You need to determine the max amount of connections you need at any one time. If you exceed this number, the connection pool will throw an exception when you try to acquire a connection.

If you are using session beans to access the connection pool, you can easily prevent this from happening. Make sure you have a number of session beans in the pool that is less than the max amount of connections. This way, if you get a sudden burst of usage, some of the threads will block (rather than having the connection pool throw an exception) when they try to access the session bean. They will unblock as soon as the other threads have completed.