Chapter 7   Handling Transactions with EJBs


This chapter describes the transaction support built into the Enterprise JavaBean programming model.

This chapter begins by introducing the EJB transaction model and explains the notion of container and bean managed transactions. Then it explains the semantics of all transaction attributes and use of the Java Transaction API for bean managed transactions. Finally, it discusses the restrictions on various combinations of EJB types and transaction attributes.

This chapter includes the following sections:

• Understanding the Transaction Model

• Specifying Transaction Attributes in an EJB

• Using Bean Managed Transactions

Understanding the Transaction Model

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One of the primary advantages of using Enterprise JavaBeans (EJBs) is the support they provide for declarative transactions. In the declarative transaction model, attributes are associated with beans at deployment time and it is the container's responsibility, based on the attribute value, to demarcate and transparently propagate transactional context. The container is also responsible, in conjunction with a Transaction Manager, for ensuring that all participants in the transaction see a consistent outcome.

Declarative transactions free the programmer from explicitly demarcating transactions. They facilitate component-based applications where multiple components, potentially distributed and updating heterogeneous resources, can participate in a single transaction. The EJB specification also supports programmer-demarcated transactions using javax.transactions.UserTransaction().

It is necessary to understand the distinction between global and local transactions in order to understand the iAS support for transactions. Global transactions are managed and coordinated by a Transaction Manager and can span multiple databases and processes. The Transaction Manager typically uses the XA protocol to interact with the database backends. Local transactions, on the other hand, are native to a database and are restricted within a single process. Local transactions can work against only a single backend. Local transactions are typically demarcated using JDBC APIs.

In iAS, all transactions are started by the container or by using javax.transaction.UserTransaction().

The EJB specification requires support for flat (as opposed to nested) transactions. In this model each transaction is decoupled from and independent of other transactions in the system. Flat transactions are by far the most prevalent model and are supported by most commercial database systems.

Note If your application uses global transactions, your should configure and enable the corresponding iAS Resource Managers. See the Administration and Deployment Guide for further details.

Specifying Transaction Attributes in an EJB

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Transactional attributes can be specified on a bean-wide basis or on a per-method basis for a bean's remote interface. If attributes are specified at both levels, method-specific values take precedence over bean-wide values. These two should be mixed with care since some combinations are invalid as documented in the restrictions section.

Transactional attributes are specified as part of the bean's XML Deployment Descriptor file (see "EJB iAS XML DTD" in Chapter 10 "Packaging for Deployment").

Using Bean Managed Transactions

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While it is preferable to use container-managed transactions, you may find that your application requirements necessitate the use of bean-managed transactions. To manage transactions programmatically, refer to the Enterprise JavaBeans Specification, v1.1 for this interface at the following URL:

http://java.sun.com/products/ejb/javadoc-1.1/javax/ejb/EJBContext.h tml