In this post , we will learn to implement inheritance by using hibernate JOINED strategy .
In this example we have a Person class which will have two fields (firstName and lastName)
Person Class is Extended by Student and Teacher classes , which will share the fields of Person apart from having their own fields.
Here is the code for Base class Person . Please note that we have used @Inheritance annotation in this base class along with strategy defined as JOINED.
package com.javaroots.model;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
/**
*
*
* @author Abhishek Somani
*
*/
@Entity
@Table(name="person")
@Inheritance(strategy=InheritanceType.JOINED)
public class Person {
@Id
@GeneratedValue
@Column(name="person_id")
private Long id;
@Column(name="firstName")
private String firstName;
@Column(name="lastName")
private String lastName;
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
}
This is the Student class which extends Person class. We are using @PrimaryKeyJoinColumn annotation which will create a primary key named student_id for student table which will reference to person table primary key(person_id).
package com.javaroots.model;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;
/**
*
*
* @author Abhishek Somani
*
*/
@Entity
@Table(name = "student")
@PrimaryKeyJoinColumn(name = "student_id", referencedColumnName = "person_id")
public class Student extends Person {
@Column(name = "standard")
private String standard;
@Column(name = "instructor")
private String instructor;
public String getStandard() {
return standard;
}
public void setStandard(String standard) {
this.standard = standard;
}
public String getInstructor() {
return instructor;
}
public void setInstructor(String instructor) {
this.instructor = instructor;
}
}
Similar to Student , we have another class called Teacher which will also get it's primary key(teacher_id) from person table .
package com.javaroots.model;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;
/**
*
*
* @author Abhishek Somani
*
*/
@Entity
@Table(name = "teacher")
@PrimaryKeyJoinColumn(name = "teacher_id", referencedColumnName = "person_id")
public class Teacher extends Person{
@Column(name = "main_subject")
private String mainSubject;
@Column(name = "salary")
private int salary;
@Column(name = "type")
private String type; // Primary or Secondary School teacher
public String getMainSubject() {
return mainSubject;
}
public void setMainSubject(String mainSubject) {
this.mainSubject = mainSubject;
}
public int getSalary() {
return salary;
}
public void setSalary(int salary) {
this.salary = salary;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
}
This is the test class to create entries in Student , Teacher and Person Table.
package com.javaroots.main;
import org.hibernate.Session;
import com.javaroots.model.Student;
import com.javaroots.model.Teacher;
import com.javaroots.util.HibernateUtil;
public class HibernateTest {
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().openSession();
session.beginTransaction();
Student st = new Student();
st.setInstructor("Joshua");
st.setFirstName("James");
st.setLastName("java roots");
session.save(st);
Teacher te = new Teacher();
te.setMainSubject("java");
te.setSalary(1000);
te.setFirstName("Gosling");
te.setLastName("opensource");
session.save(te);
session.getTransaction().commit();
}
}
These are the structures of tables(person , teacher , student) . teacher and student table's primary key references to person table primary key .
CREATE TABLE `person` ( `person_id` bigint(20) NOT NULL AUTO_INCREMENT, `firstName` varchar(255) DEFAULT NULL, `lastName` varchar(255) DEFAULT NULL, PRIMARY KEY (`person_id`) ) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8$$ delimiter $$ CREATE TABLE `student` ( `instructor` varchar(255) DEFAULT NULL, `standard` varchar(255) DEFAULT NULL, `student_id` bigint(20) NOT NULL, PRIMARY KEY (`student_id`), KEY `FK8FFE823B961B9B25` (`student_id`), CONSTRAINT `FK8FFE823B961B9B25` FOREIGN KEY (`student_id`) REFERENCES `person` (`person_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8$$ delimiter $$ CREATE TABLE `teacher` ( `main_subject` varchar(255) DEFAULT NULL, `salary` int(11) DEFAULT NULL, `type` varchar(255) DEFAULT NULL, `teacher_id` bigint(20) NOT NULL, PRIMARY KEY (`teacher_id`), KEY `FKAA31CBE28C839C1E` (`teacher_id`), CONSTRAINT `FKAA31CBE28C839C1E` FOREIGN KEY (`teacher_id`) REFERENCES `person` (`person_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8$$These are the queries fired by hibernate . Whenever a teacher or student object is created , hibernate first create an entry in person table , and then creates entry in student or teacher table with the primary id of person .
Hibernate:
alter table student
drop
foreign key FK8FFE823B961B9B25
Hibernate:
alter table teacher
drop
foreign key FKAA31CBE28C839C1E
Hibernate:
drop table if exists person
Hibernate:
drop table if exists student
Hibernate:
drop table if exists teacher
Hibernate:
create table person (
person_id bigint not null auto_increment,
firstName varchar(255),
lastName varchar(255),
primary key (person_id)
)
Hibernate:
create table student (
instructor varchar(255),
standard varchar(255),
student_id bigint not null,
primary key (student_id)
)
Hibernate:
create table teacher (
main_subject varchar(255),
salary integer,
type varchar(255),
teacher_id bigint not null,
primary key (teacher_id)
)
Hibernate:
alter table student
add index FK8FFE823B961B9B25 (student_id),
add constraint FK8FFE823B961B9B25
foreign key (student_id)
references person (person_id)
Hibernate:
alter table teacher
add index FKAA31CBE28C839C1E (teacher_id),
add constraint FKAA31CBE28C839C1E
foreign key (teacher_id)
references person (person_id)
Hibernate:
insert
into
person
(firstName, lastName)
values
(?, ?)
Hibernate:
insert
into
student
(instructor, standard, student_id)
values
(?, ?, ?)
Hibernate:
insert
into
person
(firstName, lastName)
values
(?, ?)
Hibernate:
insert
into
teacher
(main_subject, salary, type, teacher_id)
values
(?, ?, ?, ?)
This is how the inheritance is implemented in hibenrate JOINED inheritance strategy.
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