Java Reference
In-Depth Information
sume each message. The first receiver to fetch the message will get it, while every-
one else will not:
A message sent to a
topic
, on the other hand, may be received by multiple parties.
Messages published on a specific topic are sent to all message consumers that have
registered (subscribed) to receive messages on that topic. A subscription can be
durable
or
nondurable
. A nondurable subscriber can only receive messages that
are published while it is
active
. A nondurable subscription does not guarantee the
delivery of the message or may deliver the same message more than once. A dur-
able subscription, on the other hand, guarantees that the consumer receives the
message exactly once:
As far as message consumption is concerned, even though JMS is inherently asyn-
chronous, the JMS specification allows messages to be consumed in either of the
following two ways:
•
Synchronously
: A subscriber or a receiver explicitly fetches the mes-
sage from the destination by calling the
receive()
method of any
Mes-
sageConsumer
instance. The
receive()
method can block until a
message arrives or can time out if a message does not arrive within a
specified time limit.
•
Asynchronously
: With the asynchronous mode, the client must imple-
ment the
javax.jms.MessageListener
interface and overwrite the
onMessage()
method. Whenever a message arrives at the destination,
the JMS provider delivers the message by calling the listener's
onMes-
sage
method, which acts on the contents of the message.
