The Life Cycle of an Entity Bean

The Life Cycle of an Entity Bean

• Does Not Exist. In this state, the bean instance simply does not exist.
• Pooled. When WebLogic server is first started, several bean instances are created and placed in the pool. A bean instance in the pooled state is not tied to particular data, that is, it does not correspond to a record in a database table. Additional bean instances can be added to the pool as needed, and a maximum number of instances can be set
• Ready. A bean instance in the ready state is tied to particular data, that is, it represents an instance of an actual business object.

/**
 * @ejbgen:ejb-local-ref link="Recording" 
 * 
 * ...
 */
abstract public class BandBean extends GenericEntityBean implements EntityBean
{
  private EntityContext ctx;
  private RecordingHome recordingHome;


  public void setEntityContext(EntityContext c) {
      // store the reference to the EntityContext
      ctx = c;
		// look up the home interface of the RecordingBean
      try {
         javax.naming.Context ic = new InitialContext();
         recordingHome = (RecordingHome)ic.lookup("java:/comp/env/ejb/Recording"); 
      }
      catch(Exception e) {
         System.out.println("Unable to obtain RecordingHome: " + e.getMessage());
      }
  } 

  ...

The Pooled State

When a bean instance is in the pooled state, it is not tied to any particular business object. When in the pooled state, the methods defined in the home interface can be invoked, effectively transitioning it from the pooled to the ready state, with the exception of ejbHome methods. When a home method is invoked, a result that is not bean instance specific is returned to the caller, and the bean instance remains in the pooled state. Home methods in turn often invoke ejbSelect methods to query bean instances.

Moving from the Pooled to the Ready State

The following methods move a bean instance from the pooled to the ready state to represent a business object:

• ejbCreate and ejbPostCreate. When the create method is invoked on the bean's home interface, the ejbCreate and ejbPostCreate methods are invoked. The bean instance moves to the ready state and represents this newly created business object. After creation, a (local or remote) reference to this object is returned to the caller, enabling the caller to invoke business methods on this instance. The ejbPostCreate method is used to set references to other entity beans as part of the creation of a new bean instance.
• findByPrimaryKey. When this method is invoked on the bean's home interface with the (compound) primary key as the parameter, the bean instance moves to the ready state and represents the business object uniquely identified by the parameter. Also a (local or remote) reference to this object is returned to the caller, enabling the caller to invoke business methods on this instance.
• Finder methods. When a Finder method is invoked on the bean's home interface, one or a set of references to objects matching the queries are returned to the caller. In many cases the corresponding bean instance(s) are not loaded with data and move to the ready state until at a later point when a business method is actually invoked on this object, a concept known as lazy loading. However, you can specify eager loading, that is, tell the EJB container to load (part of) the data and move the entity bean instance(s) to the ready state as a part of the finder method execution.

1. Before a business method is executed, the EJB container updates the fields of the bean instance with the latest data from the database table to ensure that the bean instance has the latest data. Just after the data is updated, the EJB container invokes the callback method ejbLoad. If your entity bean needs to execute some custom logic as part of this synchronization step, you can use implement it using this callback method.
2. The business method executes and completes.
3. The EJB container now updates the database table to ensure that it contains the latest data from the entity bean instance. In other words, if the business method changes data values, this synchronization step ensures these changes are stored. Just prior to updating the database table, the EJB container invokes the callback method ejbStore. If your entity bean needs to execute some custom logic as part of this synchronization step, you can implement it using this callback method.

Because a record in a database table can be accessed by multiple bean instances at the same time, these synchronization steps ensure that each bean instance always has the latest data. However, in some cases these synchronization steps might be overkill and unnecessarily slow down performance. For instance, an entity bean might be read-only, reading data that is changed rarely if at all. In these cases one can safely bypass the synchronization steps without risking violations to data integrity.

• You are responsible for synchronizing the entity bean instance and the underlying database record before and after a business method invocation. To do so you must implement the callback methods ejbLoad and ejbStore. (The EJB container invokes the various callback methods for CMP and BMP entity beans alike.)
• The ejbCreate method(s) must take care of storing the new data in the database.
• The findByPrimaryKey method must be explicitly defined and must take care of locating the data corresponding to the (compound) primary key in the database.
• Removing the data from the database when a bean instance is removed must be handled by the bean class. To do so you must implement the ejbRemove callback method.
• Query methods cannot be implemented using EJB QL or WebLogic QL. Instead the finder or select method(s) must define the database queries.

• To create a new BMP entity bean file, the first step is creating it as you would a CMP entity bean. When the file is created and opened, go to source view and add persistence-type="bmp" to the @ejbgen:entity tag. When you close and re-open the file, you will notice that design view is no longer available and, correspondingly, that the various wizards to for instance add an entity relation or a create method are no longer available.
• You need to add an @ejbgen:resource-ref tag to the datasource (the database). Because the EJB container doesn't handle persistence, you should remove the attributes data-source-name and table-name from the @ejbgen:entity tag.
• The database table for the BMP entity bean is not created for you by WebLogic. You must define the table in the database.
• You must define the property fields and accessor methods.
• You must define the findByPrimaryKey method and the various callback methods as mentioned in the previous section.
• WebLogic will still generate the various interfaces for you. To that effect, you also still use the appropriate @ejbgen tags to ensure that a particular method is added to the appropriate interface. The findByPrimaryKey method will automatically be added to the defined home interface(s).