I'm trying to implement a Push Notification process with return from a Java server for my Android application, but unfortunately I'm not getting it. At first I will run a process on the server and, when necessary this process will send a notification to Android. When you receive the notification on your phone, the user can continue the process in your application.
The point is that the mobile app will not always be open. You can take as an example WhatsApp where the mobile receives the notification without the app is open.
An important factor is that notification will always be sent to a specific user among many, ie it will not be sent to all users.
Please could anyone give me a help how could I proceed to implement this function? I do not know any API or function for these processes and I have no idea how to get started. I did some research, found something, but nothing helped me in this case.
Thank you in advance. Many thanks