The Java EE 6 Tutorial, Volume I

The Single Table per Class Hierarchy Strategy

With this strategy, which corresponds to the default InheritanceType.SINGLE_TABLE, all classes in the hierarchy are mapped to a single table in the database. This table has a discriminator column, a column that contains a value that identifies the subclass to which the instance represented by the row belongs.

The discriminator column can be specified by using the javax.persistence.DiscriminatorColumn annotation on the root of the entity class hierarchy.

Table 19–2 @DiscriminatorColumn Elements






The name of the column in the table to be used as the discriminator column. The default is DTYPE. This element is optional.



The type of the column to be used as a discriminator column. The default is DiscriminatorType.STRING. This element is optional.



The SQL fragment to use when creating the discriminator column. The default is generated by the Persistence provider, and is implementation-specific. This element is optional. 



The column length for String-based discriminator types. This element is ignored for non-String discriminator types. The default is 31. This element is optional.

The javax.persistence.DiscriminatorType enumerated type is used to set the type of the discriminator column in the database by setting the discriminatorType element of @DiscriminatorColumn to one of the defined types. DiscriminatorType is defined as:

public enum DiscriminatorType {

If @DiscriminatorColumn is not specified on the root of the entity hierarchy and a discriminator column is required, the Persistence provider assumes a default column name of DTYPE, and column type of DiscriminatorType.STRING.

The javax.persistence.DiscriminatorValue annotation may be used to set the value entered into the discriminator column for each entity in a class hierarchy. You may only decorate concrete entity classes with @DiscriminatorValue.

If @DiscriminatorValue is not specified on an entity in a class hierarchy that uses a discriminator column, the Persistence provider will provide a default, implementation-specific value. If the discriminatorType element of @DiscriminatorColumn is DiscriminatorType.STRING, the default value is the name of the entity.

This strategy provides good support for polymorphic relationships between entities and queries that cover the entire entity class hierarchy. However, it requires the columns that contain the state of subclasses to be nullable.