The Servlet init Vs Constructor

Generally Servlets don’t have the Constructor, because a servlet is just like an applet in the respect that it has an init() method that acts as a constructor, an initialization code you need to run should be place in the init(), since it get called when the servlet is first loaded.

You cann’t use a Constructor instead of init() method to initialize a servlet.The original reason for init() was that ancient versions of Java couldn’t dynamically invoke constructors with arguments, so there was no way to give the constructor a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg constructor. So you won’t have access to a ServletConfig or ServletContext. 

Even while you trying to override the init() method you should pass the argument a ServletContext object because the container passes the ServletConfig object to the servlet only when it calls the init method. So ServletConfig will not be accessible in the constructor.