Java Enterprise Best Practices

Use an Email Management Framework

Single applications should have as few points of contact with the external email layer as possible. This allows the centralization of logic to deal with concurrency issues—only one thread ever accesses the mailbox. To help you out with this, I present a basic framework for handling incoming messages. The framework is based on the Strategy design pattern, which allows you to plug in different kinds of processing behavior.^[41] Having this framework will allow you to easily extend your applications to handle different kinds of messages by creating new instances of a monitoring thread pointed at different mailboxes and passing in handler objects that know how to deal with particular message types.

Start by declaring an interface that can be implemented by classes that will be able to handle incoming messages. I will keep it simple for this example, so you will define just one kind of interface, a BlockingMailHandler , which handles a particular message and returns when processing for that message has completed. (See Example 10-4.)

Example 10-4. BlockingMailHandler

public interface BlockingMailHandler 
{
  public boolean handleMessage(javax.mail.Message message);
}

A more complete implementation would result in a BlockingMailHandler from a MailHandler , and probably define an AsynchronousMailHandler as well.

To handle incoming messages, you create a class that implements the BlockingMailHandler interface and does something interesting in the handleMessage( ...