3.2.1. Data Model

Data model entities are divided into two categories:

  • Persistent – instances of such entities are stored in the database using ORM.

  • Non-persistent – instances exist only in memory, or are stored somewhere via different mechanisms.

The entities are characterized by their attributes. An attribute corresponds to a field and a pair of access methods (get / set) of the field. If the setter is omitted, the attribute becomes read-only.

Persistent entities may include attributes that are not stored in the database. For a non-persistent attribute, the field is optional and you can create only access methods.

The entity class should meet the following requirements:

  • Be inherited from one of the base classes provided by the platform (see below).

  • Have a set of fields and access methods corresponding to attributes.

  • The class and its fields (or access methods if the attribute has no corresponding field) must be annotated to provide information for the ORM (in case of a persistent entity) and metadata frameworks.

The following types can be used for entity attributes:

  • java.lang.String

  • java.lang.Boolean

  • java.lang.Integer

  • java.lang.Long

  • java.lang.Double

  • java.math.BigDecimal

  • java.util.Date

  • java.sql.Date

  • java.sql.Time

  • java.util.UUID

  • byte[]

  • enum

  • Entity

Base entity classes (see below) override equals() and hashCode() methods to check entity instances equivalence by comparing their identifiers. I.e., instances of the same class are considered equal if their identifiers are equal.