JPA List Mapping

A List is an interface which is used to insert and delete elements on the basis of index. It can be used when there is a requirement of retrieving elements in a user-defined order.

List Mapping Example

In this example, we embed an object in an entity class and define it as a collection type List.

This example contains the following steps: –

Create an entity class Employee.java under com.tutoraspire.jpa package that contains employee id, name and embedded object (employee Address). The annotation @ElementCollection represents the embedded object.

Employee.java

  package com.tutoraspire.jpa;  import java.util.*;    import javax.persistence.*;  @Entity    public class Employee {    @Id  @GeneratedValue(strategy=GenerationType.AUTO)  private int e_id;  private String e_name;    @ElementCollection  private List<Address> address=new ArrayList<Address>();      public int getE_id() {  return e_id;  }    public void setE_id(int e_id) {  this.e_id = e_id;  }    public String getE_name() {  return e_name;  }    public void setE_name(String e_name) {  this.e_name = e_name;  }    public List<Address> getAddress() {  return address;  }    public void setAddress(List<Address> address) {  this.address = address;  }      }  

Now, create a class of embedded object Address.java under com.tutoraspire.jpa package. The annotation @Embeddable represents the embeddable object.

Address.java

  package com.tutoraspire.jpa;  import javax.persistence.*;    @Embeddable  public class Address {    private int e_pincode;  private String e_city;  private String e_state;  public int getE_pincode() {  return e_pincode;  }  public void setE_pincode(int e_pincode) {  this.e_pincode = e_pincode;  }  public String getE_city() {  return e_city;  }  public void setE_city(String e_city) {  this.e_city = e_city;  }  public String getE_state() {  return e_state;  }  public void setE_state(String e_state) {  this.e_state = e_state;  }        }  

Now, map the entity class and other databases confiuguration in Persistence.xml file.

Persistence.xml

  <persistence>  <persistence-unit name=”Collection_Type”>    <class>com.tutoraspire.jpa.Employee</class>  <class>com.tutoraspire.jpa.Address</class>        <properties>  <property name=”javax.persistence.jdbc.driver” value=”com.mysql.jdbc.Driver”/>  <property name=”javax.persistence.jdbc.url” value=”jdbc:mysql://localhost:3306/collection_mapping”/>  <property name=”javax.persistence.jdbc.user” value=”root”/>  <property name=”javax.persistence.jdbc.password” value=””/>  <property name=”eclipselink.logging.level” value=”SEVERE”/>  <property name=”eclipselink.ddl-generation” value=”create-or-extend-tables”/>  </properties>    </persistence-unit>  </persistence>  

Create a persistence class ListMapping.java under com.tutoraspire.collection package to persist the entity object with data.

ListMapping.java

  package com.tutoraspire.collection;  import javax.persistence.*;      import com.tutoraspire.jpa.*;  public class ListMapping{    public static void main(String[] args) {    EntityManagerFactory emf=Persistence.createEntityManagerFactory(“Collection_Type”);  EntityManager em=emf.createEntityManager();    em.getTransaction().begin();          Address a1=new Address();  a1.setE_pincode(201301);  a1.setE_city(“Noida”);  a1.setE_state(“Uttar Pradesh”);            Address a2=new Address();  a2.setE_pincode(302001);  a2.setE_city(“Jaipur”);  a2.setE_state(“Rajasthan”);        Employee e1=new Employee();  e1.setE_id(1);  e1.setE_name(“Vijay”);  e1.getAddress().add(a1);    Employee e2=new Employee();  e2.setE_id(2);  e2.setE_name(“John”);  e2.getAddress().add(a2);    em.persist(e1);  em.persist(e2);    em.getTransaction().commit();    em.close();  emf.close();        }    }  

Output:

After the execution of the program, the following table are generated under MySQL workbench.

Employee table – This table contains the employee details. To fetch data, run select * from employee query in MySQL.

Employee_address table – This table represents the mapping between employee and address table. To fetch data, run select * from employee_address query in MySQL.

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