Nobody knows this fundamental question - CMP, CMT, BMP, BMT. Surprised !!

Question

Hello,
I've talked to many experts and read books on EJB but none of them gave me a clear cut answers. What are the definitions of CMP, CMT, BMP and BMT? No one could explain "Persistent" from "Transaction" properly. No books clearly define them at all. Some say BMP means BMT and CMP means CMT. Some others say they are totally different concept. Others even say different stories. Actually, when we create EJBs (in Visual Cafe as an example), we can tell whether the bean is CMP or BMP but there os no place where we can define the transaction mode (BMT or CMT). How come do so many people get confused about this fundamental concept? Can anybody explain them clearly please?

Answer 1

CMP = Container Managed Persistence
This means that the container (part of the EJB Server) automatically handles persisting your bean to a DB. So if you have an Entity Bean, for example, that has some properties (like name, size) and a table in the database with the fields name (as a varchar), size (as an integer), then when you set the bean's properties to "Joe" and "14", you can then look in your database and those values will be there. You (the developer) do not have to write any code to put those values into the DB.
BMP = Bean Managed Persistence
This means that (in the above example of an EJB with properties name, size), if you set your bean's name to "joe" and size to 14, those values will NOT go into the DB unless you (the developer) put them there.
CMT = Container Managed Transactions
A transaction is really either one statement (or action) or a bunch of them that are grouped together. If it is a group, then for any one of them to be executed / committed, they must all execute successfully. If even just one of the actions / statements fails, they all fail (or are all "rolled back", i.e. un-done)
CMT means that the container (EJB Server) will handle making sure that all the actions or statements from a transaction (i.e. a particular context of actions kicked off when an EJB is called) are completed successfully. If they are, the container will commit (execute) all the statements and return "success". If any one of them fails, the container will handle rolling back (un-doing) all the statements. This is VERY useful, as you do not need any special if statements or try-catches, the container does it for you.
BMT = Bean Managed Transaction
This means that you (the developer) will have to logically group together all that actions that you consider for a transaction. If any one of them fails, you will need to make sure all of them are rolled back and that "failure" is returned to the caller. If they are all successful, you will have to return success.
You can do this by using the below code:
(in all the code below, they may wrap from one line to the next, just pay attention to where the ";" is)
//import Transaction
import javax.transaction.UserTransaction
//make a new transaction:
UserTransaction userTran = ( UserTransaction ) initialContext.lookup("java:comp/UserTransaction");
try
//And then marking the beginning of the transaction:
userTran.begin();
//Then Do your stuff (i.e. the actions)
myPersonalBusinessMethods();
//commit the transaction cause it's all ok
userTran.commit();
catch (Exception e)
//rollback (un-do) all the stuff cause error occurred
userTran.rollback();
Hope all this helped and finally explained it! Feel free to throw some duke dollars my way if it did! ;P
Robert