Suggestion:

Improve log message to include

• the host name that was attempted in lookup
• The error code/message
• Suggested course of action, like "Check DNS / name resolution"

These will assist the user in isolating and resolving the root causes.

@koobs we show java.net.UnknownHostException in log, that will give us enough information about problem. IMHO it should be a special article on a website that will show jabref users how to solve such a problem. Also, I don't see any warning message mechanism

Like the idea, an error message could be indeed improved. Regarding info, I would link to a help page under https://docs.jabref.org/

Please have a look at the checkstyle issues. You can import JabRef's code style into intellij and then use the formatter
https://devdocs.jabref.org/getting-into-the-code/guidelines-for-setting-up-a-local-workspace#using-jabrefs-code-style

Thanks for your contribution. Issues by Tobias Diez have been resolved. Checks are green. Code looks good to me. Merging...