8.6 Transactions in Web Components

A servlet or JSP page can use JNDI to lookup a UserTransaction object, then use the UserTransaction interface to demarcation transactions. This is useful in a two-tier application where a Web component needs to access enterprise information systems under the scope of a JTA transaction.

Code Example 8.1 illustrates the use of the JTA interface to demarcate transactions within a Web component:

Context ic = new InitialContext();
UserTransaction ut =
	(UserTransaction) ic.lookup("java:comp/UserTransaction");
ut.begin();
// perform transactional work here
ut.commit();

A Web component may only start a transaction in its service method. A transaction that is started by a servlet or JSP page must be completed before the service method returns. In other words, transactions may not span Web requests.

There are many subtle and complex interactions between the use of JTA transactions, threads, and JDBC connections. Web components should follow the guidelines stated in the transaction management chapter of the J2EE specification:

• JTA transactions should be started and completed only from the thread in which the service method is called. If the Web component creates additional threads for any purpose, these threads should not attempt to start JTA transactions.
• JDBC connections may be acquired and released by a thread other than the service method thread, but should not be shared between threads.
• JDBC Connection objects should not be stored in static fields.
• For Web components implementing SingleThreadModel, JDBC Connection objects may be stored in class instance fields.
• For Web components not implementing SingleThreadModel, JDBC Connection objects should not be stored in class instance fields, and should be acquired and released within the same invocation of the service method.

In a multitier environment, servlets and JSP pages are mainly responsible for the presentation of the application and dealing with browser interaction. In this case, the use of JTA transactions in the Web tier is not recommended. Instead, transactional work such as database access should be delegated to enterprise beans in the EJB tier.