Using RestFB along side a JavaFX WebView, how can I post to Facebook?

Question

I've been trying a number of ways to achieve a way to post to Facebook from within my JavaFX application using a WebView to manage login.

I have read Facebook's documentation but continue to get lost with user_key, codes, access_code, tokens and what not.

What I have so far is this:

My WebView loads this URL.

public static String userLogin() {
    String state = UUID.randomUUID().toString();
    return String.format(AUTHORIZATION_URL, CLIENT_ID, REDIRECT_URI, state);
}

User logs in and authorises my Facebook application and I capture the response

System.out.println("Response: " + response);
    if (response.contains("code=")) {
        String[] finalCode = response.split("code=");
        Facebook.CODE = finalCode[1];
        System.out.println("Access Key:" + finalCode[1]);
    }
}

My webview displays the following:

SECURITY WARNING: Please treat the URL above as you would your password and do not share it with anyone. See the Facebook Help Centre for more information.

I also see my code returned: https://www.facebook.com/connect/login_success.html?code=AQA4w...

I then try to get my User Token, by directing my webView here:

public static String getUserToken() {
    return String.format(ACCESS_TOKEN_URL, CLIENT_ID, REDIRECT_URI, CLIENT_SECRET, CODE);
}

At which point my webview displays the following:

{
   "error": {
      "message": "Missing authorization code",
      "type": "OAuthException",
      "code": 1,
      "fbtrace_id": "D/0gXWRYkx4"
   }
}

How can I solve this?