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< title > ActiveMQ JMS Queue Selector Example< / title >
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< link rel = "stylesheet" type = "text/css" href = "../common/prettify.css" / >
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< h1 > JMS Queue Selector Example< / h1 >
< p > This example shows you how to selectively consume messages using message selectors with queue consumers.< / p >
< p > Message selectors are strings with special syntax that can be used in creating consumers. Message consumers
created with a message selector will only receive messages that match its selector. On message delivery, the JBoss Message
Server evaluates the corresponding message headers of the messages against each selector, if any, and then delivers
the 'matched' messages to its consumer. Please consult the JMS 1.1 specification for full details.< / p >
< p > In this example, three message consumers are created on a queue. The first consumer is created with selector
< code > 'color=red'< / code > , it only receives messages that
have a 'color' string property of 'red' value; the second is created with selector < code > 'color=green'< / code > , it
only receives messages who have a 'color' string property of
'green' value; and the third without a selector, which means it receives all messages. To illustrate, three messages
with different 'color' property values are created and sent.< / p >
< p > Selectors can be used with both queue consumers and topic consumers. The difference is that with queue consumers,
a message is only delivered to one consumer on the queue, while topic consumers the message will be delivered to every
matching consumers. In this example, if the third consumer (anyConsumer) were the first consumer created, it will
consume the first message delivered, therefore there is no chance for the next consumer to get the message, even if it
matches the selector.< / p >
< h2 > Example step-by-step< / h2 >
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< p > < i > To run the example, simply type < code > mvn verify -Pexample< / code > from this directory< / i > < / p >
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< ol >
< li > First we need to get an initial context so we can look-up the JMS connection factory and destination objects from JNDI. This initial context will get it's properties from the < code > client-jndi.properties< / code > file in the directory < code > ../common/config< / code > < / li >
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< code > InitialContext initialContext = getContext();< / code >
< / pre >
< li > We look-up the JMS queue object from JNDI< / li >
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< code > Queue queue = (Queue) initialContext.lookup("/queue/exampleQueue");< / code >
< / pre >
< li > We look-up the JMS connection factory object from JNDI< / li >
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< code > ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");< / code >
< / pre >
< li > We create a JMS connection< / li >
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< code > connection = cf.createConnection();< / code >
< / pre >
< li > We start the connection< / li >
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< code > connection.start();< / code >
< / pre >
< li > We create a JMS session. The session is created as non transacted and will auto acknowledge messages.< / li >
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< code > Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);< / code >
< / pre >
< li > We create a JMS message producer on the session. This will be used to send the messages.< / li >
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< code > MessageProducer messageProducer = session.createProducer(queue);< / code >
< / pre >
< li > We create two selectors.< / li >
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< code >
String redSelector = "color='red'";
String greenSelector = "color='green'";
< / code >
< / pre >
< li > We Create a JMS Message Consumer that receives 'red' messages.< / li >
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< code >
MessageConsumer redConsumer = session.createConsumer(queue, redSelector);
redConsumer.setMessageListener(new SimpleMessageListener("red"));
< / code >
< / pre >
< li > We Create a second JMS Message Consumer that receives 'green' messages.< / li >
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< code >
MessageConsumer greenConsumer = session.createConsumer(queue, greenSelector);
greenConsumer.setMessageListener(new SimpleMessageListener("green"));
< / code >
< / pre >
< li > We Create another JMS Message Consumer that receives all messages. Please not that the order
of consumers on a queue is of significance. If the anyConsumer is created before the above two, the
result will be totally different.< / li >
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< code >
MessageConsumer anyConsumer = session.createConsumer(queue);
anyConsumer.setMessageListener(new SimpleMessageListener("any"));
< / code >
< / pre >
< li > We Create three messages, each has a different color property.< / li >
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< code >
TextMessage redMessage = session.createTextMessage("Red");
redMessage.setStringProperty("color", "red");
TextMessage greenMessage = session.createTextMessage("Green");
greenMessage.setStringProperty("color", "green");
TextMessage blueMessage = session.createTextMessage("Blue");
blueMessage.setStringProperty("color", "blue");
< / code >
< / pre >
< li > We send the messages to the topic< / li >
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< code >
producer.send(redMessage);
System.out.println("Message sent: " + redMessage.getText());
producer.send(greenMessage);
System.out.println("Message sent: " + greenMessage.getText());
producer.send(blueMessage);
System.out.println("Message sent: " + blueMessage.getText());
< / code >
< / pre >
< li > And finally, < b > always< / b > remember to close your JMS connections and resources after use, in a < code > finally< / code > block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects< / li >
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< code > finally
{
if (initialContext != null)
{
initialContext.close();
}
if (connection != null)
{
connection.close();
}
}< / code >
< / pre >
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