O'Reilly - Java Message Service

Appendix D. Message Selectors
Java Message Service
Message selectors allow a JMS consumer to be more selective about the messages it
receives from a particular topic or queue. Message selectors use Message properties and
headers as criteria in conditional expressions. These conditional expressions use boolean
logic to declare which messages should be delivered to a client.
The message selectors are based on a subset of the SQL-92 conditional expression syntax
that is used in the WHERE clauses of SQL statements. This section is a detailed exploration
of the message selector syntax.
To illustrate how message selectors are applied, we will consider a hypothetical message
that contains three application properties: Age, Weight, and LName. Age is an int, Weight is a
double, and LName is a String property. The values of these properties depend on the
message. The message selector is used to obtain only those messages with property values
of interest to the consumer.
D.1 Identifiers
An identifier is the part of the expression that is being compared. For example, the
identifiers in the following expression are Age, Weight, and LName:
Age < 30 AND Weight >= 100.00 AND LName = 'Smith'
Identifiers can be any application-defined, JMS-defined, or provider-specific property, or
one of several JMS headers. Identifiers must match the property or JMS header name
exactly; identifiers are case sensitive. Identifiers have the same naming restrictions as
property names (see Appendix C).
The JMS headers that can be used as identifiers include JMSDeliveryMode, JMSPriority,
JMSMessageID, JMSTimestamp, JMSCorrelationID, and JMSType. The JMSDestination and
JMSReplyTo headers cannot be used as identifiers because their corresponding values are
Destination objects whose underlying value is proprietary and therefore undefined.
The JMSRedelivered value may be changed during delivery. If a consumer uses a message
selector where "JMSRedelivered = FALSE", and there was a failure delivering a message,
the JMSRedelivered flag might be set to TRUE. JMSExpiration is not supported as an
identifier because JMS providers may choose to implement this value differently. Some
may store it with the message, while others calculate it as needed.
D.2 Literals
Literals are expression values that are hard-coded into the message selector. In the message
selector shown here, 30, 100.00, and 'Smith' are all literals:
Age < 30 AND Weight >= 100.00 AND LName = 'Smith'
String literals are enclosed in single quotes. An apostrophe or single quote can be included
in a String literal by using two single quotes (e.g., 'Smith''s').
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