title: Uma Introdução a RDF e à API RDF de Jena

Prefácio

Este é um tutorial introdutório ao framework de descrição de recursos (RDF) e Jena, uma API Java para RDF. Ele é escrito para programadores que não estão familiarizados com RDF e que aprendem melhor através de prototipagem, ou, por outros motivos, desejam avançar rapidamente para a implementação. Familiaridade com XML e Java é assumido.

Introdução {# id=“ch-Introduction” }

// some definitions
static String personURI    = "http://somewhere/JohnSmith";
static String fullName     = "John Smith";

// create an empty Model
Model model = ModelFactory.createDefaultModel();

// create the resource
Resource johnSmith = model.createResource(personURI);

// add the property
johnSmith.addProperty(VCARD.FN, fullName);

Resource johnSmith =
       model.createResource(personURI)
            .addProperty(VCARD.FN, fullName);

// some definitions
String personURI    = "http://somewhere/JohnSmith";
String givenName    = "John";
String familyName   = "Smith";
String fullName     = givenName + " " + familyName;

// create an empty Model
Model model = ModelFactory.createDefaultModel();

// create the resource
//   and add the properties cascading style
Resource johnSmith
  = model.createResource(personURI)
         .addProperty(VCARD.FN, fullName)
         .addProperty(VCARD.N,
                      model.createResource()
                           .addProperty(VCARD.Given, givenName)
                           .addProperty(VCARD.Family, familyName));

Sentenças {# id=“ch-Statements” }

// list the statements in the Model
StmtIterator iter = model.listStatements();

// print out the predicate, subject and object of each statement
while (iter.hasNext()) {
    Statement stmt      = iter.nextStatement();  // get next statement
    Resource  subject   = stmt.getSubject();     // get the subject
    Property  predicate = stmt.getPredicate();   // get the predicate
    RDFNode   object    = stmt.getObject();      // get the object

    System.out.print(subject.toString());
    System.out.print(" " + predicate.toString() + " ");
    if (object instanceof Resource) {
       System.out.print(object.toString());
    } else {
        // object is a literal
        System.out.print(" \"" + object.toString() + "\"");
    }

    System.out.println(" .");
}

http://somewhere/JohnSmith http://www.w3.org/2001/vcard-rdf/3.0#N anon:14df86:ecc3dee17b:-7fff .
anon:14df86:ecc3dee17b:-7fff http://www.w3.org/2001/vcard-rdf/3.0#Family  "Smith" .
anon:14df86:ecc3dee17b:-7fff http://www.w3.org/2001/vcard-rdf/3.0#Given  "John" .
http://somewhere/JohnSmith http://www.w3.org/2001/vcard-rdf/3.0#FN  "John Smith"
.

Escrita de RDF {# id=“ch-Writing-RDF” }

// now write the model in XML form to a file
model.write(System.out);

<rdf:RDF
  xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
  xmlns:vcard='http://www.w3.org/2001/vcard-rdf/3.0#'
 >
  <rdf:Description rdf:about='http://somewhere/JohnSmith'>
    <vcard:FN>John Smith</vcard:FN>
    <vcard:N rdf:nodeID="A0"/>
  </rdf:Description>
  <rdf:Description rdf:nodeID="A0">
    <vcard:Given>John</vcard:Given>
    <vcard:Family>Smith</vcard:Family>
  </rdf:Description>
</rdf:RDF>

// now write the model in a pretty form
RDFDataMgr.write(System.out, model, Lang.RDFXML);

// now write the model in XML form to a file
RDFDataMgr.write(System.out, model, Lang.NTRIPLES);

Leitura de RDF {# id=“ch-Reading-RDF” }

// create an empty model
Model model = ModelFactory.createDefaultModel();

// use the RDFDataMgr to find the input file
InputStream in = RDFDataMgr.open( inputFileName );
if (in == null) {
    throw new IllegalArgumentException(
                                 "File: " + inputFileName + " not found");
}

// read the RDF/XML file
model.read(in, null);

// write it to standard out
model.write(System.out);

<rdf:RDF
  xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
  xmlns:vcard='http://www.w3.org/2001/vcard-rdf/3.0#'
 >
  <rdf:Description rdf:nodeID="A0">
    <vcard:Family>Smith</vcard:Family>
    <vcard:Given>John</vcard:Given>
  </rdf:Description>
  <rdf:Description rdf:about='http://somewhere/JohnSmith/'>
    <vcard:FN>John Smith</vcard:FN>
    <vcard:N rdf:nodeID="A0"/>
  </rdf:Description>
  <rdf:Description rdf:about='http://somewhere/SarahJones/'>
    <vcard:FN>Sarah Jones</vcard:FN>
    <vcard:N rdf:nodeID="A1"/>
  </rdf:Description>
  <rdf:Description rdf:about='http://somewhere/MattJones/'>
    <vcard:FN>Matt Jones</vcard:FN>
    <vcard:N rdf:nodeID="A2"/>
  </rdf:Description>
  <rdf:Description rdf:nodeID="A3">
    <vcard:Family>Smith</vcard:Family>
    <vcard:Given>Rebecca</vcard:Given>
  </rdf:Description>
  <rdf:Description rdf:nodeID="A1">
    <vcard:Family>Jones</vcard:Family>
    <vcard:Given>Sarah</vcard:Given>
  </rdf:Description>
  <rdf:Description rdf:nodeID="A2">
    <vcard:Family>Jones</vcard:Family>
    <vcard:Given>Matthew</vcard:Given>
  </rdf:Description>
  <rdf:Description rdf:about='http://somewhere/RebeccaSmith/'>
    <vcard:FN>Becky Smith</vcard:FN>
    <vcard:N rdf:nodeID="A3"/>
  </rdf:Description>
</rdf:RDF>

Controlando prefixos {# id=“ch-Prefixes” }

Definições explícitas de prefixos

Na sessão anterior, nós vimos que a saída XML declarou um prefixo namespace
vcard e o usou para abreviar URIs. Enquanto que RDF usa somente URIs completas, e não sua forma encurtada, Jena provê formas de controlar namespaces usados na saída com seu mapeamento de prefixos. Aqui vai um exemplo simples.

Model m = ModelFactory.createDefaultModel();
String nsA = "http://somewhere/else#";
String nsB = "http://nowhere/else#";
Resource root = m.createResource( nsA + "root" );
Property P = m.createProperty( nsA + "P" );
Property Q = m.createProperty( nsB + "Q" );
Resource x = m.createResource( nsA + "x" );
Resource y = m.createResource( nsA + "y" );
Resource z = m.createResource( nsA + "z" );
m.add( root, P, x ).add( root, P, y ).add( y, Q, z );
System.out.println( "# -- no special prefixes defined" );
m.write( System.out );
System.out.println( "# -- nsA defined" );
m.setNsPrefix( "nsA", nsA );
m.write( System.out );
System.out.println( "# -- nsA and cat defined" );
m.setNsPrefix( "cat", nsB );
m.write( System.out );

A saída deste fragmento são três blocos de RDF/XML, com três diferentes mapeamentos de prefixos. Primeiro o padrão, sem prefixos diferentes dos padrões:

# -- no special prefixes defined

<rdf:RDF
    xmlns:j.0="http://nowhere/else#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:j.1="http://somewhere/else#" >
  <rdf:Description rdf:about="http://somewhere/else#root">
    <j.1:P rdf:resource="http://somewhere/else#x"/>
    <j.1:P rdf:resource="http://somewhere/else#y"/>
  </rdf:Description>
  <rdf:Description rdf:about="http://somewhere/else#y">
    <j.0:Q rdf:resource="http://somewhere/else#z"/>
  </rdf:Description>
</rdf:RDF>

Nós vimos que o namespace rdf é declarado automaticamente, já que são requeridos para tags como <RDF:rdf> e <rdf:resource>. Declarações de namespace são também necessárias para o uso das duas propriedades P e Q, mas já que seus namespaces não foram introduzidos no modelo, eles recebem nomes namespaces inventados j.0 e j.1.

# -- nsA defined

<rdf:RDF
    xmlns:j.0="http://nowhere/else#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:nsA="http://somewhere/else#" >
  <rdf:Description rdf:about="http://somewhere/else#root">
    <nsA:P rdf:resource="http://somewhere/else#x"/>
    <nsA:P rdf:resource="http://somewhere/else#y"/>
  </rdf:Description>
  <rdf:Description rdf:about="http://somewhere/else#y">
    <j.0:Q rdf:resource="http://somewhere/else#z"/>
  </rdf:Description>
</rdf:RDF>

O outro namespace ainda recebe o nome criado automaticamente, mas o nome nsA é agora usado nas tags de propriedades. Não há necessidade de que o nome do prefixo tenha alguma relação com as variáveis do código Jena:

# -- nsA and cat defined

<rdf:RDF
    xmlns:cat="http://nowhere/else#"
    xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
    xmlns:nsA="http://somewhere/else#" >
  <rdf:Description rdf:about="http://somewhere/else#root">
    <nsA:P rdf:resource="http://somewhere/else#x"/>
    <nsA:P rdf:resource="http://somewhere/else#y"/>
  </rdf:Description>
  <rdf:Description rdf:about="http://somewhere/else#y">
    <cat:Q rdf:resource="http://somewhere/else#z"/>
  </rdf:Description>
</rdf:RDF>

Ambos os prefixos são usados na saída, e não houve a necessidade de prefixos gerados automaticamente.

Definições implícitas de prefixos

Assim como as declarações de prefixos definidas por chamadas a setNsPrefix, Jena vai lembrar-se dos prefixos que foram usados na entrada para model.read().

Model m2 = ModelFactory.createDefaultModel();
m2.read( "file:/tmp/fragment.rdf" );
m2.write( System.out );

Você verá que os prefixos da entrada são preservados na saída. Todos os prefixos são escritos, mesmo se eles não forem usados em lugar algum. Você pode remover um prefixo com removeNsPrefix(String prefix) se você não o quiser na saída.

Pacotes Jena RDF {# id=“ch-Jena-RDF-Packages” }

Navegação em Modelos {# id=“ch-Navigating-a-Model” }

// retrieve the John Smith vcard resource from the model
Resource vcard = model.getResource(johnSmithURI);

// retrieve the value of the N property
Resource name = (Resource) vcard.getProperty(VCARD.N)
                                .getObject();

// retrieve the value of the N property
Resource name = vcard.getProperty(VCARD.N)
                     .getResource();

String fullName = vcard.getProperty(VCARD.FN)
                        .getString();

// add two nickname properties to vcard
vcard.addProperty(VCARD.NICKNAME, "Smithy")
     .addProperty(VCARD.NICKNAME, "Adman");

// set up the output
System.out.println("The nicknames of \""
                      + fullName + "\" are:");
// list the nicknames
StmtIterator iter = vcard.listProperties(VCARD.NICKNAME);
while (iter.hasNext()) {
    System.out.println("    " + iter.nextStatement()
                                    .getObject()
                                    .toString());
}

The nicknames of "John Smith" are:
    Smithy
    Adman

Consultas em Modelos {# id=“ch-Querying-a-Model” }

// list vcards
ResIterator iter = model.listSubjectsWithProperty(VCARD.FN);
while (iter.hasNext()) {
    Resource r = iter.nextResource();
    ...
}

Selector selector = new SimpleSelector(subject, predicate, object);

Selector selector = new SimpleSelector(null, null, null);

Selector selector = new SimpleSelector(null, VCARD.FN, null);

listStatements( S, P, O )

é equivalente a

listStatements( new SimpleSelector( S, P, O ) )

// select all the resources with a VCARD.FN property
ResIterator iter = model.listSubjectsWithProperty(VCARD.FN);
if (iter.hasNext()) {
    System.out.println("The database contains vcards for:");
    while (iter.hasNext()) {
        System.out.println("  " + iter.nextResource()
                                      .getProperty(VCARD.FN)
                                      .getString());
    }
} else {
    System.out.println("No vcards were found in the database");
}

The database contains vcards for:
  Sarah Jones
  John Smith
  Matt Jones
  Becky Smith

// select all the resources with a VCARD.FN property
// whose value ends with "Smith"
StmtIterator iter = model.listStatements(
    new SimpleSelector(null, VCARD.FN, (RDFNode) null) {
        public boolean selects(Statement s)
            {return s.getString().endsWith("Smith");}
    });

The database contains vcards for:
  John Smith
  Becky Smith

// do all filtering in the selects method
StmtIterator iter = model.listStatements(
  new
      SimpleSelector(null, null, (RDFNode) null) {
          public boolean selects(Statement s) {
              return (subject == null   || s.getSubject().equals(subject))
                  &amp;&amp; (predicate == null || s.getPredicate().equals(predicate))
                  &amp;&amp; (object == null    || s.getObject().equals(object)) ;
          }
     }
     });

StmtIterator iter =
  model.listStatements(new SimpleSelector(subject, predicate, object)

Operações em Modelos {# id=“ch-Operations-on-Models” }

// read the RDF/XML files
model1.read(new InputStreamReader(in1), "");
model2.read(new InputStreamReader(in2), "");

// merge the Models
Model model = model1.union(model2);

// print the Model as RDF/XML
model.write(system.out, "RDF/XML-ABBREV");

<rdf:RDF
    xmlns:rdf="<a href="http://www.w3.org/1999/02/22-rdf-syntax-ns#">http://www.w3.org/1999/02/22-rdf-syntax-ns#</a>"
    xmlns:vcard="http://www.w3.org/2001/vcard-rdf/3.0#">
  <rdf:Description rdf:about="http://somewhere/JohnSmith/">
    <vcard:EMAIL>
      <vcard:internet>
        <rdf:value>John@somewhere.com</rdf:value>
      </vcard:internet>
    </vcard:EMAIL>
    <vcard:N rdf:parseType="Resource">
      <vcard:Given>John</vcard:Given>
      <vcard:Family>Smith</vcard:Family>
    </vcard:N>
    <vcard:FN>John Smith</vcard:FN>
  </rdf:Description>
</rdf:RDF>

Containers {# id=“ch-Containers” }

// create a bag
Bag smiths = model.createBag();

// select all the resources with a VCARD.FN property
// whose value ends with "Smith"
StmtIterator iter = model.listStatements(
    new SimpleSelector(null, VCARD.FN, (RDFNode) null) {
        public boolean selects(Statement s) {
                return s.getString().endsWith("Smith");
        }
    });
// add the Smith's to the bag
while (iter.hasNext()) {
    smiths.add(iter.nextStatement().getSubject());
}

<rdf:RDF
  xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
  xmlns:vcard='http://www.w3.org/2001/vcard-rdf/3.0#'
 >
...
  <rdf:Description rdf:nodeID="A3">
    <rdf:type rdf:resource='http://www.w3.org/1999/02/22-rdf-syntax-ns#Bag'/>
    <rdf:_1 rdf:resource='http://somewhere/JohnSmith/'/>
    <rdf:_2 rdf:resource='http://somewhere/RebeccaSmith/'/>
  </rdf:Description>
</rdf:RDF>

// print out the members of the bag
NodeIterator iter2 = smiths.iterator();
if (iter2.hasNext()) {
    System.out.println("The bag contains:");
    while (iter2.hasNext()) {
        System.out.println("  " +
            ((Resource) iter2.next())
                            .getProperty(VCARD.FN)
                            .getString());
    }
} else {
    System.out.println("The bag is empty");
}

The bag contains:
  John Smith
  Becky Smith

Mais sobre Literais e Datatypes {# id=“ch-More-about-Literals-and-Datatypes” }

// create the resource
Resource r = model.createResource();

// add the property
r.addProperty(RDFS.label, model.createLiteral("chat", "en"))
 .addProperty(RDFS.label, model.createLiteral("chat", "fr"))
 .addProperty(RDFS.label, model.createLiteral("&lt;em&gt;chat&lt;/em&gt;", true));

// write out the Model
model.write(system.out);

<rdf:RDF
  xmlns:rdf='http://www.w3.org/1999/02/22-rdf-syntax-ns#'
  xmlns:rdfs='http://www.w3.org/2000/01/rdf-schema#'
 >
  <rdf:Description rdf:nodeID="A0">
    <rdfs:label xml:lang='en'>chat</rdfs:label>
    <rdfs:label xml:lang='fr'>chat</rdfs:label>
    <rdfs:label rdf:parseType='Literal'><em>chat</em></rdfs:label>
  </rdf:Description>
</rdf:RDF>

// create the resource
Resource r = model.createResource();

// add the property
r.addProperty(RDFS.label, "11")
 .addProperty(RDFS.label, 11);

// write out the Model
model.write(system.out, "N-TRIPLE");

_:A... <http://www.w3.org/2000/01/rdf-schema#label> "11" .

Glossário {# id=“ch-Glossary” }