how can server be updated with features or changes without restarting it

I am learning some basics of server side coding in java and i know enough about how to run this server using command line (in ubuntu system) but what I don't understand is how can i manage this server..? For example, if I want to update the code in the server or add a new feature what steps do i need to take, do i have to stop the server update the codes and then restart it..? It will be helpful if i get proper links for information about managing servers and how medium to small companies manage there server,if they use some particular architecture for it please provide me with information regarding it Thanks

It’s a very broader question and very often asked in interviews as well. I am on mobile so can’t go in detail but read below...

Google Zero Downtime Deployment:

Zero down­time deploy­ment is a deploy­ment method where your web­site or appli­ca­tion is nev­er down or in an unsta­ble state dur­ing the deploy­ment process. To achieve this the web serv­er doesn’t start serv­ing the changed code until the entire deploy­ment process is complete.