Reading and Writing with Hibernate

We have an entity class mapped to the COUNTRY table and a Hibernate Session object to communicate with our database, so let's put Hibernate to work.

The COUNTRY table includes this data:

CTRY_ID	CTRY_NAME	AREA
27	Ukraine	233000
28	pain	95365
29	Columbia	440831
224	Yugoslavia	98766

Read

To return a single object for a given identifier, use either of these methods:

Country spain = (Country) session.load(Country.class, 28);

Country spain = (Country) session.get(Country.class, 28);

Both of the methods generate this SQL (or something very similar):

select
  CTRY_ID,
  CTRY_NAME,
  AREA
from
  COUNTRY
where
  CTRY_ID=?

The difference between the two methods is that load() will throw a HibernateException if the row does not exist whereas get() will return a null value.

Create

To persist an object to the database, use the save() method.

session.beginTransaction();

Country croatia = new Country();
croatia.setName("Croatia");
croatia.setArea(21831);
int croatiaId = (Integer) session.save(croatia);

session.getTransaction().commit();

This SQL is generated when used with an Oracle database:

select hibernate_sequence.nextval from dual

insert into COUNTRY (CTRY_NAME, AREA, CTRY_ID) values (?, ?, ?)

The @GeneratedValue annotation on the Country entity ensures that a sequential value is generated for the CTRY_ID column. This value is then returned by the save() method.

A transaction is started on the Session object and can be committed, using commit(), or rolled back, using rollback(), as you would expect.

Update

To update values on a persistent object, simply retrieve the entity, make the required changes to the field values and commit the transaction. There is no explicit update method to be called.

session.beginTransaction();

Country spain = (Country) session.load(Country.class, 28);
spain.setName("Spain");     // Fix capital 'S'
spain.setArea(195365);      // and correct the area

session.getTransaction().commit();

This SQL is executed when the transaction is committed:

 
update COUNTRY set CTRY_NAME=?, AREA=? where CTRY_ID=?

Delete

To permanently remove a persistent object, use the delete() method inside a transaction.

session.beginTransaction();

Country yugoslavia = (Country) session.load(Country.class, 224);
session.delete(yugoslavia);

session.getTransaction().commit();

The delete() method generates this SQL:

 
delete from COUNTRY where CTRY_ID=?

Quick Start contains a complete working set of all the SQL and Java code featured in this section.

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