Activity.java

public class HavenCoreActivity {
    public static Core core;
    public static Activity activity;
    private static Thread callbackRunner;

    public static Thread getCallbackRunner() {
        return callbackRunner;
    }

    public static void start() {
        File discordLibrary;
        try {
            discordLibrary = downloadDiscordLibrary();
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        if (discordLibrary == null) {
            System.err.println("Error downloading Discord SDK.");
            // System.exit(-1);
            FMLCommonHandler.instance().exitJava(-1, false);
        }
        // Initialize the Core
        Core.init(discordLibrary);
        initPresence();
        if (callbackRunner == null) {
            callbackRunner = new Thread(
                    () -> {
                        while (!Thread.currentThread().isInterrupted()) {
                            core.runCallbacks();
                            try {
                                Thread.sleep(100 + new Random().nextInt(50));
                            } catch (InterruptedException localInterruptedException) {
                                throw new RuntimeException(localInterruptedException);
                            }
                        }
                    },
                    "RPC-Callback-Handler");
        }
        callbackRunner.start();
    }

    public static void initPresence() {
        CreateParams params = new CreateParams();
        params.setClientID(788530816740753459L);
        params.setFlags(CreateParams.getDefaultFlags());
        // Create the Core
        core = new Core(params);

        activity = new Activity();
        activity.setDetails("About to enjoy HavenCore");
        activity.setType(ActivityType.PLAYING);
        // Setting a start time causes an "elapsed" field to appear
        activity.timestamps().setStart(Instant.now());

        // Make a "cool" image show up
        activity.assets().setLargeImage("havencorelogo");

        // Finally, update the current activity to our activity
        core.activityManager().updateActivity(activity);
    }

    public static File downloadDiscordLibrary() throws IOException {
        // Find out which name Discord's library has (.dll for Windows, .so for Linux)
        String name = "discord_game_sdk";
        String suffix;

        String osName = System.getProperty("os.name").toLowerCase(Locale.ROOT);
        String arch = System.getProperty("os.arch").toLowerCase(Locale.ROOT);

        if (osName.contains("windows")) {
            suffix = ".dll";
        } else if (osName.contains("linux")) {
            suffix = ".so";
        } else if (osName.contains("mac os")) {
            suffix = ".dylib";
        } else {
            throw new RuntimeException("cannot determine OS type: " + osName);
        }

        /*
        Some systems report "amd64" (e.g. Windows and Linux), some "x86_64" (e.g. Mac OS).
        At this point we need the "x86_64" version, as this one is used in the ZIP.
         */
        if (arch.equals("amd64")) arch = "x86_64";

        // Path of Discord's library inside the ZIP
        String zipPath = "lib/" + arch + "/" + name + suffix;

        // Open the URL as a ZipInputStream
        URL downloadUrl = new URL("https://dl-game-sdk.discordapp.net/2.5.6/discord_game_sdk.zip");
        HttpURLConnection connection = (HttpURLConnection) downloadUrl.openConnection();
        connection.setRequestProperty(
                "User-Agent", "discord-game-sdk4j (https://github.com/JnCrMx/discord-game-sdk4j)");
        ZipInputStream zin = new ZipInputStream(connection.getInputStream());

        // Search for the right file inside the ZIP
        ZipEntry entry;
        while ((entry = zin.getNextEntry()) != null) {
            if (entry.getName().equals(zipPath)) {
                // Create a new temporary directory
                // We need to do this, because we may not change the filename on Windows
                File tempDir = new File(System.getProperty("java.io.tmpdir") + "HavenCorePublic-"
                        + "java-" + name + System.nanoTime());
                if (!tempDir.mkdir()) throw new IOException("Cannot create temporary directory");
                tempDir.deleteOnExit();

                // Create a temporary file inside our directory (with a "normal" name)
                File temp = new File(tempDir, name + suffix);
                temp.deleteOnExit();

                // Copy the file in the ZIP to our temporary file
                Files.copy(zin, temp.toPath());

                // We are done, so close the input stream
                zin.close();

                // Return our temporary file
                return temp;
            }
            // next entry
            zin.closeEntry();
        }
        zin.close();
        // We couldn't find the library inside the ZIP
        return null;
    }
}