The Single threadModel Interface.

[Rakesh Mehra]

HOW the below highlighted implemented: :
1. During the lifetime of a servlet that does not implement Single threadModel interface,the container may send multiple service requests in different threads to a single instance
2. Single threadModel follow instance pooling approach to ensure each servlet instance is invoked in a separarte thread. in this approach the container maintains a pool of servlet instance

[Ben Souther]

These things are implemented by the builders of servlet containers (Tomcat, Websphere, etc..) not application developers. How they're implemented is really up to the the container builders and can vary from container to container. Luckily, if this is really of interest to you, there are several popular, open source, containers available for you to study.
Tomcat, Resin, and Jetty are the first three that come to mind.

Keep in mind that the SingleThreadModel interface is deprecated so, with the exception of needing to understand it in order to work with legacy code, there isn't much point in studying it.