Spring mvc SpringMVC不返回网页

在我的jsp表单中，我从用户那里获得月和年的值，并返回一个包含相关值的网页。这是我的表格：

    <div class="modal-content animate" >
        <div class="imgcontainer">
            <span onclick="document.getElementById('month').style.display='none'" class="close" title="Close Modal">&times;</span>
            <img src="doctor.jpg" alt="Avatar" class="avatar">
        </div>
        <form id="differentMonth" action="differentMonth"  method="post">

                <select name="month" multiple="multiple">
                    <%
                        int i;
                        for(i=1;i<=12;i++){
                            out.print("<option value=" + i +">" + Month.of(i) +  "</option>");
                        }
                    %>
                </select>

            <select id="year" multiple="multiple">
                <%  int year1 = Integer.parseInt(Year.now().toString());
                    for(i=0;i<=10;i++){
                        out.print("<option value=" + i +">" + ( year1) +  "</option>");
                        year1=year1-1;
                    }
                %>
            </select>

            <input type="submit" name="submitted" value="submit" />
        </form>

        </centre>
        </div>

    </div>

</div>

“user”也有很好的定义，因为这个div是从user.jsp本身调用的。它第一次奏效了。问题是当servlet第二次返回用户时

以下是错误：

HTTP Status 500 – Internal Server Error
Type Exception Report

M

    essage Request processing failed; nested exception is java.lang.NullPointerException

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

    Exception

    org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
        org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
        org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
        org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
        org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    Root Cause

    java.lang.NullPointerException
        com.PricolAttendance.spring.controller.HomeController.differentMonth(HomeController.java:95)
        java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        java.base/java.lang.reflect.Method.invoke(Method.java:567)
        org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205)
        org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133)
        org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:97)
        org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)
        org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)
        org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
        org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:967)
        org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:901)
        org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)
        org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
        org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
        org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    Note The full stack trace of the root cause is available in the server logs.

    Apache Tomcat/7.0.99

看起来没有逻辑错误，因为user.jsp一开始工作正常。

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请提供帮助。

在您的代码中，使用bean映射表单时出现错误

<form:select id="sel_id" multiple="multiple" path="bean_attribute_mapped" />

在jsp内部，将此添加到顶部：

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://www.springframework.org/tags/form" prefix="form"%>

---

代码验证失败。不同月份的对象应该有问题。

 <form id="form_id" action="url"  method="post" modelAttribute="bean_name">

<form:select id="sel_id" multiple="multiple" path="bean_attribute_mapped" />

@RequestMapping(value = "/differentMonth", method = RequestMethod.POST)
public String differentMonth(@Valid @ModelAttribute DifferentMonth mon, Model model) {