Difference between revisions of "ProtegeOWL API Programmers Guide"

Revision as of 15:18, May 13, 2010

Overview

The Protege-OWL API is an open-source Java library for the Web Ontology Language (OWL) and RDF(S). The API provides classes and methods to load and save OWL files, to query and manipulate OWL data models, and to perform reasoning based on Description Logic engines. Furthermore, the API is optimized for the implementation of graphical user interfaces.

The API is designed to be used in two contexts:

• For the development of components that are executed inside of the Protege-OWL editor's user interface
• For the development of stand-alone applications (e.g., Swing applications, Servlets, or Eclipse plug-ins)

Protege is a flexible, configurable platform for the development of arbitrary model-driven applications and components. Protege has an open architecture that allows programmers to integrate plug-ins, which can appear as separate tabs, specific user interface components (widgets), or perform any other task on the current model. The Protege-OWL editor provides many editing and browsing facilities for OWL models, and therefore can serve as an attractive starting point for rapid application development. Developers can initially wrap their components into a Protege tab widget and later extract them to distribute them as part of a stand-alone application.

This guide will introduce you to some basic concepts of the Protege-OWL API, no matter whether you intend to use it for stand-alone applications or Protege plug-ins. The guide tries to illustrate the API by examples, and most examples create parts of OWL ontologies such as properties and restrictions. The complete source code for most of the examples can be found in the edu.stanford.smi.protegex.owlx.examples package in the Protege-OWL source code. If you have trouble getting started, please post questions on the protege-owl mailing list.

Installation & Getting Started

The Protege-OWL editor is provided with the standard installation of Protege. In addition to installing Protege, you should download the source code from the Protege Subversion repository. The source code is the most reliable reference for the system's functionality and browsing the source code with a Java IDE is a great way to learn the API. Detailed instructions on how to download source code from our repository are available on the main Protege website. The URL to download the Protege-OWL source code using a Subversion client is:

http://smi-protege.stanford.edu/repos/protege/owl/trunk/.

You will also find Javadoc for the API useful. In particular, you will need to understand the interfaces from the edu.stanford.smi.protegex.owl.model package.

Following a successful installation, your directory structure should look approximately as follows:

Directory structure of the Protege application after a successful install

Now let us configure our Java project and write a small "Hello World" application. If you are using a Java IDE such as Eclipse or IntelliJ, select the Protege installation folder as your project home. Next, add all the JAR files from the installation to your project classpath. Set your compiler output path to plugins/classes.

Then create a Java class such as the following:

package com.demo.application;

import edu.stanford.smi.protegex.owl.model.OWLModel;
import edu.stanford.smi.protegex.owl.model.OWLNamedClass;
import edu.stanford.smi.protegex.owl.ProtegeOWL;

public class OWLAPIDemoApplication {

    public static void main(String[] args) {
        OWLModel owlModel = ProtegeOWL.createJenaOWLModel();
        owlModel.getNamespaceManager().setDefaultNamespace("http://hello.com#");
        OWLNamedClass worldClass = owlModel.createOWLNamedClass("World");
        System.out.println("Class URI: " + worldClass.getURI());
    }
}

Execute this program stand-alone. The output should be "Class URI: http://hello.com#World".

Basics

Working with OWL Models

The Protege-OWL API is centered around a collection of Java interfaces from the model package. These interfaces provide access to the OWL model and its elements like classes, properties, and individuals. Application developers should not access the implementation of these interfaces (such as DefaultRDFIndividual) directly, but only operate on the interfaces. Using these interfaces you don't have to worry about the internal details of how Protege stores ontologies. Everything is abstracted into interfaces and your code should not make any assumptions about the specific implementation.

The most important model interface is OWLModel, which provides access to the top-level container of the resources in the ontology. You can use OWLModel to create, query, and delete resources of various types and then use the objects returned by the OWLModel to do specific operations. For example, the following snippet creates a new OWLNamedClass (which corresponds to owl:Class in OWL), and then gets its URI:

OWLModel owlModel = ProtegeOWL.createJenaOWLModel();
OWLNamedClass worldClass = owlModel.createOWLNamedClass("World");
System.out.println("Class URI: " + worldClass.getURI());

Note that the class ProtegeOWL provides a couple of convenient static methods to create OWLModels, also from existing OWL files. For example, you can load an existing ontology from the web using

String uri = "http://www.co-ode.org/ontologies/pizza/2007/02/12/pizza.owl";
OWLModel owlModel = ProtegeOWL.createJenaOWLModelFromURI(uri);

Names, Namespace prefixes, and URIs

OWL and RDF resources are globally identified by their URIs, such as http://www.owl-ontologies.com/travel.owl#Destination. However, since URIs are long and often inconvenient to handle, the primary access and identification mechanism for ontological resources in the Protege-OWL API is their name. A name is a short form consisting of local name and an optional prefix. Prefixes are typically defined in the ontology to abbreviate names of imported resources. For example, instead of writing http://www.w3.org/2002/07/owl#Class, we can write owl:Class because "owl" is a prefix for the namespace "http://www.w3.org/2002/07/owl#". Similarly, if an ontology imports the travel ontology from above, it can define a prefix to access all imported classes and properties with "travel", e.g. travel:Destination. If we are inside the default namespace of the ontology, the prefix is empty, i.e., the name of the resource is only Destination. Application developers can take control of namespace prefixes using the NamespaceManager object. In order to access the current NamespaceManager of an OWLModel, you can use OWLModel.getNamespaceManager(). Assuming we have loaded the travel ontology as a default namespace, we can access the resources contained in the OWLModel using the following example calls:

OWLNamedClass destinationClass = owlModel.getOWLNamedClass("Destination");
OWLObjectProperty hasContactProperty = owlModel.getOWLObjectProperty("hasContact");
OWLDatatypeProperty hasZipCodeProperty = owlModel.getOWLDatatypeProperty("hasZipCode");
OWLIndividual sydney = owlModel.getOWLIndividual("Sydney");

... and use the objects to perform further queries or operations on the corresponding resources. For example, in order to extract the URI for a named object, you can use the RDFResource.getURI() method.