C M P   2 . 0  

Container Managed Relationships 

Container Managed Relationships (CMRs) are a powerful new feature of CMP 2.0.  

Programmers have been creating relationships between entity objects since EJB 1.0 was 

introduced (not to mention since the introduction of databases), but before CMP 2.0 the 

programmer had to write a lot of code for each relationship in order to extract the primary 

key of the related entity and store it in a pseudo foreign key field. The simplest relationships 

were tedious to code, and complex relationships with referential integrity required many 

hours to code. With CMP 2.0 there is no need to code relationships by hand. The container 

can manage one to one, one to many and many to many relationships, with referential 

integrity. One restriction with CMRs is that they are only defined between local interfaces. 

This means that a relationship cannot be created between two entities in different virtual 

machines.

 3

There are two basic steps to create a container managed relationship: create the cmr field 

abstract accessors and declare the relationship in the ejb jar.xml file. The following two 

sections describe these steps. 

CMR Field Abstract Accessors 

A cmr field abstract accessor has the same signatures as cmp field, except a single valued 

cmr field must return the local interface of the related entity, and a multi valued cmr field 

can only return a java.util.Collection (or java.util.Set) object. For example, to declare a 

one to many relationship between Organization and Gangster, first add the following to the 

OrganizationBean class: 

public abstract class OrganizationBean implements EntityBean { 

   public 

abstract

Set

getMemberGangsters

(); 

   public 

abstract

 void 

setMemberGangsters

(

Set

 gangsters); 

} 

Listing 9 6, Collection Valued cmr field Abstract Accessor Declaration 

Second, add the following to the GangsterBean class: 

public abstract class GangsterBean implements EntityBean { 

   public 

abstract

Organization

getOrganization

(); 

   public 

abstract

 void 

setOrganization

(

Organization

 org); 

} 

Listing 9 7, Single Valued cmr field Abstract Accessor Declaration 

Although in Listing 9 6 and Listing 9 7 each bean declared a cmr field, only one of the two 

beans in a relationship must have a set of accessors. As with cmp fields, a cmr field is 

required to have both a getter and a setter method. 

3

 The EJB specification does not even allow for relationships between entities in different applications within the same VM. 

88