Question

<% Integer i=(Integer)request.getAttribute("count"); %>

我想通过按下next / prev链接来增加/减少我的数组计数器（i）。但到目前为止它还没有奏效。我尝试使用脚本。我的代码是：

<form action="<%=application.getContextPath()%>/Controller" method="get" id="Form">
<input type="hidden" value="" name="prev" id="prev"></input>
<a class="button_prev" href="javascript:prev('<%=(i-1)%>')">prev</a>
</form>

脚本代码是：

function prev(i)
{
if(i==-1)
    i=0;
document.getElementById("prev").value=i;
request.setAttribute("count",i);
document.getElementById("Form").submit();
}

我的servlet代码是：

Integer num=(Integer)request.getAttribute("chapter");
    if(num==1)
        request.getRequestDispatcher("/jsp/Page2.jsp").forward(request, response);
    else if(num==2)
        request.getRequestDispatcher("/jsp/Page3.jsp").forward(request, response);
    else if(num==3)
        request.getRequestDispatcher("/jsp/Page4.jsp").forward(request, response);

请你仔细看看，告诉我出了什么问题，或者你能告诉我一个更好的方法来实现这个吗？

我的错误是：控制器请求未定义。

我的web-xml文件是

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee"       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
<display-name>Website</display-name>
<servlet>
    <description>
    </description>
    <display-name>Controller</display-name>
    <servlet-name>Controller</servlet-name>
    <servlet-class>
    com.tcs.website.Controller</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>Controller</servlet-name>
    <url-pattern>/Controller</url-pattern>
</servlet-mapping>
<welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
</welcome-file-list>
  </web-app>

Answer 1

看起来你正在混淆服务器端和客户端代码。在服务器上评估<%=(i-1)%>等表达式，并且变量i与javascript上的变量不同

您需要检查您的业务需求，计数器应存储在何处？在服务器/客户端？

Answer 2

表单值作为请求参数传递给servlet。您只需获取值request.getParameter（“prev”）; 您正在分配值request.setAttribute（i）;这不是必需的。

Answer 3

试试这个

<form action="/Controller" method="get" id="Form"> <input type="hidden" value="" name="prev" id="prev"></input> <a class="button_prev" href="javascript:prev()">prev</a> </form>

Javascript代码

`function prev(){`

`var i = parseInt(document.getElementById("prev").value)-1;
if(i==-1)i=0;
document.getElementById("prev").value=i;
document.getElementById("Form").submit();
}`

Servlet代码

Integer num=Integer.valuesOf(request.getParameter("prev"));
if(num==1)
    request.getRequestDispatcher("/jsp/Page2.jsp").forward(request, response);
else if(num==2)

Answer 4

1）你为什么称我为“阵列计数器”？

2）你的servlet代码检索'chapter'属性，但你的html或javascript中没有提到'chapter'。

3）您的javascript代码：

function prev(i)
{
    if(i==-1)
        i=0;

    document.getElementById("prev").value=i;
    request.setAttribute("count",i);
    document.getElementById("Form").submit();
}

...在浏览器中执行。并且在浏览器中执行的代码不知道服务器上存在哪些变量和方法。同样，在服务器上执行的servlet也不知道在浏览器中执行的某些javascript代码中存在哪些变量和方法。那么在你的javascript代码中，变量请求引用了什么？

4）以下是您可以尝试的完整示例。您有许多活动部件，每个组件的路径由不同的规则控制。

进入浏览器的网址：

http://localhost:8181/myapp2/Go/fish/in/a/lake

（我已将Tomcat配置为侦听端口8181）

Tomcat中的目录结构：

webapps/myapp2/
   js/
      jquery-1.9.1.js
      jquery-ui-1.10.2.js
      myjs.js
   jsp/
      Initial.jsp
      Page2.jsp
      Page3.jsp
   WEB-INF/
      web.xml
      classes/
         com/
            tcs/
               website/
                   Controller.class

的web.xml ------------------------------------------- < / p>

<?xml version="1.0" encoding="UTF-8"?>
<web-app id="WebApp_ID" version="2.4" 
  xmlns="http://java.sun.com/xml/ns/j2ee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"  
  xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

<servlet>
    <servlet-name>MySuperAwesomeController</servlet-name>
    <servlet-class>com.tcs.website.Controller</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>MySuperAwesomeController</servlet-name>
    <url-pattern>/Go/fish/in/a/lake</url-pattern>
</servlet-mapping>

</web-app>

myjs.js ----------------------------------------

$(document).ready( function() {

  var chapter_val = $("#chapter_div").text();
  chapter_val = parseInt(chapter_val);
  chapter_val = (--chapter_val < 0) ? 0 : chapter_val;

  alert("Assigning onclick handler to the link...\n" +
        "When the link is clicked, the onclick function \n" +
        "will change the link's url to include the chapter val => " + chapter_val);


  $(".button_prev").on("click", function() {
    this.href += "?chapter=" + chapter_val;
  });


});

这是一个纯粹的javascript版本（但你真的应该使用jquery来避免几个潜在的问题）：

window.onload = function () {

  var chapter_val = document.getElementById("chapter_div").childNodes[0].nodeValue;
  chapter_val = parseInt(chapter_val);
  chapter_val = (--chapter_val < 0) ? 0 : chapter_val;

  alert("Assigning onclick handler to the link...\n" +
        "When the link is clicked, the onclick function \n" +
        "will change the link's url to include the chapter val => " + chapter_val);

  document.getElementsByClassName("button_prev")[0].onclick = function() {
    this.href += "?chapter=" + chapter_val;
  };

};

Controller.java ------------------

package com.tcs.website;

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class Controller extends HttpServlet {

    public void doGet(HttpServletRequest req, HttpServletResponse resp)
                                throws IOException,
                                       ServletException {

        String chapter = req.getParameter("chapter");
        String jsp_path = null;

        if (chapter == null) {
            jsp_path = "/jsp/Initial.jsp";
        }
        else {
            int chapter_num = 0;

            try {
                chapter_num = Integer.parseInt(req.getParameter("chapter"));

                if ( (chapter_num == 3) || (chapter_num == 4) ) {
                     jsp_path = "/jsp/Page" + 
                                Integer.toString(--chapter_num) + 
                                ".jsp";
                }
            }
            catch (NumberFormatException e) {
                System.out.println("Bad chapter number");
            }
        }

        System.out.println("*********jsp_path=" + jsp_path);  #goes to logs/catalina.out
        req.getRequestDispatcher(jsp_path).forward(req, resp);
    }
}

Initial.jsp ----------------------

<!DOCTYPE html>
<html>
<head>
<title>Initial.jsp</title>

<script type='text/javascript' src='/myapp2/js/jquery-1.9.1.js'></script>
<script type='text/javascript' src='/myapp2/js/jquery-ui-1.10.2.js'></script>
<script type='text/javascript' src='/myapp2/js/myjs.js'></script>

</head>
<body>

  <div id="chapter_div">5</div>
  <a class="button_prev" 
     href="<%= application.getContextPath() %>/Go/fish/in/a/lake">prev</a>

</body>

</html>

Page2.jsp ------------------------

<!DOCTYPE html>
<html>
<head>
<title>Page2.jsp</title>

<script type='text/javascript' src='/myapp2/js/jquery-1.9.1.js'></script>
<script type='text/javascript' src='/myapp2/js/jquery-ui-1.10.2.js'></script>
<script type='text/javascript' src='/myapp2/js/myjs.js'></script>

</head>
<body>

  <div id="chapter_div"><%= request.getParameter("chapter") %></div>
  <a class="button_prev" 
     href="<%= application.getContextPath() %>/Go/fish/in/a/lake">prev</a>

</body>

</html>

Page3.jsp ------------------

<!DOCTYPE html>
<html>
<head>
<title>Page3.jsp</title>

<script type='text/javascript' src='/myapp2/js/jquery-1.9.1.js'></script>
<script type='text/javascript' src='/myapp2/js/jquery-ui-1.10.2.js'></script>
<script type='text/javascript' src='/myapp2/js/myjs.js'></script>


</head>
<body>

  <div id="chapter_div"><%= request.getParameter("chapter") %></div>
  <a class="button_prev" 
     href="<%= application.getContextPath() %>/Go/fish/in/a/lake">prev</a>

</body>

</html>

通过使用链接递增计数器

4 个答案: