Hibernate one to many mapping annotation example

Hibernate one to many mapping is made between two entities where first entity can have relation with multiple second entity instances but second can be associated with only one instance of first entity. Its 1 to N relationship.

For example, in any company an employee can register multiple bank accounts but one bank account will be associated with one and only one employee. In this hibernate one to many mapping annotation example, we will learn to make such mapping in database using hibernate.

Table of Contents

When to use one to many mapping
Hibernate one to many mapping solutions
1. Hibernate one to many mapping with foreign key association
2. Hibernate one to many mapping with join table

When to use one to many mapping

Use one to mapping to create 1..N relationship between entities or objects.

For example, we have to write two entities i.e. EmployeeEntity and AccountEntity such that multiple accounts can be associated with a single employee, but one single account can not be shared between two or more employees.

Hibernate one to many mapping solutions

This problem can be solved in two different ways.

1. One is to have a foreign key column in account table i.e. EMPLOYEE_ID. This column will refer to primary key of Employee table. This way no two accounts can be associated with multiple employees. Obviously, account number needs to be unique for enforcing this restriction.
2. Second approach is to have a common join table lets say EMPLOYEE_ACCOUNT. This table will have two column i.e. EMP_ID which will be foreign key referring to primary key in EMPLOYEE table and similarly ACCOUNT_ID which will be foreign key referring to primary key of ACCOUNT table.

1. Hibernate one to many mapping with foreign key association

In this approach, both entity will be responsible for making the relationship and maintaining it. EmployeeEntity should declare that relationship is one to many, and AccountEntity should declare that relationship from its end is many to one.

1.1. Design one to many mapping relationship

Lets first see the schema design.

1.2. Entity classes

Write entity classes.

package hibernate.test.oneToMany.foreignKeyAsso;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "ForeignKeyAssoEntity")
@Table(name = "Employee", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID"),
@UniqueConstraint(columnNames = "EMAIL") })
public class EmployeeEntity implements Serializable {

	private static final long serialVersionUID = -1798070786993154676L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer employeeId;

	@Column(name = "EMAIL", unique = true, nullable = false, length = 100)
	private String email;

	@Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
	private String firstName;

	@Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
	private String lastName;

	@OneToMany(cascade=CascadeType.ALL)
	@JoinColumn(name="EMPLOYEE_ID")
	private Set<AccountEntity> accounts;

	//Getters and setters
}

Write AccountEntity.java.

package hibernate.test.oneToMany.foreignKeyAsso;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "ForeignKeyAssoAccountEntity")
@Table(name = "ACCOUNT", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID")})
public class AccountEntity implements Serializable
{
	private static final long serialVersionUID = -6790693372846798580L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer accountId;

	@Column(name = "ACC_NUMBER", unique = true, nullable = false, length = 100)
	private String accountNumber;

	@ManyToOne
	private EmployeeEntity employee;

	//Getters and setters
}

1.3. Demo

package hibernate.test.oneToMany;

import hibernate.test.HibernateUtil;
import hibernate.test.oneToMany.foreignKeyAsso.AccountEntity;
import hibernate.test.oneToMany.foreignKeyAsso.EmployeeEntity;

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

import org.hibernate.Session;

public class TestForeignKeyAssociation
{

	public static void main(String[] args)
	{
		Session session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();

		AccountEntity account1 = new AccountEntity();
		account1.setAccountNumber("Account detail 1");

		AccountEntity account2 = new AccountEntity();
		account2.setAccountNumber("Account detail 2");

		AccountEntity account3 = new AccountEntity();
		account3.setAccountNumber("Account detail 3");

		//Add new Employee object
		EmployeeEntity firstEmployee = new EmployeeEntity();
		firstEmployee.setEmail("demo-user-first@mail.com");
		firstEmployee.setFirstName("demo-one");
		firstEmployee.setLastName("user-one");

		EmployeeEntity secondEmployee = new EmployeeEntity();
		secondEmployee.setEmail("demo-user-second@mail.com");
		secondEmployee.setFirstName("demo-two");
		secondEmployee.setLastName("user-two");

		Set<AccountEntity> accountsOfFirstEmployee = new HashSet<AccountEntity>();
		accountsOfFirstEmployee.add(account1);
		accountsOfFirstEmployee.add(account2);

		Set<AccountEntity> accountsOfSecondEmployee = new HashSet<AccountEntity>();
		accountsOfSecondEmployee.add(account3);

		firstEmployee.setAccounts(accountsOfFirstEmployee);
		secondEmployee.setAccounts(accountsOfSecondEmployee);
		//Save Employee
		session.save(firstEmployee);
		session.save(secondEmployee);

		session.getTransaction().commit();
		HibernateUtil.shutdown();
	}
}

Program Output:

Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?

2. Hibernate one to many mapping with join table

This approach uses a join table to store the associations between account and employee entities. @JoinTable annotation has been used to make this association.

2.1. Design

Lets see how the database schema will look like:

2.2. Entity classes

package hibernate.test.oneToMany.joinTable;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "JoinTableEmployeeEntity")
@Table(name = "Employee", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID"),
@UniqueConstraint(columnNames = "EMAIL") })
public class EmployeeEntity implements Serializable
{
	private static final long serialVersionUID = -1798070786993154676L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer employeeId;

	@Column(name = "EMAIL", unique = true, nullable = false, length = 100)
	private String email;

	@Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
	private String firstName;

	@Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
	private String lastName;

	@OneToMany(cascade=CascadeType.ALL)
	@JoinTable(name="EMPLOYEE_ACCOUNT", joinColumns={@JoinColumn(name="EMPLOYEE_ID", referencedColumnName="ID")}
	, inverseJoinColumns={@JoinColumn(name="ACCOUNT_ID", referencedColumnName="ID")})
	private Set<AccountEntity> accounts;

	//Getters and setters
}

package hibernate.test.oneToMany.joinTable;

import java.io.Serializable;

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

@Entity(name = "JoinTableAccountEntity")
@Table(name = "ACCOUNT", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID")})
public class AccountEntity implements Serializable
{

	private static final long serialVersionUID = -6790693372846798580L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer accountId;

	@Column(name = "ACC_NUMBER", unique = true, nullable = false, length = 100)
	private String accountNumber;

	//Getters and setters
}

2.3. Hibernate configuration

We have available both entities to runtime, we have to add them in hibernate.cfg.xml file. Please note that only one set of entities should be configured in configuration file otherwise unexpected results can occur.

< ?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">com.mysql.jdbc.Driver</property>
		<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernatetest</property>
		<property name="hibernate.connection.password">XXXXXX</property>
		<property name="hibernate.connection.username">root</property>
		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
		<property name="show_sql">true</property>
		<property name="hbm2ddl.auto">create</property>
		<mapping clas="hibernate.test.oneToMany.foreignKeyAsso.AccountEntity"></mapping>
		<mapping clas="hibernate.test.oneToMany.foreignKeyAsso.EmployeeEntity"></mapping>
	</session-factory>
</hibernate-configuration>

2.4. Demo

Now, its time to test the code. I have written following code to test above entities.

package hibernate.test.oneToMany;

import hibernate.test.HibernateUtil;
import hibernate.test.oneToMany.joinTable.AccountEntity;
import hibernate.test.oneToMany.joinTable.EmployeeEntity;

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

import org.hibernate.Session;

public class TestJoinTable
{
	public static void main(String[] args)
	{
		Session session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();

		AccountEntity account1 = new AccountEntity();
		account1.setAccountNumber("123-345-65454");

		AccountEntity account2 = new AccountEntity();
		account2.setAccountNumber("123-345-6542222");

		//Add new Employee object
		EmployeeEntity emp = new EmployeeEntity();
		emp.setEmail("demo-user@mail.com");
		emp.setFirstName("demo");
		emp.setLastName("user");

		Set<AccountEntity> accounts = new HashSet<AccountEntity>();
		accounts.add(account1);
		accounts.add(account2);

		emp.setAccounts(accounts);
		//Save Employee
		session.save(emp);

		session.getTransaction().commit();
		HibernateUtil.shutdown();
	}
}

Program Output:

Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)
Hibernate: insert into ACCOUNT (ACC_NUMBER) values (?)
Hibernate: insert into EMPLOYEE_ACCOUNT (EMPLOYEE_ID, ACCOUNT_ID) values (?, ?)
Hibernate: insert into EMPLOYEE_ACCOUNT (EMPLOYEE_ID, ACCOUNT_ID) values (?, ?)

In this hibernate one to many mapping annotation example using list, we learned to create 1..N relationships between two entities using foreign key association and join table techniques.

Happy Learning !!

Read More:

Hibernate one to one mapping annotation example
Hibernate many to many mapping annotation example

Further Reading:

Hibernate One to One Mapping Annotation Example Hibernate many to many mapping annotation example Defining Hibernate Association Mappings