Upload a file using Java HttpURLConnection

I tried uploading a file from a Java client to the server implemented in the following article, so I will describe the procedure. I tried uploading a file from httpclient to the Ruby Sinatra server

It can't be helped compared to Ruby, but the code has become longer.

I have corrected the code by pointing out from the comment section. </ font>


import java.io.OutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.charset.StandardCharsets;
import java.util.UUID;

public class Uploader {
    private static final String EOL = "\r\n";

    public static int Send(String filename, String url, String method) throws IOException {
        try (FileInputStream file = new FileInputStream(filename)) {
            HttpURLConnection con = (HttpURLConnection) new URL(url).openConnection();
            final String boundary = UUID.randomUUID().toString();
            con.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
            try (OutputStream out = con.getOutputStream()) {
                out.write(("--" + boundary + EOL +
                    "Content-Disposition: form-data; name=\"file\"; " +
                    "filename=\"" + filename + "\"" + EOL +
                    "Content-Type: application/octet-stream" + EOL + EOL)
                byte[] buffer = new byte[128];
                int size = -1;
                while (-1 != (size = file.read(buffer))) {
                    out.write(buffer, 0, size);
                out.write((EOL + "--" + boundary + "--" + EOL).getBytes(StandardCharsets.UTF_8));
                return con.getResponseCode();
            } finally {

    public static void main(String[] args) throws IOException {
        if (args.length < 2) {
            System.err.println("Input [Upload file path] [Upload URL]");
        String filename = args[0];
        String url = args[1];
        int res = Uploader.Send(filename, url, "POST");
        if (res == HttpURLConnection.HTTP_OK) {
        } else {
            System.err.printf("Failed %d\n", res);

In addition, this source was created so that the communication that uploaded the file with curl was confirmed with Wireshark and the contents were the same in java.

$ curl -X POST http://localhost:3000/upload -F "file=@./filepath"

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