Java Data Persistence Using Hibernate - People

Java Data Persistence
Using Hibernate

Jack Gardner

October 2004

2

Overview


What is Hibernate


What is ORM


Create Example


Read Examples


Persistable Classes


Database Table


Mapping Class to Table


Configuration


Configuration Properties



Session


Transaction


Object Lifecycle


Query Options


Associations


References

3

What is Hibernate?


An object/relational mapping (ORM)
implementation


Open source


Development started late 2001

4

What is Object/Relational Mapping?


Provides a transparent bridge between
objects and database tables


Allows source code to work with objects and
their attributes vs. tables and columns


Eliminates need for most/all SQL


Eliminates use of query result sets


5

Create Example


// create a new object

Widget w =
new

Widget();

w.setName(
“WidgetA”
);

w.setValue(
10
);


// persist it

session.save(w);

6

Read Examples


// get a known widget

Widget w = (Widget) session.load(Widget.
class
,
“WidgetA”
);


// get all widgets

List widgets = session.find(
“from Widget”
);

7

Persistable Classes


Classes are simply JavaBeans


public class

Widget

{


private

String

name;


private

int

value;



public

Widget() {}


public

String

getName() {
return

name;}


public

void

setName(
String

s) {name = s;}


public

int

getValue() {
return

value;}


public

void

setValue(
int

i) {value = i;}

}


8

Database Table


Persistable classes have an associated table

9

Mapping Class to Table


widget.hbm.xml


<
hibernate
-
mapping
>


<
class

name
=
“mypackage.Widget”

table
=
“WIDGET”
>



<
id

name
=
“name”

column
=
“NAME”
>




<
generator

class
=
“assigned”
/>



</
id
>



<
property

name
=
“value”

column
=
“VALUE”
/>


</
class
>

</
hibernate
-
mapping
>



10

Configuration


import

net.sf.hibernate.cfg.Configuration;

import

net.sf.hibernate.SessionFactory;


// build configuration based on properties

Configuration config =
new

Configuration();


// add persistable classes to configuration

config.addClass(Widget.
class
);


// build a session factory based on configuration

SessionFactory sessionFactory =
config.buildSessionFactory();


11

Configuration Properties


hibernate.properties


hibernate.dialect
=net.sf.hibernate.dialect.Oracle9Dialect

hibernate.connection.driver_class
=oracle.jdbc.driver.OracleDriver

hibernate.connection.url
=jdbc:oracle:thin:@oracle.cis.ksu.edu:1521:ORACLE

hibernate.connection.username
=<myusername>

hibernate.connection.password
=<mypassword>

12

Session


Lightweight and inexpensive to create/destroy


Not threadsafe
–

each thread needs its own


Obtained from SessionFactory instance


Session session = sessionFactory.openSession();

// perform persistence operations

session.close();

13

Transaction


Set of operations that are all committed or all
rolled back


Obtained from Session instance


Can perform multiple transactions within
session


Transaction tx = session.beginTransaction();

// perform persistence operations

tx.commit();


14

Object Lifecycle


Transient


Newly created object


Persistent


New object has been “saved”


Previously saved object has been “read”


Detached


Session closed


Persistent object serialized


Can be reattached later to become persistent



15

Query Options


Hibernate Query Language (HQL)


Query q = session.createQuery(
“from Widget w where

w.value > :value”
);


q.setParameter(
“value”
, someValue);


List widgets = q.list();



Criteria API


Criteria c = session.createCriteria(Widget.
class
);


c.add(Expression.gt(
“value”
, someValue);


List widgets = c.list();



SQL


Query q = session.createSQLQuery(
“select {w.*} from

WIDGET {w} where VALUE > :value”
,

“w”
,

Widget.
class
);


q.setParameter(
“value”
, someValue);


List widgets = q.list();


16

Associations


Supports all association types


one
-
to
-
one


one
-
to
-
many


many
-
to
-
many


Inherently unidirectional


Supports bidirectional

17

One
-
to
-
Many Example:
Unidirectional


Option 1: Group


Member



public class

Group



public class

Member


{





{



private

String

name;




private

String

name;



private

Collection members;



…



…





}


}



Option 2: Group


Member



public class

Group



public class

Member


{





{



private

String

name;




private

String

name;



…






private

Group group;


}






…







}

18

One
-
to
-
Many Example:
Bidirectional


Group


Member



public class

Group



public class

Member


{





{



private

String

name;




private

String

name;



private

Collection members;



private

Group group;



…






…


}





}



Application responsible for maintaining each
end of association

19

One
-
to
-
Many Table Structure


Underlying table structure not affected by
directionality


Standard implementation


Foreign key in many
-
side table


Alternate implementation


Use join table, with no
-
duplicate constraint on
many
-
side foreign key

20

One
-
to
-
Many Table Structure

21

Many
-
to
-
Many Example:
Unidirectional


Option 1: Group


Member



public class

Group



public class

Member


{





{



private

String

name;




private

String

name;



private

Collection members;



…



…





}


}



Option 2: Group


Member



public class

Group



public class

Member


{





{



private

String

name;




private

String

name;



…






private

Collection groups;


}






…







}

22

Many
-
to
-
Many Example:
Bidirectional


Group


Member



public class

Group



public class

Member


{





{




private

String

name;




private

String

name;




private

Collection members;



private

Collection groups;




…






…


}





}



Application responsible for maintaining each
end of association


23

Many
-
to
-
Many Table Structure


Underlying table structure not affected by
directionality


Implemented using join table

24

Many
-
to
-
Many Table Structure

25

References


Bauer, Christian and Gaven King,
Hibernate
in Action
, Manning, 2005.


Hibernate Reference Documentation
, Version
2.1.6, Hibernate, 2004.