>>You could just have the servlet write "Hello World" directly.
It's been years since I've had to write a Java webapp, but from what I remember using the servlet code to directly write the webpage output was considered bad. That was the job of the jsp file. e.g., "views" in bottlepy and Rails: http://guides.rubyonrails.org/layouts_and_rendering.html
Yes, it was considered bad style because of separation of concerns etc, but nothing necessitated JSP, you could just use servlets and any of tons of template libraries.
And for a hello world, a servlet would do just fine.
It's been years since I've had to write a Java webapp, but from what I remember using the servlet code to directly write the webpage output was considered bad. That was the job of the jsp file. e.g., "views" in bottlepy and Rails: http://guides.rubyonrails.org/layouts_and_rendering.html