How to properly design the server?

Question

Java

Hello!
I chose Java as a server part for the application (I am writing for self-study) and I do not quite understand which technology. choose.
The essence of the project:
There are many modules - we can consider that this is a mini program that runs on the server, and the result is visible on the client + the module of one user can access the methods of the module of another user through the server.
1) The clients themselves are android + web.
2) Each has its own set of modules, its own settings. All this is stored in the database.
It turns out that there is something simple here: the client asked the server to give him some JSON data.
There are situations where one client wants to contact another through the server: That is, the server must be able to notify clients.
But I have doubts from a misunderstanding of the issue of server varieties. As I understand it, there are 2 options for mutual exchange: the client constantly listens to the server and the server itself notifies the client.
The second option is preferable, but as I understand it, it requires a permanent connection, which is not very good.
As a result, we got a number of requirements for the server (correct if I'm wrong):

• Receive command and response in JSON format
  
• Processing modules in threads in "user space" (Storing an active list of users, modules and settings)
  
• Notification of clients at the request of modules (for example, notification at a certain time)
  
• All this in multi-threaded mode (Many requests - many answers)
  
• Some kind of protection against left requests
  

Can you tell me what is the best way to use this?