Java Enterprise Best Practices

Retrieve Incoming Mail Efficiently

Inefficient access to email message stores can result in performance degradation.

If you’re using messaging middleware, J2EE provides a convenient way to receive incoming messages via message-driven EJBs. The J2EE container will deliver messages to EJBs as they arrive from the external messaging server. While some message queue vendors might have “email bridge” support built into their products, there is no specific J2EE mechanism for receiving messages.

In most cases, including whenever a POP3 message store is involved, you have to explicitly check for mail, and J2EE applications have a few methods for doing so. The first is to check in response to some user activity, either in an EJB session bean or a Java servlet. In most cases, this is not a terribly useful thing to do. Users will not respond well to a three- or four-second (or potentially much longer) wait while the system takes care of housekeeping that is not related to the task at hand.

Instead of holding users hostage to the mail component, most applications deal with mail retrieval by launching one or more background worker threads. This generally has to be done at the application’s controller layer, usually by launching a thread from a servlet’s initialization method or when the servlet context is being initialized. Remember that EJBs are explicitly forbidden from creating or maintaining threads of any kind. For more information, see Section 10.3.7 later in this chapter.

JavaMail supports ...