When coding with J2EEpatterns we have to comply with its standards.
So for the following archives the deployment descriptors are mandatory.
these deployment descriptors are of two types application specific / server specific
here web.xml/ejb-jar.xml are the application specific descriptors where the file name is same irrespective of the server/tool where its deployed . and [weblogic.xml/weblogic-ejb-jar.xml] are server specific descriptors where the file name will change according to the server where its deployed.
Takes care of the parameters required for application configuration like [ servlet declarations/mappings/Ejb-mappigs/env-variable/welcome messagess/filter settings etc]
Defines the configuration setup for the application module in the server [ context-root/replication model/security-permissions/role mappings/version/ etc..]