如何通过java服务器使用FCM向Android设备发送推送通知？

我用Java服务器代码编写了使用fcm发送通知的代码。但是它是抛出异常服务器返回的HTTP响应代码：500的URL： https：//fcm.googleapis .com / fcm / send

I have write this code for send notification using fcm by java server code. But it is throwing exception Server returned HTTP response code: 500 for URL: https://fcm.googleapis.com/fcm/send

public static void pushFCMNotification(String userDeviceIdKey) throws     Exception{

    String authKey = AUTH_KEY_FCM;   // You FCM AUTH key
    String FMCurl = API_URL_FCM;     

    URL url = new URL(FMCurl);
    HttpURLConnection conn = (HttpURLConnection) url.openConnection();

    conn.setUseCaches(false);
    conn.setDoInput(true);
    conn.setDoOutput(true);

    conn.setRequestMethod("POST");
    conn.setRequestProperty("Authorization","key="+authKey);
    conn.setRequestProperty("Content-Type","application/json");

    JSONObject json = new JSONObject();
    json.put("to",userDeviceIdKey.trim());
    JSONObject info = new JSONObject();
    info.put("title", "Notificatoin Title");   // Notification title
    info.put("body", "Hello Test notification"); // Notification body
    json.put("notification", info);

    OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
    wr.write(json.toString());
        wr.flush();
        wr.close();

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

        BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
        String inputLine;
        StringBuffer response = new StringBuffer();

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();


    /*OutputStreamWriter wr = new OutputStreamWriter(conn.getOutputStream());
    wr.write(json.toString());
    wr.flush();
    conn.getInputStream();*/
    }