Understanding the EJB Entity Beans With CMP Architecture

External JDBC Pools

By default, TopLink manages its own connection pools. You can also configure TopLink to use connection pooling offered by the host application server. This feature is useful for shared connection pools and is required for JTA/JTS integration (see "Configuring External Connection Pooling").

JTA/JTS Integration

JTA and JTS are standard Java components that enable sessions to participate in distributed transactions. You must configure TopLink to use JTA/JTS to use session beans in the architecture (see "Integrating the Unit of Work With an External Transaction Service").

Cache Coordination

If you choose to use multiple servers to scale your application, you may require TopLink cache coordination (see "Understanding Cache Coordination").

Maintaining Bidirectional Relationships

When one-to-one or many-to-many relationship is bidirectional, you must maintain the back pointers as the relationships change.

TopLink automatically maintains the relationship between two entity beans.

To set the back pointer manually, do one of the following:

• Code the entity bean to maintain the back pointer when the relationship is established or modified (recommended).

• Code the client to explicitly set the back pointer.

If you code the entity bean to set back pointers, the client is freed of this responsibility. This has the advantage of encapsulating this maintenance implementation in the bean.

In a one-to-many relationship, a source bean might have several dependent target objects. For example, an EmployeeBean might own several dependent PhoneNumber instances. When you add a new dependent object (a PhoneNumber, in this example) to an employee, you must set the PhoneNumber instance's back pointer to its owner (the employee). Maintaining a one-to-many relationship in the entity bean involves getting the local object reference from the context of the EmployeeBean, and then updating the back pointer as Example 2-1 shows.

// obtain owner and phoneNumber
owner = empHome.findByPrimaryKey(ownerId); 
phoneNumber = new PhoneNumber("cell", "613", "5551212");
// add phoneNumber to the phoneNumbers of the owner
owner.addPhoneNumber(phoneNumber);

// Maintain the relationship in the Employee's addPhoneNumber method 
public void addPhoneNumber(PhoneNumber newPhoneNumber) {
    // get, then set the back pointer to the owner
    Employee owner = (Employee)this.getEntityContext().getEJBLocalObject();
    newPhoneNumber.setOwner(owner); 
    // add new phone
    getPhoneNumbers().add(newPhoneNumber);
}

For more information, see the following:

• Configuring Bidirectional Relationship

• "Directionality"

Managing Dependent Objects

Unlike EJB, TopLink dependent persistent objects can be sent back and forth between a client and a server. When objects are serialized, the risk exists the objects can cause the cache to lose the identity of the objects or attempt to cache duplicate identical objects. To avoid potential problems, use the bean setter methods when adding dependent objects to relationship collections as Example 2-2 shows. This enables TopLink to handle merging of objects in the cache.

addPhoneNumber(PhoneNumber phone) { 
    Collection phones = this.getPhoneNumbers(); 
    Vector newCollection = new Vector();
    newCollection.addAll(phones); 
    newCollection.add(phone); 
    this.setPhones(newCollection); 
}

Managing Collections of EJBObject Objects

Collections generally use the equals method to compare objects. This is not a problem in the case of an object that contains a collection of EJBObject objects, because the EJB container collection handles equality appropriately.