This file is invoked generally by server. In case if a developer does not specify any name of the file. Default starting page of a website is configured within Welcome-Configuration file.
Welcome-file-list element of a web-app is automatically generated by server and this contains definition of welcome files.
Server look at following welcome-file file order by default :-
A 404 not found message to requested browser is sent if any of these files does not exists in a application by container as http response.
Incase of welcome-file is defined within web.xml and all prioritised welcome-file exists they it will display following file and if not it will look for index.html. Index.htm and index.jsp in listed order.
Here is a example how welcome-file is defined within web.xml
<web-app> .... <welcome-file-list> <welcome-file>home.html</welcome-file> <welcome-file>home.jsp</welcome-file> </welcome-file-list> </web-app>