When we click the submit button as the following form, my
understanding is it will submit the form to the web server,
and then open page2.jsp. My question is web server has
some area to store the name-value pairs?? I just want to understand
how it works internally.

<FORM ACTION="page2.jsp" METHOD="POST">
<INPUT TYPE="hidden" name="username" value="joe">
<INPUT TYPE="submit" value="submit form">
</FORM>

In page2.jsp
<%= request.getParameter("username") %>


Please advise. Thanks!!

that would all depend on the server implementation.
But typically it would just hand over the raw http request data to the JSP engine.

so the JSP engine doesn't store any form data? People told me when we do the following, it will submit the current form data to page2.jsp, and nothing is stored in web server.

<form action="page2.jsp" method="post">

please advise. thanks!!