20+ TOP JMS Interview Questions & Answers
1. 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.
2. 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.
3. 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.
4. 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.All the J2EE compliant application servers come built with thier own implementation of JMS.
5. 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.
6. What are the different messaging paradigms JMS supports?
A: Publish and Subscribe i.e. pub/suc and Point to Point i.e. p2p.
7. What is the difference between topic and queue?
A: A topic 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 Messaging.
8. 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 need 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.
9. What is the use of Message object?
A: Message is a light weight message having only header and properties and no payload. Thus if theIf the receivers are to be notified abt an event, and no data needs to be exchanged then using Message can be very efficient.
10. 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.
11. What is the use of Bytes Message?
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.
12. 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.
13. 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.
14. 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 developer.
15. 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.
16. 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.
17. 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.
18. 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.
19. What is the advantage of persistent message delivery compared to nonpersistent delivery?
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 .
20. 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 topic on the server.