JMS Interview Questions

What is JMS?

Java Message Service: An interface implemented by most J2EE containers to provide point-to-point queueing and topic (publish/subscribe) behavior. JMS is frequently used by EJB's that need to start another process asynchronously. For example, instead of sending an email directly from an Enterprise JavaBean, the bean may choose to put the message onto a JMS queue to be handled by a Message-Driven Bean (another type of EJB) or another system in the enterprise. This technique allows the EJB to return to handling requests immediately instead of waiting for a potentially lengthy process to complete.

What are the advantages of JMS?

You can use it in the context of mutithreading but it means JMS is not meant for Multithreading. Its basically meant for object communication. It will be useful when you are writing some event based applications like Chat Server which needs a publish kind of event mechanism to send messages between the server to the clients who got connected with the server. Moreover JMS gives Loosely-coupled kind of mechanism when compared with RMI which is tightly-coupled. In JMS there is no need for the destination object to be available online while sending a message from the client to the server. But in RMI it is necessary. So we can use JMS in place of RMI where we need to have loosely-coupled mechanism.

What are major JMS products available in the market?

IBM's MQ Series, SonicMQ, iBus etc. All the J2EE compliant application servers come built with thier own implementation of JMS.

What are the different types of messages available in the JMS API?

Message, TextMessage, BytesMessage, StreamMessage, ObjectMessage, MapMessage are the different messages available in the JMS API.

What is the difference between Point to Point and Publish/Subscribe Messaging Domains

A point-to-point (PTP) product or application is built around the concept of message queues, senders, and receivers. Each message is addressed to a specific queue, and receiving clients extract messages from the queue(s) established to hold their messages. Queues retain all messages sent to them until the messages are consumed or until the messages expire In a publish/subscribe (pub/sub) product or application, clients address messages to a topic. Publishers and subscribers are generally anonymous and may dynamically publish or subscribe to the content hierarchy. The system takes care of distributing the messages arriving from a topic's multiple publishers to its multiple subscribers. Topics retain messages only as long as it takes to distribute them to current subscribers.

What is the diffrence between JAVA Mail and JMS Queue

Java Mail – API siting on top of e-mail protocols like SMTP, POP, IMAP – essentially same stuff e-mail clients like MS outlook use .. hence make sense if at least on one side of conversation we have human. JMS Queue – is asynchronous point-to-point communication between systems

What is the difference between ic and queue?

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 Messaging.

What is the role of JMS in enterprise solution development?

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.

What is the use of Message object?

Message is a light weight message having only header and properties and no payload. Thus if the receivers are to be notified about an event, and no data needs to be exchanged then using Message can be very efficient.

What are the three components of a Message ?

A JMS message consists of three parts: Message header For message identification. For example, the header is used to determine if a given message is appropriate for a "subscriber" Properties For application-specific, provider-specific, and optional header fields Body Holds the content of the message. Several formats are supported, including TextMessage, which wrap a simple String, that wrap arbitrary Java objects (which must be serializable). Other formats are supported as well.

What kind of information found in the header of a Message ?

The header of a message contains message identification and routing information. This includes , but is not limited to : JMSDestination JMSDeliveryMode JMSMessageID JMSTimeStamp JMSExpiration JMSReplyTO JMSCorrelationID JMSType JMSRedelivered

What is the basic difference between Publish Subscribe model and P2P model?

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.

What is the use of BytesMessage?

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.

What is the use of StreamMessage?

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.

What is the use of TextMessage?

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.

What is the use of ObjectMessage?

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.

What is the use of MapMessage?

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.

What is the difference between BytesMessage and StreamMessage?

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.

What is point-to-point messaging?

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.

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?

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.

What is the advantage of persistent message delivery compared to nonpersistent delivery?

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 .

Give an example of using the publish/subscribe model.

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.

Why doesn't the JMS API provide end-to-end synchronous message delivery and notification of delivery?

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 ensure 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.

What are the various message types supported by JMS?

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

How is a java object message delivered to a non-java Client?

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.

What is MDB and What is the special feature of that?

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.

What are the types of messaging?

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.

What are the core JMS-related objects required for each JMS-enabled application?

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 receiving • 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)