Defining Primary Identifier Mappings

Primary identifier mapping is mandatory in Hibernate. Primary identifiers of classes are mapped to primary keys of master tables in data sources. If not defined, a default primary identifier mapping will be generated, but this may not work properly.

There are three kinds of primary identifier mapping in Hibernate:

Mapped classes must declare the primary key column of the database table. Most classes will also have a Java-Beans-style property holding the unique identifier of an instance.

• Simple Identifier Mapping
  When a primary key is attached to a single column, only one attribute in the primary identifier can be mapped. This kind of primary key can be generated automatically. You can define the generator class and parameters. There are many generator class types, such as increment, identity, sequence, etc. Each type of generator class may have parameters that are meaningful to it. See your Hibernate documentation for detailed information.
• Composite Identifier Mapping
  If a primary key comprises more than one column, the primary identifier can have multiple attributes mapped to these columns. In some cases, the primary key column could also be the foreign key column.
• Component Primary Identifier Mapping
  For more convenience, a composite identifier can be implemented as a separate value type class. The primary identifier has just one attribute with the class type. The separate class should be defined as a value type class. Component class mapping will be generated then.