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.

What happens to messages that are not selected for delivery to the consumer by its message selector? This depends on the message model used. In the pub/sub model, the message is simply not delivered to that consumer, but it is delivered to other consumers. This is true of both nondurable and durable subscriptions. In the p2p model, the messages remain in the queue, so other consumers of the queue can see them, but they are not visible to the consumer that used the message selector.