The conversion from UNKNOWN to UNKNOWN is unsupported

This error can occur, if you use jdbcTemplate.update(sql, List<Object[]>, int[]), but instead you wanted to use jdbcTemplate.batchUpdate(sql, List<Object[]>, int[]).

It compiles, because there is a jdbcTemplate.update(sql, Object...) method in jdbcTemplate, but it will yield "The conversion from UNKNOWN to UNKNOWN is unsupported.", because it cannot use List<Object[]> as parameter.

I found it. Clockwork-Muse put me on the path. The char type does not convert to an Object when you set the parameters. The following will work:

     try (PreparedStatement st = con.prepareStatement(query)) {
        int n = 1;
        for (Object o : params) {

            if (o instanceof Character) {
                o = "" + o;
            }

            st.setObject(n, o);
            n++;
        }
        st.executeQuery();
    }