A client uses a TopicSubscriber object to receive messages that
have been published to a topic. A TopicSubscriber object is the
publish/subscribe form of a message consumer. A MessageConsumer
can be created by using Session.createConsumer.
A TopicSession allows the creation of multiple
TopicSubscriber objects per topic. It will deliver each
message for a topic to each
subscriber eligible to receive it. Each copy of the message
is treated as a completely separate message. Work done on one copy has
no effect on the others; acknowledging one does not acknowledge the
others; one message may be delivered immediately, while another waits
for its subscriber to process messages ahead of it.
Regular TopicSubscriber objects are not durable. They
receive only messages that are published while they are active.
Messages filtered out by a subscriber's message selector will never
be delivered to the subscriber. From the subscriber's perspective, they
do not exist.
In some cases, a connection may both publish and subscribe to a topic.
The subscriber NoLocal attribute allows a subscriber to inhibit
the
delivery of messages published by its own connection.
If a client needs to receive all the messages published on a topic,
including the ones published while the subscriber is inactive, it uses
a durable TopicSubscriber. The JMS provider retains a record of
this durable
subscription and insures that all messages from the topic's publishers
are retained until they are acknowledged by this durable
subscriber or they have expired.
Sessions with durable subscribers must always provide the same client
identifier. In addition, each client must specify a name that uniquely
identifies (within client identifier) each durable subscription it creates.
Only one session at a time can have a TopicSubscriber for a
particular durable subscription.
A client can change an existing durable subscription by creating a
durable TopicSubscriber with the same name and a new topic
and/or message
selector. Changing a durable subscription is equivalent to unsubscribing
(deleting) the old one and creating a new one.
The unsubscribe method is used to delete a durable
subscription. The unsubscribe method can be used at the
Session or TopicSession level.
This method deletes the state being
maintained on behalf of the subscriber by its provider.
Creating a MessageConsumer provides the same features as
creating a TopicSubscriber. To create a durable subscriber,
use of Session.CreateDurableSubscriber is recommended. The
TopicSubscriber is provided to support existing code. | 