JMS QueueRequestor Example

This example shows you how to use a QueueRequestor with ActiveMQ.

JMS is mainly used to send messages asynchronously so that the producer of a message is not waiting for the result of the message consumption. However, there are cases where it is necessary to have a synchronous behavior: the code sending a message requires a reply for this message before continuing its execution.
A QueueRequestor facilitates this use case by providing a simple request/reply abstraction on top of JMS.

The example consists in two classes:

TextReverserService

A JMS MessageListener which consumes text messages and sends replies containing the reversed text

QueueRequestorExample

A JMS Client which uses a QueueRequestor to send text requests to a queue and receive replies with the reversed text in a synchronous fashion

Example step-by-step

To run the example, simply type mvn verify -Pexample from this directory

1. First we need to get an initial context so we can look-up the JMS connection factory from JNDI. This initial context will get it's properties from the client-jndi.properties file in the directory ../common/config

           InitialContext initialContext = getContext();
        

2. We look up the JMS queue from JNDI

           Queue queue = (Queue)initialContext.lookup("/queue/exampleQueue");
        

3. We look up JMS queue connection factory from JNDI

           QueueConnectionFactory cf = (QueueConnectionFactory)initialContext.lookup("/ConnectionFactory");
        

4. We create the TextReverserService which will consume messages from the queue

            TextReverserService reverserService = new TextReverserService(cf, queue);
        

5. We Create a JMS queue connection

           connection = cf.createQueueConnection();
        

6. We start the connection. In order for delivery to occur on any consumers or subscribers on a connection, the connection must be started

           connection.start();
        

7. We create a JMS queue session. The session is created as non transacted and will auto acknowledge messages (this is mandatory to use it to create a QueueRequestor)

           QueueSession session = connection.createQueueSession(false, Session.AUTO_ACKNOWLEDGE);
        

8. We create a JMS QueueRequestor using the session and the queue

           QueueRequestor queueRequestor = new QueueRequestor(session, queue);
        

9. We create a JMS text message to send as a request

           TextMessage request = session.createTextMessage("Hello, World!");
        

10. We use the queue requestor to send the request and block until a reply is received.
    Using the queue requestor simplify request/reply use case by abstracting boilerplate JMS code (creating a temporary queue, a consumer and a producer, setting the JMS ReplyTo header on the request, sending the request with the producer, consuming the message from the consumer). All this code is replaced by a single call to QueueRequestor.request() method.

           TextMessage reply = (TextMessage)queueRequestor.request(request);
        

11. The reply's text contains the reverse of the request's text

           System.out.println("Send request: " + request.getText());
           System.out.println("Received reply:" + reply.getText());
        

12. We close the queue requestor to release all the JMS resources it created to provide request/reply mechanism

           queueRequestor.close()
        

13. We do the same for the text reverser service

           reverserService.close()
        

14. And finally, always remember to close your JMS connections and resources after use, in a finally block. Closing a JMS connection will automatically close all of its sessions, consumers, producer and browser objects

           finally
           {
              if (initialContext != null)
              {
                initialContext.close();
              }
              if (connection != null)
              {
                 connection.close();
              }
           }