Working with Databases in Java - Beginning Java Programming: The Object-Oriented Approach - page 333

3.

Then the Employee class was created. It contains the same four attributes as in the MySQL
database: EmployeeID , Name , Gender , and DNR . The mapping to the Employee MySQL table is
performed using the @Entity and @Id annotations. The @Entity annotation is included before
the class definition and maps the class to the relational MySQL Employee table. The @Id annota-
tion then defines the identifier of the class that will be mapped to the primary key of the MySQL
Employee table. Note that the Java class may have a different name than the MySQL table. In
this case, the @Table annotation can be used to define the mapping. If no @Table annotation is
used, the name of the class is used, which is fine in this case since it is identical to the name of the
MySQL table. Likewise, the @Column annotation can be used to map Java properties to MySQL
columns. Refer to http://hibernate.org/ for other examples of annotations.

4.

In Step 7 of the Try It Out, the myDBApp class was created to perform some basic database opera-
tions. It starts with creating a new employee object Myemp and setting its characteristics. Then a
sessionFactory object was created, which will in turn allow the creation of a session object.
The latter represents a single-threaded wrapped JDBC connection with the database. Note that
Hibernate often introduces new classes and methods to create both sessionFactory and session
objects, and it is thus recommended to check http://hibernate.org/ regularly for updates.

5.

The session object serves as a factory for creating database transactions. The statement session.
beginTransaction(); initiates a database transaction that represents an atomic unit of database
operations. If needed, multiple transactions per session can be created. The Myemp employee object
will then be made persistent in the MySQL database using the method session.save(Myemp) fol-
lowed by session.getTransaction().commit() . Then the insert is verified by issuing the HQL
query: Query query = session.createQuery("from Employee where EmployeeID = 6") . The
results of the query are stored in a list object List<?> list = query.list() . The query will return
one result, which will be stored in the object emp as follows: Employee emp = (Employee)list.get(0) .
This is followed by retrieving the various attributes and displaying them on the console. The program
finishes by closing the session and sessionFactory objects.

6. Since the show_SQL property was set to true in the configuration file, you can see the SQL state-

ments that are being generated by Hibernate as follows:
Hibernate: insert into Employee (DNR, gender, name, employeeID) values (?, ?, ?, ?)
Hibernate: select employee0_.employeeID as employee1_0_, employee0_.DNR as DNR2_0_,
employee0_.gender as gender3_0_, employee0_.name as name4_0_
from Employee employee0_
where EmployeeID=6

The result of the query is then:
Hibernate dude
Male
2

This is also confirmed by inspecting the table directly in MySQL.

In most applications, Java objects and their corresponding relational tables will be associated using
relationships. Hibernate also has facilities for managing the object relational mapping of these asso-
ciations. Obviously, the mapping depends on the type and the multiplicity of the association.