Monday, February 12, 2007

Java Message Service API - FAQ

Below is JMS FAQ from Sun. Lots of answers to lots of common JMS questions .

FREQUENTLY ASKED QUESTIONS

General Questions

Technical questions

General questions

Q: What is the Java Message Service?
A: The Java Message Service (JMS) API is an API for accessing enterprise messaging systems. It is part of the Java 2 Platform, Enterprise Edition (J2EE).

The Java Message Service makes it easy to write business applications that asynchronously send and receive critical business data and events.

The Java Message Service defines a common enterprise messaging API that is designed to be easily and efficiently supported by a wide range of enterprise messaging products.

The Java Message Service supports both message queueing and publish-subscribe styles of messaging.

Q: Is the Java Message Service another mail API?
A: No. The term messaging is broadly defined in computing. It is used for describing various operating system concepts; it is used to describe email and fax systems; and with the JMS API, it is used to describe asynchronous communication between enterprise applications.

JMS messages are asynchronous requests, reports, or events that are consumed by enterprise applications, not humans. They contain vital information needed to coordinate these systems. They contain precisely formatted data that describe specific business actions. Through the exchange of these messages, each application tracks the progress of the enterprise.

Q: Is the Java Message Service a product?
A: No, the Java Message Service is the specification of a common API for enterprise messaging. A JMS provider supplied by an enterprise messaging vendor is required to use it.

Q: Where can I find the Java Message Service specification?
A: The Java Message Service 1.0.2 specification is available at http://java.sun.com/products/jms/docs.html. A link to the online JMS API documentation is at the same location.

Q: Who was involved with Sun in the creation of the specification of Java Message Service 1.0.2?
A: A number of important industry players initially collaborated with Sun to define the first draft of the Java Message Service specification. In addition, many comments were received from other companies, government and educational organizations, and others during the three-month public review period.

Q: What is compelling about the Java Message Service?
A: The Java Message Service is compelling for these reasons:

  • It is the first enterprise messaging API that has achieved wide industry support.
  • It simplifies the development of enterprise applications by providing standard messaging concepts and conventions that apply across a wide range of enterprise messaging systems.
  • It leverages existing, enterprise-proven messaging systems.

Q: Why should developers use the Java Message Service?
A: The Java Message Service makes it easy to do the following:

  • Write portable, message-based business applications
  • Extend existing message-based applications by adding new JMS clients that interoperate fully with their existing non-JMS clients

Q: What does a programmer need to learn to use the Java Message Service?
A: Message-based applications are fundamentally different from remote procedure call based applications. Once a developer understands how best to employ both technologies, the JMS API makes writing message-based applications as easy as learning a few additional interfaces.

Q: What is the relationship between the Java Message Service and Enterprise JavaBeans components?
A: Since J2EE version 1.2 was released, Enterprise JavaBeans components have been able to use the JMS API to send enterprise messages and receive them synchronously. The Java 2 SDK, Enterprise Edition, version 1.3, now available, provides a new kind of enterprise bean, the message-driven bean, that allows an enterprise application to receive messages asynchronously.

Q: What is the relationship between the Java Message Service and the Java Naming and Directory Interface (JNDI) API?
A: The JMS API, like the other Java Enterprise APIs, uses the JNDI API for administration. The JMS API defines ConnectionFactories and Destinations as administered objects that are configured and placed in a JNDI naming context. JMS clients then look up and use these preconfigured objects. This insures that JMS applications are easy to deploy and administer.

Q: What is the relationship between the Java Message Service and the Java DataBase Connectivity (JDBC) API?
A: JMS clients may also use the JDBC API. They may use both the JMS API and the JDBC API in the same transaction. In most cases, they will achieve this automatically by implementing these clients as enterprise beans. They may also use the Java Transaction API.

Q: What is the relationship between the Java Message Service, the Java Transaction API, and the Java Transaction Service?
A: The Java Transaction API (JTA) provides a client API for delimiting distributed transactions and an API for accessing a resource's ability to participate in a distributed transaction. A JMS client may use JTA to delimit distributed transactions. A JMS provider can optionally support distributed transactions via JTA.

The Java Transaction Service (JTS) can be used with the JMS API to form distributed transactions that combine message sends and receives with database updates and other JTS aware services. These services should be handled automatically when a JMS client is run from within an application server such as a J2EE server; however, it is also possible for JMS clients to program them explicitly.

Q: Where can I find a JMS API discussion group?
A: You can find a JMS API discussion forum at http://java.sun.com/j2ee/community/forums/index.html. You must be a member of the Java Developer Connection to join this forum.

Q: Why is the JMS API promoted as a technology for providing communication between components within an enterprise, but not for business-to-business (B2B) communication between enterprises over the Internet? What prevents its use over the Internet?
A: The main issue in providing Internet-based JMS API messaging is that JMS API vendors would have to agree on a common (interoperable) wire format, and the JMS specification would have to specify a router-router API.

B2B messaging is an important area with its own unique requirements. This area is being addressed by emerging standards such as the Electronic Business XML initiative, ebXML (see http://www.ebxml.org/), where Sun is playing an active role. The ebXML initiative currently defines a Messaging Service (see the ebXML Specifications page), which supports XML messaging over the Internet. Current ebXML specifications focus on providing point-to-point asynchronous communication between parties over the Internet using multiple transport protocols such as HTTP, mail, and FTP. Work is also planned in ebXML to provide publish/subscribe XML messaging. Sun and its partners are currently defining a new Java API for XML Messaging, JAXM (see JSR #000067, Java APIs for XML Messaging 1.0), which supports the ebXML Messaging Service specification. An early-access version of JAXM is available via the Java Developer Connection at The M Project 1.0 Early Access.

In summary, the JMS API is intended for use within an enterprise, while XML messaging between independent enterprises over the Internet is being supported by the ebXML initiative and the new Java APIs (including JAXM) that support various ebXML specifications.

Technical questions

Q: What JDK release does the JMS API need?
A: The JMS API requires JDK/JRE release 1.1.x or higher.

Q: Does the JMS API provide a version of a distributed Java event model?
A: The JMS API can be used, in general, as a notification service; however, it does not define a distributed version of Java Events. One alternative for implementing distributed Java Events would be as JavaBeans components that transparently, to the event producer and listener beans, distribute the events via the JMS API.

Q: Why are there two separate JMS API domains, point-to-point and publish/subscribe, instead of just one?
A: Even though there are many similarities, providing separate domains still seems to be important. It means that vendors aren't forced to support facilities out of their domain and that client code can be a bit more portable because products more fully support a domain (as opposed to supporting less defined subsets of a merged domain).

Q: Why doesn't the JMS API specify a set of JavaBeans components?
A: The JMS API is a low-level API, and like other low-level Java APIs, it doesn't lend itself to direct representation as JavaBeans components.

Q: How is the JMS API aligned with the CORBA Notification Service?
A: The Notification Service adds filtering, delivery guarantee semantics, durable connections, and the assembly of event networks to the CORBA Event Service. It gets its delivery guarantee semantics from the CORBA Messaging Service (which defines asynchronous CORBA method invocation).

Java technology is well integrated with CORBA. It provides Java IDL and COS Naming. In addition, OMG has recently defined RMI over IIOP.

It is expected that most use of IIOP from Java will be via RMI. It is expected that most use of COS Naming from Java will be via the JNDI API (Java Naming and Directory Service). The JMS API is a Java specific API designed to be layered over a wide range of existing and future Message Oriented Middleware (MOM) systems (just as the JNDI API is layered over existing name and directory services).

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.

These issues are more clearly understood when they are examined in the context of publish/subscribe applications. In this context, synchronous delivery and/or system acknowledgment of receipt are not an effective mechanism for implementing reliable applications (because producers by definition are not, and do not want to be, responsible for end-to-end message delivery).

Q: Why doesn't the JMS API provide a send-to-list mechanism?
A: Currently the JMS API provides a number of message send options; however, messages can only be sent to one Destination at a time.

The benefit of send-to-list is slightly less work for the programmer and the potential for the JMS provider to optimize the fact that several destinations are being sent the same message.

The down side of a send-to-list mechanism is that the list is, in effect, a group that is implemented and maintained by the client. This would complicate the administration of JMS clients.

Instead of the JMS API providing a send-to-list mechanism, it is recommended that providers support configuring destinations that represent a group. This allows a client to reach all consumers with a single send, while insuring that groups are properly administrable.

Q: Why doesn't the JMS API provide subscription notification? If it were possible for a publisher to detect when subscribers for a topic existed, it could inhibit publication on unsubscribed topics.
A: Although there may be some benefit in providing publishers with a mechanism for inhibiting publication to unsubscribed topics, the complexity this would add to the JMS API and the additional provider overhead it would require are not justified by its potential benefits. Instead, JMS providers should insure that they minimize the overhead for handling messages published to an unsubscribed topic.

Q: Can a message be acknowledged after its message consumer is closed?
A: Yes. Since message acknowledgment processing is performed at the session level, message acknowledgement is still relevant after a consumer is closed. All messages consumed by the session are acknowledged for the following two examples: 

// CLIENT_ACKNOWLEDGE session
Message msg1 = topicSubscriber1.receive();
Message msg2 = topicSubscriber2.receive();
topicSubscriber1.close();
msg2.acknowledge();

// transacted session
Message msg1 = queueReceiver.receive();
queueReceiver.close();
session.commit();


After the session has been closed, however, a call to the Message.acknowledge or Session.commit method throws an IllegalStateException. If close is called on a transacted session before the transaction in progress is committed, the transaction is rolled back.



Note that in order to prevent duplicate delivery of a message from a durable subscription or queue, a message that can still be acknowledged by a session cannot be redelivered to another message consumer. The message can only be redelivered to another message consumer when it can no longer be acknowledged by the session that initially received the message.



Q: What message distribution policy does the JMS API specify when there are two or more QueueReceivers for the same queue at the same time?


A: The JMS API does not specify a message distribution policy when two or more QueueReceivers are registered with a destination at the same time. JMS API semantics account for only one QueueReceiver at any point in time. The JMS API does not prohibit multiple QueueReceivers for a queue; it just does not define the behavior for this case.



Q: How does CLIENT_ACKNOWLEDGE mode work when there are two or more TopicSubscribers to the same topic within a session?


A: From Section 6.11 of the JMS 1.0.2 Specification:



A TopicSession allows the creation of multiple TopicSubscribers per destination, it will deliver each message for a destination to each TopicSubscriber eligible to receive it. Each copy of the message is treated as a completely separate message. Work done on one copy has no affect on the other; acknowledging one does not acknowledge the other; one message may be delivered immediately while another waits for its consumer to process messages ahead of it.


The Message.acknowledge method is documented to acknowledge the receipt of all messages consumed by the session. Thus, if msgA is delivered to topic subscribers TS1 and TS2 of the same session, and TS1 synchronously receives its copy of msgA and acknowledges it, TS2 will still be able to synchronously receive its copy of the message. However, if TS1 receives its copy of msgA and then TS2 receives its copy of msgA, acknowledging receiving the message from TS2 will acknowledge both the msgAs received by TS1 and TS2, based on the fact that TS1's copy of msgA was received by the session before TS2's copy of msgA was received by the session.



Q: What is the recoverability of messages for closed consumers?


A: For messages consumed from a queue or durable subscription, a rollback ensures that the messages are not acknowledged and that they will be delivered to the next consumer of these persistent entities. The JMS specification states that messages delivered to non-durable subscribers may be dropped. This statement means that the JMS API does not define the recoverability of messages consumed from a non-durable subscriber.



Q: Why can't I run the JMS API sample programs using the J2EE 1.2.x SDK?


A: The J2EE 1.2.x Reference Implementation did not include an implementation of the JMS API. It included only the interfaces. The JMS Reference Implementation is available as part of J2EE 1.3 and you can run the sample programs with the J2EE 1.3 server. See the JMS Tutorial for information on running simple JMS client programs with the J2EE 1.3 server.



Please see Java Message Service API Licensees for a list of vendors that provide implementations of the JMS API. Many of these vendors have demonstration copies of their JMS API implementations that you can use to run the JMS API samples that you downloaded from the website.



Q: According to the specification, "Getting a property value for a name which has not been set is handled as if the the property exists with a null value." In the case of getStringProperty, does this mean a null value or a null string?


A: Either is permissible. The reason for this ambiguity is that some MOM systems do not have the native concept of a null string, so they convert all null-valued strings at send time to empty strings.



Q: I used the Connection.setExceptionListener(ExceptionListener) method to try to handle connection problems, but when I stop the server, no JMSException seems to be triggered; the onException method is not called. Is this a bug?


A: The JMS specification does not specify exactly what exceptions are delivered to an ExceptionListener or when they are delivered, so providers vary in how they handle connection problems. Check with your JMS provider if you have questions.



Q: According to Section 3.4 of the JMS Specification, a client can effectively set only three of the message header fields. All the others are set either by the JMS provider or by the send or publish method. Why do all the fields have a setter method, when any client setting will be overridden when the message is sent or when the JMS provider delivers it? Wouldn't it make sense to allow only getter methods for most of the fields?


A: The specification requires setter methods mainly for consistency, so that each message header field has both a getter and setter method. Another reason is to allow a receiving client to change a field after a message is received but before it is passed to some other part of an application for processing. Although most setter methods are used very rarely, they are provided to give programmers flexibility.



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? [new!]


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.




Java Message Service API - FAQ

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