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| <h1>Basics - Transactions</h1> |
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| <div id="preamble"> |
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| <div class="paragraph"> |
| <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> |
| </div> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_participants">Participants</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <p>Various components and parts of the EJB system can be part of a |
| transaction. Examples are</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>Session bean</p> |
| </li> |
| <li> |
| <p>Message Driven Bean</p> |
| </li> |
| <li> |
| <p>EntityManager (a.k.a. Persistence context)</p> |
| </li> |
| </ol> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_behaviour">Behaviour</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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> |
| </div> |
| </div> |
| </div> |
| <div class="sect1"> |
| <h2 id="_configuration">Configuration</h2> |
| <div class="sectionbody"> |
| <div class="paragraph"> |
| <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 <code>@TransactionAttribute</code>(value = TransactionAttributeType.X), where X |
| is one of…​</p> |
| </div> |
| <div class="olist arabic"> |
| <ol class="arabic"> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>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.</p> |
| </li> |
| <li> |
| <p>MANDATORY - a transaction must always exist prior to the call, and it |
| will be used. It is an error otherwise</p> |
| </li> |
| <li> |
| <p>NOT_SUPPORTED - component not included in the transaction</p> |
| </li> |
| <li> |
| <p>SUPPORTS - transaction will be used if it exists, but will not be |
| created if it does not exist</p> |
| </li> |
| <li> |
| <p>NEVER - if a transaction exists, it is an error to call the method</p> |
| </li> |
| </ol> |
| </div> |
| <div class="paragraph"> |
| <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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