This could be caused by incorrect network configuration. The android emulator instances use a specific network setup, and iirc you'll need to change that, or at least keep it in mind.

Thanks a lot for the answer, but I'm not sure that is a network problem. If it was a network configuration problem, the app should not even connect with rosmaster. Instead the app connects to rosmaster normally.

is your ros master also running on your PC? Or does the rosjava app start it? How is the ros master uri configured in your rosjava app? If it's set to localhost it should try to connect to the localhost of the android device. It should be set to the IP of the host PC. Don't change localhost.

Master and server running on PC. Also the client, but using an emulator. Initially i start to run the master and the server. After this I start to run the client, then the client connects to the master using localhost(it works fine). The connection with server failed instead (also using localhost).

How did you verify that the connection from client -> master is ok? Can you rosnode ping .. or rosnode info .. the node on the android device?

I cannot call these commands from the android device, but the connection between client and master is ok, I'm sure. If the connection between client and master fails the app stops the execution.

I know you can't use those commands on the android device, but you can run them on the host, in a terminal. It would tell us if there are any basic networking issues.

If I run rosnode list from my desktop, I see all three nodes (client, server, master), everything works fine between the client try to connect the server. If I use the node rosnode info .. i see all the nodes. But i cannot connect to client (because i cannot connect with the android device).

I tried to run the client application on a real android embedded device, but i received the same error (Connection refused). I connected the Client with the Master using a wifi connection, and it works. But after that the client tries to connect with the localhost and not the real server address.

So what is the value of ROS_MASTER_URI on the client device? You should set it to the IP of the host that is running your ROS master. Also: make sure that your PC is reporting its IP and not its hostname, unless you have working DNS for your PC and android device.

The ROS_MASTER_URI on the client device was correct? The problem was ROS_MASTER_URI on the server, it was set to localhost. I corrected this error and now the application works correctly. Thanks a lot for your help!!

So just for my own understanding: what is not the value of ROS_MASTER_URI on the android device, and what is it on the PC?

The problem was the ROS_IP variable of the Client, it was set to localhost. But the client device has 10.0.2.2 as localhost (android emulator). So when I tried to contact the server, the connection failed because the system saw two different networks, although I used the same PC.