Spring MVC not returning webpage - spring-mvc

Question

In my jsp form, I get the value of month and year from the user, and return a webpage with relevant values. Here is my form:
<div class="modal-content animate" >
<div class="imgcontainer">
<span onclick="document.getElementById('month').style.display='none'" class="close" title="Close Modal">×</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>
This form takes the value from the user. In the dispatcher,
#RequestMapping(value = "/differentMonth", method = RequestMethod.POST)
public String differentMonth(#Validated DifferentMonth mon, Model model) {
System.out.println("In controller");
AccessDatabase ac = new AccessDatabase(AccessDatabase.id);
System.out.println(AccessDatabase.id);
ac.setStatusOfMonth(AccessDatabase.id,Integer.toString(mon.getMonth()),Integer.toString(mon.getYear()));
System.out.println(user.getUserName()+" "+mon.getMonth()+" "+mon.getYear());
model.addAttribute("database", ac);
return "user"; // Only this is important
}
The 'user' is also well defined, as this div is called from user.jsp itself. It worked the first time. Issue is when user is return by the servlet the second time
Here is the error:
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
Doesnt look like there is a logical error, as user.jsp works fine at first.
Please help.

Answer 1

In your code there are errors on mapping the form with the bean.
inside jsp add this on top:
<%# taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%# taglib uri="http://www.springframework.org/tags/form" prefix="form"%>
Write you open tag for form like this. ModelAttribute attribute map the element of the form with your bean.
<form id="form_id" action="url" method="post" modelAttribute="bean_name">
Add your form elements. the attribute path map the element inside form with the attribute of the bean.
<form:select id="sel_id" multiple="multiple" path="bean_attribute_mapped" />
Inside bean class put your annotations for validations.
Write the signature of your endpoint. #Valid enable validation and #ModelAttribute map the parameter with your bean.
#RequestMapping(value = "/differentMonth", method = RequestMethod.POST)
public String differentMonth(#Valid #ModelAttribute DifferentMonth mon, Model model) {