Java - Hosting a file via server/client architecture

If you are willing to obtain a code signing certificate, you could use WebStart

1. Check Java WebStart - this covers all your needs regarding checking and downloading.

2. Build a simple webapp that contains / hosts two things: Your .jnlp (Web Start descriptor file) and your game .jar

3. Sign your jar using a valid code signing certificate (this is required for webstart)

4. Deploy your webapp or jnlp/jar combination to whatever webserver you can get a hold of. As you're only uploading two files, there are a lot of "free" webhosting sites where you can upload some files.

If you really, really like to run the server for the downloads yourself, you should possibly check out Tomcat or Jetty or run Java SE HttpServer like this:

    public static void main(String[] args) throws Exception {
        HttpServer server = HttpServer.create(new InetSocketAddress(8000), 0);
        server.createContext("/game.jnlp", new FileHandler("game.jnlp"));
        server.createContext("/game.jar", new FileHandler("game.jar"));
        server.setExecutor(null); 
        server.start();
    }

    static class FileHandler implements HttpHandler {
        String file = "";
        public FileHandler(String string) {
            file = string;
        }
        @Override
        public void handle(HttpExchange t) throws IOException {
            byte[] data = IOUtils.toByteArray(getClass().getResourceAsStream(file));
            t.sendResponseHeaders(200, data.length);
            OutputStream os = t.getResponseBody();
            os.write(data);
            os.close();
        }
    }

If you believe Webstart is overkill, which I'd understand in your situation, you would need two jar files in the client: One to check and do the download of the other and then start the newly loaded file up. Sequence of steps would be:

• Check if .jar is reachable by network.
• If it is, load it and store it in defined path
• Start a new java Process (-> ProcessBuilder) to run that java-program
• Wait for game to finish (downloader will finish right after)

In that case your server would only need to host the game.jar