Understanding Java messaging and JMS
Foreword
Java and Java-related technologies continue to be introduced and
continue to mature. As Java becomes more enterprise ready, as
opposed to application-platform centric, we are seeing the
introduction of more sophisticated Java-related middleware. This
month we discuss one such type of middleware, JMS. The JMS
specification will no doubt result in many new products in the
market. We describe what JMS is all about and how it compares with
alternate solutions. IT architects will now need to consider this
technology when designing messaging infrastructures.
--Kara Kapczynski
In October 1998, JavaSoft released the Java Message Service (JMS)
specification v1.0.1, an API for accessing enterprise
messaging systems from Java. Because JMS is equipped to handle both
synchronous and asynchronous messaging, it represents a possible
alternative to strictly synchronous RMI.
This article will attempt both to define JMS and to compare it with other
Java messaging or traditional message-oriented middleware (MOM) messaging paradigms. The main
objective is to help you understand where JMS might fit into a
Java-based, enterprise-wide application architecture.
JMS is a set of interfaces and associated semantics that define how
a JMS client accesses the facilities of an enterprise messaging
product. Enterprise messaging is recognized as an essential tool for
building enterprise applications and ecommerce systems, and JMS
provides a common way for Java programs to create, send, receive,
and read an enterprise messaging system's messages.
With a solid implementation of JMS, one can implement client/server or
server/server asynchronous applications without CORBA. However, most ISP
implementations of JMS are by small firms that are trying to implement a
fairly comprehensive spec. Sun, though, has clearly stated that a vendor
does not need to implement the entire spec, only domains within the spec.
Sun has also indicated that there are domains outside of the scope of the
spec that should be implemented in order to provide a truly enterprise-wide
and robust messaging environment.
EAI = MOM = JMS
Enterprise Application Integration (EAI) using MOM products is becoming an essential component for integrating
intracompany operations (see August 1999's IT Architect column for more details). While JMS can be used by
itself and not just as a wrapper for MOMs, Sun's primary intention
is for corporations to use it in conjunction with such MOM products
as TIBCO or MQSeries.
The strategic intention of some JMS vendors is to provide Java
interfaces to MQSeries and TIBCO -- interfaces that allow the
developer to program in pure JMS using Java APIs, while using
MQSeries, TIBCO, and the like as the low-level transport. This
means an organization might not need to change its underlying
transport middleware; JMS gives developers a standard Java API so
messages can be sent transparently across the organization via TIBCO
or MQSeries.
JMS is clearly the missing piece in the enterprise Java
architecture, and Sun has already introduced a number of major
service APIs -- JDBC (for database access), JNDI (for naming and
directory services), and a Java mail API -- that are required in order to
write applications that integrate across the enterprise.
The JMS object model
The two dominant messaging approaches in use today and defined in the spec
are the Topic, or point-to-point messages, and the Queue, or
publish-and-subscribe messages. Because many message
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