automatical assignment of unique values
As mentioned in Tutorial1 OJB provides a mechanism to assign
unique values for primary key attributes. You just have to enable it
with a declaration in the respective FieldDescriptor of the XML
repository as follows:
 | | |
|
<field-descriptor id="1"
name="articleId"
column="Artikel_Nr"
jdbc-type="INTEGER"
primarykey="true"
autoincrement="true"
/>
| |
| | |
This definitions contains the following information: The attribute
_id is mapped on the table's
column ID. The JDBC Type of
this column is INTEGER. This
is a primary key column. OJB shall automatically assign unique values
to this attribute.
This mechanism works for columns of type INTEGER, CHAR and
VARCHAR. This mechanism helps you to keep your business logic free
from code that computes unique Ids for primary key attributes.
force computation of unique values
By default OJB triggers the computation
of unique ids during calls to PersistenceBroker.store(...). Sometimes
it will be necessary to have the ids computed in advance. This can be
done by simply obtaining the Identity of the respective object as
follows:
Identity oid = new Identity(object);
See the Tutorial2
for an example.
Alternative SequenceManagers
OJB internally uses a sequence manager for the generation of
unique ids. This SequenceManager is not very sophisticated and
provides only a minimal feature set. It's implemented in
ojb.broker.util.SequenceManagerDefaultImpl.
Another implementation is ojb.broker.util.SequenceManagerHighLowImpl,
which implements a more sophisticated aproach, by using a High/Low algorithm, as
described in Mapping Objects To Relational Databases.
It is able to generate IDs unique to a given Table as well IDs as unique over all objects
that use the same property for their id (for example each class that uses "oid" as its
id-attribute). This can be toggled by changing SequenceManagerGlobalIDs
in OJB.properties. One mayor drawback of this
implementation is, that it doesn't interact cleanly with legacy data.
If you still think something is missing you can just write your own
implementation of the interface ojb.broker.util.SequenceManager.
OJB provides a simple mechanism to integrate your implementation:
OJB obtains SequenceManager objects from the factory
ojb.broker.util.SequenceManagerFactory. This Factory can be
configured to generate instances of your specific implementation by
changing the following entry in the configuration file
ojb.broker.util.PersistenceBrokerFactoryConfiguration.properties:
SequenceManagerClass=ojb.broker.util.SequenceManagerDefaultImpl
to
SequenceManagerClass=acme.com.MyOwnSequenceManager.
Of course I'm interested in your solutions! If you have
implemented something interesting, just contact me.
