Jsp $ {param. id} usage

Its value range is Page, Request, Session, and Application.
$ {Param. id} is related to the input, relative to request. getParameter ("id "). Obtain the input parameter id.
Because the id of a. jsp submitted to B. jsp does not belong to any range of Page, Request, Session, and Application, it cannot be obtained.
Id is a parameter passed by a. jsp to B. jsp, so $ {param. id} gets the value.
The Request range refers to the range in which a jsp page sends a Request to another jsp page.
In a. jsp, <% request. setAttribute ("key", "Test") %>
When a. jsp is switched to B. jsp, B. jsp can get the variable key value of a. jsp, but other jsp pages cannot get the value.
The key here is. jsp should be switched to B. jsp, in. <jsp: forward page = "B. jsp "/> indicates to switch to B. jsp. otherwise, B. jsp cannot get. the value of the key set in jsp.