Question

I’ve been asked to output a CSV file from a view in MySQL. The app I currently am writing uses Spring and Hibernate to create the database, but the view is just handed to me.

Hibernate doesn’t know anything about this view, but I’d want to do something like this:

public List<Object> getCsvView() {
  return (List<Object>) getHibernateTemplate().find("from myView");
}

My guess was that I could map a native query so that hibernate knows about the view. This got a little tricky when I read the docs:

You can also map a native query[…]To
achieve that, you need to describe the
SQL resultset structure using
@SqlResultSetMapping[…].

Now, I’m really not interested in mapping the structure of the result. I’m happy to have the structure just be a bunch of objects.

Furthermore they might change this view at any time. I’m really not thrilled about my app even knowing about the view.

So, is there an easy way to do this in the Spring/Hibernate world, or am I attacking this problem the hard way?

Answer 1

Just perform a native query on the view and you’ll get a List of Object[] with scalar values for each column as result. From the documentation:

16.1.1. Scalar queries

The most basic SQL query is to get a
list of scalars (values).

sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();

These will return a List of Object
arrays (Object[]) with scalar values
for each column in the CATS table.
Hibernate will use ResultSetMetadata
to deduce the actual order and types
of the returned scalar values.

To avoid the overhead of using
ResultSetMetadata, or simply to be
more explicit in what is returned, one
can use addScalar():

sess.createSQLQuery("SELECT * FROM CATS")
 .addScalar("ID", Hibernate.LONG)
 .addScalar("NAME", Hibernate.STRING)
 .addScalar("BIRTHDATE", Hibernate.DATE)

This query specified:

• the SQL query string
• the columns and types to return

This will return Object arrays, but
now it will not use
ResultSetMetadata but will instead
explicitly get the ID, NAME and
BIRTHDATE column as respectively a
Long, String and a Short from the
underlying resultset. This also means
that only these three columns will be
returned, even though the query is
using * and could return more than the
three listed columns.

It is possible to leave out the type
information for all or some of the
scalars.

sess.createSQLQuery("SELECT * FROM CATS")
 .addScalar("ID", Hibernate.LONG)
 .addScalar("NAME")
 .addScalar("BIRTHDATE")

This is essentially the same query as
before, but now ResultSetMetaData is
used to determine the type of NAME and
BIRTHDATE, where as the type of ID is
explicitly specified.

How the java.sql.Types returned from
ResultSetMetaData is mapped to
Hibernate types is controlled by the
Dialect. If a specific type is not
mapped, or does not result in the
expected type, it is possible to
customize it via calls to
registerHibernateType in the
Dialect.

Looks perfect to generate a CVS file without knowing anything about the view 🙂