O'Reilly - Java Message Service

Java Message Service
The following is a list of the nine JMS-defined properties, which are described in more
detail in Appendix C:
• JMSXUserID
• JMSXAppID
• JMSXProducerTXID
• JMSXConsumerTXID
• JMSXRcvTimestamp
• JMSXDeliveryCount
• JMSXState
• JMSXGroupID
• JMSXGroupSeq
3.2.3 Provider-Specific Properties
Every JMS provider can define a set of proprietary properties that can be set by the client
or the provider automatically. Provider-specific properties must start with the prefix
"JMS_" followed by the property name (JMS_). The purpose of the
provider-specific properties is to support proprietary vendor features.
3.3 Message Selectors
A message selector allows a JMS consumer to be more selective about the messages it
receives from a particular destination (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 JMS consumer.
For example, in the chat client developed in Chapter 2, message selectors could be used to
filter out messages from specific people. To accomplish this we would first declare a new
property in the message that identifies the username of the JMS client publishing the
message:
protected void writeMessage(String text) throws JMSException{
TextMessage message = session.createTextMessage( );
message.setText(text);
message.setStringProperty("username",username);
publisher.publish(message);
}
JMS clients can now use that property to filter messages. Message selectors are declared
when the message consumer is created:
TopicSubscriber subscriber =
session.createSubscriber(chatTopic, " username != 'William' ",false);
In this code, the message selector (shown in bold) tells the message server to deliver to the
consumer only those messages that do not have a username property equal to 'William'.
The message selectors are based on a subset of the SQL-92 conditional expression syntax,
which is used in the WHERE clauses of SQL statements. If you are familiar with SQL 92,
the conditional expressions used in message selectors will be familiar to you. The syntax
used for conditional expressions is covered in detail in Appendix D.
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