The Flowers Are Still Standing

21 April 2015

File uploads via an HTTP POST to an end point are pretty easy using the RestTemplate and FileResource classes of the Spring Framework. However, this approach assumes that the file to be uploaded resides on disk. What happens if you want to dynamically create the uploaded file in memory or want to use a file on the classpath? The answer is to create a slight tweak to the ByteArrayResource implementation so that the proper from data header attributes are set in the POST body. To do this, first extend the ByteArrayResource so that you can set the filename of the resource:

public class FileMessageResource extends ByteArrayResource {

    /**
     * The filename to be associated with the {@link MimeMessage} in the form data.
     */
    private final String filename;

    /**
     * Constructs a new {@link FileMessageResource}.
     * @param byteArray A byte array containing data from a {@link MimeMessage}.
     * @param filename The filename to be associated with the {@link MimeMessage} in
     * 	the form data.
     */
    public FileMessageResource(final byte[] byteArray, final String filename) {
        super(byteArray);
        this.filename = filename;
    }

    @Override
    public String getFilename() {
        return filename;
    }
}

Next, create your new resource and add it to the POST body. From there, simply pass the POST body to the RestTemplate to send the file to the end point:

final MultiValueMap<String,Object> data = new LinkedMultiValueMap<String,Object>();
data.add("file", new FileMessageResource(fileBytes, "upload.txt"));
final HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<MultiValueMap<String, Object>>(data);
final ResponseEntity<Map<String,String>> response = getRestTemplate().exchange(url, HttpMethod.POST, requestEntity, new ParameterizedTypeReference<Map<String,String>>() {});

The result is a form POST to the end point with the proper section header:

Content-Disposition: form-data; name="file"; filename="upload.txt"

Upload File from Classpath/In-Memory Using Spring RestTemplate