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The J2EE specification given by Sun Microsystems allows the transactions to be declaratively defined through dedicated attributes, which are defined in deployment descriptors. These attributes are applied to EJB methods and parameterize the behavior of the container regarding the transaction manager when these methods are invoked. These attributes are the following: TX_NOT_SUPPORTED: Indicates that the method is not transactional. If this method is invoked within a transaction, this transaction is suspended during the method execution. TX_SUPPORTS: Indicates that the bean that supports the method will be included in the transaction as a participant. If the client does not define any transaction, the container can create a new transaction depending on the implementation. TX_REQUIRED: Indicates that the method must be executed within the context of a transaction. If not, the container automatically creates a new one. TX_REQUIRES_NEW: Indicates that the method triggers the creation of a new transaction, even if the client has already defined one. In this later case, the transaction of the client is replaced by the newly created transaction. TX_BEAN_MANAGED: Indicates that the EJB method manually implements the transaction management by using JTA, for instance. TX_MANDATORY: Indicates that the method has to be executed within a transaction. If not, the container throws a TransactionRequired exception. Figure 11-2 shows the messages that are sent from the application to the transaction manager, and to the transactional resources when the transferFunds method is set to TX_REQUIRES_NEW which means that a transaction will automatically be created by the container, even if a transaction has already been defined by the client and that the methods Account.setBalance and AccountHome.create are set to TX_REQUIRED.

Element/Attribute Name schema Description Database schema that contains the tables to which entities in the persistence unit are mapped. Database catalog that contains the tables to which entities in the persistence unit are mapped. Access type for all entities in the persistence unit, i.e., field or property. Specifies cascade persist for relationships for all entities. Default entity listeners for the persistence unit.

named-query Corresponding annotation: @javax.persistence.NamedQuery A named query element is used to define a named query using JPQL. The named query is global to the persistence unit.

In many enterprise situations, you will need to inform a user of specific events or notices by using an email message. Sending an email from a Java application has become a simple process now that developers can use the javax.mail package. The Java mail API is usually more than enough to satisfy the email needs of developers.

Element/Attribute Name name query hint Description Name of the named query. Must be unique in the persistence unit. Query in JPQL. One or more vendor-specific query hints.

The call to the transferFunds method triggers the begin message to the manager, which in turn creates a new transaction. This transaction is added to the current thread s context and is automatically transmitted to the participating EJBs. When the Account EJBs are used and the Tx bank-level transactions are created, they are added to the current transaction as participants. The successful end of the transferFund method triggers the commit, while an EJBException catch implies a rollback.

named-native-query Corresponding annotation: the @javax.persistence.NamedNativeQuery A named native query element is used to define a named query using native SQL.

To provide the ability to send email from a session bean, we need a new method, sendEmail(). Listing 4.7 contains a simple stateless session bean that exposes the sendEmail() method in its remote interface (the interface is not shown).

Element/Attribute Name name query hint result-class result-set-mapping Description Name of the named query. Must be unique in the persistence unit. Query in native SQL. One or more vendor-specific query hints. Name of the entity class returned as a result of the native query. Name of ResultSetMapping returned by the query.

sql-result-set-mapping Corresponding annotation: @javax.persistence.SqlResultSetMapping A native query may return a result set mapping that is defined using multiple entity classes.

Figure 11-2. Declarative transactions mechanism in J2EE The existence of container-managed transactions allows the program s code to remain free from the transaction concern. However, this technique has its limitations. The attributes defined by the EJB standard are too simple to cover all the possible cases that can arise when developing complex applications. For instance, the programmer may need to retry a transaction if it did not complete successfully in the first place. It is also common to optimize the application by not applying a transaction within some contexts, which depends on the application or environment state. Nevertheless, when it comes to optimization, it is sometimes useful to limit the transaction application scope to an instruction group rather than to the whole method code. In this case, the only way is to use the TX_BEAN_MANAGED attribute and to manually code the transactional logic. Even in the cases when the provided attributes allow the description of the transactional logic, this technique still requires some code to be written in order to handle the client-side exceptions and some rollback management cases. In particular, the programmer may need to force the rollback of a transaction within a participant. In this case, the setRollbackOnly method shall be used (see line 7 of Listing 11-21) in order to cancel the transaction, even if higher-level participants try to commit the transaction. As an example, the code in Listing 11-21 prevents a negative setBalance from occurring in any transactional context:

import javax.mail.*; public class SendEmailBean implements SessionBean { //other bean methods not shown private void sendEmail(String recipient, String text) { Session mailSession = null; javax.mail.Message msg = null; try{ System.out.println( "Sending Email to: " + rcpt ); mailSession = (Session) ctx.lookup("BookMailSession");