SRV.2.3.4 End of Service The servlet container is not required to keep a servlet loaded for any particular period of time. A servlet instance may be kept active in a servlet container for a period of milliseconds, for the lifetime of the servlet container (which could be a number of days, months, or years), or any amount of time in between. When the servlet container determines that a servlet should be removed from service, it calls the destroy method of the Servlet interface to allow the servlet to release any resources it is using and save any persistent state. For example, the container may do this when it wants to conserve memory resources, or when it is being shut down. Before the servlet container calls the destroy method, it must allow any threads that are currently running in the service method of the servlet to complete execution, or exceed a server-defined time limit. Once the destroy method is called on a servlet instance, the container may not route other requests to that instance of the servlet. If the container needs to enable the servlet again, it must do so with a new instance of the servlet’s class. After the destroy method completes, the servlet container must release the servlet instance so that it is eligible for garbage collection.
Source Servelt-2.4 specs document