The Oracle JDBC driver has proper support for the
setFetchSize() method on
java.sql.Statement, which allows you to control how many rows the driver will fetch in one go.
RowMapper as used by Spring works by reading each row into memory, getting the
RowMapper to translate it into an object, and storing each row's object in one big list. If your result set is huge, then this list will get big, regardless of how JDBC fetches the row data.
If you need to handle large result sets, then RowMapper isn't scaleable. You might consider using
RowCallbackHandler instead, along with the corresponding methods on JdbcTemplate.
RowCallbackHandler doesn't dictate how the results are stored, leaving it up to you to store them.