Constants.java class vs context param

Hi,

I'm just wondering whether to keep some of my variables in Constants class or keep it in web.xml

Say, I want to keep a variable of facebook graph api prefix or api_key, client_id

From my understand, the difference between Constants.java and web.xml is web.xml is easier to rewrite on compile using ant.
So you can replace your variables in web.xml according to what environment you are building you app for. (client_id varies by dev environment/prod environment, for example)

If I get it right, then facebook graph api prefix should be kept in Constants.java (because it always is "https://graph.facebook.com/" ) and api_key,client_id should be kept in web.xml ?

Please guide me what's the proper way to use them. Thank you.