java - ArrayList上的for循环更改对象

我有一个jsp页面（例如，MyJspPage.jsp）-

<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<%
    ArrayList<Person> ownerList = (ArrayList<Person>) request
            .getAttribute("ownerList");
    //set again ..
        request.setAttribute("ownerList",ownerList) ;

%>
</head>
<body>
        <%
            //itr on all the persons ..;
            for (Person person : ownerList) {
        %>
                    // some HTML code..
        <%
            }
        %>
            <form action="servlet123" method="POST">
                   // some fields ..
                   <input type="submit" value="join" />
    </form>

</body>
</html>

还有一个servlet-

@WebServlet("/servlet123")
public class servlet123 extends HttpServlet {
    private static final long serialVersionUID = 1L;


    protected void doPost(HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {
        // get the then set ..
             ArrayList<Person> ownerList = (ArrayList<Person)request.getAttribute("ownerList");
        request.setAttribute("ownerList", ownerList);
             // forward to `MyJspPage.jsp`
            dispather.forward(request, response);
        }

}

首先，另一个servlet转发到MyJspPage.jsp，并且运行良好，然后MyJspPage.jsp和servlet123之间像乒乓球一样。问题是当第二次到达MyJspPage.jsp时会引发异常-

type Exception report

message java.lang.NullPointerException

description The server encountered an internal error (java.lang.NullPointerException) that prevented it from fulfilling this request.

exception

org.apache.jasper.JasperException: java.lang.NullPointerException
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:549)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:470)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:390)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:334)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

应该注意的是，当我从MyJspPage.jsp省略for循环并将其更改为-

   <html>
    <head>
    <meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
    <%
        ArrayList<Person> ownerList = (ArrayList<Person>) request
                .getAttribute("ownerList");
        //set again ..
            request.setAttribute("ownerList",ownerList) ;

    %>
    </head>
    <body>
                <form action="servlet123" method="POST">
                       // some fields ..
                       <input type="submit" value="join" />
        </form>

    </body>
    </html>

MyJspPage.jsp和servlet123之间的所有关系都可以正常工作。

最佳答案

这是一种方法。

JSP代码如下

您可以在会话中进行如下设置，而不是在请求中再次设置arraylist
session.setAttribute("ownerList",ownerList) ;

您可以在for循环中使用arraylist之前检查它是否为null。

        if (ownerList != null)
        {
          for (Person person : ownerList) {
    %>
                // some HTML code..
    <%
          }
        }
    %>

在servlet中，您可以将代码编写为

 HttpSession session = request.getSession(false);
 ArrayList<Person> ownerList = (ArrayList<Person)session.getAttribute("ownerList");
 request.setAttribute("ownerList", ownerList);
 session.setAttribute("ownerList", null); // toremove unnecessary code from the session

也可能有其他方法。这只是您选择的那个。