Value handlers make it trivial to map any type that you can break down into one or more simple values. All value handlers implement the org.apache.openjpa.jdbc.meta.ValueHandler interface; see its Javadoc for details. Also, examine the built-in handlers in the src/openjpa/jdbc/meta/strats directory of your OpenJPA source distribution. Use these functional implementations as examples when you create your own value handlers.

OpenJPA interacts with persistent fields through the org.apache.openjpa.jdbc.meta.FieldStrategy interface. You can implement this interface yourself to create a custom field strategy, or extend one of the existing abstract or concrete strategies in the org.apache.openjpa.jdbc.meta.strats package. Creating a custom field strategy is more difficult than writing a custom value handler, but gives you more freedom in how you interact with the database.

OpenJPA gives you two ways to configure your custom field mappings. The FieldStrategies property of the built-in MappingDefaults implementations allows you to globally associate field types with their corresponding custom value handler or strategy. OpenJPA will automatically use your custom strategies when it encounters a field of the associated type. OpenJPA will use your custom value handlers whenever it encounters a field of the associated type. Section 4, “ Mapping Defaults ” described mapping defaults in detail.

Your other option is to explicitly install a custom value handler or strategy on a particular field. To do so, specify the full name of your implementation class in the proper mapping metadata extension. OpenJPA includes the org.apache.openjpa.persistence.jdbc.Strategy annotation. You can configure the named strategy or handler's bean properties in these extensions using OpenJPA's plugin format (see Section 4, “ Plugin Configuration ”).