我解决了这个问题，可能对某人有帮助，

I solved this problem may it help someone ,

要通过api将Firebase的通知发送到单个设备，如下所示:

In order to send Notification of firebase through api to single device is as follows:

1:为每个设备生成唯一的实例ID

public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {
private static final String TAG = "MyFirebaseIIDService";

@Override
public void onTokenRefresh() {

    //Getting registration token
    String refreshedToken = FirebaseInstanceId.getInstance().getToken();

    //Displaying token on logcat
    Log.d(TAG, "Refreshed token: " + refreshedToken);

    sendRegistrationToServer(refreshedToken);

}

private void sendRegistrationToServer(String token) {
    //You can implement this method to store the token on your server
    //Not required for current project




}

}

注意:您可以从应用程序中的任何位置调用实例ID，如下所示:

Note: you can call Instance Id from any where in your Application Like this:

String tkn = FirebaseInstanceId.getInstance().getToken();
        Toast.makeText(Doc_users.this, "Current token ["+tkn+"]",
                Toast.LENGTH_LONG).show();
        Log.d("App", "Token ["+tkn+"]");

此令牌必须用于将通知发送到单个设备，它是您可以将其存储在Firebase数据库中的设备的ID

this token must be used to send notification to single device it is the id of the device you can store it in your firebase database

 reference.child(UserDetails.username).child("token").setValue(tokun);

2:现在，您可以构建逻辑来获取这些ID并构建您的请求

public static String makeRequest(String id) throws JSONException {
        HttpURLConnection urlConnection;
        JSONObject json = new JSONObject();
        JSONObject info = new JSONObject();
        info.put("title", "Notification Title");   // Notification title
        info.put("body", "Notification body"); // Notification body
        info.put("sound", "mySound"); // Notification sound
        json.put("notification", info);
        json.put("to","INSTANCE ID FETCHED FOR SIGNLE DEVICE HERE");
        Log.e("deviceidkey==> ",id+"");
        Log.e("jsonn==> ",json.toString());
        String data = json.toString();
        String result = null;
        try {
            //Connect
            urlConnection = (HttpURLConnection) ((new URL("https://fcm.googleapis.com/fcm/send").openConnection()));
            urlConnection.setDoOutput(true);
            urlConnection.setRequestProperty("Content-Type", "application/json");
            urlConnection.setRequestProperty("Authorization", "key=YOUR FIREBASE SERVER KEY");
            urlConnection.setRequestMethod("POST");
            urlConnection.connect();

            //Write
            OutputStream outputStream = urlConnection.getOutputStream();
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
            writer.write(data);
            writer.close();
            outputStream.close();

            //Read
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream(), "UTF-8"));

            String line = null;
            StringBuilder sb = new StringBuilder();

            while ((line = bufferedReader.readLine()) != null) {
                sb.append(line);
            }

            bufferedReader.close();
            result = sb.toString();

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return result;
    }

通知会在从您的设备发送的那一台设备上生成，您可以调试以放置自己的设备密钥并使用邮递员进行有效的请求:祝您好运

Notification will generate on that single device sent from your device you can debug to put your own device key and use postman for valid request: Good Luck