Cannot get two android emulators connectable

I create one socket server on AE1, and then create a client on AE2. I want to access the server on AE1 from AE2. So I follow the introduction. And still cannot get it work.

I understand that I should let the server in AE1 listen on 10.0.2.15:8080, and then I use redir add tcp:8080:8080 command in AE1's console to redirect all tcp connections to localhost:8080 to a private address 10.0.2.15:8080 which is in AE1. And in AE2, I should let AE2 connect to 10.0.2.2:8080 which is actually the localhost of host machine.

I understand this structure, but not matter how I try, I just cannot make it work. Don't know what's wrong with it. Further, I cannot even access the server in AE1 from my host machine. I guess I just haven't really redirect the localhost:8080 to 10.0.2.15:8080 successfully.

My OS is: Linux Mint 19

Android SDK: 28

And help is apperciated!

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