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JPA One-to-Many, Many-to-One Bidirectional Association Mapping
Many-To-One mapping is an association between collection of same persistence objects and their related persistence object. Many persistence objects mapped to one related persistence object.
One-to-many mapping is an association between one persistence object holding the collection of same related persistence objects. One persistence object mapped to many persistence objects.
Tables of both the persistence classes will be related in database.
If one persistence object uses other and in back if other using the first persistence object then it becomes bidirectional. If it is bidirectional, then one-to-many bidirectional association mapping and many-to-one bidirectional association mapping both become one and same.
As an example, the association between Employee and Department. Here Employee needs Department information and as well as Department needs Employee information and hence it becomes bidirectional maaping.
JPA One-to-Many, Many-to-One Bidirectional Association Mapping Example
Now let us see how this mapping is done in persistence class by going through an example. The following code shows how to do one-to-many bidirectional association mapping and many-to-one bidirectional association mapping. We created two entities, Employee and Department. One Employee belongs to one Department but one Department can have many Employee. In Employee class we annotate the Department field with @ManyToOne annotation. In Department class we annotate the collection of Employee by @OneToMany annotation.
Database script (MySQL)
DROP TABLE EMPLOYEE;
COMMIT;
DROP TABLE DEPARTMENT;
COMMIT;
CREATE TABLE DEPARTMENT(
DEPTID INT(5) PRIMARY KEY AUTO_INCREMENT,
DNAME VARCHAR(30)
);
CREATE TABLE EMPLOYEE(
EMPID INT(5) PRIMARY KEY AUTO_INCREMENT,
ENAME VARCHAR(30),
EMAIL VARCHAR(30),
DEPTID INT(5),
CONSTRAINT FOREIGN KEY (DEPTID) REFERENCES DEPARTMENT (DEPTID)
);
pom.xml
<project xmlns="https://maven.apache.org/POM/4.0.0" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="https://maven.apache.org/POM/4.0.0 https://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.java4coding</groupId>
<artifactId>12M_M21_BidirectionalAssociationMapping</artifactId>
<packaging>jar</packaging>
<version>1.0-SNAPSHOT</version>
<name>12M_M21_BidirectionalAssociationMapping</name>
<url>https://maven.apache.org</url>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>3.8.1</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.eclipse.persistence</groupId>
<artifactId>javax.persistence</artifactId>
<version>2.0.0</version>
</dependency>
<dependency>
<groupId>org.hibernate</groupId>
<artifactId>hibernate-entitymanager</artifactId>
<version>4.2.8.Final</version>
</dependency>
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.11</version>
</dependency>
</dependencies>
</project>
persistence.xml
<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="https://java.sun.com/xml/ns/persistence"
xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="https://java.sun.com/xml/ns/persistence
https://java.sun.com/xml/ns/persistence/persistence_2_0.xsd"
version="2.0">
<persistence-unit name="EmployeePU">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<properties>
<property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/study" />
<property name="hibernate.connection.driver_class" value="com.mysql.cj.jdbc.Driver" />
<property name="hibernate.connection.username" value="root" />
<property name="hibernate.connection.password" value="root" />
<property name="hibernate.archive.autodetection" value="class" />
<property name="hibernate.show_sql" value="true" />
<property name="hibernate.format_sql" value="true" />
<property name="hbm2ddl.auto" value="update" />
</properties>
</persistence-unit>
</persistence>
Employee.java
package com.java4coding;
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.ManyToOne;
import javax.persistence.Table;
@Entity
@Table(name = "EMPLOYEE")
public class Employee {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "EMPID")
private int empid;
@Column(name = "ENAME")
private String ename;
@Column(name = "EMAIL")
private String email;
/* @Column(name = "DEPTID")
private int deptid;*/
@ManyToOne
@JoinColumn(name = "DEPTID")
private Department department;
public int getEmpid() {
return empid;
}
public void setEmpid(int empid) {
this.empid = empid;
}
public String getEname() {
return ename;
}
public void setEname(String ename) {
this.ename = ename;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Department getDepartment() {
return department;
}
public void setDepartment(Department department) {
this.department = department;
}
}
Department.java
package com.java4coding;
import java.util.HashSet;
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.OneToMany;
import javax.persistence.Table;
@Entity
@Table(name = "DEPARTMENT")
public class Department {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@Column(name = "DEPTID")
private int deptid;
@Column(name = "DNAME")
private String dname;
@OneToMany(mappedBy = "department", cascade = CascadeType.ALL)
private Set<Employee> employees = new HashSet<Employee>(0);
public int getDeptid() {
return this.deptid;
}
public void setDeptid(int deptid) {
this.deptid = deptid;
}
public String getDname() {
return this.dname;
}
public void setDname(String dname) {
this.dname = dname;
}
public Set<Employee> getEmployees() {
return this.employees;
}
public void setEmployees(Set<Employee> employees) {
this.employees = employees;
}
}
Test.java
package com.java4coding;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
public class Test {
public static void main(String[] args) {
EntityManagerFactory emf = Persistence.createEntityManagerFactory("EmployeePU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
Department dept = new Department();
dept.setDname("Engineering");
Employee emp1 = new Employee();
emp1.setEname("Manu Manjunatha");
emp1.setEmail("manu.m@java4coding.com");
//ASSOCIATE DEPARTMENT WITH EMPLOYEE
emp1.setDepartment(dept);
Employee emp2 = new Employee();
emp2.setEname("Advith");
emp2.setEmail("advith@java4coding.com");
//ASSOCIATE DEPARTMENT WITH EMPLOYEE
emp2.setDepartment(dept);
Employee emp3 = new Employee();
emp3.setEname("Likitha Tyagraj");
emp3.setEmail("likitha@java4coding.com");
//ASSOCIATE DEPARTMENT WITH EMPLOYEE
emp3.setDepartment(dept);
Set<Employee> s = new HashSet<>();
s.add(emp1);
s.add(emp2);
s.add(emp3);
//ASSOCIATE EMPLOYEES WITH DEPARTMENT
dept.setEmployees(s);
em.persist(dept);
em.getTransaction().commit();
em.getTransaction().begin();
// DEPARTMENT CAN ACCESS EMPLOYEE
Department d = em.find(Department.class, 1);
System.out.println("Department id is " + d.getDeptid());
System.out.println("Department name is " + d.getDname());
Set<Employee> e = d.getEmployees();
for (Employee emp : e) {
System.out.println("employee id is " + emp.getEmpid());
System.out.println("employee name is " + emp.getEname());
System.out.println("employee email is " + emp.getEmail());
}
// EMPLOYEE CAN ACCESS DEPARTMENT BECAUSE IT IS BIDRECTIONAL
Employee emp = em.find(Employee.class, 2);
System.out.println("Employee id is " + emp.getEmpid());
System.out.println("Employee name is " + emp.getEname());
System.out.println("Employee email is " + emp.getEmail());
System.out.println("Employee Department is "+emp.getDepartment().getDeptid());
System.out.println("Employee Department is "+emp.getDepartment().getDname());
em.getTransaction().commit();
}
}
Project directory structure
Output
Hibernate:
insert
into
DEPARTMENT
(DNAME)
values
(?)
Hibernate:
insert
into
EMPLOYEE
(DEPTID, EMAIL, ENAME)
values
(?, ?, ?)
Hibernate:
insert
into
EMPLOYEE
(DEPTID, EMAIL, ENAME)
values
(?, ?, ?)
Hibernate:
insert
into
EMPLOYEE
(DEPTID, EMAIL, ENAME)
values
(?, ?, ?)
Department id is 1
Department name is Engineering
employee id is 1
employee name is Advith
employee email is advith@java4coding.com
employee id is 2
employee name is Likitha Tyagraj
employee email is likitha@java4coding.com
employee id is 3
employee name is Manu Manjunatha
employee email is manu.m@java4coding.com
Employee id is 2
Employee name is Likitha Tyagraj
Employee email is likitha@java4coding.com
Employee Department is 1
Employee Department is Engineering
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