Explain how Spring MVC supports file upload.

Question

Accepted Answer

Spring MVC supports file uploads through the MultipartFile abstraction, which represents an uploaded file as a bound method parameter once the underlying multipart request has been parsed by a multipart resolver.

Key Points:
• The HTML form must set enctype="multipart/form-data" so the browser sends the file as part of a multipart request body.
• @RequestParam binds a MultipartFile parameter to the name attribute of the file input on the form.
• MultipartFile exposes methods like getBytes(), getOriginalFilename(), getSize(), and transferTo() for working with the uploaded content.
• Spring Boot auto-configures a StandardServletMultipartResolver, so no manual bean registration is needed in most cases.
• Multiple files can be uploaded at once by binding to a List parameter with matching input names.

Example:
A profile picture upload form posts to /uploadAvatar with an input named "file"; the controller method declares @RequestParam("file") MultipartFile file, checks its size and content type, and calls file.transferTo(destination) to save it to disk.

Code Example:
```
@PostMapping("/uploadAvatar")
public String upload(@RequestParam("file") MultipartFile file) throws IOException {
    if (!file.isEmpty()) {
        file.transferTo(new File("/uploads/" + file.getOriginalFilename()));
    }
    return "redirect:/profile";
}
```

Interview Tip:
A concise interview answer is:

"Spring MVC represents an uploaded file as a MultipartFile bound via @RequestParam, as long as the form uses enctype multipart/form-data. From there I can read the file's bytes, name, and size, or call transferTo() to save it — Spring Boot wires up the multipart resolver automatically."