The following does not work to inject an arbitrary JNDI entry for primitive/primitive wrapper/String fields:
@Resource( mappedName = "java:/blah/blah" )
String myVar;

In older versions, a "NYI" exception was thrown (then changed to a different exception before getting to the user).
In newer versions, the literal value in mappedName is injected.

For all other types, the JNDI entry injection works. For example, the following does inject that same String from JNDI:
@Resource( mappedName = "java:/blah/blah" )
Object myVar;