EJB3.0 Transactions

The object of a transaction is to execute a unit-of-work that results in a reliable exchange. Here are some types of business systems that employ transactions:

ATM - The ATM (automatic teller machine) you use to deposit, withdraw, and transfer funds executes these units-of-work as transactions. In an ATM withdrawal, for example, the ATM checks to make sure you don't overdraw; then it debits your account and spits out some money.

Online book order - You've probably purchased many of your Java booksmaybe even this bookfrom an online bookseller. This type of purchase is also a unit-of-work that takes place as a transaction. In an online book purchase, you submit your credit card number, it is validated, and a charge is made for the price of the book. Then an order to ship the book is sent to the bookseller's warehouse.

Transactions must be atomic, consistent, isolated, and durable (ACID).

Atomic - An atomic transaction must execute completely or not at all. This means that every task within a unit-of-work must execute without error. If any of the tasks fail, the entire unit-of-work or transaction is aborted, meaning that any changes to the data are undone. If all the tasks execute successfully, the transaction is committed, which means that the changes to the data are made permanent or durable.

Consistent - Consistency refers to the integrity of the underlying data store. It must be enforced by both the transactional system and the application developer. The transactional system fulfills this obligation by ensuring that a transaction is atomic, isolated, and durable. The application developer must ensure that the database has appropriate constraints (primary keys, referential integrity, and so forth) and that the unit-of-workthe business logicdoesn't result in inconsistent data (i.e., data that is not in harmony with the real world it represents). In an account transfer, for example, the debit to one account must equal the credit to another account.

Isolated - Isolation means that a transaction must be allowed to execute without interference from other processes or transactions. In other words, the data that a transaction accesses cannot be affected by any other part of the system until the transaction or unit-of-work is completed.

Durable - Durability means that all the data changes made during the course of a transaction must be written to some type of physical storage before the transaction is successfully completed. This ensures that the changes are not lost if the system crashes.

One of the primary advantages of Enterprise JavaBeans is that it allows for declarative transaction management .

With declarative transaction management, the transactional behavior of EJBs can be controlled using the @javax.ejb.TransactionAttribute annotation or the EJB deployment descriptor, both of which can set transaction attributes for individual enterprise bean methods.

you can set its runtime transaction attribute in the @javax.ejb.TransactionAttribute annotation or deployment descriptor to one of several values:

NotSupported
Supports
Required
RequiresNew
Mandatory
Never