I have servlets that caches user information rather then retrieving it from the user store on every request (shared Ehcache). The issue I have is that if a client is multi-threaded and they make more then one simultaneous request, before they have been authenticated, then I get this in my log:

Retrieving User [Bob]
Retrieving User [Bob]
Retrieving User [Bob]
Returned [Bob] ...caching
Returned [Bob] ...caching
Returned [Bob] ...caching 

What I would want is that the first request would call the user service, while the other two requests get blocked - and when the first request returns, and then caches the object, the other two requests go through:

Retrieving User [Bob]
blocking...
blocking...
Returned [Bob] ...caching
[Bob] found in cache
[Bob] found in cache

I've thought about locking on the String "Bob" (because due to interning it's always the same object right?). Would that work? And if so how do I keep track of the keys that actually exist in the cache and build a locking mechanism around them that would then return the valid object once it's retrieved. Thanks.