On Ubuntu, on fresh installs, I frequently tend to get UnknownHostException(s) while running Java such as below.
INFO: I/O exception (java.net.UnknownHostException) caught when processing request: repository.springsource.com.s3.amazonaws.com
This normally happens for me when running 32-bit Java on 64-bit Ubuntu. The solution is simply to install lib32nss-mdns as follows and then retry.
sudo apt-get install lib32nss-mdns