In this example we shall show you how to create a many to one mapping in JPA. The Java Persistence API provides Java developers with an object/relational mapping facility for managing relational data in Java applications.
Here, we are using the JPA to define a many to one mapping between two entities.
ManyToOneMappingInJPA class
In ManyToOneMappingInJPA we create an EntityManagerFactory interface to interact with the entity manager factory for MyPeristenceUnit, that is defined in persistence.xml file. We create an EntityManager, using the createEntityManager() API method. Then, we create new Employee and Department objects. The new objects are written to the database, using the persist(java.lang.Object entity) API method of EntityManager. The getTransaction().begin() and getTransaction().commit() methods are used before and after the EntityManager invokes a method so that a transaction begins and ends. The Employee and Department objects can be retrieved using the find(java.lang.Class<T> entityClass, java.lang.Object primaryKey) API method of EntityManager.
package com.javacodegeeks.snippets.enterprise;
import java.util.Date;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public class ManyToOneMappingInJPA {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("MyPersistenceUnit");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
Department department = new Department();
department.setName("HR");
em.persist(department);
Employee employee = new Employee();
employee.setName("Jack");
employee.setSurname("Thomson");
employee.setTitle("QA Engineer");
employee.setCreated(new Date());
employee.setDepartment(department);
em.persist(employee);
long employeeId = employee.getId();
em.getTransaction().commit();
em.getTransaction().begin();
Employee dbEmployee =em.find(Employee.class, employeeId);
System.out.println("dbEmployee " + dbEmployee);
em.getTransaction().commit();
em.close();
emf.close();
}
}
Employee class and Department class
The Employee class is an entity class, annotated with the javax.persistence.Entity annotation. It uses the @Id annotation to define its id property, and the @GeneratedValue annotation with strategy set to GenerationType.AUTO so that the id gets auto-generated values. In another property, it uses the @ManyToOne annotation to define a many-to-one relationship to another entity, that is Department class.
package com.javacodegeeks.snippets.enterprise;
import java.util.Date;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
@Entity
public class Employee {
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
private Long id;
private String name;
private String surname;
private String title;
private Date created;
@ManyToOne
private Department department;
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 getSurname() {
return surname;
}
public void setSurname(String surname) {
this.surname = surname;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public Date getCreated() {
return created;
}
public void setCreated(Date created) {
this.created = created;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
@Override
public String toString() {
return "Employee [id=" + id + ", name=" + name + ", surname=" + surname
+ ", title=" + title + ", department=" + department + "]";
}
}
package com.javacodegeeks.snippets.enterprise;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
@Entity
public class Department {
@Id @GeneratedValue(strategy=GenerationType.AUTO)
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String deptName) {
this.name = deptName;
}
@Override
public String toString() {
return "Department [id=" + id + ", name=" + name + "]";
}
}
persistence.xml
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="MyPersistenceUnit" transaction-type="RESOURCE_LOCAL">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" />
<property name="hibernate.hbm2ddl.auto" value="update" />
<property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver" />
<property name="hibernate.connection.username" value="jcg" />
<property name="hibernate.connection.password" value="jcg" />
<property name="hibernate.connection.url" value="jdbc:mysql://localhost/companydb" />
</properties>
</persistence-unit>
</persistence>
Output:
dbEmployee Employee [id=1, name=Jack, surname=Thomson, title=QA Engineer, department=Department [id=1, name=HR]]
This was an example of how to create a many to one mapping in JPA.
