spring 提交表单时的Spring绑定异常

Spring binding exception when a form is submitted

提问by java_dude

Stuck and have no clue why Spring Form is not able to submit successfully [gives binding issue] when pre-populated in get Requestcall loadForm, but works fine when populated in a method setupFormObjectwith @ModelAttributeannotation tag. I can provide a simple example in github to test out if asked for :)

卡住并且不知道为什么 Spring Form 在 get Requestcall 中预填充时无法成功提交[给出绑定问题] loadForm，但在setupFormObject带有@ModelAttribute注释标记的方法中填充时工作正常。我可以在 github 中提供一个简单的例子来测试是否需要:)

Example below

下面的例子

@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 githubexample.

根据要求进行堆栈跟踪。此异常来自github示例。

`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

非常感谢您的帮助

Thanks

谢谢

回答by Biju Kunjummen

The issue is actually that UserEntity does not have a default constructor, if you add the constructor it will work cleanly:

问题实际上是 UserEntity 没有默认构造函数，如果添加构造函数，它将正常工作：

public UserEntity(){
    //
}

回答by Shivan Dragon

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 UserEntityinstance at position 0 in the usersList of the UserForminstance, but it actually stumbles over a null reference.

我认为正在发生的事情是您的表单在 JSP 中声明的方式，当提交它时，它会假设在UserEntity实例users列表中的位置 0 处有一个UserForm实例，但它实际上偶然发现了一个空引用。

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.

是的，您确实在显示表单时向模型添加了一些 UserEntity 实例（使用映射到 HTTP Get 的 loadForm 方法），但是当您提交表单时，会创建一个新的模型对象，而这个对象只有空值在用户列表中。

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.

尝试将模型会话设置为作用域，这样您在显示表单时创建的模型实例将在提交表单时重用。或者创建您自己的 List 实现，当执行 get(int position) 并且没有任何内容时，它将返回 UserEntity 的新实例。