Hibernate – Many-to-Many example (XML Mapping)

Many-to-many relationships occur when each record in an entity may have many linked records in another entity and vice-versa.

In this tutorial, we show you how to work with many-to-many table relationship in Hibernate, via XML mapping file (hbm).

Tools and technologies used in this tutorials :

Hibernate 3.6.3.Final

MySQL 5.1.15

Maven 3.0.3

Eclipse 3.6

Project Structure

Project structure of this tutorial.

Project Dependency

Get latest

hibernate.jar

from JBoss repository.

File :

pom.xml

<project ...>

<repositories>
<repository>
<id>JBoss repository</id>
<url>http://repository.jboss.org/nexus/content/groups/public/</url>
</repository>
</repositories>

<dependencies>

<!-- MySQL database driver -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.15</version>
</dependency>

<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-core</artifactId>
<version>3.6.3.Final</version>
</dependency>

<dependency>
<groupId>javassist</groupId>
<artifactId>javassist</artifactId>
<version>3.12.1.GA</version>
</dependency>

</dependencies>
</project>

1. “Many-to-many” example

This is a many-to-many relationship table design, a

STOCK

table has more than one

CATEGORY

, and

CATEGORY

can belong to more than one

STOCK

, the relationship is linked with a third table called

STOCK_CATEGORY

.

Table

STOCK_CATEGORY

only consist of two primary keys, and also foreign key reference back to

STOCK

and

CATEGORY

.



MySQL table scripts

CREATE TABLE `stock` (
`STOCK_ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`STOCK_CODE` varchar(10) NOT NULL,
`STOCK_NAME` varchar(20) NOT NULL,
PRIMARY KEY (`STOCK_ID`) USING BTREE,
UNIQUE KEY `UNI_STOCK_NAME` (`STOCK_NAME`),
UNIQUE KEY `UNI_STOCK_ID` (`STOCK_CODE`) USING BTREE
)

CREATE TABLE `category` (
`CATEGORY_ID` int(10) unsigned NOT NULL AUTO_INCREMENT,
`NAME` varchar(10) NOT NULL,
`DESC` varchar(255) NOT NULL,
PRIMARY KEY (`CATEGORY_ID`) USING BTREE
)

CREATE TABLE  `stock_category` (
`STOCK_ID` int(10) unsigned NOT NULL,
`CATEGORY_ID` int(10) unsigned NOT NULL,
PRIMARY KEY (`STOCK_ID`,`CATEGORY_ID`),
CONSTRAINT `FK_CATEGORY_ID` FOREIGN KEY (`CATEGORY_ID`) REFERENCES `category` (`CATEGORY_ID`),
CONSTRAINT `FK_STOCK_ID` FOREIGN KEY (`STOCK_ID`) REFERENCES `stock` (`STOCK_ID`)
)

2. Hibernate Model Class

Create two model classes –

Stock.java

and

Category.java

, to represent the above tables. No need to create an extra class for table ‘

stock_category

‘.

File :

Stock.java

package com.mkyong.stock;

import java.util.HashSet;
import java.util.Set;

public class Stock implements java.io.Serializable {

private Integer stockId;
private String stockCode;
private String stockName;
private Set<Category> categories = new HashSet<Category>(0);

//getter, setter and constructor
}

File :

Category.java

package com.mkyong.stock;

import java.util.HashSet;
import java.util.Set;

public class Category implements java.io.Serializable {

private Integer categoryId;
private String name;
private String desc;
private Set<Stock> stocks = new HashSet<Stock>(0);

//getter, setter and constructor
}

3. Hibernate XML Mapping

Now, create two Hibernate mapping files (hbm) –

Stock.hbm.xml

and

Category.hbm.xml

. You will noticed the third ‘

stock_category

‘ table is reference via “many-to-many” tag.

File :

Stock.hbm.xml

<?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="com.mkyong.stock.Stock" table="stock" catalog="mkyongdb">
<id name="stockId" type="java.lang.Integer">
<column name="STOCK_ID" />
<generator class="identity" />
</id>
<property name="stockCode" type="string">
<column name="STOCK_CODE" length="10" not-null="true" unique="true" />
</property>
<property name="stockName" type="string">
<column name="STOCK_NAME" length="20" not-null="true" unique="true" />
</property>
<set name="categories" table="stock_category"
inverse="false" lazy="true" fetch="select" cascade="all" >
<key>
<column name="STOCK_ID" not-null="true" />
</key>
<many-to-many entity-name="com.mkyong.stock.Category">
<column name="CATEGORY_ID" not-null="true" />
</many-to-many>
</set>
</class>
</hibernate-mapping>

File :

Category.hbm.xml

<?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="com.mkyong.stock.Category" table="category" catalog="mkyongdb">
<id name="categoryId" type="java.lang.Integer">
<column name="CATEGORY_ID" />
<generator class="identity" />
</id>
<property name="name" type="string">
<column name="NAME" length="10" not-null="true" />
</property>
<property name="desc" type="string">
<column name="DESC" not-null="true" />
</property>
<set name="stocks" table="stock_category" inverse="true" lazy="true" fetch="select">
<key>
<column name="CATEGORY_ID" not-null="true" />
</key>
<many-to-many entity-name="com.mkyong.stock.Stock">
<column name="STOCK_ID" not-null="true" />
</many-to-many>
</set>
</class>
</hibernate-mapping>

4. Hibernate Configuration File

Now, puts

Stock.hbm.xml

and

Category.hbm.xml

and MySQL detail in

hibernate.cfg.xml

.

File :

hibernate.cfg.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/mkyongdb</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">password</property>
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="show_sql">true</property>
<property name="format_sql">true</property>
<mapping resource="com/mkyong/stock/Stock.hbm.xml" />
<mapping resource="com/mkyong/stock/Category.hbm.xml" />
</session-factory>
</hibernate-configuration>

5. Run It

Run it, Hibernate will insert a record into the

STOCK

table, two records into the

CATEGORY

table, and also two records into the

STOCK_CATEGORY

table.

File :

App.java

package com.mkyong;

import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import com.mkyong.stock.Category;
import com.mkyong.stock.Stock;
import com.mkyong.util.HibernateUtil;

public class App {
public static void main(String[] args) {

System.out.println("Hibernate many to many (XML Mapping)");
Session session = HibernateUtil.getSessionFactory().openSession();

session.beginTransaction();

Stock stock = new Stock();
stock.setStockCode("7052");
stock.setStockName("PADINI");

Category category1 = new Category("CONSUMER", "CONSUMER COMPANY");
Category category2 = new Category("INVESTMENT", "INVESTMENT COMPANY");

Set<Category> categories = new HashSet<Category>();
categories.add(category1);
categories.add(category2);

stock.setCategories(categories);

session.save(stock);

session.getTransaction().commit();
System.out.println("Done");
}
}

Output …result should be self-explanatory

Hibernate many to many (XML Mapping)
Hibernate:
insert
into
mkyongdb.stock
(STOCK_CODE, STOCK_NAME)
values
(?, ?)
Hibernate:
insert
into
mkyongdb.category
(NAME, `DESC`)
values
(?, ?)
Hibernate:
insert
into
mkyongdb.category
(NAME, `DESC`)
values
(?, ?)
Hibernate:
insert
into
stock_category
(STOCK_ID, CATEGORY_ID)
values
(?, ?)
Hibernate:
insert
into
stock_category
(STOCK_ID, CATEGORY_ID)
values
(?, ?)

Done