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Hibernate Many-To-One Mapping Tutorial

06.14.2012
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In this example you will learn how to map many-to-one relationship using Hibernate. Consider the following relationship between Student and Address entity.

According to the relationship many students can have the same address.

To create this relationship you need to have a STUDENT and ADDRESS table. The relational model is shown below.

 

To create the STUDENT and ADDRESS table you need to create the following hibernate mapping files.

Student.hbm.xml is used to create the STUDENT table.

<?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.vaannila.student.Student" table="STUDENT">
        <meta attribute="class-description">This class contains student details.</meta>
        <id name="studentId" type="long" column="STUDENT_ID">
            <generator class="native" />
        </id>
        <property name="studentName" type="string" length="100" not-null="true" column="STUDENT_NAME" />
        <many-to-one name="studentAddress" class="com.vaannila.student.Address" column="STUDENT_ADDRESS" cascade="all" not-null="true" />
    </class>
</hibernate-mapping>

The many-to-one element is used to create the many-to-one relationship between the Student and Address entities. The cascade option is used to cascade the required operations to the associated entity. If the cascade option is set to all then all the operations will be cascaded. For instance when you save a Student object, the associated Address object will also be saved automatically.

Address.hbm.xml is used to create the ADDRESS table.

<?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.vaannila.student.Address" table="ADDRESS">
		<meta attribute="class-description">This class contains the student's address
			details.</meta>
		<id name="addressId" type="long" column="ADDRESS_ID">
			<generator class="native" />
		</id>
		<property name="street" column="ADDRESS_STREET" type="string" length="250" />
		<property name="city" column="ADDRESS_CITY" type="string" length="50" />
		<property name="state" column="ADDRESS_STATE" type="string" length="50" />
		<property name="zipcode" column="ADDRESS_ZIPCODE" type="string" length="10" />
	</class>
</hibernate-mapping>

Now create the hibernate configuration file and add all the mapping files.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
		"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class"> org.hsqldb.jdbcDriver </property>
        <property name="hibernate.connection.url"> jdbc:hsqldb:hsql://localhost</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="connection.password"></property>
        <property name="connection.pool_size">1</property>
        <property name="hibernate.dialect"> org.hibernate.dialect.HSQLDialect </property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create-drop</property>
        <mapping resource="com/vaannila/student/Student.hbm.xml"/>
        <mapping resource="com/vaannila/student/Address.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

After creating the configuration file, generate java class files using Hibernate Tools.(To generate code using Hibernate Tools refer this example )

The following classes will be generated.

package com.vaannila.student;

// Generated Sep 3, 2009 7:20:37 PM by Hibernate Tools 3.2.4.GA

/**
 * This class contains student details.
 */
public class Student implements java.io.Serializable {

	private long studentId;
	private String studentName;
	private Address studentAddress;

	public Student() {
	}

	public Student(String studentName, Address studentAddress) {
		this.studentName = studentName;
		this.studentAddress = studentAddress;
	}

	public long getStudentId() {
		return this.studentId;
	}

	public void setStudentId(long studentId) {
		this.studentId = studentId;
	}

	public String getStudentName() {
		return this.studentName;
	}

	public void setStudentName(String studentName) {
		this.studentName = studentName;
	}

	public Address getStudentAddress() {
		return this.studentAddress;
	}

	public void setStudentAddress(Address studentAddress) {
		this.studentAddress = studentAddress;
	}

}
package com.vaannila.student;

// Generated Sep 3, 2009 7:20:37 PM by Hibernate Tools 3.2.4.GA

/**
 * This class contains the student's address
 * 			details.
 */
public class Address implements java.io.Serializable {

	private long addressId;
	private String street;
	private String city;
	private String state;
	private String zipcode;

	public Address() {
	}

	public Address(String street, String city, String state, String zipcode) {
		this.street = street;
		this.city = city;
		this.state = state;
		this.zipcode = zipcode;
	}

	public long getAddressId() {
		return this.addressId;
	}

	public void setAddressId(long addressId) {
		this.addressId = addressId;
	}

	public String getStreet() {
		return this.street;
	}

	public void setStreet(String street) {
		this.street = street;
	}

	public String getCity() {
		return this.city;
	}

	public void setCity(String city) {
		this.city = city;
	}

	public String getState() {
		return this.state;
	}

	public void setState(String state) {
		this.state = state;
	}

	public String getZipcode() {
		return this.zipcode;
	}

	public void setZipcode(String zipcode) {
		this.zipcode = zipcode;
	}

}
package com.vaannila.student;

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.vaannila.util.HibernateUtil;

public class Main {

	public static void main(String[] args) {
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
		try {
			transaction = session.beginTransaction();
			Address address = new Address("OMR Road", "Chennai", "TN", "600097");
			//By using cascade=all option the address need not be saved explicitly when the student object is persisted the address will be automatically saved.
            //session.save(address);
			Student student1 = new Student("Eswar", address);
			Student student2 = new Student("Joe", address);
			session.save(student1);
			session.save(student2);
			transaction.commit();
		} catch (HibernateException e) {
			transaction.rollback();
			e.printStackTrace();
		} finally {
			session.close();
		}

	}

}

On executing the Main class you will see the following output.

The Student table has two records.

The Address table has one record.

Both the student records points to the same address record, this illustrates the many-to-one mapping.

The folder structure of the example is shown below.

 

You can download the source code of this example here.

Source :Download

 

Published at DZone with permission of its author, Meyyappan Muthuraman.

(Note: Opinions expressed in this article and its replies are the opinions of their respective authors and not those of DZone, Inc.)

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Comments

Shyam Shyre replied on Fri, 2012/07/27 - 1:15pm

Hi,

Thanks for your tutorials.

Can you help me in lettinng me know why there is no JOINTABLE in many to one and why join table exists in one to many.

 

Thanks

Shyam

 

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