Spring3.X <form:form>标签数据绑定常见异常解决方法

异常：Neither BindingResult nor plain target object for bean name 'command' available as request attribute

这个异常在使用标签<form:form>时经常遇到，原因是因为model中没有绑定表单对象，而spring默认绑定的表单对象名为"command"，或者是因为Controller中声明的绑定对象与form标签中声明的绑定对象名不一致而导致出现的异常。

示例：

1.配置文件略过（使用标注的spring的配置文件非常简单，本例不需要特殊的配置）

2.Controller

@Controller@RequestMapping("/form")@SessionAttributes("formBean")public class FormController {// Invoked on every request@ModelAttributepublic void ajaxAttribute(WebRequest request, Model model) {model.addAttribute("ajaxRequest", AjaxUtils.isAjaxRequest(request));}// Invoked initially to create the "form" attribute// Once created the "form" attribute comes from the HTTP session (see @SessionAttributes)@ModelAttribute("command")（注1）public FormBean createFormBean() {return new FormBean();}@RequestMapping(method=RequestMethod.GET)public void form() {}@RequestMapping(method=RequestMethod.POST)public String processSubmit(@Valid FormBean formBean, BindingResult result, @ModelAttribute("ajaxRequest") boolean ajaxRequest, Model model, RedirectAttributes redirectAttrs) {if (result.hasErrors()) {return null;}// Typically you would save to a db and clear the "form" attribute from the session // via SessionStatus.setCompleted(). For the demo we leave it in the session.String message = "Form submitted successfully.  Bound " + formBean;// Success response handlingif (ajaxRequest) {// prepare model for rendering success message in this requestmodel.addAttribute("message", message);return null;} else {// store a success message for rendering on the next request after redirect// redirect back to the form to render the success message along with newly bound valuesredirectAttrs.addFlashAttribute("message", message);return "redirect:/form";}}}

3.form.jsp

<form:form id="form" method="post" commandName="command"（注2）  cssClass="cleanform"><div class="header">  <h2>Form</h2>  <c:if test="${not empty message}"><div id="message" class="success">${message}</div>  </c:if>  <s:bind path="*">  <c:if test="${status.error}">  <div id="message" class="error">Form has errors</div>  </c:if>  </s:bind></div>  <fieldset>  <legend>Personal Info</legend>  <form:label path="name">  Name <form:errors path="name" cssClass="error" /> </form:label>  <form:input path="name" />  <form:label path="age">  Age <form:errors path="age" cssClass="error" /> </form:label>  <form:input path="age" />    <form:label path="birthDate">  Birth Date (in form yyyy-mm-dd) <form:errors path="birthDate" cssClass="error" /> </form:label>  <form:input path="birthDate" />     <form:label path="phone">  Phone (in form (###) ###-####) <form:errors path="phone" cssClass="error" />  </form:label>  <form:input path="phone" />  <form:label path="currency">  Currency (in form $#.##) <form:errors path="currency" cssClass="error" />  </form:label>  <form:input path="currency" />  <form:label path="percent">  Percentage (in form ##%) <form:errors path="percent" cssClass="error" />  </form:label>  <form:input path="percent" />  </fieldset><fieldset><legend>Inquiry</legend><form:label path="inquiry">Type (select one)</form:label><form:select path="inquiry"><form:option value="comment">Comment</form:option><form:option value="feedback">Feedback</form:option><form:option value="suggestion">Suggestion</form:option></form:select>  <form:label path="inquiryDetails">  Details  </form:label>  <form:textarea path="inquiryDetails" />  </fieldset><fieldset class="checkbox"><legend>Request Additional Info</legend><label><form:checkbox path="additionalInfo[mvc]" value="true" />on Spring MVC</label><label><form:checkbox path="additionalInfo[java]" value="true" />on Java (4-ever)</label></fieldset>    <fieldset class="radio"><legend>Subscribe to Newsletter?</legend><label><form:radiobutton path="subscribeNewsletter" value="true" />Yes</label><label><form:radiobutton path="subscribeNewsletter" value="false" /> No</label></fieldset><p><button type="submit">Submit</button></p></form:form>

Controller里必须使用@ModelAttribute("command")来说明绑定对象名（注1），Spring默认的绑定对象名为command,如果Controller使用了"command"为绑定对象名，则form标签中可以省略commandName="command"。（如示例代码）

如果想要使用其他名称作为绑定名，例如@ModelAttribute("user")，则必须在form标签中显式声明commandName="user"或者声明modelAttribute="user"（注2）。