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Transcript jms 

A Study in JMS
(Java Messaging
Service)
Chad Beaudin
CS 522
Fall Semester 2002
JMS Overview
The Java Message Service is a Java API that
allows applications to create, send, receive,
and read messages
The JMS API minimizes the set of concepts a
programmer must learn to use messaging
products but provides enough features to
support sophisticated messaging applications
JMS Overview (cont)
The JMS API enables communication that is
not only loosely coupled but also:
Asynchronous. A JMS provider can deliver
messages to a client as they arrive; a client does
not have to request messages in order to receive
them.
Reliable. The JMS API can ensure that a
message is delivered once and only once. Lower
levels of reliability are available for applications
that can afford to miss messages or to receive
duplicate messages.
JMS Overview (cont)
Messages can be consumed in either of two ways:
Synchronously. A subscriber or a receiver explicitly fetches
the message from the destination by calling the receive
method. The receive method can block until a message
arrives or can time out if a message does not arrive within a
specified time limit.
Asynchronously. A client can register a message listener
with a consumer. A message listener is similar to an event
listener. Whenever a message arrives at the destination, the
JMS provider delivers the message by calling the listener's
onMessage() method, which acts on the contents of the
message.
JMS And JNDI
JMS does not not define a standard address syntax by which
clients communicate with each other. Instead JMS utilizes Java
Naming & Directory Interface(JNDI).
JNDI provides a mechanism by which clients can perform a
“lookup” to find the correct information to connect to each other.
Using JNDI provides the following advantages:
It hides provider-specific details from JMS clients.
It abstracts JMS administrative information into Java objects that
are easily organized and administrated from a common
management console.
Since there will be JNDI providers for all popular naming services,
this means JMS providers can deliver one implementation of
administered objects that will run everywhere. Thereby eliminating
deployment and configuration issues.
JMS Advantages over RPC
RPC relies on the physical connection of the client
and server to the network; it is a synchronous
protocol.
What happens if the client is disconnected?
Network could go down
Client could be a laptop that is used on the road.
In this case, the end user might still want to carry on
working, but can't if an RPC model is being used—at
least not without a great deal of work by the
programmer.
Messaging Benefits
JMS vs RPC
Messaging provides the ability to send data
asynchronously and in a disconnected manner.
Two messaging models
Point-to-point
Publish-and-Subscribe
In an email like model, these would be equivalent to
sending and receiving e-mails directly (point-to-point),
or subscribing to a list server and sending and
receiving e-mails through the list server (publish-andsubscribe).
What is the cost?
JMS vs RPC
Applications become asynchronous by nature
What if we require a method to give us a return
value?
What if we require the data (the messages) to be
delivered in a specific order?
Using messaging, JMS, we have to deal with
these problems ourselves. RPC handled
these issues for the programmer.
Messages Explained
A message typically consists of a header and a body.
The message header contains vendor-specified
values, but could also contain application-specific
data as well.
Headers are typically name/value pairs.
The body contains data; the type of the data is
defined by the specification.
Text
A serialized Java object
One of a number of other types of data.
Publisher Sample
See MyTopicPublisher.java for source.
1. Perform a JNDI API lookup of the TopicConnectionFactory and topic
topic = (Topic) jndiContext.lookup(topicName);
2. Create a connection and a session
topicConnection = topicConnectionFactory.createTopicConnection();
topicSession = topicConnection.createTopicSession(false,
Session.AUTO_ACKNOWLEDGE);
3. Create a TopicPublisher
topicPublisher = topicSession.createPublisher(topic);
4. Create a TextMessage
Message = topicSession.createTextMessage();
message.setText("This is message " + (i + 1));
5. Publishe one or more messages to the topic
topicPublisher.publish(message);
6. Close the connection, which automatically closes the session and
TopicPublisher
Subscriber Sample
See MyTopicSubscriber.java for source.
1.Perform a JNDI API lookup of the TopicConnectionFactory and
topic (same as publisher)
2.Create a connection and a session (same as publisher)
3.Create a TopicSubscriber
topicSubscriber = topicSession.createSubscriber(topic);
4.Create an instance of the TextListener class and registers it as
the message listener for the TopicSubscriber
topicListener = new TextListener();
topicSubscriber.setMessageListener(topicListener);
5.Start the connection, causing message delivery to begin
topicConnection.start();
6.Close the connection, which automatically closes the session and
TopicSubscriber
topicConnection.close();
TextListener Sample
1. public void onMessage(Message message) {
2.
TextMessage msg = null;
3.
4.
try {
5.
if (message instanceof TextMessage) {
6.
msg = (TextMessage) message;
7.
System.out.println("Reading message: " + msg.getText());
8.
} else {
9.
System.out.println("Message of wrong type: " +
10.
message.getClass().getName());
11.
}
12.
} catch (JMSException e) {
13.
System.out.println("JMSException in onMessage(): " + e.toString());
14.
} catch (Throwable t) {
15.
System.out.println("Exception in onMessage():" + t.getMessage());
16.
}
17. }
Running The Sample
1.
Start the JMS provider. In this case the J2EE SDK
From a command prompt run the following command:
j2ee –verbose
Wait until the server displays the message "J2EE server startup
complete
2.
Create the Administered Object. This is the object to which
you will publish and subscribe.
From a second command prompt run the following command
j2eeadmin -addJmsDestination CS522Topic topic
To verify the topic was created, view the list of Administered
Objects by typing:
j2eeadmin –listJmsDestination
Running The Sample (cont)
3.
Run the subscriber program
From a command prompt run the following command within the
directory that contains the MyTopicSubscriber.class:
java Djms.properties=%J2EE_HOME%\config\jms_client.pr
operties MyTopicSubscriber -topic=CS522Topic
4.
Run the Publisher program
From a command prompt run the following command within the
directory that contains the MyTopicPublisher.class:
java Djms.properties=%J2EE_HOME%\config\jms_client.pr
operties MyTopicPublisher -topic=CS522Topic count=500 -delay=500
5.
You will see text output in both the Publisher and Subscriber
windows
References
http://java.sun.com/products/jms/tutorial/1_
3_1-fcs/doc/jms_tutorialTOC.html
http://java.sun.com/products/jndi/