The init() method and destroy() method of a HttpServlet are called only once in the Servlet life cycle. But the service methods such as doGet() or doPost() are getting called for every client request and since servlet uses multithreading, we should provide thread safety in these methods.

To make sure that a servlet is thread safe, the service() method should not access any member variables, unless these member variables are thread safe themselves. The service() method should not reassign member variables, as this may affect other threads executing inside the service() method. If there is a compelling need to reassign a member variable, make sure this is done inside a synchronized block. If there are any local variables in service methods, no need to worry because local variables are always thread safe.