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An Introduction to RDF and the Jena RDF API

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Outline

Introduction Statements Writing RDF Reading RDF Navigating a Graph Querying a Graph Operations on Graphs

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Introduction An example of RDF :

3-Tuple representation : {creator ， [http://a.b.com] ， [John]}

Graph representation :

http://a.b.com

creator

John

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Introduction The Resource Description Framework (RDF)

is a standard for describing resources. A resource is anything we can identify. The resources in this tutorial will be about p

eople. They use an RDF representation of VCARDS.

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Introduction (contd.) RDF is best thought of in the form of

node and arc diagrams. A simple vcard might look like this in RDF:

resource

property

value◆ Fig 1.

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Introduction (contd.) Jena is a Java API which can be used to

create and manipulate RDF graphs. Jena has object classes to represent

graphs, resources and properties.

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Introduction (contd.) The code to create the model above is : // some definitions static String personURI = "http://somewhere/JohnSmit

h"; static String fullName = "John Smith";

// create an empty graph Model model = new ModelMem();

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

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

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Introduction (contd.) The code above can be more compactly written in a c

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

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

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Introduction (contd.) RDF properties can also take other

resources as their value.

property

Value (blank node)

◆ Fig 2.

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Introduction (contd.) The Jena code to construct the example above is :// some definitionsString personURI = "http://somewhere/JohnSmith";String givenName = "John";String familyName = "Smith";String fullName = givenName + " " + familyName;

// create an empty graphModel model = new ModelMem();

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

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Statements Each arc in an RDF graph is called a

statement. Each statement asserts a fact about a resource.

A statement has three parts: ˙subject

˙predicate ˙object A statement is sometimes called a triple, because of its three parts.

John Smith

http://.../JohnSmith

vcard:FN

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Statements (contd.) We can run a program to get the statements in the Fig2. anon:150bd4d:1015b258a60:-7fff http://www.w3.org/2001/vcard-rdf/3.0#Family "Smit

h" . http://somewhere/JohnSmith http://www.w3.org/2001/vcard-rdf/3.0#FN "John Smith". anon:150bd4d:1015b258a60:-7fff http://www.w3.org/2001/vcard-rdf/3.0#Given "John" . http://somewhere/JohnSmith http://www.w3.org/2001/vcard-rdf/3.0#N anon:150bd4d:1015b258a60:-7fff .

1234

1

2

3

4

◆ Fig 2.

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Writing RDF Jena has methods for reading and writing R

DF as XML. These can be used to save an RDF model to a file and later, read it back in again.

It can be easily done by creating a PrintWriter and call the model.write method.

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

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Reading RDF

Jena also has methods for reading the statements recorded in RDF XML form into a model.

The code is : // use the class loader to find the input file InputStream in = Tutorial05.class .getClassLoader() .getResourceAsStream(inputFileName); // read the RDF/XML file model.read(new InputStreamReader(in), "");

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Navigating a Graph Given the URI of a resource, the resource object ca

n be retrieved from a model. // retrieve the John Smith vcard resource from the model Resource vcard = model.getResource(johnSmithURI); Also can accessing a property of the resource. // retrieve the value of the N property Resource name = vcard.getProperty(VCARD.N) .getResource(); // retrieve the given name property String fullName = vcard.getProperty(VCARD.FN) .getString();

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Navigating a Graph (contd.)

The graph of the code above is :

http://...

John Smith

John Smith

vcard:FN

vcard:Familyvcard:Given

vcard:N

Resource vcard

http://...

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Navigating a Graph (contd.)

RDF permits a resource to repeat a property :

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

http://...

John Smith

John Smith

vcard:FN

vcard:Familyvcard:Given

vcard:NSmithy

Adman

vcard:NICKNAMEvcard:NICKNAME

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Querying a Graph For example, we can select all the resources

with a VCARD.FN property in the model :

John Smith

Tom Smith

Tom Taylor

vcard:FN

vcard:FN

vcard:N

vcard:FN

http://...

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Querying a Graph (contd.) We also can select all the resources with a

VCARD.FN property whose value ends with “Smith” :

John Smith

Tom Smith

Tom Taylor

vcard:FN

vcard:FN

vcard:N

vcard:FN

http://...

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Operations on Graphs Jena provides three operations for

manipulating graphs as a whole : ˙ union, intersection and difference. Consider the following two graphs :

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Operations on Graphs (contd.)

When the two models above are merged, the

graph will be :