private AmazonS3 getAmazons3() {
        return AmazonS3ClientBuilder.standard()
                .withCredentials(new AWSStaticCredentialsProvider(new BasicAWSCredentials(accessKey, secretKey)))
                .withRegion(region)
                .build();
    }

public String putObject(MultipartFile file, String fileName) {
        try {
            InputStream inputStream = file.getInputStream();
            String path = 『保存する名前、ここでランダムの名前生成など』
            ObjectMetadata objectMetadata = new ObjectMetadata();
            objectMetadata.setContentType(file.getContentType());
            objectMetadata.setContentLength(Integer.valueOf(inputStream.available()).longValue());
            getAmazons3().putObject(bucketName, path, inputStream, objectMetadata);

            GeneratePresignedUrlRequest generatePresignedUrlRequest = new GeneratePresignedUrlRequest(bucketName, path)
                    .withMethod(HttpMethod.GET);

            final URL fileUrl = getAmazons3().generatePresignedUrl(generatePresignedUrlRequest);
            return fileUrl.toString();
        }catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException(e);
     }
}