File file = new File("src/test/resources/input.txt");
FileInputStream input = new FileInputStream(file);
MultipartFile multipartFile = new MockMultipartFile("file",
            file.getName(), "text/plain", IOUtils.toByteArray(input));

I propose this method which allows converting a File to a MultipartFile:

public static MultipartFile buildMultipartFile(@NonNull final File file,
    @NonNull final String multipartFileParameterName) throws IOException {
       MultipartFile multipartFile = null;

       try (final FileInputStream input = new FileInputStream(file)) {
           multipartFile = new MockMultipartFile(
                   multipartFileParameterName,
                   file.getName(),
                   Files.probeContentType(file.toPath()),
                   IOUtils.toByteArray(input));
       }

       return multipartFile;
}

    File file = new File("src/test/resources/validation.txt");
    DiskFileItem fileItem = new DiskFileItem("file", "text/plain", false, file.getName(), (int) file.length() , file.getParentFile());
    fileItem.getOutputStream();
    MultipartFile multipartFile = new CommonsMultipartFile(fileItem);

You need the

    fileItem.getOutputStream();

because it will throw NPE otherwise.

This question has already been answered above. Recently i was working on the requirement to convert byte array object to multipartfile object. There are two ways to achieve this.

Approach 1:

Use the default CommonsMultipartFile where you to use the FileDiskItem object to create it.

Example:

FileItem fileItem = new DiskFileItem("fileData", "application/pdf",true, outputFile.getName(), 100000000, new java.io.File(System.getProperty("java.io.tmpdir")));              
MultipartFile multipartFile = new CommonsMultipartFile(fileItem);

Approach 2:

Create your own custom multipart file object and convert the byte array to multipartfile.

public class CustomMultipartFile implements MultipartFile {

private final byte[] fileContent;

private String fileName;

private String contentType;

private File file;

private String destPath = System.getProperty("java.io.tmpdir");

private FileOutputStream fileOutputStream;

public CustomMultipartFile(byte[] fileData, String name) {
    this.fileContent = fileData;
    this.fileName = name;
    file = new File(destPath + fileName);

}

@Override
public void transferTo(File dest) throws IOException, IllegalStateException {
    fileOutputStream = new FileOutputStream(dest);
    fileOutputStream.write(fileContent);
}

public void clearOutStreams() throws IOException {
if (null != fileOutputStream) {
        fileOutputStream.flush();
        fileOutputStream.close();
        file.deleteOnExit();
    }
}

@Override
public byte[] getBytes() throws IOException {
    return fileContent;
}

@Override
public InputStream getInputStream() throws IOException {
    return new ByteArrayInputStream(fileContent);
}
}

This how you can use above CustomMultipartFile object.

String fileName = "intermediate.pdf";
CustomMultipartFile customMultipartFile = new CustomMultipartFile(bytea, fileName);
try {
customMultipartFile.transferTo(customMultipartFile.getFile());
        
} catch (IllegalStateException e) {
    log.info("IllegalStateException : " + e);
} catch (IOException e) {
    log.info("IOException : " + e);
}

This will create the required PDF and store that into

java.io.tmpdir with the name intermediate.pdf

Thanks.