java.net.UnknownHostException: Invalid hostname for server: local

What the exception is really saying is that there is no known server with the name "local". My guess is that you're trying to connect to your local computer. Try with the hostname "localhost" instead, or perhaps 127.0.0.1 or ::1 (the last one is IPv6).

From the javadocs:

Thrown to indicate that the IP address of a host could not be determined.

127.0.0.1or ::1 or "localhost" should always be the loopback interface, so if that doesn't work I'd be really surprised.

If there really is a server called "local" on your network - examine your DNS settings or add it to your hosts file.

I was having the same issue on my mac. I found the issue when I pinged my $HOSTNAME from terminal and it returned ping: cannot resolve myHostName: Unknown host.

To resolve:

1. Do echo $HOSTNAME on your terminal.

2. Whatever hostname it shows (lets say myHostName), try to ping it : ping myHostName. If it returns ping: cannot resolve myHostName: Unknown host then add an entry into your /etc/hosts file.

3. For that edit /etc/hosts file and add following:

   127.0.0.1 myHostName

java.net.UnknownHostException: Host is unresolved:

Thrown to indicate that the IP address of a host could not be determined.

This exception is also raised when you are connected to a valid wifi but router does not receive the internet. Its very easy to reproduce this:

1. Connect to a valid wifi
2. Now remove the cable from the router while router is pluged-in

You will observe this error!!

You can't really solve this, You can only notify the user gracefully. (something like - "Unable to make a connection")