This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
Servlets get loaded once, either at startup or the first time they are hit (depending on how you've configured them in web.xml).
If you restart your application or container, you will see the new version.
Some containers such as Tomcat have an reloadable attribute that you can configure which will cause the application to automatically reload itself if a loaded class file is updated.