When you give a SQL Query a candidate class, it
will return persistent instances of that class. At a minimum, your
SQL must select the
class' primary key columns, discriminator column (if mapped), and
version column (also if mapped). The JPA runtime uses the values
of the primary key columns to construct each result object's identity,
and possibly to match it with a persistent object already in the
EntityManager's cache. When an object is
not already cached, the
implementation creates a new object to represent the current result
row. It might use the discriminator column value to make sure it
constructs an object of the correct subclass. Finally, the query
records available version column data for use in optimistic concurrency
checking, should you later change the result object and flush it back
to the database.
Aside from the primary key, discriminator, and version columns, any columns you select are used to populate the persistent fields of each result object. JPA implementations will compete on how effectively they map your selected data to your persistent instance fields.
Let's make the discussion above concrete with an example. It uses the following simple mapping between a class and the database:
Example 11.2. Retrieving Persistent Objects
Query query = em.createNativeQuery ("SELECT ISBN, TITLE, PRICE, "
+ "VERS FROM MAG WHERE PRICE > 5 AND PRICE < 10", Magazine.class);
List<Magazine> results = (List<Magazine>) query.getResultList ();
for (Magazine mag : results)
processMagazine (mag);
The query above works as advertised, but isn't very flexible. Let's update it to take in parameters for the minimum and maximum price, so we can reuse it to find magazines in any price range:
Example 11.3. SQL Query Parameters
Query query = em.createNativeQuery ("SELECT ISBN, TITLE, PRICE, "
+ "VERS FROM MAG WHERE PRICE > ?1 AND PRICE < ?2", Magazine.class);
query.setParameter (1, 5d);
query.setParameter (2, 10d);
List<Magazine> results = (List<Magazine>) query.getResultList ();
for (Magazine mag : results)
processMagazine (mag);
Like JDBC prepared statements, SQL queries represent parameters with question marks, but are followed by an integer to represent its index.
