JMS Interview Questions and Answers

Q: What is JMS?
A: JMS is an acronym used for Java Messaging Service. It is Java's answer to creating
software using asynchronous messaging. It is one of the official specifications of the J2EE
technologies and is a key technology.
Q: How JMS is different from RPC?
A: In RPC the method invoker waits for the method to finish execution and return the control
back to the invoker. Thus it is completely synchronous in nature. While in JMS the message
sender just sends the message to the destination and continues it's own processing. The sender
does not wait for the receiver to respond. This is asynchronous behavior.
Q: What are the advantages of JMS?
A: JMS is asynchronous in nature. Thus not all the pieces need to be up all the time for the
application to function as a whole. Even if the receiver is down the MOM will store the messages
on it's behalf and will send them once it comes back up. Thus at least a part of application can
still function as there is no blocking.
Q: Are you aware of any major JMS products available in the market?
A: IBM's MQ Series is one of the most popular product used as Message Oriented
Middleware. Some of the other products are SonicMQ, iBus etc. Weblogic application server also
comes with built in support for JMS messaging.
Q: What are the different types of messages available in the JMS API?
A: Message, TextMessage, BytesMessage, StreamMessage, ObjectMessage,
MapMessage are the different messages available in the JMS API.
Q: What are the different messaging paradigms JMS supports?
A: Publish and Subscribe i.e. pub/suc and Point to Point i.e. p2p.
Q: What is the difference between ic and queue?
A: A ic is typically used for one to many messaging i.e. it supports publish subscribe model
of messaging. While queue is used for one-to-one messaging i.e. it supports Point to Point
Q: What is the role of JMS in enterprise solution development?
A: JMS is typically used in the following scenarios
1. Enterprise Application Integration: - Where a legacy application is integrated with a new
application via messaging.
2. B2B or Business to Business: - Businesses can interact with each other via messaging
because JMS allows organizations to cooperate without tightly coupling their business systems.
3. Geographically dispersed units: - JMS can ensure safe exchange of data amongst the
geographically dispersed units of an organization.
4. One to many applications: - The applications that have to push data in packet to huge number
of clients in a one-to-many fashion are good candidates for the use JMS. Typical such
applications are Auction Sites, Stock Quote Services etc.
Q: What is the use of Message object?
A: Message is a light weight message having only header and properties and no payload.
Thus if the received are to be notified abt an event, and no data needs to be exchanged then
using Message can be very efficient.
Q: What is the basic difference between Publish Subscribe model and P2P model?
A: Publish Subscribe model is typically used in one-to-many situation. It is unreliable but
very fast. P2P model is used in one-to-one situation. It is highly reliable.
Q: What is the use of BytesMessage?
A: BytesMessage contains an array of primitive bytes in it's payload. Thus it can be used for
transfer of data between two applications in their native format which may not be compatible with
other Message types. It is also useful where JMS is used purely as a transport between two
systems and the message payload is opaque to the JMS client. Whenever you store any primitive
type, it is converted into it's byte representation and then stored in the payload. There is no
boundary line between the different data types stored. Thus you can even read a long as short.
This would result in erroneous data and hence it is advisable that the payload be read in the
same order and using the same type in which it was created by the sender.
Q: What is the use of StreamMessage?
A: StreamMessage carries a stream of Java primitive types as it's payload. It contains some
conveient methods for reading the data stored in the payload. However StreamMessage prevents
reading a long value as short, something that is allwed in case of BytesMessage. This is so
because the StreamMessage also writes the type information alonwgith the value of the primitive
type and enforces a set of strict conversion rules which actually prevents reading of one primitive
type as another.
Q: What is the use of TextMessage?
A: TextMessage contains instance of java.lang.String as it's payload. Thus it is very useful
for exchanging textual data. It can also be used for exchanging complex character data such as
an XML document.
Q: What is the use of ObjectMessage?
A: ObjectMessage contains a Serializable java object as it's payload. Thus it allows
exchange of Java objects between applications. This in itself mandates that both the applications
be Java applications. The consumer of the message must typecast the object received to it's
appropriate type. Thus the consumer should before hand know the actual type of the object sent
by the sender. Wrong type casting would result in ClassCastException. Moreover the class
definition of the object set in the payload should be available on both the machine, the sender as
well as the consumer. If the class definition is not available in the consumer machine, an attempt
to type cast would result in ClassNotFoundException. Some of the MOMs might support dynamic
loading of the desired class over the network, but the JMS specification does not mandate this
behavior and would be a value added service if provided by your vendor. And relying on any such
vendor specific functionality would hamper the portability of your application. Most of the time the
class need to be put in the classpath of both, the sender and the consumer, manually by the
Q: What is the use of MapMessage?
A: A MapMessage carries name-value pair as it's payload. Thus it's payload is similar to the
java.util.Properties object of Java. The values can be Java primitives or their wrappers.
Q: What is the difference between BytesMessage and StreamMessage??
A: BytesMessage stores the primitive data types by converting them to their byte
representation. Thus the message is one contiguous stream of bytes. While the StreamMessage
maintains a boundary between the different data types stored because it also stores the type
information along with the value of the primitive being stored. BytesMessage allows data to be
read using any type. Thus even if your payload contains a long value, you can invoke a method to
read a short and it will return you something. It will not give you a semantically correct data but
the call will succeed in reading the first two bytes of data. This is strictly prohibited in the
StreamMessage. It maintains the type information of the data being stored and enforces strict
conversion rules on the data being read.
Q: What is point-to-point messaging?
A: With point-to-point message passing the sending application/client establishes a named
message queue in the JMS broker/server and sends messages to this queue. The receiving client
registers with the broker to receive messages posted to this queue. There is a one-to-one
relationship between the sending and receiving clients.
Q: Can two different JMS services talk to each other? For instance, if A and B are two
different JMS providers, can Provider A send messages directly to Provider B? If not, then
can a subscriber to Provider A act as a publisher to Provider B?
A: The answers are no to the first question and yes to the second. The JMS specification
does not require that one JMS provider be able to send messages directly to another provider.
However, the specification does require that a JMS client must be able to accept a message
created by a different JMS provider, so a message received by a subscriber to Provider A can
then be published to Provider B. One caveat is that the publisher to Provider B is not required to
handle a JMSReplyTo header that refers to a destination that is specific to Provider A.
Q: What is the advantage of persistent message delivery compared to nonpersistent
A: If the JMS server experiences a failure, for example, a power outage, any message that it
is holding in primary storage potentially could be lost. With persistent storage, the JMS server
logs every message to secondary storage. (The logging occurs on the front end, that is, as part of
handling the send operation from the message producing client.) The logged message is
removed from secondary storage only after it has been successfully delivered to all consuming
clients .
Q: Give an example of using the publish/subscribe model.
A: JMS can be used to broadcast shutdown messages to clients connected to the Weblogic
server on a module wise basis. If an application has six modules, each module behaves like a
subscriber to a named ic on the server.
Q: Why doesn't the JMS API provide end-to-end synchronous message delivery and
notification of delivery?
A: Some messaging systems provide synchronous delivery to destinations as a mechanism
for implementing reliable applications. Some systems provide clients with various forms of
delivery notification so that the clients can detect dropped or ignored messages. This is not the
model defined by the JMS API.
JMS API messaging provides guaranteed delivery via the once-and-only-once delivery semantics
of PERSISTENT messages. In addition, message consumers can insure reliable processing of
messages by using either CLIENT_ACKNOWLEDGE mode or transacted sessions. This
achieves reliable delivery with minimum synchronization and is the enterprise messaging model
most vendors and developers prefer.
The JMS API does not define a schema of systems messages (such as delivery notifications). If
an application requires acknowledgment of message receipt, it can define an application-level
acknowledgment message.
Q: What are the various message types supported by JMS?
A: Stream Messages ? Group of Java Primitives
Map Messages ? Name Value Pairs. Name being a string& Value being a java primitive
Text Messages ? String messages (since being widely used a separate messaging Type has
been supported)
Object Messages ? Group of serialize able java object
Bytes Message ? Stream of uninterrupted bytes
Q: How is a java object message delivered to a non-java Client?
A: It is according to the specification that the message sent should be received in the same
format. A non-java client cannot receive a message in the form of java object. The provider in
between handles the conversion of the data type and the message is transferred to the other end.
Q: What is MDB and What is the special feature of that?
A: MDB is Message driven bean, which very much resembles the Stateless session bean.
The incoming and out going messages can be handled by the Message driven bean. The ability
to communicate asynchronously is the special feature about the Message driven bean.
Q: What are the types of messaging?
A: There are two kinds of Messaging.
Synchronous Messaging: Synchronous messaging involves a client that waits for the server to
respond to a message.
Asynchronous Messaging: Asynchronous messaging involves a client that does not wait for a
message from the server. An event is used to trigger a message from a server.
Q: What are the core JMS-related objects required for each JMS-enabled application?
A: Each JMS-enabled client must establish the following:
• A connection object provided by the JMS server (the message broker)
• Within a connection, one or more sessions, which provide a context for message sending and
• Within a session, either a queue or ic object representing the destination (the message staging
area) within the message broker
• Within a session, the appropriate sender or publisher or receiver or subscriber object
(depending on whether the client is a message producer or consumer and uses a point-to-point or
publish/subscribe strategy, respectively)
Within a session, a message object (to send or to receive)

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