Spring boot MultipartFile transferTo not working

Question

The line file1.createNewFile(); creates a new file, but the file.transferTo(file1); does not transfer the contents of the MultipartFile to file1. Is there a way to make the transferTo method work?

@PostMapping("/form3")
public String handleFormUpload3(@RequestParam("file") MultipartFile file) throws IOException {
    String fileName = file.getOriginalFilename();
    File file1 = new File(fileName);
    if(!file1.exists())
        file1.createNewFile();
    file.transferTo(file1);
    System.out.println(file1.exists());
    return "redirect:uploadSuccess";
}

When I use FileOutputStream to save the MultipartFile to a regular file by converting into to a byte array, the transfer works fine. But transferTo() is much simpler and less verbose. How do I make it work?

Answer 1

I did a little digging on the source code itself to find out why this is happening.

In the class org.springframework.web.multipart.support.StandardMultipartHttpServletRequest two functions exists

        @Override
        public void transferTo(File dest) throws IOException, IllegalStateException {
            this.part.write(dest.getPath());
            if (dest.isAbsolute() && !dest.exists()) {
                FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest.toPath()));
            }
        }

        @Override
        public void transferTo(Path dest) throws IOException, IllegalStateException {
            FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest));
        }
    }

Definition of part.write() method is written in class org.apache.catalina.core.ApplicationPart

    @Override
    public void write(String fileName) throws IOException {
        File file = new File(fileName);
        // testing whether the path is absolute or not, OP's code is relative path
        if (!file.isAbsolute()) {
          // HIGHLIGHTED PART
            file = new File(location, fileName);
        }
        try {
            fileItem.write(file);
        } catch (Exception e) {
            throw new IOException(e);
        }
    }

In the write method absolute path is being created by using location variable. Variable location hold the value of ROOT directory of TOMCAT as example

/tmp/tomcat.8080.12533880900101227658/work/Tomcat/localhost/ROOT/

So, when you do not give an absolute path to write, function simply decide to move the file to ROOT directory. In other word, file is being moved, working properly, OP just does not know where to look for the moved file.

What can be done to resolve OP''s issue.

class ControllerClass {
  // Create an absolute base path where your uploaded file will be moved. 
  // All file will be moved here 
  private final String uploadFilePath = "/a/suitable/path/";
  @PostMapping("/form3")
  public String handleFormUpload3(@RequestParam("file") MultipartFile file) throws IOException {
      String fileName = file.getOriginalFilename();
      File file1 = new File(uploadFilePath, fileName);
      //if(!file1.exists())
      //    file1.createNewFile();
      file.transferTo(file1);
      System.out.println(file1.exists());
      return "redirect:uploadSuccess";
  }
}

PS This is poor judgement of coding by Spring Boot development team. They have overwridden function that has different behvior. This is bad principle for writing library. It is unwise to force programmer to be hyper aware of how functions with same name behave. It just create distrust towards the library. Both of the functions should have behave exactly the same. My apporach would be simpley call the other function with proper value instead of writing a whole new function with different nature.

Answer 2

I have reproduced your case and can confirm that transferTo() with File object doesn't work. I don't know why. But transferTo() with Path works fine.

This code works fine:

String fileName = file.getOriginalFilename();
File file1 = new File(fileName);
file.transferTo(file1.toPath());

EDIT: I have checked the source code (Java 20) and there is an obvious difference between (1) transferTo(File file) and (2) transferTo(Path path): in (1) the file will only be copied if it is an absolute path. It means starts with '/' on Unix or drive letter or network drive on Windows. Method (2) doesn't check it and just copy the file. In our case to the working directory as the file name is not prefixed with anything.