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| <h1>Basics - Transactions |
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| <p>One of the many benefits of EJB, is that transactions within the EJB |
| container are generally managed entirely automatically. Any EJB component |
| will, by default, partake in that transaction.</p> |
| |
| <p>Here are some basic rules to understand about transactions. Keep note that |
| this is the default behaviour, and the system can be configured to behave |
| differently, depending on the needs of your system, bean, or individual |
| methods of your beans.</p> |
| |
| <p><a name="Basics-Transactions-Participants"></a></p> |
| |
| <h2>Participants</h2> |
| |
| <p>Various components and parts of the EJB system can be part of a |
| transaction. Examples are</p> |
| |
| <ol> |
| <li>Session bean</li> |
| <li>Message Driven Bean</li> |
| <li>EntityManager (a.k.a. Persistence context)</li> |
| </ol> |
| |
| <p><a name="Basics-Transactions-Behaviour"></a></p> |
| |
| <h2>Behaviour</h2> |
| |
| <p>The basic default behaviours are |
| 1. A transaction starts at the beginning of the first EJB method call, in a |
| chain of calls that are participating in the given transaction |
| 1. A transaction ends at the end of the first EJB method, in the same chain |
| 1. If a bean that has started a transaction, uses another bean, that bean |
| will automatically use the same transaction as the calling bean.</p> |
| |
| <p><a name="Basics-Transactions-Configuration"></a></p> |
| |
| <h2>Configuration</h2> |
| |
| <p>You can configure your beans in a variety of ways. Generally speaking, a |
| transaction is started when a method is called, but can be configured using |
| @TransactionAttribute(value = TransactionAttributeType.X), where X is one |
| of...</p> |
| |
| <ol> |
| <li>REQUIRED - the default, which is to start a transaction if one does not exist, but to use the existing one if it has already been started.</li> |
| <li>REQUIRES_NEW - the transaction is created on every call, and ends when the call is completed. Beans don't partake in transactions created by other parts of the system.</li> |
| <li>MANDATORY - a transaction must always exist prior to the call, and it will be used. It is an error otherwise</li> |
| <li>NOT_SUPPORTED - component not included in the transaction</li> |
| <li>SUPPORTS - transaction will be used if it exists, but will not be created if it does not exist</li> |
| <li>NEVER - if a transaction exists, it is an error to call the method</li> |
| </ol> |
| |
| <p>@TransactionAttribute applies to both methods and entire beans. You may |
| set one type of transaction behaviour (as seen above) on the bean, and a |
| different one on a specific method of that same bean, which overrides the |
| one configured for the overall bean. For instance, maybe you want to make |
| an audit entry in the database that you are about to attempt a credit card |
| payment. It really needs to be in it's own transaction so that it is |
| IMMEDIATELY committed for audit purposes, if something goes wrong with the |
| credit card payment. So, perhaps you use MANDATORY on the bean, and |
| REQUIRES_NEW on the method for audit logging. As soon as the method that |
| does the audit logging is complete, the transaction is committed, and the |
| credit card payment transaction continues on it's way.</p> |
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