Spring MVC中的文件上传给出了NullPointerException
[英]File upload in Spring MVC gives NullPointerException
问题描述
I have a simple JSP form as follows: 
我有一个简单的JSP表单如下：
<p>Please select a file and <i>click</i> <i>Upload file</i> to upload the file to the server:</p>
<c:url value="/upload/display" var="displayUploadedFileURL" />
<form:form action="${displayUploadedFileURL}" method="post" modelAttribute="upload" enctype="multipart/form-data">
<input type="file" name="file" />
<input type="submit" value="Upload file" /> <form:errors path="file" />
<input type="Reset" value="Reset">
</form:form>
Which is for a user to upload a file to the server. 这是供用户将文件上传到服务器的。 The controller's method is as follows: 控制器的方法如下：
@Controller
@RequestMapping("/upload")
public class UploadController {
@Autowired
private UploadValidator uploadValidator;
@RequestMapping(value="/display", method=RequestMethod.POST)
public String displayUploadedFile(@ModelAttribute("upload") Upload upload,
BindingResult bindingResult,
Model model) {
// Validate Upload.
uploadValidator.validate(upload, bindingResult);
if (bindingResult.hasErrors()) {
return ("view/upload/select");
}
else {
String fileName = upload.getFile().getOriginalFilename();
System.out.println("Here: " + upload.getFile().getOriginalFilename());
model.addAttribute("fileName", fileName);
return ("view/upload/display");
}
}
... ...
But when I select a file and use the Upload file button I get the following: 但是当我选择一个文件并使用上传文件按钮时，我得到以下内容：
Your page request has caused a NullPointerException: error:
library.validator.UploadValidator.validate(UploadValidator.java:29)
library.controller.upload.UploadController.displayUploadedFile(UploadController.java:45)
The validator concerned is very simple: 有关的验证人很简单：
package library.validator;
import library.model.Upload;
import org.apache.log4j.Logger;
import org.springframework.stereotype.Component;
import org.springframework.validation.Errors;
import org.springframework.validation.Validator;
@Component
public class UploadValidator implements Validator {
private static final Logger logger = Logger.getLogger(UploadValidator.class);
public UploadValidator() {
}
@Override
public boolean supports(Class cls) {
return Upload.class.isAssignableFrom(cls);
}
@Override
public void validate(Object target, Errors errors) {
logger.info(UploadValidator.class.getName() + ".validate() method called.");
Upload upload = (Upload) target;
if (upload.getFile().getSize() == 0) {
errors.rejectValue("file", "file.required");
}
}
} }
I have all the relevant .jar files in the application's .lib folder, and I'mincluding the following: 我在应用程序的.lib文件夹中包含所有相关的.jar文件，并且我将包括以下内容：
<bean id="uploadValidator" class="library.validator.UploadValidator" />
<!-- Spring multipartResolver. -->
<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />
In the DispatcherServlet . 在DispatcherServlet 。
The Upload object type is as follows: Upload对象类型如下：
import org.springframework.web.multipart.MultipartFile;
public class Upload {
private MultipartFile file;
And appropriate getter and setter. 适当的吸气剂和固定剂。
So why is my code not working? 那为什么我的代码不起作用？
Stacktrace for current problem, ie allowing for changes to controller method in answer below, is: 针对当前问题的Stacktrace，即允许在下面的答案中更改控制器方法，是：
org.springframework.util.Assert.notNull(Assert.java:112) org.springframework.web.method.annotation.RequestParamMethodArgumentResolver.resolveName(RequestParamMethodArgumentResolver.java:171) org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver.resolveArgument(AbstractNamedValueMethodArgumentResolver.java:89) org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:79) org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:157) org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:124) org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:104) org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandleMethod(RequestMappingHandlerAdapter.java:749) org.sp
2 个解决方案
解决方案1
1 2014-09-17 16:16:13
Configuring the multipartResolver bean is important thing, but also you should check that your .xml config file with this bean is imported in your general applicationContext.xml , if you have some. 配置multipartResolver bean很重要，但是如果你有一些，你应该检查你的.xml配置文件是否在你的常规applicationContext.xml导入。 I had same problem and this did the thing. 我遇到了同样的问题，这就完成了。
解决方案2
0 2014-08-02 14:31:59
This has now been solved. 这已经解决了。 The problem was that the following was incorrectly defined in the DispatcherServlet : 问题是在DispatcherServlet错误地定义了以下内容：
<!-- Spring multipartResolver. -->
<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />
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