How to construct correct MultipartEntity to send a multipart/related request in java?

Question

I want to send to OCR image from local computer, not some remote url with image there (this case working great). But the problem is, I don't know how construct correctly payload of multipart entity. My payload should be looking as it described below.

This's what described in documentation of API. Decode the image data given directly in the multipart/related data. The order is important, and the first part should be the JSON, which tells it which OCR engine to use. The schema for thi JSON is documented in the /ocr endpoint. The img_url parameter of the JSON will be ignored in this case.

The image attachment should be the second part, and it should work with any image content type (eg, image/png, image/jpg, etc).

   Request (multipart/related; boundary=---BOUNDARY)

    -----BOUNDARY
    Content-Type: application/json

    {"engine":"tesseract"}
    -----BOUNDARY

    -----BOUNDARY
    Content-Disposition: attachment;
    Content-Type: image/png
    filename="attachment.txt".

    PNGDATA.........
    -----BOUNDARY

Here's what I've tried. To perform multipart/related request I am using org.apache.httpcomponents

    CloseableHttpClient httpClient = HttpClients.createDefault();

    MultipartEntityBuilder multipartEntityBuilder =
            MultipartEntityBuilder.create().setBoundary(BOUNDARY).setContentType(ContentType.APPLICATION_JSON).addTextBody("engine", "tesseract")
                                                    .setBoundary(BOUNDARY).setBoundary(BOUNDARY);

    multipartEntityBuilder.addBinaryBody("file_upload", new File(fileTextPath), ContentType.create(CONTENT_TYPE), fileTextPath).setBoundary(BOUNDARY);

    HttpEntity entity = multipartEntityBuilder.build();


    HttpPost httpPost = new HttpPost(URL);
    httpPost.setHeader(HttpHeaders.CONTENT_TYPE, CONTENT_TYPE_MULTIPART+";boundary="+BOUNDARY);
    httpPost.setEntity(entity);

Answer 1

I resolved this problem a few weeks ago and for convenience created a lightweight java web application for both cases (remote and file upload).

You can find the particular answer for the question above here. The source code for a multipart request is shown below:

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.UUID;

public class MultipartUtility {
    private static final Logger logger = LoggerFactory.getLogger(MultipartUtility.class);

    private final String boundary = UUID.randomUUID().toString();
    private static final String LINE_FEED = "\r\n";
    private HttpURLConnection httpConn;
    private String charset;
    private OutputStream outputStream;
    private PrintWriter writer;

    /**
     * This constructor initializes a new HTTP POST request with content type
     * is set to multipart/form-data
     *
     * @param requestURL
     * @param charset
     * @throws IOException
     */
    public MultipartUtility(String requestURL, String charset) {
        this.charset = charset;

        try {
            URL url = new URL(requestURL);
            httpConn = (HttpURLConnection) url.openConnection();
            httpConn.setUseCaches(false);
            httpConn.setDoOutput(true); // indicates POST method
            httpConn.setDoInput(true);
            httpConn.setRequestProperty("Content-Type", "multipart/related; boundary=\"" + boundary + "\"");
            httpConn.setRequestProperty("Accept-Encoding", "gzip");
            outputStream = httpConn.getOutputStream();
            //outputStream = System.out;
            writer = new PrintWriter(new OutputStreamWriter(outputStream, charset), true);
        } catch (IOException ex) {
            logger.error("Error during creation of MultiPart: ", ex);
        }
    }

    /**
     * Adds a form field to the request
     */
    public void addFormField(String jsonBody) {
        writer.append("--").append(boundary).append(LINE_FEED);
        writer.append("Content-Type: application/json;").append(LINE_FEED);
        writer.append(LINE_FEED);
        writer.append(jsonBody).append(LINE_FEED);
        writer.flush();
    }

    /**
     * Adds a upload file section to the request
     *
     * @param uploadFile a File to be uploaded
     * @throws IOException
     */
    public void addFilePart(File uploadFile) {
        String fileName = uploadFile.getName();
        writer.append("--").append(boundary).append(LINE_FEED);
        writer.append("Content-Disposition: attachment;");
        writer.append(" filename=\"" + fileName + "\".").append(LINE_FEED);
        writer.append("Content-Type: image/*").append(LINE_FEED);
        writer.append(LINE_FEED);
        writer.flush();

        try {
            FileInputStream inputStream = new FileInputStream(uploadFile);
            //byte[] buffer = new byte[(int) uploadFile.length()];
            byte[] buffer = Files.readAllBytes(Paths.get(uploadFile.getPath()));
            int bytesRead = -1;
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }
            outputStream.flush();
            inputStream.close();
        } catch (IOException ex) {
            logger.error("File transformation to bytes went wrong: {}", ex);
        }

        writer.append(LINE_FEED);
        writer.flush();
    }

    /**
     * Adds a header field to the request.
     *
     * @param name  - name of the header field
     * @param value - value of the header field
     */
    public void addHeaderField(String name, String value) {
        writer.append(name + ": " + value).append(LINE_FEED);
        writer.flush();
    }

    /**
     * Completes the request and receives response from the server.
     *
     * @return a list of Strings as response in case the server returned
     * status OK, otherwise an exception is thrown.
     * @throws IOException
     */
    public String finish() {
        String response = "";
        int status = 0;

        writer.flush();
        writer.append("--").append(boundary).append("--").append(LINE_FEED);
        writer.println();
        writer.close();

        try {
            // checks server's status code first
            status = httpConn.getResponseCode();

            if (status == HttpURLConnection.HTTP_OK) {
                BufferedReader reader = new BufferedReader(new InputStreamReader(httpConn.getInputStream()));

                String line;
                while ((line = reader.readLine()) != null) {
                    response += line;
                }

                reader.close();
                httpConn.disconnect();
            } else {
                logger.error("OCR API returned error stream: {}", printErrorStream());
                throw new IOException("Server returned non-OK status: " + status + " : " + httpConn.getResponseMessage());
                //logger.error("Server returned non-OK status: " + status + " : " + httpConn.getResponseMessage());
            }
        } catch(IOException ex) {
            logger.error("Response message in Multipart finish has been received with problems: ", ex);
        }

        return response;
    }

    private String printErrorStream() throws IOException {
        //System.out.print("DEBUG System out ocr API error stream: ");
        InputStream errorStream = httpConn.getErrorStream();
        BufferedReader reader = new BufferedReader(new InputStreamReader(errorStream));
        String errLine = "", tempLine;
        while ((tempLine = reader.readLine()) != null) {
            errLine += tempLine;
        }

        return errLine;
    }

    private void getRequestHeaders(HttpURLConnection httpURLConnection) {
        for (Map.Entry<String, List<String>> entries : httpURLConnection.getRequestProperties().entrySet()) {
            String values = "";
            for (String value : entries.getValue()) {
                values += value + ",";
            }
            System.out.println("Request" + " " + entries.getKey() + " - " +  values );
        }
    }
}