This will show u how to send the messages

MessagingEndpoint.java

/*
   For step-by-step instructions on connecting your Android application to this backend module,
   see "App Engine Backend with Google Cloud Messaging" template documentation at
   https://github.com/GoogleCloudPlatform/gradle-appengine-templates/tree/master/GcmEndpoints
*/

package com.dotdex.squattn;

import com.dotdex.squattn.models.RegistrationRecord;
import com.dotdex.squattn.util.Utility;
import com.google.android.gcm.server.Constants;
import com.google.android.gcm.server.Message;
import com.google.android.gcm.server.Result;
import com.google.android.gcm.server.Sender;
import com.google.api.server.spi.config.Api;
import com.google.api.server.spi.config.ApiMethod;
import com.google.api.server.spi.config.ApiNamespace;

import java.io.IOException;
import java.util.List;
import java.util.logging.Logger;

import javax.inject.Named;

import static com.dotdex.squattn.OfyService.ofy;


/**
 * An endpoint to send messages to devices registered with the backend
 * <p/>
 * For more information, see
 * https://developers.google.com/appengine/docs/java/endpoints/
 * <p/>
 * NOTE: This endpoint does not use any form of authorization or
 * authentication! If this app is deployed, anyone can access this endpoint! If
 * you'd like to add authentication, take a look at the documentation.
 */
@Api(
  name = "messaging",
  version = "v1",
  namespace = @ApiNamespace(
    ownerDomain = "squattn.dotdex.com",
    ownerName = "squattn.dotdex.com",
    packagePath=""
  )
)
public class MessagingEndpoint {
    private static final Logger log = Logger.getLogger(MessagingEndpoint.class.getName());

    /** Api Keys can be obtained from the google cloud console */
    private static final String API_KEY = System.getProperty("gcm.api.key");

    /**
     * Send to the first 10 devices (You can modify this to send to any number of devices or a specific device)
     *
     * @param message The message to send
     */
    @ApiMethod(name = "sendMessage")
    public void sendMessage(
            @Named("message") String message
    ) throws IOException {
        if(message == null || message.trim().length() == 0) {
            log.warning("Not sending message because it is empty");
            return;
        }
        // crop longer messages
        if (message.length() > 1000) {
            message = message.substring(0, 1000) + "[...]";
        }
        Sender sender = new Sender(API_KEY);
        Message msg = new Message.Builder()
                .addData("type",Utility.PUSH_NOTIFICATION)
                .addData("message", message)
                .build();
        List<RegistrationRecord> records = ofy().load().type(RegistrationRecord.class).limit(100).list();
        for(RegistrationRecord record : records) {
            Result result = sender.send(msg, record.getRegId(), 5);
            if (result.getMessageId() != null) {
                log.info("Message sent to " + record.getRegId());
                String canonicalRegId = result.getCanonicalRegistrationId();
                if (canonicalRegId != null) {
                    // if the regId changed, we have to update the data store
                    log.info("Registration Id changed for " + record.getRegId() + " updating to " + canonicalRegId);
                    record.setRegId(canonicalRegId);
                    ofy().save().entity(record).now();
                }
            } else {
                String error = result.getErrorCodeName();
                if (error.equals(Constants.ERROR_NOT_REGISTERED)) {
                    log.warning("Registration Id " + record.getRegId() + " no longer registered with GCM, removing from data store");
                    // if the device is no longer registered with Gcm, remove it from the datastore
                    ofy().delete().entity(record).now();
                }
                else {
                    log.warning("Error when sending message : " + error);
                }
            }
        }
    }


    /**
     * Send a notification to the owner of the space
     *
     * @param name The message to send
     */

    @ApiMethod(name = "sendRequestNotification")
    public void sendRequestNotification(
            @Named("requesterName") String name,
            @Named("requestFrom") String requestFrom,
            @Named("requestKey") String requestKey,
            @Named("requestTo") String requestTo,
            @Named("spaceKey") String spaceKey,
            @Named("spaceNum") String spaceNum,
            @Named("spaceLoc") String spaceLoc
    ) throws IOException{
        Sender sender = new Sender(API_KEY);
        String big_msg = name +" wants to rent your parking space located @ "+spaceLoc+" with SQN Number  *"+spaceNum+"*";
        String small_msg = name + " wants to rent your parking space!";
        Message msg = new Message.Builder()
                .addData("type", Utility.GCM_MSG_TYPE_NOTIFICATION)
                .addData("small_msg",small_msg)
                .addData("big_msg", big_msg)
                .addData("name",name)
                .addData("request_from",requestFrom)
                .addData("request_to",requestTo)
                .addData("space_key",spaceKey)
                .addData("space_num",spaceNum)
                .addData("space_loc",spaceLoc)
                .addData("request_key",requestKey)
                .build();
        RegistrationRecord record = ofy().load().type(RegistrationRecord.class).filter("uId ==", requestTo).first().now();
            Result result = sender.send(msg, record.getRegId(), 5);

            if (result.getMessageId() != null) {
                log.info("Message sent to " + record.getRegId());
                String canonicalRegId = result.getCanonicalRegistrationId();
                if (canonicalRegId != null) {
                    // if the regId changed, we have to update the data store
                    log.info("Registration Id changed for " + record.getRegId() + " updating to " + canonicalRegId);
                    record.setRegId(canonicalRegId);
                    ofy().save().entity(record).now();
                }
            } else {
                String error = result.getErrorCodeName();
                if (error.equals(Constants.ERROR_NOT_REGISTERED)) {
                    log.warning("Registration Id " + record.getRegId() + " no longer registered with GCM, removing from data store");
                    // if the device is no longer registered with Gcm, remove it from the data store
                    ofy().delete().entity(record).now();
                } else {
                    log.warning("Error when sending message : " + error);
                }
            }
        }

    @ApiMethod(name = "sendRequestConfirmationNotification")
    public void sendRequestConfirmationNotification(
                                        @Named("requesterName") String name,
                                        @Named("requestFrom") String requestFrom,
                                        @Named("requestKey") String requestKey,
                                        @Named("requestTo") String requestTo,
                                        @Named("spaceKey") String spaceKey,
                                        @Named("spaceNum") String spaceNum,
                                        @Named("spaceLoc") String spaceLoc,
                                        @Named("confirmationType") String type
    ) throws IOException {
        Sender sender = new Sender(API_KEY);
        StringBuilder msgBuilder = new StringBuilder();
        msgBuilder.append(name + ", Your request to rent a  parking space located @ " + spaceLoc + " with SQN Number  *" + spaceNum + "* " +
                "has being");
        String small_msg = "";
        if (type.equals(Utility.CONFIRMATION_TYPE_APPROVED))
        {
            msgBuilder.append(" approved by the space owner and manager. You can now proceed and make use of the space");
            small_msg = "Your request for a paring space has been approved!";

        }else if (type.equals(Utility.CONFIRMATION_TYPE_DISAPPROVED)){

            msgBuilder.append(" disapproved by the space owner and manager. Please try again!");
            small_msg = " Your request for a paring space has been disapproved!";
        }
        Message msg = new Message.Builder()
                .addData("type", Utility.GCM_MSG_TYPE_APPROVAL_NOTIFICATION)
                .addData("small_msg", small_msg)
                .addData("big_msg", msgBuilder.toString())
//                .addData("requester_name", name)//shall be used to decrease his credit
                .addData("request_from", requestFrom)//shall be used to decrease his credit
                .addData("request_from", requestTo) //shall be used to increment his credit
//                .addData("space_key", spaceKey) //shall be used to to sent the toggle the request status
//                .addData("space_num", spaceNum) //
                .addData("request_key", requestKey)
                .build();
        RegistrationRecord record = ofy().load().type(RegistrationRecord.class).filter("uId ==", requestFrom).first().now();
        Result result = sender.send(msg, record.getRegId(), 5);

        if (result.getMessageId() != null) {
            log.info("Message sent to " + record.getRegId());
            String canonicalRegId = result.getCanonicalRegistrationId();
            if (canonicalRegId != null) {
                // if the regId changed, we have to update the data store
                log.info("Registration Id changed for " + record.getRegId() + " updating to " + canonicalRegId);
                record.setRegId(canonicalRegId);
                ofy().save().entity(record).now();
            }
        } else {
            String error = result.getErrorCodeName();
            if (error.equals(Constants.ERROR_NOT_REGISTERED)) {
                log.warning("Registration Id " + record.getRegId() + " no longer registered with GCM, removing from datastore");
                // if the device is no longer registered with Gcm, remove it from the datastore
                ofy().delete().entity(record).now();
            } else {
                log.warning("Error when sending message : " + error);
            }
        }
    }

}