After jsp been translated into servlet, service method has nine object definitions and initialization is complete, we can use these nine objects directly in the jsp scripts
out (output page), Reques (acquired client information), Response (response information to the client), config (configuration server, the initialization parameter can be obtained), the session (used to store user information), file application (for all users to share information ), page (after the servlet class instance of the current page conversion), pageContext (jsp pages container), exception
Range from small to large domain objects arranged:
page域（pageContext）< request域 < session域 < application域