It sounds like you are interested in 'Reverse-Ajax', where the client is notified when an event happens server side. This is different than standard Ajax, where an asynchronous event is sent to the server based on some client action. Reverse Ajax is possible, and one framework that does a very good job of allowing this is and simplifying the underlying complexity is DWR.
http://directwebremoting.org/dwr/reverse-ajax
You'll want to read up on the performance implications of the various ways to implement based on your expected load, webapp container, etc. regardless of which framework you use.
As for whether or not it is good practice, that really depends on your application. If it is important to get near-real time data pushed back to the client and you don't want to use something like Flex or other heavier frameworks, then I'd say you are on the right track. If the data does not need to be real time, or if your load is extremely high, then perhaps a more simple approach like a scheduled page refresh will save you some complexity and help with performance.