To make a http request with multipart/form-data, we need to define a boundary separator, which supports uploading images and other files. Using the HttpURLConnection class with multipart/form-data Content-Type is similar to HTML form below.

<body>
    <form action="/test" method="post" enctype="multipart/form-data">
        <input type="password" name="password">
        <input type="username" name="username">
        <input type="file" name="imgFile">
        <input type="submit" value="Submit">
    </form>
</body>

Example

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.Iterator;
import java.util.Map;
import java.util.UUID;

public class HttpPostMultipart {
    private final String boundary;
    private static final String LINE = "\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
     * @param headers
     * @throws IOException
     */
    public HttpPostMultipart(String requestURL, String charset, Map<String, String> headers) throws IOException {
        this.charset = charset;
        boundary = UUID.randomUUID().toString();
        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/form-data; boundary=" + boundary);
        if (headers != null && headers.size() > 0) {
            Iterator<String> it = headers.keySet().iterator();
            while (it.hasNext()) {
                String key = it.next();
                String value = headers.get(key);
                httpConn.setRequestProperty(key, value);
            }
        }
        outputStream = httpConn.getOutputStream();
        writer = new PrintWriter(new OutputStreamWriter(outputStream, charset), true);
    }

    /**
     * Adds a form field to the request
     *
     * @param name  field name
     * @param value field value
     */
    public void addFormField(String name, String value) {
        writer.append("--" + boundary).append(LINE);
        writer.append("Content-Disposition: form-data; name=\"" + name + "\"").append(LINE);
        writer.append("Content-Type: text/plain; charset=" + charset).append(LINE);
        writer.append(LINE);
        writer.append(value).append(LINE);
        writer.flush();
    }

    /**
     * Adds a upload file section to the request
     *
     * @param fieldName
     * @param uploadFile
     * @throws IOException
     */
    public void addFilePart(String fieldName, File uploadFile)
            throws IOException {
        String fileName = uploadFile.getName();
        writer.append("--" + boundary).append(LINE);
        writer.append("Content-Disposition: form-data; name=\"" + fieldName + "\"; filename=\"" + fileName + "\"").append(LINE);
        writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(fileName)).append(LINE);
        writer.append("Content-Transfer-Encoding: binary").append(LINE);
        writer.append(LINE);
        writer.flush();

        FileInputStream inputStream = new FileInputStream(uploadFile);
        byte[] buffer = new byte[4096];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.flush();
        inputStream.close();
        writer.append(LINE);
        writer.flush();
    }

    /**
     * Completes the request and receives response from the server.
     *
     * @return String as response in case the server returned
     * status OK, otherwise an exception is thrown.
     * @throws IOException
     */
    public String finish() throws IOException {
        String response = "";
        writer.flush();
        writer.append("--" + boundary + "--").append(LINE);
        writer.close();

        // checks server's status code first
        int status = httpConn.getResponseCode();
        if (status == HttpURLConnection.HTTP_OK) {
            ByteArrayOutputStream result = new ByteArrayOutputStream();
            byte[] buffer = new byte[1024];
            int length;
            while ((length = httpConn.getInputStream().read(buffer)) != -1) {
                result.write(buffer, 0, length);
            }
            response = result.toString(this.charset);
            httpConn.disconnect();
        } else {
            throw new IOException("Server returned non-OK status: " + status);
        }
        return response;
    }
}

Use it

try {
    // Set header
    Map<String, String> headers = new HashMap<>();
    headers.put("User-Agent", "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36");
    HttpPostMultipart multipart = new HttpPostMultipart("http://localhost/index", "utf-8", headers);
    // Add form field
    multipart.addFormField("username", "test_name");
    multipart.addFormField("password", "test_psw");
    // Add file
    multipart.addFilePart("imgFile", new File("/Users/apple/Desktop/test.png"));
    // Print result
    String response = multipart.finish();
    System.out.println(response);
} catch (Exception e) {
    e.printStackTrace();
}

Use charles to capture packets, we can see data such as request headers and post parameters.

POST /index HTTP/1.1
Host: localhost
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_4) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/79.0.3945.88 Safari/537.36
Content-Type: multipart/form-data; boundary=--eab528da-8c70-424d-8746-c18ee89f036c

--eab528da-8c70-424d-8746-c18ee89f036c
Content-Disposition: form-data; name="username"
Content-Type: text/plain; charset=utf-8

test_name
--eab528da-8c70-424d-8746-c18ee89f036c
Content-Disposition: form-data; name="password"
Content-Type: text/plain; charset=utf-8

test_psw
--eab528da-8c70-424d-8746-c18ee89f036c
Content-Disposition: form-data; name="imgFile"; filename="test.png"
Content-Type: image/png
Content-Transfer-Encoding: binary

(data)
--eab528da-8c70-424d-8746-c18ee89f036c--

HttpURLConnection multipart/form-data example

Example

Use it