4.2.1. Data Model

Problem domain is modeled with the help of interconnected Java classes, which are called entity classes or simply entities.

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. To make an attribute immutable (read only), it is enough to omit "set" method.

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 JPA (in case of a persistent entity) and the metadata framework.

  • To enable support of potential extension of the entity, fields should be declared with the modifier protected, 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 provide entity instance matching by comparing their identifiers. I.e., instances are considered equal, if their identifiers match. 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.

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