What exactly is inside a web server worker thread?

For example, if you are running Spring on top of Jetty, I'm trying to figure out what each worker thread contains. Surely its not an instance of the entire Spring binary, and I'm assuming it is a combination of local and global variables, along with some processes.

It seems like requests go into this neat bundle that does everything for us, and I'm just curious how the engine works underneath, especially the threading part.