从 Java 发送数组列表时出错 class 显示在 JSP

Question

我正在练习基于 Java 的 web 开发，使用 Tomcat 作为应用程序服务器并遵循 MVC 模式，没有任何框架。 我想从数据库发送一些字段并以 JSP 形式显示。 为此，我在 java class 中创建数据的数组列表，然后将其发送到 controller，然后将其发送到 JSP 文件，该文件将显示在表格中。

但是，数组列表在 controller 中很好（在 ArrayList 上使用了 .size() 方法来确认数据库记录数相等），但是当到达 JSP 页面时，数据显然是 null。 我尝试过许多链接，但大多数链接不适用于我的情况（不是 maven），并且是唯一一个不提供解决方案的链接。 感谢任何帮助！

menu_model.java

package Created;

public class menu_model {
    private String menuid;
    private String menuscr;
    private String menuprnt;
    
    public void setmid(String menuid) {
        this.menuid = menuid;
    }

    public void setmscr(String menuscr) {
        this.menuscr = menuscr;
    }
    
    public void setmprnt(String menuprnt) {
        this.menuprnt = menuprnt;
    }
    
    public String getmid() {
        return menuid;
    }

    public String getmscr() {
        return menuscr;
    }

    public String getmprnt() {
        return menuprnt;
    }    
}

menu_DAO.java

package Created;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.util.ArrayList;

public class menu_DAO {
        public static String cond = "jdbc:oracle:thin:@localhost:1535:DATABASE";
        public static String usnm = "username";
        public static String pwrd = "password";
        
        public static ArrayList<menu_model> menu_rtrv(){
        try {
        PreparedStatement pst = null; 
        
        menu_model po;
        
        Class.forName("oracle.jdbc.OracleDriver");
        Connection conn = DriverManager.getConnection(cond,usnm,pwrd);
        pst = conn.prepareStatement("select m_id,m_scrnm,m_prnt from menu_menus where m_id in "
                + "(select um_id from um_menus where user_id=? and umenu_enable=?)");
        pst.setString(1,"TEST");
        pst.setString(2, "YES");
        ResultSet rs=pst.executeQuery();
        
        ArrayList<menu_model> rsarr = new ArrayList<>();
                while (rs.next())
                {
                    po = new menu_model();
                    po.setmid(rs.getString("m_id"));
                    po.setmscr(rs.getString("m_scrnm"));
                    po.setmprnt(rs.getString("m_prnt"));
                    rsarr.add(po);
                }
                conn.close();
                return rsarr;
            }
        
        catch (ClassNotFoundException | SQLException e)
        {
            e.getMessage();
        }
            return null;
}
}

menu_cntrlr.java（控制器）

package Created;

import jakarta.servlet.RequestDispatcher;
import java.io.IOException;
import jakarta.servlet.ServletException;
import jakarta.servlet.annotation.WebServlet;
import jakarta.servlet.http.*;
import java.util.ArrayList;

@WebServlet (name = "menu_cntrlr", urlPatterns = {"/mctrl"})
public class menu_cntrlr extends HttpServlet {

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        ArrayList alist = menu_items.menu_rtrv();
        request.setAttribute("list", alist);
        RequestDispatcher dispatcher = request.getRequestDispatcher("m_menu.jsp");
        dispatcher.forward(request, response);
        }

@Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

@Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }
}

m_menu.jsp

<%@page import="java.util.*,Created.menu_model"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE html>
<html>
   <head>
    <link href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/css/bootstrap.min.css" rel="stylesheet">
    <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0-alpha1/dist/js/bootstrap.bundle.min.js"></script>
    <link rel="stylesheet" href="static/vendor/fontawesome/all.min.css" />
    <link rel="stylesheet" href="bs_ovrd.css" />
    <title>Main menu</title>
   </head>
    <body>
        <table border=2>
    <tr>
        <th>Menu ID</th>
        <th>Menu Scr Name</th>
        <th>Menu Parent</th>
    </tr>
        <%
        ArrayList<menu_model> dlist=(ArrayList)request.getAttribute("list");
        Iterator it=dlist.iterator();
        while(it.hasNext())
        {
            menu_model m=(menu_model)it.next();  
        %>
    <tr>
        <td><%=m.getmid() %></td>
        <td><%=m.getmscr() %></td>
        <td><%=m.getmprnt() %></td>
    </tr>
    <%
        }
    %>
    </table>
    </body>
</html>

HTTP 状态 500 – 内部服务器错误（加载时 http://localhost:8080/WebApplication1/m_menu.jsp）

Type Exception Report

Message An exception occurred processing [/m_menu.jsp] at line [30]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.apache.jasper.JasperException: An exception occurred processing [/m_menu.jsp] at line [30]

28:     
29:         
30:         Iterator it=dlist.iterator();
31:         while(it.hasNext())
32:         {
33:             menu_model m=(menu_model)it.next();


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:610)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:499)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:380)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:328)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Root Cause

java.lang.NullPointerException: Cannot invoke "java.util.ArrayList.iterator()" because "dlist" is null
    org.apache.jsp.m_005fmenu_jsp._jspService(m_005fmenu_jsp.java:148)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:466)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:380)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:328)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Note The full stack trace of the root cause is available in the server logs.

尝试使用 JSTL 和 JSP 页面加载但 if 条件返回并且没有列表：

<c:if test="${empty list}">
            it is null
            </c:if>
        <c:forEach var="mitem" items="${list}">   
         <td><c:out value="${mitem.menuid}"/></td>
       <td><c:out value="${mitem.menuscr}"/></td>
       <td><c:out value="${mitem.menuprnt}"/></td>
     </c:forEach>

编辑：我尝试将 controller 中声明的 integer 传递给 JSP，发现以下错误，我认为它与前一个错误有关：

Exception Report

Message An exception occurred processing [m_menu.jsp] at line [21]

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

org.apache.jasper.JasperException: An exception occurred processing [m_menu.jsp] at line [21]

18:    </head>
19:     <body>
20:      <%//ArrayList<menu_model> dlist=(ArrayList)request.getAttribute("list");
21:         int sc = (int) request.getAttribute("list");
22:         out.print(sc);%>
23:         <table border=2>
24:     <tr>


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:610)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:499)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:380)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:328)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Root Cause

java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "jakarta.servlet.http.HttpServletRequest.getAttribute(String)" is null
    org.apache.jsp.m_005fmenu_jsp._jspService(m_005fmenu_jsp.java:145)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:466)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:380)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:328)
    jakarta.servlet.http.HttpServlet.service(HttpServlet.java:792)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)

下面是我添加的代码：

//in the Controller
int c=2;
        request.setAttribute("list", c);

//in the JSP
<%  int sc = (int) request.getAttribute("list");
    out.print(sc);
%>

Answer 1

当加载 URL http://localhost:8080/WebApplication1/m_menu.jsp 时，这将直接调用 JSP，而无需先通过menu_cntrlr servlet。 这意味着它永远没有机会向请求添加属性。 然后，当 JSP 尝试访问这些属性时，它们将作为null返回。

为了调用servlet，需要访问其@WebServlet映射的urlPatterns参数对应的URL：http://localhost:8080/WebApplication1/mctrl

这导致 servlet 首先被调用，设置请求属性，然后转发到 JSP，然后它将能够成功读取属性。

当使用 Java Servlet API 遵循 MVC 模式时，URL 不应直接访问 JSP。它们被视为 controller 的实现细节，并且为了满足其先决条件，必须始终首先调用 controller。 出于这个原因，通常将 JSP 文件放入 webapp 的WEB-INF目录中，因为这可以防止它们被 URL 从 web 浏览器或其他 HTTP 客户端直接寻址。 Servlet 规范要求WEB-INF中的文件不直接提供给客户端。