EJB BMP Starter

Question

I have some key basic questions on BMP Entity Beans. I have been looking at the sample application at http://developers.sun.com/tools/javatools/documentation/s1s5/accountapp.html.
1. I have a database that is pre-populated by some external ways other than EJBs. Does my BMP Entity Bean that i mapped to a specific table know about the previous data. In the sample application above, there is a loadRow method (Adding private methods to perform database calls, step 10). In it, it says:
String selectStatement = "select balance from account where id = ? ";
PreparedStatement prepStmt = con.prepareStatement(selectStatement);
prepStmt.setString(1,this.id);
Where does this.id came from? It is not set anywhere but in the ejbCreate() method. For example, if my client program just wants to list the balance from accounts table bound to an id without inserting new rows into the database, how would that happen? It confuses me since ejbCreate method is called automatically everytime i use a bean and it contains SQL insert statements.
2. In the same example above, in section (implementing the bean's other methods, steps 3 and 4) it says:
public void ejbActivate() {
id = (String) context.getPrimaryKey();
} What is this id variable? There is no type definition. Is this the id that was defined in AcccountEJB (from db mapping)? If so how do i reference to it without any class names?
3. After all, i am looking for a clear BMP sample(with session beans using bmp beans, and servlets using session beans to access database data) that can guide me. If someone knows the place of such an example I would be very much greatful.
I seem to be unable to grab the logic of EJB BMP. Answers to these questions would be helpful.
Thanks in advance.

Answer 1

Greetings,
1. I have a database that is pre-populated by some external ways other than EJBs. Does my BMP
Entity Bean that i mapped to a specific table know about the previous data. In the sampleThe container will not have any beans representing the new data until the client attempts to find them.
String selectStatement = "select balance from account where id = ? ";
PreparedStatement prepStmt = con.prepareStatement(selectStatement);
prepStmt.setString(1,this.id);
Where does this.id came from? It is not set anywhereAs shown below, it is grabbed out of the EJBObject (which gets it as a consequence of either a create or finder method call), when a bean instance is "activated" - or, 'swapped in' from the free pool and associated with the EJBObject.
but in the ejbCreate() method. For example, if myThe ejbCreate method returns an instance of the Primary Key class which the container stores in the EJBObject. In this case, the PK class is type String and referenced by the variable 'id'.
client program just wants to list the balance from accounts table bound to an id without inserting
new rows into the database, how would that happen? ItPresumably, the client knows the "primary key" of the entity (the unique tuple from the accounts table which the EB represents), it wants to access and uses that as the argument to a findByPrimaryKey call against the home object.
confuses me since ejbCreate method is called automatically everytime i use a bean and it contains
SQL insert statements.Er, no. The bean's ejbCreate method is called by the container as a consequence of the client calling the corresponding create method on the home. It does not happen "automatically", but only as a consequence of the client wanting to create a new entity in the application - and, accordingly, in the underlying persistence resource. If the client does not want to create a new entity but, rather, use an existing one, then it calls an appropriate finder method to locate the instance and get a reference to it.
2. In the same example above, in section (implementing the bean's other methods, steps 3 and 4)
it says:
public void ejbActivate() {
id = (String) context.getPrimaryKey();
Refer again to the above...
What is this id variable? There is no type definition. Is this the id that was defined in AcccountEJB
(from db mapping)? If so how do i reference to it without any class names?It is indeed the private member of AccountBean (see step 7 under "Adding a create method to the bean" of section 4).
3. After all, i am looking for a clear BMP sample(with session beans using bmp beans, and
servlets using session beans to access database data) that can guide me. If someone knows the
place of such an example I would be very much greatful.It would appear that you require more grounding in the fundamentals of EJB generally - particularly with regard to their lifecycle and state transitions. Start with the specification. Then take a look at J2EE core design patterns...
I seem to be unable to grab the logic of EJB BMP.
Answers to these questions would be helpful.
Thanks in advance.Regards,
Tony "Vee Schade" Cook