I want to learn the architecture of JMS

Question

Hi
I'm doing a school project and I have to understand the architecture of JMS. Can someone explain it as simple as possible, as if I were a 10 year old.
1.) I want to know what the topics destination is...is it a class? what does it do?
2.) Topic factory...what it is...what it do...
3.) The session...when is it created...when a user log on?
Can someone explain the whole architecture i would be very grateful
Thanx in advance

Answer 1

1. Create a topic connection factory
This would be more aptly said, "GET a connection factory." As YOU don't create one. Instead, you typically look it up in JNDI like so:
Context context = new IntialContext();
ConnectionFactory factory = (ConnectionFactory)context.lookup("ConnnectionFactory");
The reason you do this is becuase ConnectionFactories are considered "Administered objects." This means that they are configured on the server by and administrator. They encapsulates provider specific settings that generally relate to the creation of connections like what protocol to use, what port, what the server IP address is, etc. When an administrator creates a ConnectionFactory it is bound to the JNDI tree.
2. create topic (destination) (What is this...whats the use of it?)
Like a ConnectionFactory, a destination is an administered object. So your question is more aptly termed "GET" a destination. Administrators create and configure destinations on the server and the JMS client looks them up just like it does the ConnectionFactory. See the example above. Like a ConnectionFactory, the administrator configures the destination with provider specific information--perhaps security, etc. It too is bound to the JNDI tree.
3. uses topic connection factory object to create connection object
A Connection represents a physical connection between the client and the server. When you call createConnection on the ConnectionFactory the provider is opening a connection to the server using the provider specific information configured in the ConnectionFactory. Connections are heavy weight objects because they are typically backed by Sockets.
4. uses topic connection objectto create topic session object
A Session object plays the central roll in JMS--it defines a serial inbound and outbound flow of messages (possibly to/from multiple destinations), and also maintains the transactional boundaries. This means that you can have several different Consumers and several different Producers and messages can be sent and received and then committed or rolled back as a transactional unit. If you rollback the transacted session, all received messages will be negatively acknowledged (causing their redelivery) and all sent messages will be destroyed (which will cause them never to be delivered to thier intended destination). A Session is also a factory for creating messages.
5. creates topic publisher or topic subscriber using session object
You use the Session as a factory for creating Consumers or Producers. As their name implies, they either producer or consume messages. The Producer only exposes a few methods which all just send messages. The Consumer, however, exposes both asynch and synch operations. If you register a MessageListener with the Consumer, then you are requesting notification from the Server. When the server wants to deliver a Message to you, it will call the OnMessage method of your implementation of MessageListener. If you want to simply block for messages in a synch fashion then you use the Receive methods.
6. starts the connection
Starting the Connection is only necessary when you are dealing with Consumers. You can send Messages on a stopped Connection. What the start method is really for is to provide the client a mechanism for telling the Connection when it is ready for message delivery to start. If the provider just started delivery as soon as your Connection was created, then you'd start receiving messages before you had your Session and Consumers setup properly.
7. Publishes or subsscribes to messages
Again, if you are only publishing (producing) you don't need to do number 6 before you do number 7. If you are subscribing (consuming) you do.
8. Close the session and the connection
When you close the Connection it will close all of the Session it created. The spec requires that the Session be shut down cleanly. An interesting thing about closing a Session is that even though you use a Session as a factory for creating (and I mean creating this time) Temporary Destinations, the lifecycle of these Destinations is NOT tied to the Session which created them. Instead, their lifecycle is tied to the Connection on which the Session was tied. That means that you can close a Session and the TemporaryDestination will not be destroyed. This is just a little oddity in my mind...