Ideally, this would be something that you could get the IDE to validate for you...
As an alternative, there's a question on stackoverflow that discusses several alternatives, when one is trying to use Java constants in JSP EL expressions:
http://stackoverflow.com/questions/3732608/how-to-reference-constants-in-el
The suggestion in the question, though, is one way in which you could get compile-time validation that you are indeed defining all the constants you'd need: you can use
<%=Addresses.URL%>
to reference Java constants by their name.
You could then define a class that defines a Java String constant for each key you intend to use, then reference keys in your JSPs by using references to the Java constants in your class.
If you now compile all your JSPs before deploying the application, any reference to a String constant that is not in your class will result in a compile-time exception when compiling that JSP...
Granted, this wouldn't help you, if you forget to define the value for that key...
Moises