Firebase有两个向设备发送消息的功能：

• Firebase控制台中的"通知"面板允许您向特定设备、用户组或用户订阅的主题发送通知。
• 通过调用Firebase Cloud Messaging API，您可以使用您喜欢的任何定位策略发送消息。调用FCM API需要访问您的服务器密钥，您不应在客户端设备上暴露该密钥。这就是您应始终在应用服务器上运行此类代码的原因。

Firebase documentation直观地显示了这一点：

Firebase Cloud Messaging客户端SDK不支持将消息从一台设备直接发送到另一台设备。

• • 更新**：我写了一篇详细介绍how to send notifications between Android devices using Firebase Database, Cloud Messaging and Node.js的博客文章。
• • 更新2**：现在，您还可以使用Cloud Functions for Firebase安全地发送消息，而无需启动服务器。请参阅此示例用例以开始使用。如果您不想使用云函数，您可以在任何已拥有的可信环境（如您的开发计算机或您控制的服务器）上运行相同的逻辑。

• • 警告**我们没有在任何地方提到这种方法有一个非常重要的原因。这会暴露您放置在每个客户端设备上的APK中的服务器密钥。它可以（因此将）从那里被拿走，并可能导致滥用您的项目。我强烈建议不要采取这种方法，除非您只放置在自己的设备上。-Frank van Puffelen

Frank的回答是正确的，Firebase本身不支持设备到设备的消息传递，但是其中有一个漏洞。Firebase服务器无法识别您是从实际的服务器发送请求还是从您的设备发送请求。
因此，您只需将Post Request与服务器密钥一起发送到Firebase的消息传递服务器。请记住，服务器密钥不应位于设备上，但如果您希望使用Firebase Messaging进行设备到设备的消息传递，则没有其他选项。
我使用OkHTTP而不是默认的方式调用Rest API。代码如下所示-

public static final String FCM_MESSAGE_URL = "https://fcm.googleapis.com/fcm/send";
OkHttpClient mClient = new OkHttpClient();
public void sendMessage(final JSONArray recipients, final String title, final String body, final String icon, final String message) {

        new AsyncTask<String, String, String>() {
            @Override
            protected String doInBackground(String... params) {
                try {
                    JSONObject root = new JSONObject();
                    JSONObject notification = new JSONObject();
                    notification.put("body", body);
                    notification.put("title", title);
                    notification.put("icon", icon);

                    JSONObject data = new JSONObject();
                    data.put("message", message);
                    root.put("notification", notification);
                    root.put("data", data);
                    root.put("registration_ids", recipients);

                    String result = postToFCM(root.toString());
                    Log.d(TAG, "Result: " + result);
                    return result;
                } catch (Exception ex) {
                    ex.printStackTrace();
                }
                return null;
            }

            @Override
            protected void onPostExecute(String result) {
                try {
                    JSONObject resultJson = new JSONObject(result);
                    int success, failure;
                    success = resultJson.getInt("success");
                    failure = resultJson.getInt("failure");
                    Toast.makeText(getCurrentActivity(), "Message Success: " + success + "Message Failed: " + failure, Toast.LENGTH_LONG).show();
                } catch (JSONException e) {
                    e.printStackTrace();
                    Toast.makeText(getCurrentActivity(), "Message Failed, Unknown error occurred.", Toast.LENGTH_LONG).show();
                }
            }
        }.execute();
    }

String postToFCM(String bodyString) throws IOException {
        RequestBody body = RequestBody.create(JSON, bodyString);
        Request request = new Request.Builder()
                .url(FCM_MESSAGE_URL)
                .post(body)
                .addHeader("Authorization", "key=" + SERVER_KEY)
                .build();
        Response response = mClient.newCall(request).execute();
        return response.body().string();
    }

我希望Firebase将来会有更好的解决方案，但在此之前，我认为这是唯一的方法，另一种方法是发送主题消息或组消息，但这不在问题的范围之内。

• • 更新日期：**

JSONArray的定义如下：

JSONArray regArray = new JSONArray(regIds);

regIds是一个包含注册ID的字符串数组，您希望将此消息发送到这些ID。请记住，注册ID必须始终位于数组中，即使您希望将其发送到单个收件人。

我也一直在我的原型中使用直接设备到设备的gcm消息传递。它一直工作得很好。我们没有任何服务器。我们使用sms/text交换GCM reg id，然后使用GCM进行通信。我在这里放了与GCM处理相关的代码

*发送GCM消息

//Sends gcm message Asynchronously
public class GCM_Sender extends IntentService{
    final String API_KEY = "****************************************";

    //Empty constructor
    public GCM_Sender() {
        super("GCM_Sender");
    }

    //Processes gcm send messages
    @Override
    protected void onHandleIntent(Intent intent) {  

        Log.d("Action Service", "GCM_Sender Service Started");
        //Get message from intent
        String msg = intent.getStringExtra("msg");
        msg =  "\"" + msg + "\"";
        try{
            String ControllerRegistrationId = null;                 
            //Check registration id in db       
            if(RegistrationIdAdapter.getInstance(getApplicationContext()).getRegIds().size() > 0 ) {
                String controllerRegIdArray[] = RegistrationIdAdapter.getInstance(getApplicationContext()).getRegIds().get(1);
                if(controllerRegIdArray.length>0)
                    ControllerRegistrationId = controllerRegIdArray[controllerRegIdArray.length-1];

                if(!ControllerRegistrationId.equalsIgnoreCase("NULL")){
                    // 1. URL
                    URL url = new URL("https://android.googleapis.com/gcm/send");
                    // 2. Open connection
                    HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
                    // 3. Specify POST method
                    urlConnection.setRequestMethod("POST");
                    // 4. Set the headers
                    urlConnection.setRequestProperty("Content-Type", "application/json");
                    urlConnection.setRequestProperty("Authorization", "key=" + API_KEY);
                    urlConnection.setDoOutput(true);
                    // 5. Add JSON data into POST request body
                    JSONObject obj = new JSONObject("{\"time_to_live\": 0,\"delay_while_idle\": true,\"data\":{\"message\":" + msg + "},\"registration_ids\":[" + ControllerRegistrationId + "]}");
                    // 6. Get connection output stream
                    OutputStreamWriter out = new OutputStreamWriter(urlConnection.getOutputStream());
                    out.write(obj.toString());
                    out.close();
                    // 6. Get the response
                    int responseCode = urlConnection.getResponseCode();

                    BufferedReader in = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
                    String inputLine;
                    StringBuffer response = new StringBuffer();
                    while ((inputLine = in.readLine()) != null){
                        response.append(inputLine);
                    }
                    in.close();
                    Log.d("GCM getResponseCode:", new Integer(responseCode).toString());
                }else{
                    Log.d("GCM_Sender:","Field REGISTRATION_TABLE is null");
                }
            }else {
                Log.d("GCM_Sender:","There is no Registration ID in DB ,please sync devices");
            }
        } catch (Exception e) {
            e.printStackTrace();
            //MessageSender.getInstance().sendMessage(msg, Commands.SMS_MESSAGE);
        } 
    }

    //Called when service is no longer alive
    @Override
    public void onDestroy() {
        super.onDestroy();
        //Do a log that GCM_Sender service has been destroyed
        Log.d("Action Service", "GCM_Sender Service Destroyed");
    }
}

*正在接收GCM消息

public class GCM_Receiver extends WakefulBroadcastReceiver {
    public static final String RETRY_ACTION ="com.google.android.c2dm.intent.RETRY";
    public static final String REGISTRATION ="com.google.android.c2dm.intent.REGISTRATION";
    public SharedPreferences preferences;

    //Processes Gcm message .
    @Override
    public void onReceive(Context context, Intent intent) {
        ComponentName comp = new ComponentName(context.getPackageName(),
                GCMNotificationIntentService.class.getName());
        //Start GCMNotificationIntentService to handle gcm message asynchronously
        startWakefulService(context, (intent.setComponent(comp)));
        setResultCode(Activity.RESULT_OK);

        /*//Check if DatabaseService is running .
        if(!DatabaseService.isServiceRunning) {
            Intent dbService = new Intent(context,DatabaseService.class);
            context.startService(dbService);
        }*/
        //Check if action is RETRY_ACTION ,if it is then do gcm registration again .
        if(intent.getAction().equals(RETRY_ACTION)) {
            String registrationId = intent.getStringExtra("registration_id");

            if(TextUtils.isEmpty(registrationId)){
                DeviceRegistrar.getInstance().register(context);
            }else {
                //Save registration id to prefs .
                preferences = PreferenceManager.getDefaultSharedPreferences(context);
                SharedPreferences.Editor editor = preferences.edit();
                editor.putString("BLACKBOX_REG_ID",registrationId);
                editor.commit();
            }
        } else if (intent.getAction().equals(REGISTRATION)) {
        }

    }
}

//Processes gcm messages asynchronously .
public class GCMNotificationIntentService extends IntentService{
    public static final int NOTIFICATION_ID = 1;
    private NotificationManager mNotificationManager;
    String gcmData;
    private final String TAG = "GCMNotificationIntentService";

    //Constructor with super().
    public GCMNotificationIntentService() {
        super("GcmIntentService");
    }

    //Called when startService() is called by its Client .
    //Processes gcm messages .
    @Override
    protected void onHandleIntent(Intent intent) {

        Log.d("GCMNotificationIntentService", "GCMNotificationIntentService Started");
        Bundle extras = intent.getExtras();
        //Get instance of GoogleCloudMessaging .
        GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(this);
        //Get gcm message type .
        String messageType = gcm.getMessageType(intent);

        if (!extras.isEmpty()) {
            if (GoogleCloudMessaging.MESSAGE_TYPE_SEND_ERROR
                    .equals(messageType)) {
                sendNotification("Send error: " + extras.toString());
            } else if (GoogleCloudMessaging.MESSAGE_TYPE_DELETED
                    .equals(messageType)) {
                sendNotification("Deleted messages on server: "
                        + extras.toString());
            } else if (GoogleCloudMessaging.MESSAGE_TYPE_MESSAGE
                    .equals(messageType)) {
                Log.i(TAG, "Completed work @ " + SystemClock.elapsedRealtime());

                gcmData = extras.getString("message");
                Intent actionService = new Intent(getApplicationContext(),Action.class);    
                actionService.putExtra("data", gcmData);
                //start Action service .
                startService(actionService);

                //Show push notification .
                sendNotification("Action: " + gcmData);
                //Process received gcmData.

                Log.d(TAG,"Received Gcm Message from Controller : " + extras.getString("message"));
            }
        }
        GCM_Receiver.completeWakefulIntent(intent);
    }

    //Shows notification on device notification bar .
    private void sendNotification(String msg) {
        mNotificationManager = (NotificationManager) this.getSystemService(Context.NOTIFICATION_SERVICE);
        Intent notificationIntent = new Intent(this, BlackboxStarter.class);
        //Clicking on GCM notification add new layer of app.
        notificationIntent.setFlags( Intent.FLAG_ACTIVITY_CLEAR_TASK | Intent.FLAG_ACTIVITY_NEW_TASK);
        PendingIntent contentIntent = PendingIntent.getActivity(this, 0,notificationIntent, PendingIntent.FLAG_CANCEL_CURRENT);
        NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(
                this).setSmallIcon(R.drawable.gcm_cloud)
                .setContentTitle("Notification from Controller")
                .setStyle(new NotificationCompat.BigTextStyle().bigText(msg))
                .setContentText(msg);
        mBuilder.setContentIntent(contentIntent);
        mNotificationManager.notify(NOTIFICATION_ID, mBuilder.build());
        //Play default notification
        try {
            Uri notification = RingtoneManager.getDefaultUri(RingtoneManager.TYPE_NOTIFICATION);
            Ringtone r = RingtoneManager.getRingtone(getApplicationContext(), notification);
            r.play();
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    //Called when service is no longer be available .
    @Override
    public void onDestroy() {
        // TODO Auto-generated method stub
        super.onDestroy();
        Log.d("GCMNotificationIntentService", "GCMNotificationIntentService Destroyed");
    }

}

根据在October 2, 2018上更新的新文档，您必须发送以下发布请求

https://fcm.googleapis.com/fcm/send
Content-Type:application/json
Authorization:key=AIzaSyZ-1u...0GBYzPu7Udno5aA //Server key

{
    "to": "sent device's registration token",
    "data": {
       "hello": "message from someone",
    }
}

要获取设备的注册令牌，请扩展FirebaseMessagingService并覆盖onNewToken(String token)有关更多信息，请参阅doc https://firebase.google.com/docs/cloud-messaging/android/device-group

我迟到了，但上面的解决方案帮助我写下了这个简单的答案，你可以直接发送你的消息到Android设备从Android应用程序，这里是我做了简单的实现，它对我来说很好.
1.编译android volley库

compile 'com.android.volley:volley:1.0.0'

1.只是复制粘贴这个简单的功能;）你的生活会变得像黄油里的刀一样光滑。

public static void sendPushToSingleInstance(final Context activity, final HashMap dataValue /*your data from the activity*/, final String instanceIdToken /*firebase instance token you will find in documentation that how to get this*/ ) {

final String url = "https://fcm.googleapis.com/fcm/send";
StringRequest myReq = new StringRequest(Request.Method.POST,url,
        new Response.Listener<String>() {
            @Override
            public void onResponse(String response) {
                Toast.makeText(activity, "Bingo Success", Toast.LENGTH_SHORT).show();
            }
        },
        new Response.ErrorListener() {
            @Override
            public void onErrorResponse(VolleyError error) {
                Toast.makeText(activity, "Oops error", Toast.LENGTH_SHORT).show();
            }
        }) {

    @Override
    public byte[] getBody() throws com.android.volley.AuthFailureError {
        Map<String, Object> rawParameters = new Hashtable();
        rawParameters.put("data", new JSONObject(dataValue));
        rawParameters.put("to", instanceIdToken);
        return new JSONObject(rawParameters).toString().getBytes();
    };

    public String getBodyContentType()
    {
        return "application/json; charset=utf-8";
    }
    @Override
    public Map<String, String> getHeaders() throws AuthFailureError {
        HashMap<String, String> headers = new HashMap<String, String>();
        headers.put("Authorization", "key="+YOUR_LEGACY_SERVER_KEY_FROM_FIREBASE_CONSOLE);
        headers.put("Content-Type","application/json");
        return headers;
    }

};

Volley.newRequestQueue(activity).add(myReq);
}

注意如果要将消息发送到主题，以便可以将参数instanceIdToken更改为类似**/topics/topicName的内容。对于组**，实现是相同的，但您只需要注意参数。checkout Firebase documentation and you can pass those parameters.如果遇到任何问题，请告诉我。