firebase (fcm) says 401 unauthorized
<pre><code>private void sendMsg() {
    DBManager dbManager = DBManager.getInstance();
    ArrayList&lt;String&gt; firebaseIds;

    try {
        ResultSet rs= dbManager.getRegisteredFirebaseDevice();
        while(rs.next()){
            System.out.println(rs.getString(1));
            firebaseIds.add(rs.getString(1));
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }

    String url = "https://fcm.googleapis.com/fcm/send";
    URL obj = new URL(url);
    HttpURLConnection con = (HttpURLConnection) obj.openConnection();

    // add reuqest header
    con.setRequestMethod("POST");
    con.setRequestProperty("Authorization: key", "AIzaSyAl6S936qt_NKKFwwbd-NEmiSGIL7G_yJc");
    con.setRequestProperty("Content-Type", "application/json");
    // String msg="New design added in "+getCategory(designCategory)+". Design no."+designNo;
    // String urlParameters = "data.msg="+msg+"&amp;registration_id="+firebaseIds.get(0);

    JSONObject msg=new JSONObject();
    msg.put("msg","New design added in "+getCategory(designCategory)+". Design no."+designNo);


    JSONObject parent=new JSONObject();

    parent.put("to", firebaseIds.get(0));
    parent.put("data", msg);

    // String urlParameters = "registration_id="+firebaseIds.get(0);
    // Send post request
    con.setDoOutput(true);


    OutputStreamWriter wr= new OutputStreamWriter(con.getOutputStream());
    wr.write(parent.toString());

    // DataOutputStream wr = new DataOutputStream(con.getOutputStream());
    // wr.writeBytes(urlParameters);
    // wr.flush();
    // wr.close();

    int responseCode = con.getResponseCode();
    System.out.println("\nSending 'POST' request to URL : " + url);
    System.out.println("Post parameters : " + parent.toString());
    System.out.println("Response Code : " + responseCode+" "+con.getResponseMessage());

}
</code></pre>

<p>When I call the above code, it gives me response as <code>401 Unauthorized</code>. I am not able to understand why I am getting this error. I have used proper server key. Is there any syntax error or anything wrong in the strategy used by me. </p>

<p>I have followed the <a href="https://firebase.google.com/docs/cloud-messaging/server#implementing-http-connection-server-protocol" rel="noreferrer">https://firebase.google.com/docs/cloud-messaging/server#implementing-http-connection-server-protocol</a> documentation</p>