Although this might be basic , but i do not know whether we can write our own constructor in servlet and if we can write down that , what can be its uses and disadvantes?
No, you should not create your own constructor. While you're free to create your own constructor, if it is not the no-arg constructor it will never be called by the container.
Whatever you might want to write in constructor, you can write in init() method, in addition you have ServletConfig and ServletContext in init() method.
