My nodelet code is

PLUGINLIB_EXPORT_CLASS(example_pkg::MyNodeletClass, nodelet::Nodelet)

namespace example_pkg
{
    void MyNodeletClass::messageCb(const std_msgs::UInt8::ConstPtr& msg)
    {
        ROS_INFO("NODE 1 CALLBACK");
    }   
    void MyNodeletClass::onInit()
    {   
        ros::NodeHandle n = getMTPrivateNodeHandle();
        ROS_INFO("NODELET 1 INITIALIZATION");
        n.subscribe("msg_in", 10, &MyNodeletClass::messageCb, this);

    }

}

and my main node code is

int main(int argc, char **argv)
{
  ros::init(argc, argv, "main");
  nodelet::M_string remap(ros::names::getRemappings());
  nodelet::V_string nargv;
  nodelet::Loader nodelet1;
  nodelet1.load("one", "example_pkg/MyNodeletClass", remap, nargv);
  ROS_INFO("I AM HERE");
  ros::spin();
  return 0;
}

After I run the main code, I do rostopic pub msgin stdmsgs/UInt8 1 but the output remains

[ INFO] [1542015803.740819742]: Initializing nodelet with 6 worker threads.
[ INFO] [1542015803.789365470]: INITIALIZATION OF NODE 1 COMPLETE
[ INFO] [1542015803.821358176]: I AM HERE

I have also tried to use a launch file as below

<launch>
<node pkg="nodelet" type="nodelet" name="standalone_nodelet"  args="manager" output="screen"
></node>
<node pkg="nodelet" type="nodelet" name="MyNodeletClass" args="load example_pkg/MyNodeletClass standalone_nodelet" output="screen"></node>
</launch>

and after launching that, I run the same rostopic pub msgin stdmsgs/UInt8 1 command and get the following output

[ INFO] [1542019185.420793556]: Initializing nodelet with 6 worker threads.
[ INFO] [1542019185.440188632]: NODELET 1 INITIALIZATION

Why isn't the nodelet going into callback? I am not supposed to put ros::spin() in the onInit() function of the nodelet right? Thanks in advance.