默认已经知道：MultipartFile  这个类一般是用来接受前台传过来的文件。

那transferTo方法有什么作用呢？

打开源码：

    public void transferTo(File dest) throws IOException, IllegalStateException {
        if (!this.isAvailable()) {
            throw new IllegalStateException("File has already been moved - cannot be transferred again");
        } else if (dest.exists() && !dest.delete()) {
            throw new IOException("Destination file [" + dest.getAbsolutePath() + "] already exists and could not be deleted");
        } else {
            try {
                this.fileItem.write(dest);
                if (logger.isDebugEnabled()) {
                    String action = "transferred";
                    if (!this.fileItem.isInMemory()) {
                        action = this.isAvailable() ? "copied" : "moved";
                    }

                    logger.debug("Multipart file '" + this.getName() + "' with original filename [" + this.getOriginalFilename() + "], stored " + this.getStorageDescription() + ": " + action + " to [" + dest.getAbsolutePath() + "]");
                }

            } catch (FileUploadException var3) {
                throw new IllegalStateException(var3.getMessage(), var3);
            } catch (IOException | IllegalStateException var4) {
                throw var4;
            } catch (Exception var5) {
                throw new IOException("File transfer failed", var5);
            }
        }
    }

意思是说：把dest这个文件路径所指向的文件上传到对应的目录下。