Thanks for creating an issue with config files. I've been doing some testing with jdbc resource from context.xml and straight forward jdbc resource should work from context.xml in web apps. You did mention that you're trying to access the resource from a managed bean so the issue might be with access from a managed bean.
BTW, before going further with this, can you check if your resource has been created and is available? You can do this using glassfish admin via Resources> JDBC> JDBC Resources and check if your resource is listed.
Thanks,
Amy
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