A JSP container manages two phases of a JSP page s lifecycle. In the translation
phase, the container validates the syntactic correctness of the JSP pages and tag
files and determines a JSP page implementation class that corresponds to the JSP
page. In the execution phase the container manages one or more instances of this
class in response to requests and other events.
and furthermore about the translation phase:
The translation of a JSP source page into its implementation class can occur at
any time between initial deployment of the JSP page into the JSP container and
the receipt and processing of a client request for the target JSP page.
When you define a JSP page in a servlet element with a load-on-startup element, it will be instantiated at the start of the application, otherwise the container decides