In this tutorial, we will learn about how to use Hibernate Many-To-One Unidirectional mapping using annotation based configuration.

In Many-To-One Unidirectional mapping, one table has a foreign key column that references the primary key of associated table.By Unidirectional relationship means only one side navigation is possible (STUDENT to UNIVERSITY in this example).

We are discussing an example of Student and University relationship. Many student can enroll at one University. And one University can have many students. Let’s get going.

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Following technologies being used:

• Hibernate 4.3.6.Final
• MySQL Server 5.6
• Maven 3.1.1
• JDK 1.6
• Eclipse JUNO Service Release 2

Let’s begin.

Step 1: Create required Database Table

Open MySQL terminal / workbench terminal and execute following MySQL script :

create table UNIVERSITY (
   university_id BIGINT NOT NULL AUTO_INCREMENT,
   name VARCHAR(30) NOT NULL,
   country  VARCHAR(30) NOT NULL,
   PRIMARY KEY (university_id)
);

create table STUDENT (
   student_id BIGINT NOT NULL AUTO_INCREMENT,
   university_id BIGINT NOT NULL,
   first_name VARCHAR(30) NOT NULL,
   last_name  VARCHAR(30) NOT NULL,
   section    VARCHAR(30) NOT NULL,
   PRIMARY KEY (student_id),
   CONSTRAINT student_university FOREIGN KEY (university_id) REFERENCES UNIVERSITY (university_id) ON UPDATE CASCADE ON DELETE CASCADE
);

Here we have first created University table followed by student table as student table contains a foreign key referring to University table.
Please visit MySQL installation on Local PC in case you are finding difficulties in setting up MySQL locally.

Step 2: Create project directory structure

Following will be the final project structure:

Now let’s add/update the content mentioned in above project structure.

Step 3: Update pom.xml to include required Hibernate and MySQL dependency

Following is the updated minimalistic pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>

	<groupId>com.websystique.hibernate</groupId>
	<artifactId>ManyToOneUniDirectional</artifactId>
	<version>1.0.0</version>
	<packaging>jar</packaging>

	<name>ManyToOneUniDirectional</name>

	<properties>
		<hibernate.version>4.3.6.Final</hibernate.version>
		<mysql.connector.version>5.1.31</mysql.connector.version>
	</properties>

	<dependencies>
		<!-- Hibernate -->
		<dependency>
			<groupId>org.hibernate</groupId>
			<artifactId>hibernate-core</artifactId>
			<version>${hibernate.version}</version>
		</dependency>

		<!-- MySQL -->
		<dependency>
			<groupId>mysql</groupId>
			<artifactId>mysql-connector-java</artifactId>
			<version>${mysql.connector.version}</version>
		</dependency>
	</dependencies>
	<build>
		<pluginManagement>
			<plugins>
				<plugin>
					<groupId>org.apache.maven.plugins</groupId>
					<artifactId>maven-compiler-plugin</artifactId>
					<version>3.2</version>
					<configuration>
						<source>1.6</source>
						<target>1.6</target>
					</configuration>
				</plugin>
			</plugins>
		</pluginManagement>
	</build>

</project>

On saving pom.xml with above content, Eclipse will download all the dependencies.

Step 4: Create Model classes

Model class Student & University are simple POJO class which is annotated with JPA annotations to map it to a database tables(created in step 1).

package com.websystique.hibernate.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "UNIVERSITY")
public class University {

	@Id
	@GeneratedValue
	@Column(name = "UNIVERSITY_ID")
	private long id;

	@Column(name = "NAME")
	private String name;

	@Column(name = "COUNTRY")
	private String country;

	public University() {

	}

	public University(String name, String country) {
		this.name = name;
		this.country = country;
	}

	public long getId() {
		return id;
	}

	public void setId(long id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getCountry() {
		return country;
	}

	public void setCountry(String country) {
		this.country = country;
	}

	@Override
	public String toString() {
		return "University [id=" + id + ", name=" + name + ", country="	+ country + "]";
	}

}

package com.websystique.hibernate.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Entity
@Table(name = "STUDENT")
public class Student {

	@Id
	@GeneratedValue
	@Column(name = "STUDENT_ID")
	private long id;

	@Column(name = "FIRST_NAME")
	private String firstName;

	@Column(name = "LAST_NAME")
	private String lastName;

	@Column(name = "SECTION")
	private String section;

	@ManyToOne(optional = false)
	@JoinColumn(name="UNIVERSITY_ID")
	private University university;


	public Student() {
	}

	public Student(String firstName, String lastName, String section) {
		this.firstName = firstName;
		this.lastName = lastName;
		this.section = section;
	}

	public long getId() {
		return id;
	}

	public void setId(long id) {
		this.id = id;
	}

	public String getFirstName() {
		return firstName;
	}

	public void setFirstName(String firstName) {
		this.firstName = firstName;
	}

	public String getLastName() {
		return lastName;
	}

	public void setLastName(String lastName) {
		this.lastName = lastName;
	}

	public String getSection() {
		return section;
	}

	public void setSection(String section) {
		this.section = section;
	}

	public University getUniversity() {
		return university;
	}

	public void setUniversity(University university) {
		this.university = university;
	}

	@Override
	public String toString() {
		return "Student [id=" + id + ", firstName=" + firstName + ", lastName="
				+ lastName + ", section=" + section + "]";
	}

}

University class is a simple pojo with no information of Student class. Student class on the other hand contains an ManyToOne association to University class.

	@ManyToOne(optional = false)
	@JoinColumn(name="UNIVERSITY_ID")
	private University university;

@ManyToOne indicates that Many student tuples can refer to one University tuple. Also note that we have provided optional=false means this relationship becomes mandatory , no student row can be saved without a university tuple reference.@JoinColumn says that there is a column UNIVERSITY_ID in Student table which will refer(foreign key) to primary key of the University table. In this example only Student to University entity navigation is possible. Not viceversa. In practice, however, you are free to use query language to find all the student for a given university.

Step 5: Create Hibernate configuration file

We need to inform hibernate about how to connect to database, which database dialect we will be using so that hibernate can generate the instruction specific to that database.

We define all these information in hibernate.cfg.xml. Create this file with below content and save it in src/main/resources folder.

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


<hibernate-configuration>
    <session-factory>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.username">myuser</property>
        <property name="hibernate.connection.password">mypassword</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/websystique</property>
        <property name="show_sql">true</property>
        <property name="format_sql">false</property>
        <mapping class="com.websystique.hibernate.model.Student"/>
        <mapping class="com.websystique.hibernate.model.University"/>
    </session-factory>
</hibernate-configuration>

Step 6: Create Hibernate Utility class

This class is well-known in hibernate community, and used for configuring hibernate on startup and managing session factory.

package com.websystique.hibernate;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;

public class HibernateUtil {
	
	private static final SessionFactory sessionFactory;
	
	static{
		try{
			sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();

		}catch (Throwable ex) {
			System.err.println("Session Factory could not be created." + ex);
			throw new ExceptionInInitializerError(ex);
		}	
	}
	
	public static SessionFactory getSessionFactory() {
		return sessionFactory;
	}
	
}

Step 7: Create executable class to Run and perform operations on Database

package com.websystique.hibernate;

import java.util.List;

import org.hibernate.Session;

import com.websystique.hibernate.model.Student;
import com.websystique.hibernate.model.University;


public class HibernateStandAlone {
	
	@SuppressWarnings("unchecked")
	public static void main(String[] args) {

		Student student1 = new Student("Sam","Disilva","Maths");
		Student student2 = new Student("Joshua", "Brill", "Science");
		Student student3 = new Student("Peter", "Pan", "Physics");
		
		University university = new University("CAMBRIDGE", "ENGLAND");

		student1.setUniversity(university);
		student2.setUniversity(university);
		student3.setUniversity(university);


		Session session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();

		session.persist(university);
		session.persist(student1);
		session.persist(student2);
		session.persist(student3);
		
		List<Student> students = (List<Student>)session.createQuery("from Student ").list();
		for(Student s: students){
			System.out.println("Details : "+s);
			System.out.println("Student University Details: "+s.getUniversity());
		}
		
		session.getTransaction().commit();
		session.close();  
	}

}

Here we have persisted University class firstly in order to meet foreign key constraint(not null), then we have set student’s address property followed by persisting student.

Execute above class as Java application. You will see following output

Hibernate: insert into UNIVERSITY (COUNTRY, NAME) values (?, ?)
Hibernate: insert into STUDENT (FIRST_NAME, LAST_NAME, SECTION, UNIVERSITY_ID) values (?, ?, ?, ?)
Hibernate: insert into STUDENT (FIRST_NAME, LAST_NAME, SECTION, UNIVERSITY_ID) values (?, ?, ?, ?)
Hibernate: insert into STUDENT (FIRST_NAME, LAST_NAME, SECTION, UNIVERSITY_ID) values (?, ?, ?, ?)
Hibernate: select student0_.STUDENT_ID as STUDENT_1_0_, student0_.FIRST_NAME as FIRST_NA2_0_, student0_.LAST_NAME as LAST_NAM3_0_, student0_.SECTION as SECTION4_0_, student0_.UNIVERSITY_ID as UNIVERSI5_0_ from STUDENT student0_
Student Details : Student [id=1, firstName=Sam, lastName=Disilva, section=Maths]
Student University Details: University [id=1, name=CAMBRIDGE, country=ENGLAND]
Student Details : Student [id=2, firstName=Joshua, lastName=Brill, section=Science]
Student University Details: University [id=1, name=CAMBRIDGE, country=ENGLAND]
Student Details : Student [id=3, firstName=Peter, lastName=Pan, section=Physics]
Student University Details: University [id=1, name=CAMBRIDGE, country=ENGLAND]

Below is the snapshot of MySQL database after execution of above program.

That’s it.

Download Source Code

References

• Hibernate Documentation – Many to one unidirectional association