Creating Jena models

Introduction

Jena is a moderately complicated system, with several different kinds of Model and ways of constructing them. This note describes the Jena ModelFactory, a one-stop shop for creating Jena models. ModelFactory lives in Java package com.hp.hpl.jena.rdf.model.

This note is an introduction, not an exhaustive description. As usual consult the Javadoc for details of the methods and classes to use.

Simple model creation

The simplest way to create a model (if not the shortest) is to call ModelFactory.createDefaultModel(). This [by default] delivers a plain RDF model, stored in-memory, that does no inference and has no special ontology interface.

Database model creation

Note: this section previously referred to creating database models with RDB, a now obsolete Jena features. For methods of creating models with SDB and TDB please see the relevant reference sections.

Inference model creation

An important feature of Jena is support for different kinds of inference over RDF-based models (used for RDFS and OWL). Inference models are constructed by applying reasoners to base models and optionally schema. The statements deduced by the reasoner from the base model then appear in the inferred model alongside the statements from the base model itself. RDFS reasoning is directly available:

• createRDFSModel(Model base) creates an inference model over the base model using the built-in RDFS inference rules and any RDFS statements in the base model.

• createRDFSModel(Model schema, Model base) creates an RDFS inference model from the base model and the supplied schema model. The advantage of supplying the schema separately is that the reasoner may be able to compute useful information in advance on the assumption that the schema won't change, or at least not change as often as the base model.

It's possible to use other reasoning systems than RDFS. For these a Reasoner is required:

• createInfModel(Reasoner reasoner, Model base) creates an inference model using the rules of reasoner over the model base.

• createInfModel(Reasoner reasoner, Model schema, Model base) Just as for the RDFS case, the schema may be supplied separately to allow the reasoner to digest them before working on the model.

From where do you fetch your reasoners? From the reasoner registry, the class ReasonerRegistry. This allows reasoners to be looked up by name, but also provides some predefined access methods for well-know reasoners:

• getOWLReasoner(): the reasoner used for OWL inference

• getRDFSReasoner(): the reasoner used for RDFS inference

• getTransitiveReasoner(): a reasoner for doing subclass and sub-property closure.

Ontology model creation

An ontology model is one that presents RDF as an ontology - classes, individuals, different kinds of properties, and so forth. Jena supports RDFS and OWL ontologies through profiles. There is extensive documentation on Jena's ontology support, so all we'll do here is summarise the creation methods.

• createOntologyModel() Creates an ontology model which is in-memory and presents OWL ontologies.

• createOntologyModel(OntModelSpec spec, Model base) Creates an ontology model according the OntModelSpec spec which presents the ontology of base.

• createOntologyModel(OntModelSpec spec, ModelMaker maker, Model base) Creates an OWL ontology model according to the spec over the base model. If the ontology model needs to construct additional models (for OWL imports), use the ModelMaker to create them. [The previous method will construct a MemModelMaker for this.]

Where do OntModelSpecs come from? There's a cluster of constants in the class which provide for common uses; to name but three: - OntModelSpec.OWL_MEM_RDFS_INF OWL ontologies, model stored in memory, using RDFS entailment only

• OntModelSpec.RDFS_MEM RDFS ontologies, in memory, but doing no additional inferences

• OntModelSpec.OWL_DL_MEM_RULE_INF OWL ontologies, in memory, with the full OWL Lite inference

Creating models from Assembler descriptions

A model can be built from a description of the required model. This is documented in the assembler howto. Access to the assembler system for model creation is provided by three ModelFactory methods:

• assembleModelFrom( Model singleRoot ): assemble a Model from the single Model description in singleRoot. If there is no such description, or more than one, an exception is thrown. If a description has to be selected from more than one available candidates, consider using the methods below.

• findAssemblerRoots( Model m ): answer a Set of all the Resources in m which are of type ja:Model, ie descriptions of models to assemble. (Note that this will include sub-descriptions of embedded models if they are present.)

• assembleModelFrom( Resource root ): answer a Model assembled according to the description hanging from root. Assemblers can construct other things as well as models, and the Assembler system is user-extensible: see the howto for details.

File-based models

The method ModelFactory.createFileModelMaker(String) returns a ModelMaker which attaches models to filing-system files. The String argument is the fileBase. When a file-ModelMaker opens a file, it reads it from a file in the directory named by the fileBase; when the model is closed (and only then, in the current implementation), the contents of the model are written back to the file.

Because the names of models in a modelMaker can be arbitrary character strings, in particular URIs, they are translated slightly to avoid confusion with significant characters of common filing systems. In the current implementation,

• colon : is converted to \_C
• slash \/ is converted to \_S
• underbar _ is converted to \_U

ModelMakers

Plain models can be given names which allows them to be "saved" and looked up by name later. This is handled by implementations of the interface ModelMaker; each ModelMaker produces Models of the same kind. The simplest kind of ModelMaker is a memory model maker, which you get by calling ModelFactory.createMemModelMaker(). The methods you'd want to use to start with on a ModelMaker are:

• createModel(String): create a model with the given name in the ModelMaker. If a model with that name already exists, then that model is used instead.

• openModel(String): open an existing model with the given name. If no such model exists, create a new empty one and give it that name. [createModel(String) and openModel(String) behave in the same way, but each has a two-argument form for which the behaviour is different. Use whichever one best fits your intention.]

• createModel(): create a fresh anonymous model.

• getModel(): each ModelMaker has a default model; this method returns that model.

There are other methods, for removing models, additional control over create vs open, closing the maker, and looking names up; for those consult the ModelMaker JavaDoc.

Miscellany

Finally, ModelFactory contains a collection of methods for some special cases not conveniently dealt with elsewhere.

createModelForGraph(Graph g) is used when an advanced user with access to the Jena SPI has constructed or obtained a Graph and wishes to present it as a model. This method wraps the graph up as a plain model. Alterations to the graph are visible in the model, and vice versa.