You must describe your factory in weblogic.properties.
From the WL5.1 docs:
ConnectionFactories allow JMS clients to create JMS connections. They are configurable so that they create connections with predefined attributes. The JMS specification classifies ConnectionFactories, Queues, and Topics as "administered" objects. They are configured by the messaging system administrator and added to the JNDI namespace to allow access to JMS clients.
To define a WebLogic JMS ConnectionFactory, add a weblogic.jms.connectionFactoryName property:
See the full documentation here: http://www.weblogic.com/docs51/classdocs/API_jms.html#configuring
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