Firebase Notifications (FCM) sending using Spring boot

Use Spring's built in RestTemplate REST abstraction to simplify it.
The FireBasePost object just contains the required fields for the POST Message API: Firebase API- and I have verified the request Entity works by posting with String.class, so the response is unmarshalled JSON.

-----------------response object-----------
package Your package;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class FirebaseResponse {
    private long multicast_id;
    private Integer success;
    private Integer failure;
    private Object canonical_ids;
    private long message_id;
    public FirebaseResponse() {
    }

    public long getMulticast_id() {
        return multicast_id;
    }

    public void setMulticast_id(long multicast_id) {
        this.multicast_id = multicast_id;
    }

    public Integer getSuccess() {
        return success;
    }

    public void setSuccess(Integer success) {
        this.success = success;
    }

    public Integer getFailure() {
        return failure;
    }

    public void setFailure(Integer failure) {
        this.failure = failure;
    }

    public Object getCanonical_ids() {
        return canonical_ids;
    }

    public void setCanonical_ids(Object canonical_ids) {
        this.canonical_ids = canonical_ids;
    }

    public long getMessage_id() {
        return message_id;
    }

    public void setMessage_id(long message_id) {
        this.message_id = message_id;
    }

    @Override
    public String toString() {
        return "FirebaseResponse [multicast_id=" + multicast_id + ", success=" + success + ", failure=" + failure
                + ", canonical_ids=" + canonical_ids + ", message_id=" + message_id + "]";
    }

   
}
---------------------------------Usage------------------------------------------------------
package Your package;

import java.util.ArrayList;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.http.HttpEntity;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.stereotype.Component;
import org.springframework.web.client.RestTemplate;

import com.conduiraonline.objects.FirebaseResponse;
import com.conduiraonline.objects.HeaderRequestInterceptor;
import com.google.gson.Gson;

@Component
public class FirebaseService {
    private static final Logger logger = LoggerFactory.getLogger(FirebaseService.class);
    private static final String firebaseUrl = "https://fcm.googleapis.com/fcm/send";
    private static RestTemplate restTemplate = new RestTemplate();
    private final String FIREBASE_SERVER_KEY = "Firebase server key";
   
    public final static String appTestingTopic = "/topics/App-Testing";
     
    public String sendNotification(){

        restTemplate.setInterceptors(getInterceptors());

             JSONObject body = new JSONObject();
            // JsonArray registration_ids = new JsonArray();
            // body.put("registration_ids", registration_ids);
            body.put("to", "cfW930CZxxxxxxxxxxxxxxxxxxxxxxxxxxipdO-bjHLacHRqQzC0aSXlRFKdMHv_aNBxkRZLNxxxxxxxxxxx59sPW4Rw-5MtwKkZxxxxxxxgXlL-LliJuujPwZpLgLpji_");
           //or some topic
           //body.put("to"appTestingTopic , );  
          body.put("priority", "high");
            // body.put("dry_run", true);

            JSONObject notification = new JSONObject();
            notification.put("body", "body string here");
            notification.put("title", "title string here");
            // notification.put("icon", "myicon");

            JSONObject data = new JSONObject();
            data.put("key1", "value1");
            data.put("key2", "value2");

            body.put("notification", notification);
            body.put("data", data);
            HttpEntity<String> request = new HttpEntity<>(body.toString());

        FirebaseResponse firebaseResponse = restTemplate.postForObject(firebaseUrl, request, FirebaseResponse.class);
        logger.info("response is: " + firebaseResponse.toString());
       
        return firebaseResponse.getMessage_id()+"";
    }
   
    public ArrayList<ClientHttpRequestInterceptor> getInterceptors(){
        ArrayList<ClientHttpRequestInterceptor> interceptors = new ArrayList<>();
        interceptors.add(new HeaderRequestInterceptor("Authorization", "key=" + FIREBASE_SERVER_KEY));
        interceptors.add(new HeaderRequestInterceptor("Content-Type", "application/json"));
        return interceptors;
    }
}