Jena Assembler howto

Introduction

This document describes the vocabulary and effect of the built-in Jena assembler descriptions for constructing models (and other things). A companion document describes the built-in assembler classes and how to write and integrate your own assemblers. If you just need a quick guide to the common model specifications, see the assembler quickstart.

This document describes how to use the Assembler classes to construct models – and other things – from RDF descriptions that use the Jena Assembler vocabulary. That vocabulary is available in assembler.ttl as an RDFS schema with conventional prefix ja for the URI http://jena.hpl.hp.com/2005/11/Assembler#; the class JA is its Java rendition.

The examples used in this document are extracted from the examples file examples.ttl. The pieces of RDF/OWL schema are extracted from the ja-vocabulary file.

The property names selected are those which are the “declared properties” (as per Jena’s listDeclaredProperties method) of the class. Only the most specialised super-classes and range classes are shown, so (for example) rdf:Resource typically won’t appear.

Overview

An Assembler specification is a Resource in some RDF Model. The properties of that Resource describe what kind of object is to be assembled and what its components are: for example, an InfModel is constructed by specifying a base model and a reasoner. The specifications for the components are themselves Assembler specifications given by other Resources in the same Model.For example, to specify a memory model with data loaded from a file:

eg:model a ja:MemoryModel
    ; ja:content [ja:externalContent <file:////home/kers/projects/jena2/doc/assembler/Data/example.n3>]
    .

The rdf:type of eg:model specifies that the constructed Model is to be a Jena memory-based model. The ja:content property specifies that the model is to be loaded with the content of the resource file:Data/example.n3. The content handler guesses from the “.n3” suffix that this file is to be read using the Jena N3 reader.

Unless otherwise specified by an application, Assembler specifications are interpreted after completion by

1. including the JA schema,
2. including (recursively) the objects of any owl:imports and ja:imports statements, and
3. doing (limited) RDFS inference.

(The supplied model is not modified.) In the example above, eg:model has to be given an explicit type, but the ja:externalContent bnode is implicitly typed by the domain of ja:externalContent. In this document, we will usually leave out inferrable types.

We can construct our example model from the specification like this (you may need to tweak the filename to make this work in your environment):

Model spec = RDFDataMgr.loadModel( "examples.ttl" );
Resource root = spec.createResource( spec.expandPrefix( "eg:opening-example" ) );
Model m = Assembler.general.openModel( root );

The model is constructed from the “root resource”, eg:opening-example in our example. general knows how to create all the kinds of objects - not just Models - that we describe in the next sections.

Specifications common to all models

Assembler specifications can describe many kinds of models: memory, inference, ontology, and file-backed. All of these model specifications share a set of base properties for attaching content and prefix mappings.

ja:Loadable a rdfs:Class ;
  rdfs:subClassOf ja:Object
.
ja:initialContent a rdf:Property ;
  rdfs:domain ja:Loadable
  rdfs:range ja:Content
.
ja:content a rdf:Property ;
  rdfs:domain ja:Loadable ;
  rdfs:range ja:Content
.

ja:Model a rdfs:Class ;
  rdfs:subClassOf ja:ContentItem ;
  rdfs:subClassOf ja:Loadable
.

ja:prefixMapping a rdf:Property ;
  rdfs:domain ja:Model ;
  rdfs:range ja:PrefixMapping
.

All of a model’s ja:content property values are interpreted as specifying Content objects and a single composite Content object is constructed and used to initialise the model. See Content for the description of Content specifications. For example:

eg:sharedContent
    ja:externalContent <http://somewhere/RDF/ont.owl>
    .
eg:common-example a ja:MemoryModel ;
      ja:content eg:sharedContent ;
      ja:content [ja:externalContent <file:////home/kers/projects/jena2/doc/assembler/Data/A.rdf>] ;
      ja:content [ja:externalContent <file:////home/kers/projects/jena2/doc/assembler/Data/B.rdf>]
    .

The model constructed for eg:A will be loaded with the contents of Data/A.n3, Data/B.rdf, and http://somewhere/RDF/ont.owl. If the model supports transactions, then the content is loaded inside a transaction; if the load fails, the transaction is aborted, and a TransactionAbortedException thrown. If the content has any prefix mappings, then they are also added to the model.

All of a model’s ja:prefixMapping, ja:prefix, and ja:namespace properties are interpreted as specifying a PrefixMapping object and a single composite PrefixMapping is constructed and used to set the prefixes of the model. See PrefixMapping for the description of Content specifications.

Content specification

A Content specification describes content that can be used to fill models. Content can be external (files and URLs) or literal (strings in the specification) or quotations (referring to RDF which is part of the specification).

ja:Content a rdfs:Class ;
  rdfs:subClassOf ja:HasFileManager
.

ja:HasFileManager a rdfs:Class ;
  rdfs:subClassOf ja:Object
.
ja:fileManager a rdf:Property ;
  rdfs:domain ja:HasFileManager ;
  rdfs:range ja:FileManager
.

A ja:Content specification may have zero or more ja:externalContent property values. These are URI resources naming an external (file or http etc) RDF object. The constructed Content object contains the union of the values of all such resources. For example:

eg:external-content-example
    ja:externalContent <file:////home/kers/projects/jena2/doc/assembler/Data/C.owl>,
                       <http://jena.hpl.hp.com/some-jena-data.rdf>
    .

The external content is located using a FileManager. If the Content resource has a ja:fileManager property, then the FileManager described by that resource is used. Otherwise, if the ContentAssembler assembling this specification was constructed with a FileManager argument, that FileManager is used. Otherwise, the default FileManager, FileManager.get(), is used.

The string literal value of the any ja:literalContent properties is interpreted as RDF in an appropriate language. The constructed Content object contains that RDF. The language is either specified by an explicit ja:contentEncoding property value, or guessed from the content of the string. The only encodings permitted are “N3” and “RDF/XML”. For example:

eg:literal-content-example
    ja:literalContent "_:it dc:title 'Interesting Times'"
    .

The literal content is wrapped so that prefix declarations for rdf, rdfs, owl, dc, and xsd apply before interpretation.

The property values of any ja:quotedContent properties should be resources. The subgraphs rooted at those resources (using the algorithm from ResourceUtils.reachableClosure()) are added to the content.

Inference models and reasoners

Inference models are specified by supplying a description of the reasoner that is used by the model and (optionally) a base model to reason over. For example:

eg:inference-example
    ja:baseModel [a ja:MemoryModel] ;
      ja:reasoner [ja:reasonerURL <http://jena.hpl.hp.com/2003/RDFSExptRuleReasoner>]
    .

describes an inference model that uses RDFS reasoning. The reasonerURL property value is the URI used to identify the reasoner (it is the value of the Jena constant RDFSRuleReasonerFactory.URI). The base model is specified as a memory model; if it is left out, an empty memory model is used.

eg:db-inference-example
    ja:baseModel eg:model-example ;
    ja:reasoner [ja:reasonerURL <http://jena.hpl.hp.com/2003/RDFSExptRuleReasoner>]
    .

The same reasoner as used as in the previous example, but now the base model is a specific model description in the same way as our earlier example.

Because Jena’s access to external reasoners goes through the same API as for its internal reasoners, you can access a DIG reasoner (such as Pellet running as a server) using an Assembler specification:

eg:external-inference-example
    ja:reasoner [<http://jena.hpl.hp.com/2003/JenaReasoner#extReasonerURL>
                   <http://localhost:2004/> ;
                 ja:reasonerURL <http://jena.hpl.hp.com/2003/DIGReasoner>]
    .

If there’s a DIG server running locally on port 2004, this specification will create a DIG inference model that uses it.

The internal rule reasoner can be supplied with rules written inside the specification, or outside from some resource (file or http: URL): eg:rule-inference-example ja:reasoner [ja:rule “[r1: (?x my:P ?y) -> (?x rdf:type my:T)]”] .

This reasoner will infer a type declaration from a use of a property. (The prefix my will have to be known to the rule parser, of course.)

ja:InfModel a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:reasoner; owl:maxCardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:baseModel; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:Model
.
ja:reasoner a rdf:Property ;
  rdfs:domain ja:InfModel ;
  rdfs:range ja:ReasonerFactory
.
ja:baseModel a rdf:Property ;
  rdfs:domain ja:InfModel ;
  rdfs:range ja:Model
.

ja:HasRules a rdfs:Class ;
  rdfs:subClassOf ja:Object
.
ja:rule a rdf:Property ;
  rdfs:domain ja:HasRules
.
ja:rulesFrom a rdf:Property ;
  rdfs:domain ja:HasRules
.
ja:rules a rdf:Property ;
  rdfs:domain ja:HasRules ;
  rdfs:range ja:RuleSet
.

An InfModel’s ja:baseModel property value specifies the base model for the inference model; if omitted, an empty memory model is used.

An InfModel’s ja:ReasonerFactory property value specifies the Reasoner for this inference model; if omitted, a GenericRuleReasoner is used.

A Reasoner’s optional ja:schema property specifies a Model which contains the schema for the reasoner to be bound to. If omitted, no schema is used.

If the Reasoner is a GenericRuleReasoner, it may have any of the RuleSet properties ja:rules, ja:rulesFrom, or ja:rule. The rules of the implied RuleSet are added to the Reasoner.

ReasonerFactory

A ReasonerFactory can be specified by URL or by class name (but not both).

ja:ReasonerFactory a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:ReasonerURL; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:HasRules
.
ja:reasonerClass a rdf:Property ;
  rdfs:domain ja:ReasonerFactory
.
ja:reasonerURL a rdf:Property ;
  rdfs:domain ja:ReasonerFactory
.
ja:schema a rdf:Property ;
  rdfs:domain ja:ReasonerFactory ;
  rdfs:range ja:Model
.

If the optional unique property ja:reasonerURL is specified, then its resource value is the URI of a reasoner in the Jena reasoner registry; the reasoner is the one with the given URI.

If the optional property ja:schema is specified, then the models specified by all the schema properties are unioned and any reasoner produced by the factory will have that union bound in as its schema (using the Reasoner::bindSchema() method).

If the optional unique property ja:reasonerClass is specified, its value names a class which implements ReasonerFactory. That class is loaded and an instance of it used as the factory.

The class may be named by the lexical form of a literal, or by a URI with the (fake) “java:” scheme.

If the class has a method theInstance, that method is called to supply the ReasonerFactory instance to use. Otherwise, a new instance of that class is constructed. Jena’s reasoner factories come equipped with this method; for other factories, see the documentation.

Rulesets

A RuleSet specification allows rules (for ReasonerFactories) to be specified inline, elsewhere in the specification model, or in an external resource.

ja:RuleSet a rdfs:Class ;
  rdfs:subClassOf ja:HasRules
.

The optional repeatable property ja:rule has as its value a literal string which is the text of a Jena rule or rules. All those rules are added to the RuleSet.

The optional repeatable property ja:rulesFrom has as its value a resource whose URI identifies a file or other external entity that can be loaded as Jena rules. All those rules are added to the RuleSet.

The optional repeatable property ja:rules has as its value a resource which identifies another RuleSet in the specification model. All those rules from that RuleSet are added to this RuleSet.

Ontology models

Ontology models can be specified in several ways. The simplest is to use the name of an OntModelSpec from the Java OntModelSpec class:

eg:simple-ont-example
    ja:ontModelSpec ja:OWL_DL_MEM_RULE_INF
    .

This constructs an OntModel with an empty base model and using the OWL_DL language and the full rule reasoner. All of the OntModelSpec constants in the Jena implementation are available in this way. A base model can be specified:

eg:base-ont-example
    ja:baseModel [a ja:MemoryModel ;
                 ja:content [ja:externalContent <http://jena.hpl.hp.com/some-jena-data.rdf>]]
    .

The OntModel has a base which is a memory model loaded with the contents of http://jena.hpl.hp.com/some-jena-data.rdf. Since the ontModelSpec was omitted, it defaults to OWL_MEM_RDFS_INF - the same default as ModelFactory.createOntologyModel().

ja:OntModel a rdfs:Class ;
  rdfs:subClassOf ja:UnionModel ;
  rdfs:subClassOf ja:InfModel
.
ja:ontModelSpec a rdf:Property ;
  rdfs:domain ja:OntModel ;
  rdfs:range ja:OntModelSpec
.

ja:OntModelSpec a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:like; owl:maxCardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:reasonerFactory; owl:maxCardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:importSource; owl:maxCardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:documentManager; owl:maxCardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:ontLanguage; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:Object
.
ja:importSource a rdf:Property ;
  rdfs:domain ja:OntModelSpec
.
ja:reasonerFactory a rdf:Property ;
  rdfs:domain ja:OntModelSpec ;
  rdfs:range ja:ReasonerFactory
.
ja:documentManager a rdf:Property ;
  rdfs:domain ja:OntModelSpec
.
ja:ontLanguage a rdf:Property ;
  rdfs:domain ja:OntModelSpec
.
ja:likeBuiltinSpec a rdf:Property ;
  rdfs:domain ja:OntModelSpec
.

OntModel is a subclass of InfModel, and the ja:baseModel property means the same thing.

The OntModelSpec property value is a resource, interpreted as an OntModelSpec description based on its name and the value of the appropriate properties:

• ja:likeBuiltinSpec: The value of this optional unique property must be a JA resource whose local name is the same as the name of an OntModelSpec constant (as in the simple case above). This is the basis for the OntModelSpec constructed from this specification. If absent, then OWL_MEM_RDFS_INF is used. To build an OntModelSpec with no inference, use eg ja:likeBuiltinSpec ja:OWL_MEM.
• ja:importSource: The value of this optional unique property is a ModelSource description which describes where imports are obtained from. A ModelSource is usually of class ja:ModelSource.
• ja:documentManager: This value of this optional unique property is a DocumentManager specification. If absent, the default document manager is used.
• ja:reasonerFactory: The value of this optional unique property is the ReasonerFactory resource which will be used to construct this OntModelSpec’s reasoner. A reasonerFactory specification is the same as an InfModel’s reasoner specification (the different properties are required for technical reasons).
• ja:reasonerURL: as a special case of reasonerFactory, a reasoner may be specified by giving its URL as the object of the optional unique reasonerURL property. It is not permitted to supply both a reasonerURL and reasonerFactory properties.
• ja:ontLanguage: The value of this optional unique property is one of the values in the ProfileRegistry class which identifies the ontology language of this OntModelSpec:
  • OWL: http://www.w3.org/2002/07/owl#
  • OWL DL: http://www.w3.org/TR/owl-features/#term_OWLDL
  • OWL Lite: http://www.w3.org/TR/owl-features/#term_OWLLite
  • RDFS: http://www.w3.org/2000/01/rdf-schema#

Any unspecified properties have default values, normally taken from those of OntModelSpec.OWL_MEM_RDFS_INF. However, if the OntModelSpec resource is in the JA namespace, and its local name is the same as that of an OntModelSpec constant, then that constant is used as the default value.

Document managers

An OntDocumentManager can be specified by a ja:DocumentManager specification which describes the OntDocumentManager’s file manager and policy settings.

eg:mapper
    lm:mapping [lm:altName "file:etc/foo.n3" ;
    lm:name "file:foo.n3"]
    .
eg:document-manager-example
    ja:fileManager [ja:locationMapper eg:mapper] ;
    ja:meta [ dm:altURL <http://localhost/RDF/my-alt.rdf>]
    .

In this example, eg:document-manager-example is a ja:DocumentManager specification. It has its own FileManager specification, the object of the ja:fileManager property; that FileManager has a location mapper, eg:mapper, that maps a single filename.

The document manager also has an additional property to link it to document manager meta-data: the sub-model of the assembler specification reachable from eg:document-manager-example is passed to the document manager when it is created. For the meanings of the dm: properties, see the Jena ontology documentation and the ontology.rdf ontology.

    ja:DocumentManager a rdfs:Class ;
      rdfs:subClassOf [owl:onProperty ja:policyPath; owl:maxCardinality 1] ;
      rdfs:subClassOf [owl:onProperty ja:fileManager; owl:maxCardinality 1] ;
      rdfs:subClassOf [owl:onProperty ja:fileManager; owl:maxCardinality 1] ;
      rdfs:subClassOf ja:HasFileManager
    .
    ja:policyPath a rdf:Property ;
      rdfs:domain ja:DocumentManager
    .

The ja:fileManager property value, if present, has as its object a ja:FileManager specification; the constructed document manager is given a new file manager constructed from that specification. If there is no ja:fileManager property, then the default FileManager is used.

The ja:policyPath property value, if present, should be a string which is a path to policy files as described in the Jena ontology documentation. If absent, the usual default path is applied.

If the sub-model of the assembler specification reachable from the DocumentManager resource contains any OntDocumentManager DOC_MGR_POLICY or ONTOLOGY_SPEC objects, they will be interpreted by the constructed document manager object.

ja:FileManager a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:locationMapper; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:Object
.
ja:locationMapper a rdf:Property ;
  rdfs:domain ja:FileManager ;
  rdfs:range ja:LocationMapper
.

A ja:FileManager object may have a ja:locationMapper property value which identifies the specification of a LocationMapper object initialising that file manager.

ja:LocationMapper a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty lm:mapping; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:Object
.
lm:mapping a rdf:Property ;
  rdfs:domain ja:LocationMapper
.

A ja:LocationMapper object may have lm:mapping property values, describing the location mapping, as described in the FileManager documentation. (Note that the vocabulary for those items is in a different namespace than the JA properties and classes.)

Union models

Union models can be constructed from any number of sub-models and a single root model. The root model is the one written to when the union model is updated; the sub-models are untouched.

ja:UnionModel a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:rootModel; owl:maxCardinality 1] ;
  rdfs:subClassOf ja:Model
.
ja:rootModel a rdf:Property ;
  rdfs:domain ja:UnionModel ;
  rdfs:range ja:Model
.
ja:subModel a rdf:Property ;
  rdfs:domain ja:UnionModel ;
  rdfs:range ja:Model
.

If the single ja:rootModel property is present, its value describes a model to use as the root model of the union. All updates to the union are directed to this root model. If no root model is supplied, the union is given an immutable, empty model as its root.

Any ja:subModel property values have objects describing the remaining sub-models of the union. The order of the sub-models in the union is undefined (which is why there’s a special rootModel property).

Prefix mappings

The PrefixMappings of a model may be set from PrefixMapping specifications.

ja:PrefixMapping a rdfs:Class ;
  rdfs:subClassOf ja:Object
.
ja:includes a rdf:Property ;
  rdfs:domain ja:PrefixMapping ;
  rdfs:range ja:PrefixMapping
.

ja:SinglePrefixMapping a rdfs:Class ;
  rdfs:subClassOf [owl:onProperty ja:namespace; owl:cardinality 1] ;
  rdfs:subClassOf [owl:onProperty ja:prefix; owl:cardinality 1] ;
  rdfs:subClassOf ja:PrefixMapping
.
ja:namespace a rdf:Property ;
  rdfs:domain ja:SinglePrefixMapping
.
ja:prefix a rdf:Property ;
  rdfs:domain ja:SinglePrefixMapping
.

The ja:includes property allows a PrefixMapping to include the content of other specified PrefixMappings.

The ja:prefix and ja:namespace properties allow the construction of a single element of a prefix mapping by specifying the prefix and namespace of the mapping.

Other Assembler directives

There are two more Assembler directives that can be used in an Assembler specification: the assembler and imports directives.

Assembler

A specification may contain statements of the form:

someResource ja:assembler "some.Assembler.class.name"

When someResource is used as the type of a root object, the AssemblerGroup that processes the description will use an instance of the Java class named by the object of the statement. That class must implement the Assembler interface. See loading assembler classes for more details.

Similarly, statements of the form:

someResource ja:loadClass "some.class.name"

will cause the named class to be loaded (but not treated as assemblers).

Imports

If a specification contains statements of the form:

anyResource owl:imports someURL

or, equivalently,

anyResource ja:imports someURL

then the specification is regarded as also containing the contents of the RDF at someURL. That RDF may in turn contain imports referring to other RDF.

Limited RDFS inference

The Assembler engine uses limited RDFS inference to complete the model it is given, so that the spec-writer does not need to write excessive and redundant RDF. (It does not use the usual Jena reasoners because this limited once-off reasoning has been faster.) The inference steps are:

• add all the classes from the JA schema.
• do subclass closure over all the classes.
• do domain and range inference.
• do simple intersection inference: if X is an instance of intersection A B C …, then X is an instance of A, B, C … (and their supertypes).

This is sufficient for closed-world assembling. Other parts of the JA schema – eg, cardinality constraints – are hard-coded into the individual assemblers.