Spring binding exception when a form is submitted - spring

Question

Stuck and have no clue why Spring Form is not able to submit successfully [gives binding issue] when pre-populated in get Request call loadForm, but works fine when populated in a method setupFormObject with #ModelAttribute annotation tag. I can provide a simple example in github to test out if asked for :)
Example below
#ModelAttribute("showForm")
public ShowForm setupFormObject() {
//Instantiate showForm with data
return showForm;
}
#RequestMapping(method = RequestMethod.GET)
public ModelAndView loadForm(#RequestParam("id") String id, HttpSession session) {
ModelAndView modelAndView = new ModelAndView(nextPage);
//Instantiate showForm with data
//modelAndView.addObject("showForm", showForm);
return modelAndView;
}
#RequestMapping(method = RequestMethod.POST)
public String post(#ModelAttribute("showForm") ShowForm showForm, BindingResult result, final RedirectAttributes redirectAttrs) {
//I see changed data here in showForm when populated using #setupFormObject
//See an exception in JSP with binding error if populated in loadForm
return "";
}
Stack Trace as requested. This exception is from github example.
`HTTP Status 500 - Request processing failed; nested exception is org.springframework.beans.InvalidPropertyException: Invalid property 'users[0]' of bean class [com.example.UserForm]: Illegal attempt to get property 'users' threw exception; nested exception is org.springframework.beans.NullValueInNestedPathException: Invalid property 'users' of bean class [com.example.UserForm]: Could not instantiate property type [com.example.UserEntity] to auto-grow nested property path: java.lang.InstantiationException: com.example.UserEntity`
`type Exception report`
`message Request processing failed; nested exception is org.springframework.beans.InvalidPropertyException: Invalid property 'users[0]' of bean class [com.example.UserForm]: Illegal attempt to get property 'users' threw exception; nested exception is org.springframework.beans.NullValueInNestedPathException: Invalid property 'users' of bean class [com.example.UserForm]: Could not instantiate property type [com.example.UserEntity] to auto-grow nested property path: java.lang.InstantiationException: com.example.UserEntity`
`description The server encountered an internal error that prevented it from fulfilling this request.`
`exception`
`org.springframework.web.util.NestedServletException: Request processing failed; nested exception is org.springframework.beans.InvalidPropertyException: Invalid property 'users[0]' of bean class [com.example.UserForm]: Illegal attempt to get property 'users' threw exception; nested exception is org.springframework.beans.NullValueInNestedPathException: Invalid property 'users' of bean class [com.example.UserForm]: Could not instantiate property type [com.example.UserEntity] to auto-grow nested property path: java.lang.InstantiationException: com.example.UserEntity`
`org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:927)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:822)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:796)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)`
`root cause`
`org.springframework.beans.InvalidPropertyException: Invalid property 'users[0]' of bean class [com.example.UserForm]: Illegal attempt to get property 'users' threw exception; nested exception is org.springframework.beans.NullValueInNestedPathException: Invalid property 'users' of bean class [com.example.UserForm]: Could not instantiate property type [com.example.UserEntity] to auto-grow nested property path: java.lang.InstantiationException: com.example.UserEntity
org.springframework.beans.BeanWrapperImpl.getPropertyValue(BeanWrapperImpl.java:829)
org.springframework.beans.BeanWrapperImpl.getNestedBeanWrapper(BeanWrapperImpl.java:556)
org.springframework.beans.BeanWrapperImpl.getBeanWrapperForPropertyPath(BeanWrapperImpl.java:533)
org.springframework.beans.BeanWrapperImpl.setPropertyValue(BeanWrapperImpl.java:894)
org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:75)
org.springframework.validation.DataBinder.applyPropertyValues(DataBinder.java:699)
org.springframework.validation.DataBinder.doBind(DataBinder.java:595)
org.springframework.web.bind.WebDataBinder.doBind(WebDataBinder.java:191)
org.springframework.web.bind.ServletRequestDataBinder.bind(ServletRequestDataBinder.java:112)
org.springframework.web.servlet.mvc.method.annotation.ServletModelAttributeMethodProcessor.bindRequestParameters(ServletModelAttributeMethodProcessor.java:153)
org.springframework.web.method.annotation.ModelAttributeMethodProcessor.resolveArgument(ModelAttributeMethodProcessor.java:106)
org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:77)
org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:162)
org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:123)
org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:746)
org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:687)
org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:80)
org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:925)
org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:856)
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:915)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:822)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:796)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)
`
Your help is highly appreciated
Thanks

Answer 1

The issue is actually that UserEntity does not have a default constructor, if you add the constructor it will work cleanly:
public UserEntity(){
//
}

Answer 2

I think what's happening is that the way your form is declared in the JSP, when submitting it it will assume that there is a UserEntity instance at position 0 in the users List of the UserForm instance, but it actually stumbles over a null reference.
Yes, you do add some UserEntity instances to the model when the form is displayed (using the loadForm method which is mapped to HTTP Get), but when you submit the form, a new model object is created, and this one will only have nulls in the users List.
Try making the model session scoped, this way the model instance you create when the form is shown will be reused when the form is submitted. Or create your own implementation of List that will return a new instance of UserEntity when a get(int position) is performed and there's nothing there.