Servlets run in a multi-threaded environment. You don't need to "implement" threading in a servlet. UNLESS of course, you are talking about using a class that extends
Thread or implements Runnable within your servlet. In which case you proceed "as usual" just like in any other program where you want to use threads. The caveat is: Some enterprise-level containers do not allow you to create your own threads.
Tomcat lets you, iPlanet App Server doesn't.
If you're talking about "are servlets mutlithreaded" and "what are the issues surrounding that?", then just search this forum for the words "multithreaded or threading".
Here's a few to get you started:
https://coderanch.com/t/355806/Servlets/java/Thread-safety https://coderanch.com/t/355472/Servlets/java/static-methods-not-thread-safe