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

By mkyong | Updated: April 28, 2011

Viewed: 2,383 (+15 pv/w)

Tags: hibernate many-to-many

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 :

1. Hibernate 3.6.3.Final
2. MySQL 5.1.15
3. Maven 3.0.3
4. 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

Reference

1. Hibernate Documentation – many to many relationship.