JPA Tutorial - JPA ManyToMany Map Example

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The following code shows how to create a many to many mapping.

It states a relation between person and department. One department can have many persons and one person can work for many departments.

Example

The following code is from Department.java.

package com.java2s.common;

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

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Department implements java.io.Serializable{
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
  
  private String name;
  
  @ManyToMany
  private Set<Person> persons = new HashSet<Person>();
  
  public Set<Person> getPersons() {
    return persons;
  }

  public void setPersons(Set<Person> persons) {
    this.persons = persons;
  }

  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;
  }

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

}

The following code is from Person.java.

package com.java2s.common;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Person {
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private long id;
   
  private String name;
  
  @ManyToMany
  private Set<Department> departments = new HashSet<Department>();
  
  public Person() {}

  public Person(String name) {
    this.name = name;
  }

 
  public Set<Department> getDepartments() {
    return departments;
  }

  public void setDepartments(Set<Department> departments) {
    this.departments = departments;
  }

  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;
  }

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


}

The following code is from PersonDaoImpl.java.

package com.java2s.common;

import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

import org.springframework.transaction.annotation.Transactional;

@Transactional
public class PersonDaoImpl {
  public void test(){
    Department d1 = new Department();
    d1.setName("Design");

    Department d2 = new Department();
    d2.setName("Testing");
    
    Person p1 = new Person("Tom");
    p1.getDepartments().add(d1);
    p1.getDepartments().add(d2);
    
    Person p2 = new Person("Jack");
    p2.getDepartments().add(d1);
    p2.getDepartments().add(d2);
    
    d1.getPersons().add(p1);
    d1.getPersons().add(p2);
    
    
    em.persist(p1);
    em.persist(p2);
    em.persist(d1);
    em.persist(d2);
  }
  @PersistenceContext
  private EntityManager em;
}

The following is the database dump.

Table Name: DEPARTMENT
 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Design

 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 2

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Testing


Table Name: DEPARTMENT_PERSON
 Row:
    Column Name: DEPARTMENT_ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: PERSONS_ID,
    Column Type: BIGINT:
    Column Value: 1

 Row:
    Column Name: DEPARTMENT_ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: PERSONS_ID,
    Column Type: BIGINT:
    Column Value: 2





Table Name: PERSON
 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Tom

 Row:
    Column Name: ID,
    Column Type: BIGINT:
    Column Value: 2

    Column Name: NAME,
    Column Type: VARCHAR:
    Column Value: Jack





Table Name: PERSON_DEPARTMENT
 Row:
    Column Name: PERSON_ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: DEPARTMENTS_ID,
    Column Type: BIGINT:
    Column Value: 1

 Row:
    Column Name: PERSON_ID,
    Column Type: BIGINT:
    Column Value: 1

    Column Name: DEPARTMENTS_ID,
    Column Type: BIGINT:
    Column Value: 2

 Row:
    Column Name: PERSON_ID,
    Column Type: BIGINT:
    Column Value: 2

    Column Name: DEPARTMENTS_ID,
    Column Type: BIGINT:
    Column Value: 1

 Row:
    Column Name: PERSON_ID,
    Column Type: BIGINT:
    Column Value: 2

    Column Name: DEPARTMENTS_ID,
    Column Type: BIGINT:
    Column Value: 2

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