Add null check in StandardMultipartHttpServletRequest `isEmpty()` to prevent NPE

[hyejungg]

Summary

Invoking MultipartFile.isEmpty() when MultipartFile is null results in 'NullPointerException'.

When an image is imported to the client, it is usually null if the image is not uploaded. In this case, developers often assume that both null and empty cases are handled by the presumption that '!image.isEmpty()' is used for brevity.

Currently, StandardMultipartHttpServletRequest#isEmpty() calls 'this.part.getSize()' directly without confirming that it is 'null', which can result in NPE.

Here’s the exception I encountered:

java.lang.NullPointerException: Cannot invoke "org.springframework.web.multipart.MultipartFile.isEmpty()" because "image" is null

Proposal

It would be helpful if isEmpty() were made more defensive by returning true when this.part == null, similar to how StringUtils.isEmpty() treats null as empty.

Suggested implementation:

@Override
public boolean isEmpty() {
    return (this.part == null || this.part.getSize() == 0);
}

For those familiar with utility-style isEmpty() methods such as StringUtils, more secure null handling will improve the developer experience.

If this change makes sense, I'm willing to open a pull request!

[whlit]

Your problem now is that the object 'image' is null. No matter how you change it, it is wrong to use image.isEmpty(). It is equivalent to executing 'null.isEmpty()'. At any time, it will throw NPE.

[hyejungg]

@whlit
Hello,
You're right — I wasn't thinking it through enough. I should have used ObjectUtils.isEmpty(image) or image != null for that case. Thank you!