JPA/EclipseLink - Component Mapping Using Annotations |@Embeddable & @Embedded

In this example, we will learn Component Mapping using JPA annotations.Component Mapping represents the has-a relationship, composition is a stronger association where the contained object has no existence of its own.For example, a student has an address, an address cannot exist separately without a student.As the Student and Address entities are strongly related, it is best to store them in a single table.

Create a Java project “JpaComponentMapping”

Folder structure:

Bookstores

Create Java classes:

StudentAddress.java

package aroundcode;
 
import javax.persistence.Column;
import javax.persistence.Embeddable;
 
/**
*
* @author aroundducode
*/
@Embeddable
public class HighAddress {
@Column(name = "POSTAL_CODE")
private int Postcode;
@Column(name = "STREET")
private String street;
 
public StudentAddress() {
}
 
public int getPostalCode() {
return PostCode;
}
 
public void setPostalCode(intPostalCode) {
this.PostalCode = PostalCode;
}
 
public String getStreet() {
return street;
}
 
public void setStreet(String street) {
this.street = street;
}
}

We have marked our EleveAddress class with the @Embeddable annotation so that this class is eligible to be an embeddable class.The @Embeddable annotation is used to specify that the EleveAddress class will be used as a component.EleveAddress cannot have its own primary key and it will use the primary key of the Eleve class.

Student.java

package aroundcode;
 
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Embedded;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
 
/**
*
* @author aroundducode
*/
@Entity
@Table(name = "student")
public class Eleve implements Serializable {
 
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
@Column(name = "ID")
private Long id;
@Column(name = "NAME", length = 20)
private String name;
@Embedded
private StudentAddress address;
 
public Student() {
}
 
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 StudentAddress getAddress() {
return address;
}
 
public void setAddress(HighAddressaddress) {
this.address = address;
}
}

We used the following JPA annotations in our Student class

@Entity – This annotation will mark our Eleve class as an Entity Bean.
@Table – The @Table annotation will map our class to the corresponding database table.You can also specify other attributes such as indexes, catalog, schema, uniqueConstraints.The @Table annotation is an optional annotation;if this annotation is not provided, the class name will be used as the table name.
@Id – The @Id annotation marks the particular field as the primary key of the entity.
@GeneratedValue – This annotation is used to specify how the primary key should be generated.
@Column – This annotation associates the corresponding fields with their respective columns in the database table.
@Embedded – The @Embedded annotation is used to specify that the EleveAddress entity should be stored in the “eleve” table as a component.

persistence.xml

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.1" xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistencehttp://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd">
<persistence-unit name="JpaComponentMappingPU" transaction-type="RESOURCE_LOCAL">
<provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
<class>autourducode.Eleve</class>
<properties>
<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/component_mapping?zeroDateTimeBehavior=CONVERT_TO_NULL"/>
<property name="javax.persistence.jdbc.user" value="autourducode"/>
<property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
<property name="javax.persistence.jdbc.password" value="autourducode"/>
<property name="javax.persistence.schema-generation.database.action" value="create"/>
</properties>
</persistence-unit>
</persistence>

The first and most important property is used to specify the JDBC driver class, in my case Driver.

<property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>

Give the Connection URL of the database and provide the Username and Password for the database connection above.

<property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/component_mapping?zeroDateTimeBehavior=CONVERT_TO_NULL"/>
<property name="javax.persistence.jdbc.user" value="autourducode"/>
<property name="javax.persistence.jdbc.password" value="autourducode"/>

If the property “javax.persistence.schema-generation.database.action” is set to “create“, the database schema will be dropped and recreated on each run.

<property name="javax.persistence.schema-generation.database.action" value="create"/>

you can also use this property

<property name="eclipselink.ddl-generation" value="create-tables"/>

Creating the database schema

After starting our application, eclipseLink creates the database schema

Thread(Thread[main,5,main])--CREATE TABLE high (ID BIGINT AUTO_INCREMENT NOT NULL, NAME VARCHAR(20), ZIPCODE INTEGER, STREET VARCHAR(255), PRIMARY KEY (ID))

The POSTAL_CODE and STREET columns of the EleveAdresse class used as a component have been integrated into the “eleve” table.

I hope this article was useful to you.Thanks for reading it.

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