My application stopped working after MariaDB JDBC driver was upgraded from version 1.1.8 to version 1.4.4. Any JDBC call against connection throws "SQLNonTransientConnectionException: Connection is close" exception. host missed in connection URL was direct reason of this e.g. "jdbc:mariadb:///schemaname" instead "jdbc:mariadb://localhost/schemaname". My application worked before drivers upgrade because host was optional in connection URL at least for 1.1.8 (same as in MySlq connection URL).
Yes this is RTFM but current driver behavior looks not good because according to javadoc java.sql.Driver#connect method should return valid connection or throws exception when can't connect to database. Otherwise it is hard to catch problem reason without driver code debugging.