Handling File Uploads

• 🐣Spring & SpringBoot

multipart/form-data

A special encoding method that enables a browser to send a request that includes binary file data.

• Unlike the transmission of plain text data, file uploads are sent using a special HTTP request format called multipart/form-data (and not the typical x-www-form-urlencoded)
• This method is designed to allow both text data and binary files to be delivered to the server within a single request.
• A single file can be processed as one MultipartFile, while multiple files can be handled as a List<MultipartFile>.

Note

Think of multipart/form-data request as a single large shipping box (the HTTP request) that contains several smaller, individually labeled boxes inside (the “parts”). Each part can have its own content type.

• This format is specifically designed to send a mix of different data types at once, like files, text, and JSON

How Browser File Uploads Work

1. A user selects a local file using an <input type="file"> element.
2. The user submits the <form>. In response, the browser sets the request’s Content-Type header to multipart/form-data.
3. Each form field and the file data are then sent as separate “parts” of the request body.

Sample code

Directory Structure

src/main/java/com/springboot/coffee
├── controller
│   └── CoffeeController.java
├── dto
│   └── CoffeeImageDto.java
├── templates
│   └── uploadForm.html
│   └── uploadResult.html

CoffeeImageDto.java

@Getter
@Setter
public class CoffeeImageDto {
    private String coffeeName;
    private String fileName;
}

CoffeeController.java

@PostMapping("/v1/coffees/upload")
public String uploadCoffeeImage(
	@RequestParam("coffeeName") String coffeeName,
	@RequestParam("image") MultipartFile file,
	Model model) throws IOException {
    
    String fileName = file.getOriginalFilename();
    Path savePath = Paths.get("./uploads/" + fileName);
    Files.createDirectories(savePath.getParent());
    file.transferTo(savePath);
 
    CoffeeImageDto dto = new CoffeeImageDto();
    dto.setCoffeeName(coffeeName);
    dto.setFileName(fileName);
 
    model.addAttribute("coffeeName", coffeeName);
    model.addAttribute("fileName", fileName);
    return "uploadResult";
}

• The directory where files will be saved is ./uploads/, and it will be automatically created at the project’s root.

Tips

• For file uploads, you must use enctype="multipart/form-data" in your HTML form.
• To prevent file name conflicts on the server, it is recommended to either use a UUID to generate a unique file name or to save files into user-specific directories.
• An image preview feature can be implemented on the front-end using JavaScript.
• Implementing security measures is essential. This includes limiting file sizes and validating file extensions.