Every java class will have aleast one constructor and servlets are no exception to this.
But while using servlets ,we never instantiate the servlet rather the container does it for us by using the newInstance() defined in the Class class; which in turn uses the empty(Default) constructor to create a new instance of the servlet.
If we define parameterised constructor in the serlet ,then container will fail in instantiating the servlet.
The rest is left for you to decide whether a servlets need a user defined constructor or not.