File upload in Spring MVC gives NullPointerException

Question

I have a simple JSP form as follows:

<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" />&nbsp;<form:errors path="file" />
        <input type="Reset" value="Reset">           
    </form:form>         

Which is for a user to upload a file to the server. The controller's method is as follows:

@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:

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:

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:

<bean id="uploadValidator" class="library.validator.UploadValidator" />        

<!-- Spring multipartResolver. -->         
<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />  

In the DispatcherServlet.

The Upload object type is as follows:

import org.springframework.web.multipart.MultipartFile;

public class Upload {

    private MultipartFile file;

And appropriate getter and setter.

So why is my code not working?

Stacktrace for current problem, i.e. allowing for changes to controller method in answer below, is:

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.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:690) org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:83) org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:945) org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:876) org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:961) org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:863) javax.servlet.http.HttpServlet.service(HttpServlet.java:641) org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:837) javax.servlet.http.HttpServlet.service(HttpServlet.java:722) org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:304) org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:240) org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:164) org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:498) org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:164) org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100) org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:562) org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118) org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:394) org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:243) org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:188) org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:166) org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:302) java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142) java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617) java.lang.Thread.run(Thread.java:744)

Answer 1

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. I had same problem and this did the thing.

Answer 2

This has now been solved. The problem was that the following was incorrectly defined in the DispatcherServlet:

<!-- Spring multipartResolver. -->         
<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" />