JSP POST传参(乱码) 为何是ISO8859_1

A 页面：
HTML code

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>

<% response.setCharacterEncoding("UTF-8"); %>

<meta http-equiv="content-type" content="text/html; charset=UTF-8" />

<form action="B.jsp" method="post">
    <input type="submit" name="submit" value="中文" />
</form>

B 页面：
HTML code

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>

<% request.setCharacterEncoding("UTF-8"); %>

<meta http-equiv="content-type" content="text/html; charset=UTF-8" />

<!-- 请问 #P1 为何乱码 -->
<p id="p1"><%=request.getParameter("submit")%></p>

<!-- 请问 #P2 为何要重建字符串？以前听说POST传参只需设置request编码，为什么是ISO8859_1？-->
<p id="p2"><%=new String(request.getParameter("submit").getBytes("ISO8859_1"), "UTF-8")%></p>

<%@ page contentType="text/html; charset=UTF-8" pageEncoding="UTF-8" %>

<% request.setCharacterEncoding("UTF-8"); %>

<meta http-equiv="content-type" content="text/html; charset=UTF-8" />


你首先要搞清楚这几个编码的作用

http://apps.hi.baidu.com/share/detail/33081081