4.2.1. Data Model

Data model entities are divided into two categories:

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

  • Non-persistent – instances exist only in memory.

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 non-persistent attribute the field is optional, creation of access methods will be sufficent.

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 the entity attributes.

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

  • To support potential entity extension, fields should be declared with the protected modifier instead of private.

The following attribute types of entities are supported:

  • 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 are considered equal, if their identifiers are equal. An identifier of the UUID type is assigned to an instance right after its creation in memory, which is why new instances can also be compared and added to collections.