Enterprise Bean (EJB) programming tutorials

Enterprise Beans Tutorial

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Enterprise Beans Tutorial

18 Transactions Because the steps within a transaction are a unified whole, a transaction is often defined as an indivisible unit of work . A transaction can end in two ways: with a commit or a rollback. When a transaction commits, the data modifications made by its statements are saved. If a statement within a transaction fails, the transaction
rolls back, undoing the effects of all statements in the transaction. the begin and commit statments mark the boundaries of the transaction. When deploying an enterprise bean, you determine how the boundaries are set by specifying either container-managed or bean-managed
transactions. ## Container-Managed Transactions ## In an enterprise bean with container-managed transactions, the EJB container sets the boundaries of the transactions. You can use container-managed transactions with both session and entity beans.
Container-managed transactions simplify development because the enterprise bean code does not explicitly mark the transaction's boundaries. The code does not include statements that begin and end the transaction.
Typically, the container begins a transaction immediately before an enterprise bean method starts. It commits the transaction just before the method exits. Each method can be associated with a single
transaction. Nested or multiple transactions are not allowed within a method.
Container-managed transactions do not require all methods to be associated with transactions. When deploying a bean, you specify which of the bean's methods are associated with transactions by setting
the transaction attributes. Transaction Attributes A transaction attribute controls the scope of a transaction. Method-A of Bean 1 begins a transaction and then invokes method-B of Bean-2. When method-B executes, does it run within the scope of the transaction
started by method-A or does it execute with a new transaction? The answer depends on the transaction
attribute of method-B. Transaction Attribute Values A transaction attribute may have one of the following values: Required RequiresNew Mandatory NotSupported Supports Never Required If the client is running within a transaction and it invokes the enterprise bean's method, the method executes within the client's transaction. If the client is not associated with a transaction, the container starts a new transaction before running the method. The Required attribute will work for most transactions. Therefore, you may want to use it as a default, at least in the early phases of development. Because transaction attributes are
declarative, you can easily change them at a later time.