yeah you can. but again its mendatory to ask why?? why you want to do that??
please dont say tht you are going to instantiate your servet inside some of your normal java class. although you can instantiate your servlet with a default constructor. and if it is like that it is very bad idea.
therefore remember that the only constructor that may make any sense to create is a no-argument constructor as that's the only one that the servlet engine will expect there to be and thus call.
But of course the correct place for servlet initialisation code is inside the servlet's init method, not the constructor!
And please don't think to do such things as opening global database connections and files in servlets! At most you could use it to load some static data like global constants but a static initialisation block is probably more suitable to that.