Java5 Language Features in JDO 1.1 and 2.0

This activity clarifies the JDO spec on persistence-capable classes using the new language features in Java 5, namely enums and generics.

Generics

JDO implementations need the information about field types that in JDO 1 is provided by the metadata element collection and map. The user can specify the type of the elements of the collection and the types of the key and value of the map. For example,

class Employee {
...
Map<Project, Integer> projectNumbers;
Set<Skill> skillSet;
...
}

Generic wildcards allow the user to bound the types of persistent collection elements, or map keys and values. For example, if we know that a Set can only contain Number elements, we might declare it as a Set<? extends Number> skills;. Mapping this persistent field to the datastore is similar to the issue of mapping a field of a superclass to the datastore, e.g. Number skill;. And it is similar to mapping Set<Number>. We believe that as far as JDO is concerned, the implementation can consider Set<? extends Number> exactly as Set<Number>.

The only implementation class for type identifiers is Class, which cannot be persistent. I (clr) propose to wait until a use-case is developed.

This seems to apply only to methods of generic classes and not to persistent behavior. I (clr) propose to wait until a use-case is developed.

This usage is not well-defined. I (clr) think that most uses would involve some kind of wrapper or holder that was type-specific. I (clr) propose to wait until a use-case is developed.

Enums

Java 5 has introduced linguistic support for enumerated types in form of enum declarations, for example:

enum Season { WINTER, SPRING, SUMMER, FALL };

In Java, enum declarations have a number (surprising) features, which exceed their counterparts in other languages:

To point out commonalities, Java enum types are no different from other user-defined classes, except that

For enum type support in JDO, we have to discuss

JDO Specific Annotations

For managed relations, we may add javax.jdo.annotation.Inverse for use in a PC class:

public class Department {
...
@javax.jdo.annotation.Inverse("department")
Set<Employee> employees;
...
}

public class Employee {
...
Department department;
...
}

This annotation may be used to generate 'mapped by' metadata.

Java5FeaturesAndJdo (last edited 2009-09-20 23:08:36 by localhost)