service和activity怎样通过handler双向通信？

handler = new Handler(handlerThread.getLooper()){
            @Override
            public void handleMessage(Message msg) {
                if(msg.what == 0x00){
                    mActivityMessenger = msg.replyTo;
                    Message message = this.obtainMessage();
                    message.what = 0x12;
                    message.arg2 = 234;
                    try {
                        mActivityMessenger.send(message);
                    } catch (RemoteException e) {
                        e.printStackTrace();
                    }
                }
这是service中的代码， mActivityMessenger = msg.replyTo;获取activity中的messager  我怎样能让 mActivityMessenger.send(message);可以在其它地方随意使用呢？上面这段程序能够正常发送到activity，但我在其它地方通过 mActivityMessenger.send(message);发送消息就不行，求大神指教