What does JdbcTemplate do when RowMapper returns null?

You can simply remove the null from the list after populating with the RowMapper.

rows = JdbcTemplate.query(sql, args, rowMapper);
rows.removeAll(Collections.singletonList(null));

This is the piece of code that adds rows to the result list

public class RowMapperResultSetExtractor<T> implements ResultSetExtractor<List<T>> {
    ...
    public List<T> extractData(ResultSet rs) throws SQLException {
        List<T> results = (this.rowsExpected > 0 ? new ArrayList<T>(this.rowsExpected) : new ArrayList<T>());
        int rowNum = 0;
        while (rs.next()) {
            results.add(this.rowMapper.mapRow(rs, rowNum++));
        }
        return results;
    }
    ...

as we can see it will really add null. However there is no reason why RowMapper should ever return null unless there is a bug in it.

When you return null then it indeed adds this null to the list, also, if you throw an SQLException it is "wrapped" in a Exception that extends RuntimeException, for you not to have to include explicitly a try-catch statement if you don't want to.