Head First Servlets and JSP, 2nd Edition

Each request runs in a separate thread!

You might hear people say things like, “Each instance of the servlet...” but that’s just wrong. There aren’t multiple instances of any servlet class, except in one special case (called SingleThreadModel, which is inherently evil), but we’re not talking about that special case yet.

The Container runs multiple threads to process multiple requests to a single servlet.

And every client request generates a new pair of request and response objects.

There are no Dumb Questions

Q:	Q: This is confusing... in the picture above you show two different clients, each with its own thread. What happens if the same client makes multiple requests? Is it one thread per client or one thread per request?
A:	A: One thread per request. The Container doesn’t care who makes the request—every incoming request means a new thread/stack.
Q:	Q: What if the Container uses clustering, and distributes the app on more than one JVM?
A:	A: Imagine the picture above is for a single JVM, and each JVM has the same picture. So for a distributed web app, there would be one instance of a particular servlet per JVM, but each JVM would still have only a single instance of that servlet.
Q:	Q: I noticed that HttpServlet is in a different package from GenericServlet... how many servlet packages are there?
A:	A: Everything related to servlets (but excluding JSP stuff) is in either javax.servlet or javax.servlet.http ...