Hibernate One-to-many Relationships
Hibernate One-to-many Relationships - One to many example code in
Hibernate using the xml file as metadata.
Here you will learn one-to-many relationships developed
using the hmb.xml file. The one-to-many tag is used to define the relationships.
One-to-Many Relationships
In case of one-to-many relationships, each row in table A linked to
multiple rows in table B. Here we will implement one-to-many relationship
between two entries Group and Story.
There exists multiple stories under a group.
We are using two tables grouptable and story and java
entities Group and Story maps respectively. Here is the ER
diagram
Java Persistence Objects:
The code for Group.java is given below:
package roseindia;
import java.util.List;
public class Group {
private int id;
private String name;
private List stories;
public Group(){
}
public Group(String name) {
this.name = name;
}
public void setId(int i) {
id = i;
}
public int getId() {
return id;
}
public void setName(String n) {
name = n;
}
public String getName() {
return name;
}
public void setStories(List sl) {
stories = sl;
}
public List getStories() {
return stories;
}
}
|
The code for Story.java is given below:
package roseindia;
public class Story {
private int id;
private String info;
private Group group;
public Story(){
}
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}
public Story(String info) {
this.info = info;
}
public void setId(int i) {
id = i;
}
public int getId() {
return id;
}
public void setInfo(String n) {
info = n;
}
public String getInfo() {
return info;
}
}
|
Mapping xml file (Group.hbm.xml):
This file contains the mapping for both the entities Stroy and Group
which maps with the tables gropuptable and story respectively.
<?
xml
version="1.0"?>
<!
DOCTYPE
hibernate-mapping
PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<
hibernate-mapping>
<class
name="roseindia.Group"
table="gropuptable">
<id
name="id"
type="int"
column="id">
<generator
class="native"
/>
</id>
<property
name="name"
type="java.lang.String"
column="name"
length="50"
/>
<list
name="stories"
cascade="all">
<key
column="parent_id"
/>
<index
column="idx"
/>
<one-to-many
class="roseindia.Story"
/>
0
</list>
</class>
1
<class
name="roseindia.Story"
table="story">
<id
name="id"
type="int"
column="id">
<generator
class="increment"
/>
2
</id>
<property
name="info"
type="java.lang.String"
column="info"
length="255"
/>
3
<many-to-one
name="group"
class="roseindia.Group"
column="parent_id"
/>
</class>
</
hibernate-mapping>
4
The following mapping tag defines the one-to-many mapping between
Group and Story entities.
5
<list
name="stories"
cascade="all">
<key
column="parent_id"
/>
<index
column="idx"
/>
6
<one-to-many
class="roseindia.Story"
/>
</list>
In Group.java we have stories variable of the list type, so we have
used the <list../> tag here. The <one-to-many ../> specifies the one to
many relationships to Story entity.
7
Running the Program:
To run and test the program you have to execute the
OneToManyRelation.java. Here is the full code of
OneToManyRelation.java
file:
8
package roseindia;
import java.awt.Stroke;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
//Goups and Stories
public class OneToManyRelation {
// Group and Multiple Stories
public static void main(String[] args) {
SessionFactory sessFact = null;
Session session = null;
try {
sessFact = new Configuration().configure()
.buildSessionFactory();
session = sessFact.openSession();
Transaction tr = session.beginTransaction();
// Group Object
Group group = new Group("Java Group 2");
// Array list to hold stories
ArrayList list = new ArrayList();
// Create Stories
Story javaHistory = new Story();
javaHistory.setInfo("Java History");
list.add(javaHistory);
// Create Stories
Story javaPlatform = new Story();
javaPlatform.setInfo("Java Platforms");
list.add(javaPlatform);
// Create Stories
Story javaNews = new Story();
javaNews.setInfo("Java News");
list.add(javaNews);
// Create Stories
Story jeeNews = new Story();
jeeNews.setInfo("JEE News");
list.add(jeeNews);
// Add the list to Group object
group.setStories(list);
// Save group
session.save(group);
// Print the generated group id
System.out.println("Group id:" +
group.getId());
/*
*
* //Load the group class Group g = (Group)
* session.load(Group.class, 1);
System.out.println("Group Name:" +
* g.getName());
*
* List listStories = g.getStories();
Iterator storiesIter =
* listStories.iterator();
while(storiesIter.hasNext()) { Story
* story = (Story)storiesIter.next();
* System.out.println("Story Info:" +
story.getInfo()); }
*/
tr.commit();
} catch (HibernateException he) {
System.out.println(he.getMessage());
} finally {
// Session close
session.close();
// SessionFactory close
sessFact.close();
}
}
}
|
The above code is self explanatory, we are creating the objects of
Group:
// Group Object
Group group = new Group("Java Group 2");
Then set the author name:
9
The create ArrayList object set and add many Story objects:
0
// Array list to
hold stories
ArrayList list =
new
ArrayList();
1
// Create Stories
Story javaHistory =
new Story();
javaHistory.setInfo(
"Java
History");
2
list.add(javaHistory);
// Create Stories
3
Story javaPlatform =
new Story();
javaPlatform.setInfo(
"Java
Platforms");
list.add(javaPlatform);
4
// Create Stories
Story javaNews =
new Story();
5
javaNews.setInfo(
"Java
News");
list.add(javaNews);
6
// Create Stories
Story jeeNews =
new Story();
jeeNews.setInfo(
"JEE
News");
7
list.add(jeeNews);
8
Finally add the book set to the group object and save the group
object:
// Add the list to
Group object
group.setStories(list);
9
// Save group
session.save(group);
0
Here
is output of the program when executed in Eclipse IDE.
1
log4j:WARN No appenders could be found for logger
(org.hibernate.cfg.Environment).
log4j:WARN Please initialize the log4j system properly.
Hibernate: insert into gropuptable (name) values (?)
2
Hibernate: select max(id) from story
Group id:2
Hibernate: insert into story (info, parent_id, id)
values (?, ?, ?)
3
Hibernate: insert into story (info, parent_id, id)
values (?, ?, ?)
Hibernate: insert into story (info, parent_id, id)
values (?, ?, ?)
Hibernate: insert into story (info, parent_id, id)
values (?, ?, ?)
4
Hibernate: update story set parent_id=?, idx=? where
id=?
Hibernate: update story set parent_id=?, idx=? where
id=?
Hibernate: update story set parent_id=?, idx=? where
id=?
5
Hibernate: update story set parent_id=?, idx=? where
id=?
In this section we learned about the one-to-many relationships in
Hibernate.
6
In next section we will learn how to create and run the many-to-many
mapping in Hibernate. Many to many relationships is interesting as one
extra table is needed to persist the relationship.
Download the code example
