Java Reference
In-Depth Information
A message selector is a
String
that contains an expression. The syntax of the expression
is based on a subset of the SQL92 conditional expression syntax. The message selector in
the example selects any message that has a
NewsType
property that is set to the value
'Sports'
or
'Opinion'
:
NewsType = 'Sports' OR NewsType = 'Opinion'
The
createConsumer
and
createDurableSubscriber
methods allow you to
specify a message selector as an argument when you create a message consumer.
The message consumer then receives only messages whose headers and properties match
the selector. (See “
Message Headers
”
on page
356
, and “
Message Properties
”
on page
body.
JMS Messages
The ultimate purpose of a JMS application is to produce and consume messages that can
then be used by other software applications. JMS messages have a basic format that is
simple but highly flexible, allowing you to create messages that match formats used by
non-JMS applications on heterogeneous platforms.
A JMS message can have three parts: a header, properties, and a body. Only the header is
required. The following sections describe these parts.
For complete documentation of message headers, properties, and bodies, see the docu-
mentation of the
Message
interface in the API documentation.
Message Headers
A JMS message header contains a number of predefined fields that contain values used by
both clients and providers to identify and route messages.
Table 20-1
lists the JMS mes-
sage header fields and indicates how their values are set. For example, every message has
a unique identifier, which is represented in the header field
JMSMessageID
. The value
of another header field,
JMSDestination
, represents the queue or the topic to which
the message is sent. Other fields include a timestamp and a priority level.
