vcards in RDF/XML form</a>. The following |
| code will read it in and write it out. <em>Note that for this application to |
| run, the input file must be in the current directory.</em></p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create an empty model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Model model <span style="color:#666">=</span> ModelFactory<span style="color:#666">.</span><span style="color:#b44">createDefaultModel</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// use the RDFDataMgr to find the input file |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>InputStream in <span style="color:#666">=</span> RDFDataMgr<span style="color:#666">.</span><span style="color:#b44">open</span><span style="color:#666">(</span> inputFileName <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">if</span> <span style="color:#666">(</span>in <span style="color:#666">==</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">throw</span> <span style="color:#a2f;font-weight:bold">new</span> IllegalArgumentException<span style="color:#666">(</span><span style="color:#b44">"File: "</span> <span style="color:#666">+</span> inputFileName <span style="color:#666">+</span> <span style="color:#b44">" not found"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// read the RDF/XML file |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">read</span><span style="color:#666">(</span>in<span style="color:#666">,</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// write it to standard out |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The second argument to the <code>read()</code> method call is the URI which will |
| be used for resolving relative URI's. As there are no relative URI |
| references in the test file, it is allowed to be empty. When run, <a |
| href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial05.java"> tutorial 5</a> will produce XML output which |
| looks like:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:vcard=</span><span style="color:#b44">'http://www.w3.org/2001/vcard-rdf/3.0#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A0"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Smith<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>John<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">'http://somewhere/JohnSmith/'</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>John Smith<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A0"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">'http://somewhere/SarahJones/'</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>Sarah Jones<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A1"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">'http://somewhere/MattJones/'</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>Matt Jones<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A2"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A3"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Smith<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>Rebecca<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A1"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Jones<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>Sarah<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A2"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Jones<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>Matthew<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">'http://somewhere/RebeccaSmith/'</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>Becky Smith<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A3"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><h2 id="ch-Prefixes">Controlling Prefixes</h2> |
| <h3 id="explicit-prefix-definitions">Explicit prefix definitions</h3> |
| <p>In the previous section, we saw that the output XML declared a namespace |
| prefix <code>vcard</code> and used that prefix to abbreviate URIs. While RDF |
| uses only the full URIs, and not this shortened form, Jena provides ways |
| of controlling the namespaces used on output with its <i>prefix mappings</i>. |
| Here’s a simple example.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Model m <span style="color:#666">=</span> ModelFactory<span style="color:#666">.</span><span style="color:#b44">createDefaultModel</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span>String nsA <span style="color:#666">=</span> <span style="color:#b44">"http://somewhere/else#"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span>String nsB <span style="color:#666">=</span> <span style="color:#b44">"http://nowhere/else#"</span><span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span>Resource root <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span> nsA <span style="color:#666">+</span> <span style="color:#b44">"root"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Property P <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createProperty</span><span style="color:#666">(</span> nsA <span style="color:#666">+</span> <span style="color:#b44">"P"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Property Q <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createProperty</span><span style="color:#666">(</span> nsB <span style="color:#666">+</span> <span style="color:#b44">"Q"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Resource x <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span> nsA <span style="color:#666">+</span> <span style="color:#b44">"x"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Resource y <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span> nsA <span style="color:#666">+</span> <span style="color:#b44">"y"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>Resource z <span style="color:#666">=</span> m<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">(</span> nsA <span style="color:#666">+</span> <span style="color:#b44">"z"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">add</span><span style="color:#666">(</span> root<span style="color:#666">,</span> P<span style="color:#666">,</span> x <span style="color:#666">).</span><span style="color:#b44">add</span><span style="color:#666">(</span> root<span style="color:#666">,</span> P<span style="color:#666">,</span> y <span style="color:#666">).</span><span style="color:#b44">add</span><span style="color:#666">(</span> y<span style="color:#666">,</span> Q<span style="color:#666">,</span> z <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span> <span style="color:#b44">"# -- no special prefixes defined"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span> System<span style="color:#666">.</span><span style="color:#b44">out</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span> <span style="color:#b44">"# -- nsA defined"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">setNsPrefix</span><span style="color:#666">(</span> <span style="color:#b44">"nsA"</span><span style="color:#666">,</span> nsA <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span> System<span style="color:#666">.</span><span style="color:#b44">out</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span> <span style="color:#b44">"# -- nsA and cat defined"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">setNsPrefix</span><span style="color:#666">(</span> <span style="color:#b44">"cat"</span><span style="color:#666">,</span> nsB <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span> System<span style="color:#666">.</span><span style="color:#b44">out</span> <span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The output from this fragment is three lots of RDF/XML, with |
| three different prefix mappings. First the default, with no |
| prefixes other than the standard ones:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span># -- no special prefixes defined |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:j.0=</span><span style="color:#b44">"http://nowhere/else#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">"http://www.w3.org/1999/02/22-rdf-syntax-ns#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:j.1=</span><span style="color:#b44">"http://somewhere/else#"</span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#root"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><j.1:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#x"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><j.1:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><j.0:Q</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#z"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>We see that the rdf namespace is declared automatically, since it |
| is required for tags such as <code><rdf:RDF></code> and |
| <code><rdf:resource></code>. XML namespace declarations are also |
| needed for using the two properties P and Q, but since their |
| prefixes have not been introduced to the model in this example, |
| they get invented namespace names: <code>j.0</code> and <code>j.1</code>.</p> |
| <p>The method <code>setNsPrefix(String prefix, String URI)</code> |
| declares that the namespace <code>URI</code> may be abbreviated |
| by <code>prefix</code>. Jena requires that <code>prefix</code> be |
| a legal XML namespace name, and that <code>URI</code> ends with a |
| non-name character. The RDF/XML writer will turn these prefix |
| declarations into XML namespace declarations and use them in its |
| output: |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span># -- nsA defined |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:j.0=</span><span style="color:#b44">"http://nowhere/else#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">"http://www.w3.org/1999/02/22-rdf-syntax-ns#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:nsA=</span><span style="color:#b44">"http://somewhere/else#"</span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#root"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><nsA:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#x"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><nsA:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><j.0:Q</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#z"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>The other namespace still gets the constructed name, but the nsA name |
| is now used in the property tags. There’s no need for the prefix name |
| to have anything to do with the variables in the Jena code:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span># -- nsA and cat defined |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:cat=</span><span style="color:#b44">"http://nowhere/else#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">"http://www.w3.org/1999/02/22-rdf-syntax-ns#"</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:nsA=</span><span style="color:#b44">"http://somewhere/else#"</span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#root"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><nsA:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#x"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><nsA:P</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/else#y"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><cat:Q</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">"http://somewhere/else#z"</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>Both prefixes are used for output, and no generated prefixes are needed.</p> |
| <h3>Implicit prefix definitions</h3> |
| <p>As well as prefix declarations provided by calls to <code>setNsPrefix</code>, |
| Jena will remember the prefixes that were used in input to |
| <code>model.read()</code>.</p> |
| <p>Take the output produced by the previous fragment, and paste it into |
| some file, with URL <b>file:/tmp/fragment.rdf</b> say. Then run the |
| code: |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>Model m2 <span style="color:#666">=</span> ModelFactory<span style="color:#666">.</span><span style="color:#b44">createDefaultModel</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span>m2<span style="color:#666">.</span><span style="color:#b44">read</span><span style="color:#666">(</span> <span style="color:#b44">"file:/tmp/fragment.rdf"</span> <span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>m2<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span> System<span style="color:#666">.</span><span style="color:#b44">out</span> <span style="color:#666">);</span> |
| </span></span></code></pre></div><p>You’ll see that the prefixes from the input are preserved in the output. |
| All the prefixes are written, even if they’re not used anywhere. You can |
| remove a prefix with <code>removeNsPrefix(String prefix)</code> if you |
| don’t want it in the output.</p> |
| <p>Since NTriples doesn't have any short way of writing URIs, it takes |
| no notice of prefixes on output and doesn't provide any on input. The |
| notation <b>N3</b>, also supported by Jena, does have short prefixed names, |
| and records them on input and uses them on output. |
| <p>Jena has further operations on the prefix mappings that a model holds, |
| such as extracting a Java <code>Map</code> of the exiting mappings, or |
| adding a whole group of mappings at once; see the documentation for |
| <code>PrefixMapping</code> for details. |
| <h2 id="ch-Jena-RDF-Packages">Jena RDF Packages</h2> |
| <p>Jena is a Java API for semantic web applications. The key RDF package for |
| the application developer is |
| <code>org.apache.jena.rdf.model</code>. The API has been defined |
| in terms of interfaces so that application code can work with different |
| implementations without change. This package contains interfaces for |
| representing models, resources, properties, literals, statements and all the |
| other key concepts of RDF, and a ModelFactory for creating models. So that |
| application code remains independent of |
| the implementation, it is best if it uses interfaces wherever possible, not |
| specific class implementations.</p> |
| <p>The <code>org.apache.jena.tutorial</code> package contains the |
| working source code for all the examples used in this tutorial.</p> |
| <p>The <code>org.apache.jena...impl</code> packages contains |
| implementation classes which may be common to many implementations. For |
| example, they defines classes <code>ResourceImpl</code>, |
| <code>PropertyImpl</code>, and <code>LiteralImpl</code> which may be |
| used directly or subclassed by different implementations. Applications |
| should rarely, if ever, use these classes directly. For example, rather |
| than creating a new instance of <code>ResourceImpl</code>, it is better to |
| use the <code>createResource</code> method of whatever model is being |
| used. That way, if the model implementation has used an optimized |
| implementation of <code>Resource</code>, then no conversions between the two |
| types will be necessary.</p> |
| <h2 id="ch-Navigating-a-Model">Navigating a Model</h2> |
| <p>So far, this tutorial has dealt mainly with creating, reading and writing |
| RDF Models. It is now time to deal with accessing information held in a |
| Model.</p> |
| <p>Given the URI of a resource, the resource object can be retrieved from a |
| model using the <code>Model.getResource(String uri)</code> method. This |
| method is defined to return a Resource object if one exists in the model, or |
| otherwise to create a new one. For example, to retrieve the John Smith |
| resource from the model read in from the file in tutorial 5:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// retrieve the John Smith vcard resource from the model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource vcard <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">getResource</span><span style="color:#666">(</span>johnSmithURI<span style="color:#666">);</</span>code<span style="color:#666">></span> |
| </span></span></code></pre></div><p>The Resource interface defines a number of methods for accessing the |
| properties of a resource. The <code>Resource.getProperty(Property |
| p)</code> method accesses a property of the resource. This method does |
| not follow the usual Java accessor convention in that the type of the object |
| returned is <code>Statement,</code> not the <code>Property</code> that you |
| might have expected. Returning the whole statement allows the application to |
| access the value of the property using one of its accessor methods which |
| return the object of the statement. For example to retrieve the |
| resource which is the value of the <code>vcard:N</code> property:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// retrieve the value of the N property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource name <span style="color:#666">=</span> <span style="color:#666">(</span>Resource<span style="color:#666">)</span> vcard<span style="color:#666">.</span><span style="color:#b44">getProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">N</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getObject</span><span style="color:#666">();</span> |
| </span></span></code></pre></div><p>In general, the object of a statement could be a resource or a literal, so |
| the application code, knowing the value must be a resource, casts the |
| returned object. One of the things that Jena tries to do is to provide |
| type specific methods so the application does not have to cast and type |
| checking can be done at compile time. The code fragment above, can be |
| more conveniently written:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// retrieve the value of the N property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource name <span style="color:#666">=</span> vcard<span style="color:#666">.</span><span style="color:#b44">getProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">N</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getResource</span><span style="color:#666">();</span> |
| </span></span></code></pre></div><p>Similarly, the literal value of a property can be retrieved:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java//" data-lang="java//"><span style="display:flex;"><span>String fullName <span style="color:#666">=</span> vcard<span style="color:#666">.</span><span style="color:#b44">getProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getString</span><span style="color:#666">();</span> |
| </span></span></code></pre></div><p>In this example, the vcard resource has only one <code>vcard:FN</code> and |
| one <code>vcard:N</code> property. RDF permits a resource to repeat a |
| property; for example Adam might have more than one nickname. Let's give him |
| two:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// add two nickname properties to vcard |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>vcard<span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">NICKNAME</span><span style="color:#666">,</span> <span style="color:#b44">"Smithy"</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">NICKNAME</span><span style="color:#666">,</span> <span style="color:#b44">"Adman"</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>As noted before, Jena represents an RDF Model as <em>set </em>of |
| statements, so adding a statement with the subject, predicate and object as |
| one already in the Model will have no effect. Jena does not define which of |
| the two nicknames present in the Model will be returned. The result of |
| calling <code>vcard.getProperty(VCARD.NICKNAME)</code> is indeterminate. Jena |
| will return one of the values, but there is no guarantee even that two |
| consecutive calls will return the same value.</p> |
| <p>If it is possible that a property may occur more than once, then the |
| Resource.listProperties(Property p) method can be used to return an iterator |
| which will list them all. This method returns an iterator which returns |
| objects of type <code>Statement</code>. We can list the nicknames like |
| this:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// set up the output |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">"The nicknames of \""</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">+</span> fullName <span style="color:#666">+</span> <span style="color:#b44">"\" are:"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// list the nicknames |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>StmtIterator iter <span style="color:#666">=</span> vcard<span style="color:#666">.</span><span style="color:#b44">listProperties</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">NICKNAME</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">" "</span> <span style="color:#666">+</span> iter<span style="color:#666">.</span><span style="color:#b44">nextStatement</span><span style="color:#666">()</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getObject</span><span style="color:#666">()</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">toString</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>This code can be found in <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial06.java"> tutorial 6</a>. |
| The statement iterator <code>iter</code> produces each and every statement |
| with subject <code>vcard</code> and predicate <code>VCARD.NICKNAME</code>, |
| so looping over it allows us to fetch each statement by using |
| <code>nextStatement()</code>, get the object field, and convert it to |
| a string. |
| The code produces the following output when run:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>The nicknames of <span style="color:#b44">"John Smith"</span> are: |
| </span></span><span style="display:flex;"><span> Smithy |
| </span></span><span style="display:flex;"><span> Adman |
| </span></span></code></pre></div><p>All the properties of a resource can be listed by using the |
| <code>listProperties()</code> method without an argument. |
| </p> |
| <h2 id="ch-Querying-a-Model">Querying a Model</h2> |
| <p>The previous section dealt with the case of navigating a model from a |
| resource with a known URI. This section deals with searching a |
| model. The core Jena API supports only a limited query primitive. The |
| more powerful query facilities of SPARQL are described elsewhere.</p> |
| <p>The <code>Model.listStatements()</code> method, which lists all the |
| statements in a model, is perhaps the crudest way of querying a model. |
| Its use is not recommended on very large Models. |
| <code>Model.listSubjects()</code> is similar, but returns an iterator over |
| all resources that have properties, <i>ie</i> are the subject of some |
| statement.</p> |
| <p><code>Model.listSubjectsWithProperty(Property p, RDFNode |
| o)</code><code></code> will return an iterator over all the resources which |
| have property <code>p</code> with value <code>o</code>. If we assume that |
| only vcard resources |
| will have <code>vcard:FN</code> property, and that in our data, all such |
| resources have such a property, then we can find all the vcards like this:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// list vcards |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>ResIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listSubjectsWithProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> Resource r <span style="color:#666">=</span> iter<span style="color:#666">.</span><span style="color:#b44">nextResource</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">...</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>All these query methods are simply syntactic sugar over a primitive query |
| method <code>model.listStatements(Selector s)</code>. This method returns an |
| iterator over all the statements in the model 'selected' by <code>s</code>. |
| The selector interface is designed to be extensible, but for now, there is |
| only one implementation of it, the class <code>SimpleSelector</code> from the |
| package <code>org.apache.jena.rdf.model</code>. Using |
| <code>SimpleSelector </code>is one of the rare occasions in Jena when it is |
| necessary to use a specific class rather than an interface. The |
| <code>SimpleSelector</code> constructor takes three arguments:</p> |
| <pre>Selector selector = new SimpleSelector(subject, predicate, object);</pre> |
| <p>This selector will select all statements with a subject that matches |
| <code>subject</code>, a predicate that matches <code>predicate</code> and an |
| object that matches <code>object</code>. If a <code>null</code> is supplied |
| in any of the positions, it matches anything; otherwise they match corresponding |
| equal resources or literals. (Two resources are equal if they have equal URIs |
| or are the same blank node; two literals are the same if all their components |
| are equal.) Thus:</p> |
| <pre>Selector selector = new SimpleSelector(null, null, null);</pre> |
| <p>will select all the statements in a Model.</p> |
| <pre>Selector selector = new SimpleSelector(null, VCARD.FN, null);</pre> |
| <p>will select all the statements with VCARD.FN as their predicate, whatever |
| the subject or object. As a special shorthand, |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>listStatements<span style="color:#666">(</span> S<span style="color:#666">,</span> P<span style="color:#666">,</span> O <span style="color:#666">)</span> |
| </span></span></code></pre></div><p>is equivalent to</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>listStatements<span style="color:#666">(</span> <span style="color:#a2f;font-weight:bold">new</span> SimpleSelector<span style="color:#666">(</span> S<span style="color:#666">,</span> P<span style="color:#666">,</span> O <span style="color:#666">)</span> <span style="color:#666">)</span> |
| </span></span></code></pre></div><p> |
| The following code, which can be found in full in <a |
| href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial07.java">tutorial 7</a> lists the full names on all the |
| vcards in the database.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// select all the resources with a VCARD.FN property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>ResIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listSubjectsWithProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">if</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">"The database contains vcards for:"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">" "</span> <span style="color:#666">+</span> iter<span style="color:#666">.</span><span style="color:#b44">nextResource</span><span style="color:#666">()</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getString</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> <span style="color:#a2f;font-weight:bold">else</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">"No vcards were found in the database"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>This should produce output similar to the following:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>The database contains vcards <span style="color:#a2f;font-weight:bold">for</span>: |
| </span></span><span style="display:flex;"><span> Sarah Jones |
| </span></span><span style="display:flex;"><span> John Smith |
| </span></span><span style="display:flex;"><span> Matt Jones |
| </span></span><span style="display:flex;"><span> Becky Smith |
| </span></span></code></pre></div><p>Your next exercise is to modify this code to use <code>SimpleSelector |
| </code>instead of <code>listSubjectsWithProperty</code>.</p> |
| <p>Lets see how to implement some finer control over the statements selected. |
| <code>SimpleSelector</code> can be subclassed and its selects method modified |
| to perform further filtering:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// select all the resources with a VCARD.FN property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// whose value ends with "Smith" |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>StmtIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listStatements</span><span style="color:#666">(</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">new</span> SimpleSelector<span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">,</span> VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">,</span> <span style="color:#666">(</span>RDFNode<span style="color:#666">)</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">public</span> <span style="color:#0b0;font-weight:bold">boolean</span> <span style="color:#00a000">selects</span><span style="color:#666">(</span>Statement s<span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">{</span><span style="color:#a2f;font-weight:bold">return</span> s<span style="color:#666">.</span><span style="color:#b44">getString</span><span style="color:#666">().</span><span style="color:#b44">endsWith</span><span style="color:#666">(</span><span style="color:#b44">"Smith"</span><span style="color:#666">);}</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">});</span> |
| </span></span></code></pre></div><p>This sample code uses a neat Java technique of overriding a method |
| definition inline when creating an instance of the class. Here the |
| <code>selects(...)</code> method checks to ensure that the full name ends |
| with "Smith". It is important to note that filtering based on the subject, |
| predicate and object arguments takes place before the |
| <code>selects(...)</code> method is called, so the extra test will only be |
| applied to matching statements.</p> |
| <p>The full code can be found in <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial08.java">tutorial |
| 8</a> and produces output like this:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>The database contains vcards <span style="color:#a2f;font-weight:bold">for</span>: |
| </span></span><span style="display:flex;"><span> John Smith |
| </span></span><span style="display:flex;"><span> Becky Smith |
| </span></span></code></pre></div><p>You might think that:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// do all filtering in the selects method |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>StmtIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listStatements</span><span style="color:#666">(</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">new</span> |
| </span></span><span style="display:flex;"><span> SimpleSelector<span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">,</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">,</span> <span style="color:#666">(</span>RDFNode<span style="color:#666">)</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">public</span> <span style="color:#0b0;font-weight:bold">boolean</span> <span style="color:#00a000">selects</span><span style="color:#666">(</span>Statement s<span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">return</span> <span style="color:#666">(</span>subject <span style="color:#666">==</span> <span style="color:#a2f;font-weight:bold">null</span> <span style="color:#666">||</span> s<span style="color:#666">.</span><span style="color:#b44">getSubject</span><span style="color:#666">().</span><span style="color:#b44">equals</span><span style="color:#666">(</span>subject<span style="color:#666">))</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">&</span>amp<span style="color:#666">;&</span>amp<span style="color:#666">;</span> <span style="color:#666">(</span>predicate <span style="color:#666">==</span> <span style="color:#a2f;font-weight:bold">null</span> <span style="color:#666">||</span> s<span style="color:#666">.</span><span style="color:#b44">getPredicate</span><span style="color:#666">().</span><span style="color:#b44">equals</span><span style="color:#666">(</span>predicate<span style="color:#666">))</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">&</span>amp<span style="color:#666">;&</span>amp<span style="color:#666">;</span> <span style="color:#666">(</span>object <span style="color:#666">==</span> <span style="color:#a2f;font-weight:bold">null</span> <span style="color:#666">||</span> s<span style="color:#666">.</span><span style="color:#b44">getObject</span><span style="color:#666">().</span><span style="color:#b44">equals</span><span style="color:#666">(</span>object<span style="color:#666">))</span> <span style="color:#666">;</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">});</span> |
| </span></span></code></pre></div><p>is equivalent to:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span>StmtIterator iter <span style="color:#666">=</span> |
| </span></span><span style="display:flex;"><span> model<span style="color:#666">.</span><span style="color:#b44">listStatements</span><span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">new</span> SimpleSelector<span style="color:#666">(</span>subject<span style="color:#666">,</span> predicate<span style="color:#666">,</span> object<span style="color:#666">)</span> |
| </span></span></code></pre></div><p>Whilst functionally they may be equivalent, the first form will list all |
| the statements in the Model and test each one individually, whilst the second |
| allows indexes maintained by the implementation to improve performance. Try |
| it on a large Model and see for yourself, but make a cup of coffee first.</p> |
| <h2 id="ch-Operations-on-Models">Operations on Models</h2> |
| <p>Jena provides three operations for manipulating Models as a whole. These |
| are the common set operations of union, intersection and difference.</p> |
| <p>The union of two Models is the union of the sets of statements which |
| represent each Model. This is one of the key operations that the design of |
| RDF supports. It enables data from disparate data sources to be merged. |
| Consider the following two Models:</p> |
| <p style="text-align: center"> |
| <img alt="figure 4" src="figures/fig4.png" width="240" height="240">and |
| <img alt="figure 5" src="figures/fig5.png" width="240" height="240"></p> |
| <p>When these are merged, the two http://...JohnSmith nodes are merged into |
| one and the duplicate <code>vcard:FN</code> arc is dropped to produce:</p> |
| <p style="text-align: center"> |
| <img alt="figure 6" src="figures/fig6.png" width="540" height="240"></p> |
| <p>Lets look at the code to do this (the full code is in <a |
| href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial09.java">tutorial 9</a>) and see what happens.</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// read the RDF/XML files |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model1<span style="color:#666">.</span><span style="color:#b44">read</span><span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">new</span> InputStreamReader<span style="color:#666">(</span>in1<span style="color:#666">),</span> <span style="color:#b44">""</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span>model2<span style="color:#666">.</span><span style="color:#b44">read</span><span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">new</span> InputStreamReader<span style="color:#666">(</span>in2<span style="color:#666">),</span> <span style="color:#b44">""</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// merge the Models |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Model model <span style="color:#666">=</span> model1<span style="color:#666">.</span><span style="color:#b44">union</span><span style="color:#666">(</span>model2<span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// print the Model as RDF/XML |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>system<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">,</span> <span style="color:#b44">"RDF/XML-ABBREV"</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The output produced by the pretty writer looks like this:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">"<a href="</span><span style="">http://www.w3.org/1999/02/22-rdf-syntax-ns#"</span><span style="color:#008000;font-weight:bold">></span>http://www.w3.org/1999/02/22-rdf-syntax-ns#<span style="color:#008000;font-weight:bold"></a></span>" |
| </span></span><span style="display:flex;"><span> xmlns:vcard="http://www.w3.org/2001/vcard-rdf/3.0#"> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:about=</span><span style="color:#b44">"http://somewhere/JohnSmith/"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:EMAIL></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:internet></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:value></span>John@somewhere.com<span style="color:#008000;font-weight:bold"></rdf:value></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></vcard:internet></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></vcard:EMAIL></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:N</span> <span style="color:#b44">rdf:parseType=</span><span style="color:#b44">"Resource"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Given></span>John<span style="color:#008000;font-weight:bold"></vcard:Given></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:Family></span>Smith<span style="color:#008000;font-weight:bold"></vcard:Family></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></vcard:N></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><vcard:FN></span>John Smith<span style="color:#008000;font-weight:bold"></vcard:FN></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>Even if you are unfamiliar with the details of the RDF/XML syntax, it |
| should be reasonably clear that the Models have merged as expected. The |
| intersection and difference of the Models can be computed in a similar |
| manner, using the methods <code>.intersection(Model)</code> and |
| <code>.difference(Model)</code>; see the |
| <a href="/documentation/javadoc/jena/org/apache/jena/rdf/model/Model.html#difference(org.apache.jena.rdf.model.Model)">difference</a> |
| and |
| <a href="/documentation/javadoc/jena/org/apache/jena/rdf/model/Model.html#intersection(org.apache.jena.rdf.model.Model)">intersection</a> |
| Javadocs for more details. |
| </p> |
| <h2 id="ch-Containers">Containers</h2> |
| <p>RDF defines a special kind of resources for representing collections of |
| things. These resources are called <i>containers</i>. The members of a |
| container can be either literals or resources. There are three kinds of |
| container:</p> |
| <ul> |
| <li>a BAG is an unordered collection</li> |
| <li>an ALT is an unordered collection intended to represent |
| alternatives</li> |
| <li>a SEQ is an ordered collection</li> |
| </ul> |
| <p>A container is represented by a resource. That resource will have an |
| rdf:type property whose value should be one of rdf:Bag, rdf:Alt or rdf:Seq, |
| or a subclass of one of these, depending on the type of the container. |
| The first member of the container is the value of the container's rdf:_1 |
| property; the second member of the container is the value of the container's |
| rdf:_2 property and so on. The rdf:_nnn properties are known as the |
| <i>ordinal properties</i>.</p> |
| <p>For example, the Model for a simple bag containing the vcards of the |
| Smith's might look like this:</p> |
| <p align="center"> |
| <img border="0" src="figures/fig3.png" alt="figure 3" width="720" height="420"></p> |
| <p align="left">Whilst the members of the bag are represented by the |
| properties rdf:_1, rdf:_2 etc the ordering of the properties is not |
| significant. We could switch the values of the rdf:_1 and rdf:_2 |
| properties and the resulting Model would represent the same information.</p> |
| <p align="left">Alt's are intended to represent alternatives. For |
| example, lets say a resource represented a software product. It might |
| have a property to indicate where it might be obtained from. The value |
| of that property might be an Alt collection containing various sites from |
| which it could be downloaded. Alt's are unordered except that the |
| rdf:_1 property has special significance. It represents the default |
| choice.</p> |
| <p align="left">Whilst containers can be handled using the basic machinery of |
| resources and properties, Jena has explicit interfaces and implementation |
| classes to handle them. It is not a good idea to have an object |
| manipulating a container, and at the same time to modify the state of that |
| container using the lower level methods.</p> |
| <p align="left">Let's modify tutorial 8 to create this bag:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create a bag |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Bag smiths <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">createBag</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// select all the resources with a VCARD.FN property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// whose value ends with "Smith" |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>StmtIterator iter <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">listStatements</span><span style="color:#666">(</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">new</span> SimpleSelector<span style="color:#666">(</span><span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">,</span> VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">,</span> <span style="color:#666">(</span>RDFNode<span style="color:#666">)</span> <span style="color:#a2f;font-weight:bold">null</span><span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">public</span> <span style="color:#0b0;font-weight:bold">boolean</span> <span style="color:#00a000">selects</span><span style="color:#666">(</span>Statement s<span style="color:#666">)</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">return</span> s<span style="color:#666">.</span><span style="color:#b44">getString</span><span style="color:#666">().</span><span style="color:#b44">endsWith</span><span style="color:#666">(</span><span style="color:#b44">"Smith"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">});</span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// add the Smith's to the bag |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span><span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> smiths<span style="color:#666">.</span><span style="color:#b44">add</span><span style="color:#666">(</span>iter<span style="color:#666">.</span><span style="color:#b44">nextStatement</span><span style="color:#666">().</span><span style="color:#b44">getSubject</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>If we write out this Model, it contains something like the following:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:vcard=</span><span style="color:#b44">'http://www.w3.org/2001/vcard-rdf/3.0#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span>... |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A3"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:type</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag'</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:_1</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">'http://somewhere/JohnSmith/'</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:_2</span> <span style="color:#b44">rdf:resource=</span><span style="color:#b44">'http://somewhere/RebeccaSmith/'</span><span style="color:#008000;font-weight:bold">/></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>which represents the Bag resource.</p> |
| <p>The container interface provides an iterator to list the contents of a |
| container:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// print out the members of the bag |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>NodeIterator iter2 <span style="color:#666">=</span> smiths<span style="color:#666">.</span><span style="color:#b44">iterator</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span><span style="color:#a2f;font-weight:bold">if</span> <span style="color:#666">(</span>iter2<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">"The bag contains:"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#a2f;font-weight:bold">while</span> <span style="color:#666">(</span>iter2<span style="color:#666">.</span><span style="color:#b44">hasNext</span><span style="color:#666">())</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">" "</span> <span style="color:#666">+</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">((</span>Resource<span style="color:#666">)</span> iter2<span style="color:#666">.</span><span style="color:#b44">next</span><span style="color:#666">())</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getProperty</span><span style="color:#666">(</span>VCARD<span style="color:#666">.</span><span style="color:#b44">FN</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">getString</span><span style="color:#666">());</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">}</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> <span style="color:#a2f;font-weight:bold">else</span> <span style="color:#666">{</span> |
| </span></span><span style="display:flex;"><span> System<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">.</span><span style="color:#b44">println</span><span style="color:#666">(</span><span style="color:#b44">"The bag is empty"</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span><span style="color:#666">}</span> |
| </span></span></code></pre></div><p>which produces the following output:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-bash" data-lang="bash"><span style="display:flex;"><span>The bag contains: |
| </span></span><span style="display:flex;"><span> John Smith |
| </span></span><span style="display:flex;"><span> Becky Smith |
| </span></span></code></pre></div><p>Executable example code can be found in <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial10.java"> |
| tutorial 10</a>, which glues together the fragments above into a complete |
| example.</p> |
| <p>The Jena classes offer methods for manipulating containers including |
| adding new members, inserting new members into the middle of a container and |
| removing existing members. The Jena container classes currently ensure that |
| the list of ordinal properties used starts at rdf:_1 and is contiguous. |
| The RDFCore WG have relaxed this constraint, which allows partial |
| representation of containers. This therefore is an area of Jena may be |
| changed in the future.</p> |
| <h2 id="ch-More-about-Literals-and-Datatypes">More about Literals and Datatypes</h2> |
| <p>RDF literals are not just simple strings. Literals may have a language |
| tag to indicate the language of the literal. The literal "chat" with an |
| English language tag is considered different to the literal "chat" with a |
| French language tag. This rather strange behaviour is an artefact of the |
| original RDF/XML syntax.</p> |
| <p>Further there are really two sorts of Literals. In one, the string |
| component is just that, an ordinary string. In the other the string |
| component is expected to be a well balanced fragment of XML. When an RDF |
| Model is written as RDF/XML a special construction using a |
| parseType='Literal' attribute is used to represent it.</p> |
| <p>In Jena, these attributes of a literal may be set when the literal is |
| constructed, e.g. in <a href="https://github.com/apache/jena/tree/main/jena-core/src-examples/jena/examples/rdf/Tutorial11.java">tutorial 11</a>:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create the resource |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource r <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// add the property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>r<span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>RDFS<span style="color:#666">.</span><span style="color:#b44">label</span><span style="color:#666">,</span> model<span style="color:#666">.</span><span style="color:#b44">createLiteral</span><span style="color:#666">(</span><span style="color:#b44">"chat"</span><span style="color:#666">,</span> <span style="color:#b44">"en"</span><span style="color:#666">))</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>RDFS<span style="color:#666">.</span><span style="color:#b44">label</span><span style="color:#666">,</span> model<span style="color:#666">.</span><span style="color:#b44">createLiteral</span><span style="color:#666">(</span><span style="color:#b44">"chat"</span><span style="color:#666">,</span> <span style="color:#b44">"fr"</span><span style="color:#666">))</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>RDFS<span style="color:#666">.</span><span style="color:#b44">label</span><span style="color:#666">,</span> model<span style="color:#666">.</span><span style="color:#b44">createLiteral</span><span style="color:#666">(</span><span style="color:#b44">"&lt;em&gt;chat&lt;/em&gt;"</span><span style="color:#666">,</span> <span style="color:#a2f;font-weight:bold">true</span><span style="color:#666">));</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// write out the Model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>system<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>produces</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-xml" data-lang="xml"><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"><rdf:RDF</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdf=</span><span style="color:#b44">'http://www.w3.org/1999/02/22-rdf-syntax-ns#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#b44">xmlns:rdfs=</span><span style="color:#b44">'http://www.w3.org/2000/01/rdf-schema#'</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdf:Description</span> <span style="color:#b44">rdf:nodeID=</span><span style="color:#b44">"A0"</span><span style="color:#008000;font-weight:bold">></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdfs:label</span> <span style="color:#b44">xml:lang=</span><span style="color:#b44">'en'</span><span style="color:#008000;font-weight:bold">></span>chat<span style="color:#008000;font-weight:bold"></rdfs:label></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdfs:label</span> <span style="color:#b44">xml:lang=</span><span style="color:#b44">'fr'</span><span style="color:#008000;font-weight:bold">></span>chat<span style="color:#008000;font-weight:bold"></rdfs:label></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"><rdfs:label</span> <span style="color:#b44">rdf:parseType=</span><span style="color:#b44">'Literal'</span><span style="color:#008000;font-weight:bold">><em></span>chat<span style="color:#008000;font-weight:bold"></em></rdfs:label></span> |
| </span></span><span style="display:flex;"><span> <span style="color:#008000;font-weight:bold"></rdf:Description></span> |
| </span></span><span style="display:flex;"><span><span style="color:#008000;font-weight:bold"></rdf:RDF></span> |
| </span></span></code></pre></div><p>For two literals to be considered equal, they must either both be XML |
| literals or both be simple literals. In addition, either both must have no |
| language tag, or if language tags are present they must be equal. For simple |
| literals the strings must be equal. XML literals have two notions of |
| equality. The simple notion is that the conditions previously mentioned are |
| true and the strings are also equal. The other notion is that they can be |
| equal if the canonicalization of their strings is equal.</p> |
| <p>Jena's interfaces also support typed literals. The old-fashioned way |
| (shown below) treats typed literals as shorthand for strings: typed |
| values are converted in the usual Java way to strings and these strings |
| are stored in the Model. For example, try (noting that for |
| simple literals, we can omit the <code>model.createLiteral</code>(...) |
| call):</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-java" data-lang="java"><span style="display:flex;"><span><span style="color:#080;font-style:italic">// create the resource |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>Resource r <span style="color:#666">=</span> model<span style="color:#666">.</span><span style="color:#b44">createResource</span><span style="color:#666">();</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// add the property |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>r<span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>RDFS<span style="color:#666">.</span><span style="color:#b44">label</span><span style="color:#666">,</span> <span style="color:#b44">"11"</span><span style="color:#666">)</span> |
| </span></span><span style="display:flex;"><span> <span style="color:#666">.</span><span style="color:#b44">addProperty</span><span style="color:#666">(</span>RDFS<span style="color:#666">.</span><span style="color:#b44">label</span><span style="color:#666">,</span> <span style="color:#666">11</span><span style="color:#666">);</span> |
| </span></span><span style="display:flex;"><span> |
| </span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic">// write out the Model |
| </span></span></span><span style="display:flex;"><span><span style="color:#080;font-style:italic"></span>model<span style="color:#666">.</span><span style="color:#b44">write</span><span style="color:#666">(</span>system<span style="color:#666">.</span><span style="color:#b44">out</span><span style="color:#666">,</span> <span style="color:#b44">"N-TRIPLE"</span><span style="color:#666">);</span> |
| </span></span></code></pre></div><p>The output produced is:</p> |
| <div class="highlight"><pre tabindex="0" style="background-color:#f8f8f8;-moz-tab-size:4;-o-tab-size:4;tab-size:4;"><code class="language-turtle" data-lang="turtle"><span style="display:flex;"><span><span style="">_</span><span style="color:#00f;font-weight:bold">:</span><span style="color:#008000;font-weight:bold">A</span>...<span style="color:#bbb"> </span><span style="color:#b8860b"><http://www.w3.org/2000/01/rdf-schema#label></span><span style="color:#bbb"> </span><span style="color:#b44">"11"</span><span style="color:#bbb"> </span>.<span style="color:#bbb"> |
| </span></span></span></code></pre></div><p>Since both literals are really just the string "11", then only one |
| statement is added.</p> |
| <p>The RDFCore WG has defined mechanisms for supporting datatypes in RDF. |
| Jena supports these using the <i>typed literal</i> mechanisms; they are |
| not discussed in this tutorial.</p> |
| <h2 id="ch-Glossary">Glossary</h2> |
| <dl> |
| <dt><a id="glos-blank node">Blank Node</a></dt> |
| <dd>Represents a resource, but does not indicate a URI for the |
| resource. Blank nodes act like existentially qualified variables in first |
| order logic.</dd> |
| <dt><a id="glos-dublin core">Dublin Core</a></dt> |
| <dd>A standard for metadata about web resources. Further |
| information can be found at the <a href="http://purl.oclc.org/dc/">Dublin Core |
| web site</a>.</dd> |
| <dt><a id="glos-Literal">Literal</a></dt> |
| <dd>A string of characters which can be the value of a property.</dd> |
| <dt><a id="glos-Object">Object</a></dt> |
| <dd>The part of a triple which is the value of the statement.</dd> |
| <dt><a id="glos-Predicate">Predicate</a></dt> |
| <dd>The property part of a triple.</dd> |
| <dt><a id="glos-Property">Property</a></dt> |
| <dd>A property is an attribute of a resource. For example |
| DC.title is a property, as is RDF.type. |
| <dt><a id="glos-Resource">Resource</a></dt> |
| <dd>Some entity. It could be a web resource such as web page, or |
| it could be a concrete physical thing such as a tree or a car. It could be an |
| abstract idea such as chess or football. Resources are named by URI's.</dd> |
| <dt><a id="glos-Statement">Statement</a></dt> |
| <dd>An arc in an RDF Model, normally interpreted as a fact.</dd> |
| <dt><a id="glos-Subject">Subject</a></dt> |
| <dd>The resource which is the source of an arc in an RDF Model</dd> |
| <dt><a id="glos-Triple">Triple</a></dt> |
|
