SCO and FCO treatment of String, Locale, etc.

[Craig L Russell <Cr...@Sun.COM>]

Javadogs,

An issue has been raised [1] with regard to storage of instances of  
immutable system object classes, e.g. Integer, String, Locale. The  
specification calls for users of the API to not depend on whether  
these instances are stored as FCO or SCO.

The intent of the specification is that applications should not  
depend on whether the instances are stored by themselves as  
persistent instances or stored as embedded within the domain class  
instances. Furthermore, the application should not depend on FCO  
behavior: uniquing of instances in the same PersistenceManager for  
the same instance. Finally, the application should not depend on  
guaranteeing SCO behavior: copying the instance upon commit and  
guaranteeing a distinct instance in memory upon reinstantiation.

The rationale of this wording was specifically to allow an  
implementation of JDO to either store instances embedded in the  
"containing" domain class instance or as a nullable reference to a  
persistence-capable System class in the datastore, as is done by some  
object databases.

The intent was not to impose restrictions on the application's use of  
the identical instance as the value of multiple fields of domain  
classes. A literal interpretation of this part of the specification  
would require users to guarantee that the same instance of any of the  
classes was never contained in instances managed by multiple  
PersistenceManagers. The effect of this interpretation is to require  
cloning for each instance of Integer, String, Locale, etc. used in  
domain classes.

I believe that this interpretation should be disallowed by the  
specification, as it imposes a great burden on users. I would like  
the Expert Group to discuss this issue and straw proposal.

<spec 6.4.3>
Immutable Object Class types
JDO implementations must support fields that reference instances of  
immutable object
classes, and may choose to support these instances as SCOs or FCOs:
•package java.lang: Boolean, Character, Byte, Short, Integer, Long,
Float, Double, and String;
•package java.util: Locale, Currency.
•package java.math: BigDecimal, BigInteger.
Portable JDO applications must not depend on whether instances of  
these classes are treat-
ed as SCOs or FCOs.
</spec 6.4.3>

<proposed 6.4.3>
Immutable Object Class types
JDO implementations must support fields that reference instances of  
immutable object
classes, and may choose to support these instances as SCOs or FCOs:
•package java.lang: Boolean, Character, Byte, Short, Integer, Long,
Float, Double, and String;
•package java.util: Locale, Currency.
•package java.math: BigDecimal, BigInteger.
Portable JDO applications must not depend on SCO or FCO uniquing  
behavior, nor on the storage mechanism in the datastore. Portable  
applications may use the same instance of these classes as field  
values in any persistence-capable class instance.
</proposed 6.4.3>

[1]http://issues.apache.org/jira/browse/JDO-397?page=all

Craig Russell
Architect, Sun Java Enterprise System http://java.sun.com/products/jdo
408 276-5638 mailto:Craig.Russell@sun.com
P.S. A good JDO? O, Gasp!