This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
You are not using the application server's database pool. You're using straightforward jdbc. So it should not be a server configuration issue. Two possible causes for the error are: the oracle jdbc driver is not in your application's classpath, or the database service defined in your connection string is not reachable from your OAS server. The errors will tell you more.