几乎相同的 deal.jsp 代码（index.jsp不变），在IDEA相同项目运行，结果却不一样，实在想不出来

index.jsp:

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

<script language="JavaScript">
    function ifNull(form_Msg){
        var userName = form_Msg.upload_UserName.value;
        var userMsg = form_Msg.upload_UserMsg.value;
        if (userName === "") {
            alert("留言人不能为空");
            form_Msg.upload_UserName.focus();
            return false;
        }
        if (userMsg === "") {
            alert("留言内容不能为空");
            form_Msg.upload_UserMsg.focus();
            return false;
        }
    }
</script>

留言板

<% if (session.isNew()){ int number_person_index = 0; session.setAttribute("number_person",number_person_index); } int i; String[] message_index = (String[])session.getAttribute("message"); int num = Integer.parseInt(session.getAttribute("number_person").toString()); if (num != 0) { out.println("123213132"); out.println("number_person: " + num + "

"); for (i=0; i < num; i++) { out.println(message_index[i]); } } %>

---

添加留言区域
留言人： 留言内容：

deal.jsp（正确可运行）:
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<%
request.setCharacterEncoding("utf-8");
String userName_deal=request.getParameter("upload_UserName"); //获取留言人
String userMsg_deal=request.getParameter("upload_UserMsg"); //获取留言内容
String message_add="["+userName_deal+"]说："+userMsg_deal+"
"; //组合留言信息

int i;
int num = Integer.parseInt(session.getAttribute("number_person").toString());
int num_add = num+1;
String[] message_old = (String[])session.getAttribute("message");
String[] message_new = new String[num_add+1];

if(session.getAttribute("number_person")!=null){
    if (num > 0) {
        for (i=0; i<num; i++) {
            message_new[i] = message_old[i];
        }
    }
    message_new[num_add-1] = message_add;
}
session.setAttribute("number_person",num_add);
session.setAttribute("message",message_new);    //将留言信息保存到session中
response.sendRedirect("index.jsp");

%>

deal.jsp（错误不可运行）:
<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8"%>
<%
request.setCharacterEncoding("utf-8");
String userName_deal = request.getParameter("upload_UserName");
String userMsg_deal = request.getParameter("upload_UserMsg");
String message_add = "["+userName_deal+"]说："+userMsg_deal+"
"; //组合留言信息

int i;
int num = Integer.parseInt(session.getAttribute("number_person").toString());
int num_add = num+1;
String[] message_old = (String[])session.getAttribute("message");
String[] message_new = new String[num_add+1];

if (session.getAttribute("number_person")!=null) {
    if (num > 0) {
        for (i=0; i<num; i++) {
            message_new[i] = message_old[i];
        }
    }
    message_new[num_add-1] = message_add;
}
session.setAttribute("number_person",num_add);
session.setAttribute("massage",message_new);
response.sendRedirect("index.jsp");

%>

原文地址：https://www.cnblogs.com/Yogile/p/11477952.html