Creating Jena models
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
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
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
reasonerover the model
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
specwhich presents the ontology of
createOntologyModel(OntModelSpec spec, ModelMaker maker, Model base)Creates an OWL ontology model according to the
basemodel. If the ontology model needs to construct additional models (for OWL imports), use the
ModelMakerto create them. [The previous method will construct a
OntModelSpecs come from? There's a cluster of
constants in the class which provide for common uses; to name but
OntModelSpec.OWL_MEM_RDFS_INF OWL ontologies, model stored in
memory, using RDFS entailment only
OntModelSpec.RDFS_MEMRDFS ontologies, in memory, but doing no additional inferences
OntModelSpec.OWL_DL_MEM_RULE_INFOWL 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
mwhich 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.
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
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
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
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.
ModelMakerhas 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.
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
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.