A JMS message comes with a number of standard properties, providing information on the associated message. For example, each message comes with a message id that could be fetched calling getJMSMessageID(). Besides, we can use custom properties to enrich a message in any way we think make sense for our specific case. An interesting aspect of custom properties is that we can use them to discriminate on which message a consumer should receive.
Think to an application where the producer generates messages for the best offers we have in our hardware store. The consumer could be interested in all or only in a subset of them. For this reason, it could be useful to put in the message payload just a generic description, and use custom properties to store structured information, as the product name and its price.
In this way the consumer could easily specify if it wants to get all the messages, or create a selector on the custom properties. The rules to create a selector are coming by simplification from the SQL conditional expressions.
The complete Java source code of this example is on github, it is written for ActiveMQ, so you should have this MOM installed and working on your development environment.
The producer delegates the most interesting part of its job to the send() method, that creates a message, fills it with specific data, and then send it to its associated queue:
TextMessage message = session.createTextMessage();
message.setText(txt);
message.setStringProperty(PROP_DES, des); // 1
message.setDoubleProperty(PROP_PRICE, price); // 2
producer.send(message);
1. Create a String property named as defined in the first parameter ("Description"), and containing the value stored in the second one ("Pins" and "Nails" in my test example).
2. The PROP_PRICE (actually, "Price") property is of type double.
In this test it is handy using a MessageListener, as discussed in a previous post, so that all the relevant messages are consumed by its onMessage() method:
logger.info("{}: {}", PROP_DES, message.getStringProperty(PROP_DES)); // 1
logger.info("{}: {}", PROP_PRICE, message.getDoubleProperty(PROP_PRICE)); // 2
logger.info("Message id: {}", message.getJMSMessageID()); // 3
logger.info("Message: {} ", ((TextMessage) message).getText());1. The PROP_DES property is retrieved from the message, as the String that it is.
2. PROP_PRICE is a double, but we could have managed it as a String, calling getStringProperty(). ActiveMQ knows how to convert the internal double value to a String, so this cast works fine. If we tried to do the other way round, extracting a double from (1), we would have got a NumberFormatException.
3. There is not much use for this line here, but it is just to show a standard JMS property at work.
We need just one fundamental step, telling the JMS Session that the consumer has to get only some message:
switch (arg) {
case "Nails":
case "Pins":
filter = PROP_DES + " = '" + arg + "'"; // 1
consumer = session.createConsumer(destination, filter);
break;
case "Cheap":
filter = PROP_PRICE + " < 15"; // 2
consumer = session.createConsumer(destination, filter);
break;
default:
consumer = session.createConsumer(destination); // 3
break;
}1. The string "Description = Nails" (or "Description = Pins") is passed in the next line to the session as a message selector for the consumer that has to be created. Only the messages true under that condition are delivered to this consumer.
2. Same as above, but the condition is now "Price < 15".
3. A "normal" consumer is created, with no associated selector.
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