inside assemblers

This document describes Jena's 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; if you want mroe details on writing assembler descriptions, see the assembler howto.

the Assembler interface

An Assembler is an object that builds objects (most importantly, Models) from RDF descriptions.

public Object open( Assembler a, Resource root, Mode mode );

public Object open( Assembler a, Resource root );

public Object open( Resource root );

public Model openModel( Resource root );

public Model openModel( Resource root, Mode mode );

The fundamental method is the first: all the others are shorthands for ways of calling it. The abstract class AssemblerBase implements Assembler leaving only that method abstract and defining the others in terms of it.

The definition of a.open(Assembler sub, Resource root, Mode mode) is that a will construct the object described by the properties of root. If this requires the construction of sub-objects from descriptions hanging off root, sub.open is to be used to construct those. If the object is to be constructed in some persistent store, mode defines whether objects can be re-used or created: see modes for more details.

builtin assemblers

Jena comes with a collection of built-in assemblers: a bunch of basic assemblers and a composite general assembler. Each of these assemblers has a constant instance declared as a field of Assembler.

assembler	result class	Type	constant
ConnectionAssembler[Resource root] ja:Connection ConnectionDescription	dbUser String
	dbUser String
	dbUserProperty String
	dbPassword String
	dbPasswordProperty String
	dbURL String
	dbURLProperty String
	dbType String
	dbTypeProperty
	dbClass Class
dbClassProperty Class
ContentAssembler[FileManager fm] ja:Content Content	content *Content
	content *Content
	externalContent Content
	quotedContent Content
	literalContent Content
contentEncoding String
DefaultModelAssembler ja:DefaultModel Model	/see/ ModelAssembler
	/see/ ModelAssembler

	DocumentManagerAssembler ja:DocumentManager OntDocumentManager
	policyPath String default [OntDocumentManager.DEFAULT_METADATA_PATH]
fileManager *FileManager
FileManagerAssembler ja:FileManager FileManager	locationMapper !LocationMapper
locationMapper !LocationMapper
FileModelAssembler ja:FileModel FileModel	/see/ ModelAssembler
	/see/ ModelAssembler
	fileEncoding String
	mapName boolean
directory String
InfModelAssembler ja:InfModel InfModel	baseModel *Model
InfModelAssembler ja:InfModel InfModel	baseModel *Model
reasoner *Reasoner
LocationMappingAssembler ja:LocationMapper LocationMapper	/see/ LocationMapper
/see/ LocationMapper
MemoryModelAssembler ja:MemoryModel Model	/see/ ModelAssembler
/see/ ModelAssembler
ModelAssemebler ja:Model Model	reificationModel ReificationStyle
	reificationModel ReificationStyle
	/see/ PrefixMapping
/see/ Content
ModelSourceAssembler ja:ModelSource ModelGetter	connection *Connection
	connection *Connection

	NamedModelAssembler [] Model
modelName String
OntModelAssembler ja:OntModel OntModel	/see/ InfModelAssembler
	/see/ InfModelAssembler
	subModel *Model
ontModelSpec *OntModelSpec
OntModelSpecAssembler ja:OntModelSpec OntModelSpec	importSource *ModelGetter
	importSource *ModelGetter
	ontLanguage String
	reasonerURL ?ReasonerFactory
	reasonerFactory *ReasonerFactory
documentManager *OntDocumentManager
PrefixMappingAssembler ja:PrefixMapping PrefixMapping	includes *PrefixMapping
PrefixMappingAssembler ja:PrefixMapping PrefixMapping	includes *PrefixMapping
prefix BNODE[prefix, namespace]
RDBModelAssembler ja:RDBModel ModelRDB	/see/ NamedModelAssembler
RDBModelAssembler ja:RDBModel ModelRDB	/see/ NamedModelAssembler
connection *ConnectionDescription
ReasonerFactoryAssembler ja:ReasonerFactory ReasonerFactory	reasonerURL ReasonerFactory
	reasonerURL ReasonerFactory
	reasonerClass Class
/see/ RuleSetAssembler
RuleSetAssembler ja:RuleSet RuleSet	rules *RuleSet
	rules *RuleSet
	rulesFrom URL for rules
rule String rule
UnionModelAssembler ja:UnionModel Model	rootModel *Model
UnionModelAssembler ja:UnionModel Model	rootModel *Model
subModel *Model

ConnectionAssembler
ContentAssembler
DefaultModelAssembler
DocumentManagerAssembler
FileManagerAssembler
FileModelAssembler
InfModelAssembler
LocationMappingAssembler
MemoryModelAssembler
ModelAssemebler
ModelSourceAssembler
NamedModelAssembler
OntModelAssembler
OntModelSpecAssembler
PrefixMappingAssembler
RDBModelAssembler
ReasonerFactoryAssembler
RuleSetAssembler
UnionModelAssembler

AssemblerGroup

produces	Class	Type	constant
default models	DefaultModelAssembler	ja:DefaultModel	defaultModel
memory models	MemoryModelAssembler	ja:MemoryModel	memoryModel
inference models	InfModelAssembler	ja:InfModel	infModel
RDB models	RDBModelAssembler	ja:RDBModel	rdbModel
reasoners	ReasonerAssembler	ja:Reasoner	reasoner
connection descriptions	ConnectionAssembler	ja:Connection	connection
content	ContentAssembler	ja:Content	content
ontology models	OntModelAssembler	ja:OntModel	ontModel
rules	RuleSetAssembler	ja:RuleSet	rules
union models	UnionModelAssembler	ja:UnionModel	unionModel
prefix mappings	PrefixMappingAssembler	ja:PrefixMapping	prefixMapping
file models	FileModelAssembler	ja:FileModel	fileModel

inside Assemblers

Assembler.general is a particular implementation of the Assembler interface. An Assembler knows how to build the objects -- not just models -- described by an Assembler specification. The normal route into an Assembler is through the method:

open( Resource root ) ? Object

The Assembler inspects the root resource properties and decides whether it can build an object with that description. If not, it throws an exception. Otherwise, it constructs and returns a suitable object.

Since the creation of Models is the reason for the existance of Assemblers, there is a convenience wrapper method:

openModel( Resource root ) ? Model

which constructs the object and checks that it's a Model before returning it.

When an Assembler requires sub-objects (for example, when an InfModel Assembler requires a Reasoner object), it uses the method:

open( Assembler sub, Resource root ) ? Model

passing in a suitable Assembler object. In fact the standard implementation of open(root) is just

open( this, root )

passing in itself as the sub-assembler and having open(Assembler,Resource) be the place where all the work is done. (Amongst other things, this makes testing easier.)

When working with named persistent objects (typically database models), sometimes you need to control whether new objects should be constructed or old models can be reused. There is an additional method

open( Assembler sub, Resource root, Mode mode )

where the Mode argument controls the creation (or not) of persistent models. The mode is passed down to all sub-object creation. The standard implementation of open(sub,root) is just:

open( sub, root, Mode.DEFAULT )

A Mode object has two methods:

permitCreateNew( Resource root, String name )
permitUseExisting( Resource root, String name )

root is the root resource describing the object to be created or reused, and name is the name given to it. The result is true iff the permission is granted. Mode.DEFAULT permits the reuse of existing objects and denies the creation of new ones.

There are four Mode constants:

Mode.DEFAULT - reuse existing objects
Mode.CREATE - create missing objects
Mode.REUSE - reuse existing objects
Mode.ANY - reuse existing objects, create missing ones

Since the Mode methods are passed the resource root and name, the user can write specialised Modes that look at the name or the other root properties to make their decision.

Note that the Modes only apply to persistent objects, so eg MemoryModels or PrefixMappings ignore their Mode arguments.

implementing your own assemblers

(Temporary documentation pasted in from email; will be integrated and made nice RSN.)

You have to implement the Assembler interface, most straightforwardly
done by subclassing AssemblerBase and overriding

    public Object open( Assembler a, Resource root, Mode mode );

    because AssemblerBase both implements the boring methods that are
    just specialisations of `open` and provides some utility methods
    such as getting the values of unique properties. The arguments are

    * a -- the assembler to use for any sub-assemblies
    * root -- the resource in the assembler description for this object
    * mode -- the persistent open vs create mode

    The pattern is to look for the known properties of the root, use
    those to define any sub-objects of the object you're assembling
    (including using `a` for anything that's itself a structured
    object) and then constructing a new result object from those
    components.

    Then you attach this new assembler object to its type in some
    AssemblerGroup using that group's `implementWith` method. You
    can attach it to the handy-but-public-and-shared group
    `Assembler.general` or you can construct your own group. The
    point about an AssemblerGroup is that it does the type-to-assembler
    mapping for you -- and when an AssemblerGroup calls a component
    assembler's `open` method, it passes /itself/ in as the `a` argument,
    so that the invoked assembler has access to all of the component
    assemblers of the Group.

basic assemblers

There is a family of basic assemblers, each of which knows how to assemble a specific kind of object so long as they're given an Assembler that can construct their sub-objects. There are defined constants in Assembler for (an instance of) each of these basic assembler classes.

produces	Class	Type	constant
default models	DefaultModelAssembler	ja:DefaultModel	defaultModel
memory models	MemoryModelAssembler	ja:MemoryModel	memoryModel
inference models	InfModelAssembler	ja:InfModel	infModel
RDB models	RDBModelAssembler	ja:RDBModel	rdbModel
reasoners	ReasonerAssembler	ja:Reasoner	reasoner
connection descriptions	ConnectionAssembler	ja:Connection	connection
content	ContentAssembler	ja:Content	content
ontology models	OntModelAssembler	ja:OntModel	ontModel
rules	RuleSetAssembler	ja:RuleSet	rules
union models	UnionModelAssembler	ja:UnionModel	unionModel
prefix mappings	PrefixMappingAssembler	ja:PrefixMapping	prefixMapping
file models	FileModelAssembler	ja:FileModel	fileModel

Assembler.general is an assembler group, which ties together those basic assemblers. general can be extended by Jena coders if required. Jena components that use Assembler specifications to construct objects will use general unless documented otherwise.

In the remaining sections we will discuss the Assembler classes that return non-Model objects and conclude with a description of AssemblerGroup.

basic assembler ContentAssembler

The ContentAssembler constructs Content objects (using the ja:Content vocabulary) used to supply content to models. A Content object has the method:

fill( Model m ) ? m

Invoking the fill method adds the represented content to the model. The supplied ModelAssemblers automatically apply the Content objects corresponding to ja:content property values.

basic assembler ConnectionAssembler

A ConnectionAssembler constructs ConnectionDescriptions according to the specification. An ConnectionDescription retains the information required to make a database connection, can constructs that connection on demand.

When a ConnectionAssembler is constructed, it may optionally be given a Resource describing (using the JA vocabulary) default values for any of the database properties. When that Assembler is used to create a Connection, missing values are filled in from the defaults. This allows sensitive information to be left out of the RDF description.

The ConnectionAssembler embedded in Assembler.general has defaults taken from the system properties jena.dbUser and jena.dbType

basic assembler RulesetAssembler

A RulesetAssembler generates lists of Jena rules.

basic assembler DefaultModelAssembler

A "default model" is a model of unspecified type which is implemented as whatever kind the assembler for ja:DefaultModel generates. The default for a DefaultModel is to create a MemoryModel with no special properties.

AssemblerGroup

The AssemblerGroup class allows a bunch of other Assemblers to be bundled together and selected by RDF type. AssemblerGroup implements Assembler and adds the methods:

implementWith( Resource type, Assembler a ) ? this
assemblerFor( Resource type ) ? Assembler

AssemblerGroup's implementation of open(sub,root) finds the most specific type of root that is a subclass of ja:Object and looks for the Assembler that has been associated with that type by a call of implementWith. It then delegates construction to that Assembler, passing itself as the sub-assembler. Hence each component Assembler only needs to know how to assemble its own particular objects.

The assemblerFor method returns the assembler associated with the argument type by a previous call of implementWith, or null if there is no associated assembler.

loading assembler classes

AssemblerGroups implement the ja:assembler functionality. The object of an (type ja:assembler "ClassName") statement is a string which is taken as the name of an Assembler implementation to load. An instance of that class is associated with type using implementWith.

If the class has a constructor that takes a single Resource object, that constructor is used to initialise the class, passing in the type subject of the triple. Otherwise the no-argument constructor of the class is used.