How do I display validation errors about an uploaded multipart file using Spring MVC

Question

I have a Spring MVC application with a file upload form.

I would like to be able to display validation errors to a user if the uploaded content is not valid (e.g. the content is not an image, etc.)

However, by definition what is posted is of type: MultipartFile (multipart/form-data) and therefore I can't have a @ModelAttribute in my form and in order to use a BindingResult, it seems I do need a @ModelAttribute just before the BindingResult.

My question is then, what is the most appropriate way of displaying validation errors to a user when all I have is a MultipartFile? I could of course manually add model attributes to the model but I am sure there is a better way.

Answer 1

If you are using Java Bean Validation (JSR 303) you can make annotation that validates content type. See code below.

import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;
/**
 * The annotated element must have specified content type.
 *
 * Supported types are:
 * <ul>
 * <li><code>MultipartFile</code></li>
 * </ul>
 *
 * @author Michal Kreuzman
 */
@Documented
@Retention(RUNTIME)
@Constraint(validatedBy = {ContentTypeMultipartFileValidator.class})
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER })
public @interface ContentType {

    String message() default "{com.kreuzman.ContentType.message}";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

/**
 * Specify accepted content types.
 *
 * Content type example :
 * <ul>
 * <li>application/pdf - accepts PDF documents only</li>
 * <li>application/msword - accepts MS Word documents only</li>
 * <li>images/png - accepts PNG images only</li>
 * </ul>
 *
 * @return accepted content types
 */
     String[] value();
}

---

/**
  * Validator of content type. This is simple and not complete implementation
  * of content type validating. It's based just on <code>String</code> equalsIgnoreCase
  * method.
  *
  * @author Michal Kreuzman
  */
 public class ContentTypeMultipartFileValidator implements ConstraintValidator<ContentType, MultipartFile> {

private String[] acceptedContentTypes;

@Override
public void initialize(ContentType constraintAnnotation) {
    this.acceptedContentTypes = constraintAnnotation.value();
}

@Override
public boolean isValid(MultipartFile value, ConstraintValidatorContext context) {
    if (value == null || value.isEmpty())
        return true;

    return ContentTypeMultipartFileValidator.acceptContentType(value.getContentType(), acceptedContentTypes);
}

private static boolean acceptContentType(String contentType, String[] acceptedContentTypes) {
    for (String accept : acceptedContentTypes) {
            // TODO this should be done more clever to accept all possible content types
        if (contentType.equalsIgnoreCase(accept)) {
            return true;
        }
    }

    return false;
}
}

---

public class MyModelAttribute {

@ContentType("application/pdf")
private MultipartFile file;

public MultipartFile getFile() {
    return file;
}

public void setFile(MultipartFile file) {
    this.file = file;
}
}
@RequestMapping(method = RequestMethod.POST)
public String processUploadWithModelAttribute(@ModelAttribute("myModelAttribute") @Validated final MyModelAttribute myModelAttribute, final BindingResult result, final Model model) throws IOException {
 if (result.hasErrors()) {
     // Error handling
     return "fileupload";
 }

 return "fileupload";
}

Answer 2

I got the answer to my question Spring community forums:

Here is the controller method:

@RequestMapping(method = RequestMethod.POST)
    public String processUploadWithModelAttribute(@ModelAttribute("myModelAttribute") final MyModelAttribute myModelAttribute, final BindingResult result, final Model model) throws IOException {
        String mimeType = determineMimeType(myModelAttribute.getFile().getBytes());
        if (mimeType.equalsIgnoreCase("application/pdf")){
            result.addError(new ObjectError("file", "pdf not accepted"));
        }
            return "fileupload";
    }

And the model attribute class:

public class MyModelAttribute {

    private MultipartFile file;

    public MultipartFile getFile() {
        return file;
    }

    public void setFile(MultipartFile file) {
        this.file = file;
    }
}

The idea is to put the MultipartFile into the ModelAttribute as an attribute.