How to write a file to an HTTP Request Java

Question

I am new to using HTTP and I have questions about writing a file and another value to an HTTP Post request in Java. I am using an public API provided by a company called Mojang to write what is known as a "skin" (a png file) to the game Minecraft for player character modles. Here is the documentation of how to use this public API for reference:https://wiki.vg/Mojang_API#Upload_Skin

Here is the code I have written. When ran I get the 415 HTTP Response code (which I assume is "unsupported media type"). Any suggestions on what I am doing wrong and how I can fix this? I found other stack overflow issues for uploading files but I need to also add a value called "variant={classic or slim}". I am a little lost on how to make all of this work. Any help is much appreciated.

(I could not get the code to properally format in the code sample using ' ', it is in a javascript snippet)

    public static void uploadSkin(String accessToken, String variant, File file) throws IOException {

    URL url = new URL("https://api.minecraftservices.com/minecraft/profile/skins");
    HttpURLConnection con = (HttpURLConnection) url.openConnection();
    con.setDoOutput(true);
    con.setRequestMethod("POST");
    con.setRequestProperty("Authorization", "Bearer " + accessToken); // The access token is provided after an
                                                                        // authentication request has been send, I
                                                                        // have done this sucessfully in another
                                                                        // method and am passing it in here

    con.addRequestProperty("variant", variant);
    
    OutputStream outputStream = con.getOutputStream();
    PrintWriter writer = new PrintWriter(new OutputStreamWriter(con.getOutputStream(), "utf-8"), true);
    String boundary = "===" + System.currentTimeMillis() + "===";
    String fileName = file.getName();
    String LINE_FEED = "\r\n";
    String fieldName = "file";

    writer.append("--" + boundary).append(LINE_FEED);
    writer.append("Content-Disposition: form-data; name=\"" + fieldName + "\"; filename=\"" + fileName + "\"")
            .append(LINE_FEED);
    writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(fileName)).append(LINE_FEED);
    writer.append("Content-Transfer-Encoding: binary").append(LINE_FEED);
    writer.append(LINE_FEED);
    writer.flush();

    FileInputStream inputStream = new FileInputStream(file);

    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_FEED);
    writer.flush();
}

You need to send a request with Content-Type: multipart/form-data. I would strongly advise to use an HTTP client library for this. — slauth
– slauth, Commented Jul 29, 2021 at 21:21
Okay, thank you, I will give that a try and look for a client library. — Tyler
– Tyler, Commented Jul 29, 2021 at 21:26

Tyler · Accepted Answer · 2021-07-29 22:31:58Z

1

Alright, found a solution to the problem. Using this maven dependency:

    <!-- https://mvnrepository.com/artifact/org.jodd/jodd-http -->
    <dependency>
        <groupId>org.jodd</groupId>
        <artifactId>jodd-http</artifactId>
        <version>5.0.2</version>
    </dependency>

And then this:

        HttpResponse response = HttpRequest.post("https://api.minecraftservices.com/minecraft/profile/skins")
            .header("Authorization", "Bearer " + accessToken).header("Content-Type", "multipart/form-data")
            .form("variant", variant).form("file", file).send();

I was able to get it to work. Hope this is helpful to anyone that needs to upload a Skin Png file to Minecraft.

answered Jul 29, 2021 at 22:31

Tyler

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