QueueSession (Java EE 6 )
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Sun provides a list of JMS vendors at http: JMS supports both point-to-point messaging and publish-subscribe. You send point-to-point messages using queues and you publish messages to subscribers via topics. Note Although JMS supports point-to-point and pub-sub messaging, the specification doesn't require a vendor to implement both types of messaging. A particular implementation might contain only point-to-point or pub-sub.
JMS seems a little complicated when you first start using it because it you must create a lot of classes just to send a message. After you understand the structure of the classes, however, it is easy to use. The QueueConnection and TopicConnection classes manage all your interactions with the message server, but to interact with the message server, you must create either a QueueSession or a TopicSession. Although the connection is your link to the message server, you do your work using other objects.
They represent your connection to a server and you use them to create other objects, but you don't normally do your work by using them directly. The QueueSession and TopicSession classes are also similar to JDBC connections in that you use them to create objects, but again, you don't interact with them directly to send and receive messages.
The session objects let you create the senders, receivers, queues, and topics you need to actually send and receive messages. One of the big benefits of JMS is that you can perform message operations as part of a transaction.Fault Tolerance Implementaion in Tibco Administrator with Subtitles
When you create a QueueSession or a TopicSession, you can specify that the session is transactional. The reason transactions are so important for messaging is that you get into situations in which you read a message off a message queue and then try to insert the message in a database.
If the database operation fails, you can't just stick the message back on the queue. If the operation is transactional, however, when the database operation fails, your message stays in the queue.
Transactions can also ensure that the messaging system delivers the messages in the order you send them. Next, you use the QueueConnection to create a QueueSession. Next, you create the Queue although you usually look for the queue in the naming service first, in case someone has already created it.
When you create the queue, you must give it a name. Every queue must have a unique name. After you have a Queue, you're almost ready. All you need is a QueueSender to send messages on the queue and one or more Message objects. You use the QueueSession to create both the QueueSender and the messages. In this case, the program sends "Hello" messages over a message queue. Of course, instead of creating a QueueSender, you create a QueueReceiver.
There are two different ways to receive messages. You can call the receive method, which waits for messages, or you can create a listener object that receives messages when they become available.
Notice that most of the setup is the same as in Listing You tell the QueueReceiver about your message listener and it will automatically let you know when a message comes in. The QLender class received the request on the LoanRequestQ queue, applied the simple business logic based on the salary to loan ratio, and sent back the response on the LoanResponseQ queue.
The message was then received by the QBorrower class, and the contents of the return message displayed on the console.
EJB - Message Driven Beans
Producers and consumers in the loan example The rest of this chapter examines the source code for the QBorrower and QLender classes, and covers several advanced subjects related to the point-to-point messaging model. The QBorrower Class The QBorrower class is responsible for sending a loan request message to a queue containing a salary and loan amount. The class is fairly straightforward: The main method instantiates the QBorrower class and, upon receiving a salary and loan amount from standard input, invokes the sendLoanRequest method to send the message to the queue.
Here is the listing for the QBorrower class in its entirety. We will be examining the JMS aspects of this class in detail after the full listing: Once the input parameters have been validated, the QBorrower class is instantiated and a loop is started that reads the salary and loan amount into the class from the console: The input loop continues until the user presses enter on the console without entering any data: The first thing the constructor does is establish a connection to the JMS provider by creating an InitialContext: When the QueueConnection is created, the connection is initially in stopped mode.
This means you can send messages to the queue, but no message consumers including the QBorrower class, which is also a message consumer may receive messages from this connection until it is started. Typically, applications will create a single JMS Connection on application startup and maintain a pool of Session objects for use whenever a message needs to be produced or consumed.
The QueueSession object is created through a factory object on the QueueConnection object. The QueueConnection variable is declared outside of the constructor in our example so that the connection can be closed in the exit method of the QBorrower class.
The Java Message Service (JMS) | JMS and the Importance of Messaging | InformIT
It is important to close the connection after it is no longer being used to free up resources. Closing the Connection object also closes any open Session objects associated with the connection. The statement in the constructor to create the QueueSession is as follows: The first parameter indicates whether the QueueSession is transacted or not. A value of true indicates that the session is transacted, meaning that messages sent to queues during the lifespan of the QueueSession will not be delivered to the receivers until the commit method is invoked on the QueueSession.
Likewise, invoking the rollback method on the QueueSession will remove any messages sent during the transacted session. The second parameter indicates the acknowledgment mode. The three possible values are Session. The acknowledgment mode is ignored if the session is transacted. In our case, the JMS destination is cast to a Queue. The argument supplied to each of the lookup methods is a String value containing the JNDI name of the queues we are using in the class: It is generally a good idea to perform all of your initialization logic before starting the connection: Another useful thing you can obtain from the JMS Connection is the metadata about the connection.
It is particularly useful for products or applications that may use multiple providers. Sending the message and receiving the response Once the QBorrower class is initialized, the salary and loan amounts are entered through the command line.
At this point, the sendLoanRequest method is invoked from the main method to send the loan request to the queue and wait for the response from the QLender class.
At the start of this method, we chose to create a MapMessage but we could have used any of the five JMS message types: Instantiating a new JMS message object using the new keyword will not work; it must be created from the Session object.
EJB Message Driven Beans
After creating and loading the message object, we are also setting the JMSReplyTo message header property to the response queue, which further decouples the producer from the consumer. After the message is created, we then create the QueueSender object, specifying the queue we wish to send messages to, and then send the message using the send method: The one we are using here accepts only the JMS message object as the single argument.
The other overridden methods allow you to specify the Queue, the delivery mode, the message priority, and finally the message expiry. Since we are not specifying any of the other values in the example just shown, the message priority is set to normal 4the delivery mode is set to persistent messages DeliveryMode. All of these parameters can be overridden by using one of the other send methods.
Once the message has been sent, the QBorrower class will block and wait for a response from the QLender on whether the loan was approved or denied. The first step in this process is to set up a message selector so that we can correlate the response message with the one we sent. This is necessary because there may be many other loan requests being sent to and from the loan request queues while we are making our loan request.
To make sure we get the proper response back, we would use a technique called message correlation. Now that we have a QueueReceiver, we can invoke the receive method to do a blocking wait until the response message is received. In this case, we are using the overridden receive method that accepts a timeout value in milliseconds: If a timeout condition does occur, the message returned from the receive method will be null.
Note that it is the entire message object that is null, not just the message payload. The receive method returns a Message object. If the message type is known, then you can cast the return message as we did in the preceding code example. However, a more failsafe technique would be to check the return Message type using the instanceof keyword as indicated here: In the example just shown, we sent the loan request using a MapMessage, yet we received the response from the receiver as a TextMessage.
While you could potentially increase the level of decoupling between the sender and receiver by including the message type as part of the application properties of the message, you would still need to know how to interpret the payload in the message.
For example, with a StreamMessage or BytesMessage you would still need to know the order of data being sent so that you could in turn read it in the proper order and data type.
The QLender Class The role of the QLender class is to listen for loan requests on the loan request queue, determine if the salary meets the necessary business requirements, and finally send the results back to the borrower. Notice that the QLender class is structured a bit differently from the QBorrower class.
In our example, the QLender class is referred to as a message listener and, as such, implements the javax. MessageListener interface and overrides the onMessage method. Here is the complete listing for the QLender class: This is evident from the fact that the QLender class implements the MessageListener interface and overrides the onMessage method. The main method of the QLender class validates the command-line arguments and invokes the constructor by instantiating a new QLender class.
It then keeps the primary thread alive until the enter key is pressed on the command line. The constructor in the QLender class works much in the same way as the QBorrower class. The first part of the constructor establishes a connection to the provider, does a JNDI lookup to get the queue, creates a QueueSession, and starts the connection: Once the connection is started, the QLender class can begin to receive messages.
However, before it can receive messages, it must be registered by the QueueReceiver as a message listener: That thread will wait until a message is received, and upon receipt of a message, will invoke the onMessage method of the listener class.
In this case, we set the message listener to the QLender class using the this keyword in the setMessageListener method. We could have easily delegated the messaging work to another class that implemented the MessageListener interface: The onMessage method first casts the message to a MapMessage the message type we are expecting to receive from the borrower. It then extracts the salary and loan amount requested from the message payload, checks the salary to loan amount ratio, then determines whether to accept or decline the loan request: Again, to make this more failsafe, it would be better to check the JMS message type using the instanceof keyword in the event another message type was being sent to that queue: It does this by first creating a JMS message to send.
The response message does not need to be the same JMS message type as the loan request message that was received by the QLender. Once the message is created, the onMessage method then sends the message to the response queue specified by the JMSReplyTo message header property. The QLender class can now use that property as the destination to send the response message to: Many borrowers may be making requests at the same time, meaning that the lender application is sending many messages to the response queue.