The Java EE 6 Tutorial, Volume I

Initializing a Servlet

After the web container loads and instantiates the servlet class and before it delivers requests from clients, the web container initializes the servlet. To customize this process to allow the servlet to read persistent configuration data, initialize resources, and perform any other one-time activities, you override the init method of the Servlet interface. If a servlet cannot complete its initialization process, it throws an UnavailableException.