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.
There are no restrictions. But service(...) method of the GenericServlet provides a fair bit of housekeeping that you'd just have to do yourself. If you have some share functionality, you can use composition to achieve reuse by creating a helper class & methods and call that at the beginning of e.g., doPost() and doGet(). If you prefer using inheritance to achieve code reuse then you can do something like following in your extending class: