J2EE Design Patterns

Message Types

Within a particular messaging approach, several different types of messages can be sent. Each of these message type patterns can be built on top of the messaging systems discussed in the previous section. They can be implemented using guaranteed or nonguaranteed messaging on a variety of different protocols and with either point-to-point or publish-subscribe delivery.

All message types share a few common elements. A basic message has a sender attribute, a recipient attribute, a set of headers and a set of user-specified attributes. The use of the sender and recipient should be obvious. Headers and attributes serve to further define the message. An email message, for instance, will generally contain headers specifying the subject, the date the message was sent, the email client that was used, the encoding type used for the text, and the various servers that relayed the message.

The JMS APIs break a message up into three major parts: a standard header section, a user-defined header section, and a message body. Only the first of these sections is required, although a message without any user-specified content cannot deliver a whole lot of data. The following message type patterns help determine which data to store in the different parts of a message.

Event Message

Event handling in Java should be familiar to you by now; but on the off chance that it isn’t, here’s the primer: an object can register itself with another object as an event listener. Once the listener has been ...