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< title > ActiveMQ Last-Value Queue Example< / title >
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< h1 > Last-Value Queue Example< / h1 >
< p > This example shows you how to configure and use last-value queues.< / p >
< p > Last-Value queues are special queues which discard any messages when a newer message with the same value for a well-defined < em > Last-Value< / em > property is put in the queue.
In other words, a Last-Value queue only retains the last value.< / p >
< p > A typical example for Last-Value queue is for stock prices, where you are only interested by the latest value for a particular stock.< / p >
< p > The example will send 3 messages with the same < em > Last-Value< / em > property to a to a Last-Value queue.< br / >
We will browse the queue and see that only the last message is in the queue, the first two messages have been discarded. < br / >
We will then consume from the queue the < em > last< / em > message.< / p >
< h2 > Example setup< / h2 >
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< p > Last-Value queues are defined in the configuration file < a href = "server0/activemq-configuration.xml" > activemq-configuration.xml< / a > :< / p >
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< code > < address-setting match="jms.queue.lastValueQueue">
< last-value-queue> true< /last-value-queue>
< /address-setting> < / code >
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< h2 > Example step-by-step< / h2 >
< p > < i > To run the example, simply type < code > mvn verify< / code > from this directory< / i > < / p >
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< 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 < a href = "server0/client-jndi.properties" > client-jndi.properties< / a > < / li >
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< code > InitialContext initialContext = getContext();< / code >
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< li > We look up the JMS queue object from JNDI< / li >
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< code > Queue queue = (Queue) initialContext.lookup("/queue/lastValueQueue");< / code >
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< li > We look up the JMS connection factory object from JNDI< / li >
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< code > ConnectionFactory cf = (ConnectionFactory) initialContext.lookup("/ConnectionFactory");< / code >
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< li > We create a JMS connection, a session and a producer for the queue< / li >
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< code > connection = cf.createConnection();
Session session = connection.createSession(false, Session.AUTO_ACKNOWLEDGE);
MessageProducer producer = session.createProducer(queue);< / code >
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< li > We will create and send a text message with the Last-Value property set to < code > STOCK_NAME< / code > < / li >
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< code > TextMessage message = session.createTextMessage("1st message with Last-Value property set");
message.setStringProperty("_HQ_LVQ_NAME", "STOCK_NAME");
producer.send(message);
System.out.format("Sent message: %s\n", message.getText());< / code >
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< p > < em > The < em > Last-Value< / em > key is defined in ActiveMQ's MessageImpl class. Its value is < code > "_HQ_LVQ_NAME"< / code > < / em > < / p >
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< li > We will create and send a < em > second< / em > text message with the Last-Value property set to < code > STOCK_NAME< / code > < / li >
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< code > message = session.createTextMessage("2nd message with Last-Value property set");
message.setStringProperty("_HQ_LVQ_NAME", "STOCK_NAME");
producer.send(message);
System.out.format("Sent message: %s\n", message.getText());< / code >
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< li > We will create and send a < em > third< / em > text message with the Last-Value property set to < code > STOCK_NAME< / code > < / li >
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< code > message = session.createTextMessage("3rd message with Last-Value property set");
message.setStringProperty("_HQ_LVQ_NAME", "STOCK_NAME");
producer.send(message);
System.out.format("Sent message: %s\n", message.getText());< / code >
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< p > < em > When the 2< sup > nd< / sup > message was sent to the queue, the 1< sup > st< / sup > message was discarded.< br / >
Similarly, when the 3< sup > rd< / sup > message was sent to the queue, the 2< sup > nd< / sup > message was discarded.< br / >
Only the 3< sup > rd< / sup > message remains in the queue.< / em > < / p >
< li > We will browse the queue. There will be a single message displayed: the 3< sup > rd< / sup > message< / li >
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< code > QueueBrowser browser = session.createBrowser(queue);
Enumeration enumeration = browser.getEnumeration();
while (enumeration.hasMoreElements())
{
TextMessage messageInTheQueue = (TextMessage)enumeration.nextElement();
System.out.format("Message in the queue: %s\n", messageInTheQueue.getText());
}
browser.close();
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< p > < em > We will now consume the message on the queue< / em > < / p >
< li > We create a JMS message consumer on the queue< / li >
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< code > MessageConsumer messageConsumer = session.createConsumer(queue);< / code >
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< li > We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started< / li >
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< code > connection.start();< / code >
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< li > We try to receive a message from the queue. It will be the 3< sup > rd< / sup > message< / li >
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< code > TextMessage messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.format("Received message: %s\n", messageReceived.getText());< / code >
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< li > We will try to receive another message but there is no other on the queue. The < code > receive< / code > method will timeout after 5 seconds< / li >
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< code > messageReceived = (TextMessage)messageConsumer.receive(5000);
System.out.format("Received message: %s\n", messageReceived);< / code >
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< 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 >
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< h2 > More information< / h2 >
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< li > User Manual's < a href = "../../../docs/user-manual/en/html_single/index.html#last-value-queues" > Last-Value Queues chapter< / a > < / li >
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