May be this link would help you
The root of the problem is found. JBoss6 defines the "Standard CMP 2.x EntityBean" configuration in standardjboss.xml by referencing the "instance per transaction" configuration, which has no transaction-independant entity bean cache:
<container-configuration extends="Instance Per Transaction CMP 2.x EntityBean"> <container-name>Standard CMP 2.x EntityBean</container-name> </container-configuration>
This was a killer for our application, since there are huge imutable entities, which get loaded in the cache once and should not be reloaded afterwards.
By replacing "Standard CMP 2.x EntityBean" to use a configuration like "Standard Pessimistic CMP 2.x EntityBean" this problem was gone and performance is good - about 50% more than with JBoss4. But some transaction issues remain as mentioned."