Entity Classes

JOPA entity classes are much like JPA entity classes - they are regular POJO classes with some mapping annotations. The following sections provide more info about the mapping and other features related to entities.

Mapping

JOPA entities use mapping based on OWL, but RDFS ontologies can be just as easily mapped. The following table shows an overview of the mapping.

Ontology	JOPA annotation	Field type
OWL/RDFS Class	@OWLClass	-
Object property	@OWLObjectProperty	Another entity, URI
Datatype property	@OWLDataProperty	Primitive type wrapper class, String, Date, Java 8 Datetime
Annotation property	@OWLAnnotationProperty	Another entity, URI, Primitive type wrapper class, String, Date, Java 8 Datetime

Additionally, unmapped ontological data can be accessed using two special fields:

• @Types - a set of ontological types besides the one mapped by @OWLClass.
• @Properties - a map of property to set-of-values allowing to access properties not mapped by the above described explicit mapping.

Example

The following snippet shows all the above mentioned mapping features in one class.

@OWLClass(iri = SKOS.CONCEPT)
public class Term {
  @Id
  private URI id;

  @OWLAnnotationProperty(iri = SKOS.PREF_LABEL)
  private MultilingualString label;

  @OWLAnnotationProperty(iri = SKOS.DEFINITION)
  private MultilingualString definition;

  @OWLObjectProperty(iri = SKOS.NARROWER)
  private Set<Term> children;

  @OWLDataProperty(iri = SKOS.NOTATION, simpleLiteral = true)
  private Set<String> notations;

  @Inferred
  @OWLObjectProperty(iri = SKOS.RELATED)
  private Set<Term> related;

  @Types
  private Set<String> types;

  @Properties
  private Map<String, Set<String>> properties;

}

Inferred Attributes

Entity attributes annotated with @Inferred can contain values inferred by the underlying repository's reasoner. The annotation has a parameter allowing you to specify whether explicit (asserted) property values should be included when the attribute value is loaded. By default, it is true. In the listing above, the related attribute will contain terms whose relatedness was asserted on the side of this term as well as those whose relatedness was asserted on their side (because skos:related is symmetric).

Since JOPA 0.20.0, inferred attribute values are editable. However, this editing has limits. Adding new values is always ok, but when a value is removed, a check is performed whether the value being removed is inferred or asserted. If it is inferred, an exception (InferredAttributeModifiedException) is thrown.

Prior to JOPA 0.20.0, attributes marked with @Inferred were always read only and any changes to their values (even adding a new value) resulted in an InferredAttributeModifiedException.

Java Enums

Java enums can be mapped in three ways that are specified via the @Enumerated annotation:

• EnumType.STRING (default) - uses the name of the enum constant as a string literal in the ontology. It is also used when no @Enumerated annotation is specified on an enum-valued attribute.
• EnumType.ORDINAL - uses the ordinal value of the enum constant in the ontology (since 0.21.0).
• EnumType.OBJECT_ONE_OF - enum constants should be mapped to OWL individuals (RDF resources). This represents the owl:ObjectOneOf (OWL 2 reference, sec. 8.1.4)(since 0.21.0).

The string and ordinal enum types are used on attributes with @OWLDataProperty and @OWLAnnotationProperty mapping, ObjectOneOf are used on @OWLObjectProperty attributes. Here is a short example of an enum:

public enum OwlPropertyType {

    @Individual(iri = OWL.DATATYPE_PROPERTY)
    DATATYPE_PROPERTY,
    @Individual(iri = OWL.ANNOTATION_PROPERTY)
    OBJECT_PROPERTY,
    @Individual(iri = OWL.ANNOTATION_PROPERTY)
    ANNOTATION_PROPERTY
}

And its use in an entity:

@Enumerated(EnumType.OBJECT_ONE_OF)
@OWLObjectProperty(iri = "http://example.org/has-property-type")
private OwlPropertyType objectOneOf;

Lifecycle Listeners

Much like JPA, JOPA supports entity lifecycle listeners. These come in two flavors:

• Lifecycle callback methods declared on entity classes or in mapped superclasses
• Entity listener classes

The following annotations are bound to the respective entity lifecycle events:

• @PrePersist
• @PostPersist
• @PreRemove
• @PostRemove
• @PreUpdate
• @PostUpdate
• @PostLoad

Lifecycle Callbacks

Lifecycle callbacks are methods declared directly in entity classes or in mapped superclasses, annotated with the aforementioned annotations. A callback may be annotated by multiple lifecycle event annotations.

The callback may not be static, but can have arbitrary visibility. It should take no argument, as it is called on the entity instance to which the lifecycle event applies. The callback's return type must be void. Method signature should thus look as follows:

void callback();

Entity Listeners

Entity listeners are non-managed classes containing lifecycle callback methods. An entity listeners is registered for a concrete entity through the @EntityListeners annotation declared on the entity. Multiple listeners can be specified for an entity, the order of declaration specifying also the order in which their callback methods are invoked.

The callback methods are similar to those defined directly in entities (see above) but they take a single argument representing the entity for which the callback is being invoked. The type of the argument can be Object or a more specific type. The method signature should look as like this:

void callback(Object entity);

The callback method are annotated with lifecycle event annotations listed above.

Lifecycle Callback Methods Semantics

The semantics of lifecycle callback methods correspond to the semantics defined in JPA, section 3.5.3, so details can be found there. In a nutshell, PrePersist and PreRemove are invoked before the respective entity manager operations are executed. PostPersist and PostRemove are invoked after the entity has been made persistent/removed.

PreUpdate and PostUpdate occur before and after new entity state has been merged into the underlying repository. Note that if the lifecycle listener modifies the state of the entity on which it was invoked (e.g. when setting provenance data on update), the listener is not invoked again. This is to prevent endless loops.

PostLoad is invoked after the entity has been loaded into the persistence context. It also applies to query results. In case of lazily loaded attributes, the PostLoad will occur when the relationship is triggered and the entity is loaded.

All the lifecycle operations are cascaded.

Note that EntityManager is not available in lifecycle callbacks and attempts to access it (e.g. via CDI in entities) are discouraged, as they may interfere with the entity processing logic.