Groups

Chapter 8 Creating J2EE Applications for BEA WebLogic

Groups

In container-managed persistence, you use groups to specify certain persistent attributes of an entity bean. A field-group represents a subset of the cmp and cmr-fields of a bean. You can put related fields in a bean into groups that are considered into memory together as a unit. You can associate a group with a query or relationship, so that when a bean is loaded as the result of executing a query or following a relationship, only the fields mentioned in the group are loaded.

A special group named "default" is used for queries and relationships that have no group specified. By default, the default group contains all of a bean's cmp-fields and any cmr-fields that add a foreign key to the persistent state of the bean.

A field can belong to multiple groups. In this case, the getXXX() method for the field will fault in the first group that contains the field.

Specifying Field Groups

Field groups are specified in the weblogic-rdbms-cmp-jar.xml file as follows:

<group-name>financial-data</group-name>
<group-name>medical-data</group-name>
	<cmr-field>patient</cmr-field>
	<cmr-field>doctors</cmr-fields>
	<cmr-field>insurance-providers</cmr-fields>

You use field groups when you want to access a subset of fields.

Using Groups

The field group is an optimizing element that should be used with care because it is possible to corrupt the database. For example:

You have the following cmp fields: A, B, and C.
A and B belong to the same group.
You set up the following scenario:

getA() // loads A and B
modify A
// then an external process modifies the row getC()

Because C is not in the group, there are two possibilities:

The container will load C and all the other fields as well. In this case, the modification that was made to A will be lost.
The container will load C and only C. When the transaction commits, the new value for A that was assigned during the transaction might overwrite the newer value in the database.

In both cases, the database will be corrupted. The reason is that you told the container that within this transaction, only A and B would be read, but instead C also was read. The correct step to take would have been to add C to the group or to specify no groups at all.