In general, a student can have an address/employee can have an address. For these kind of requirements, we can follow Component mapping. It is nothing but a class having a reference to another class as a member variable. i.e. inside the ‘student’ class, we can have the ‘address’ class as a member variable. In MySQL, only a single table is enough which holds the information of all the attributes of both classes. Here in our example, we can see ‘student’ as primary class and it has ‘address’ as member variable.
Example Project
Project Structure:
MySQL table structure for Student:
-- Student table create table Student( studentId INT NOT NULL auto_increment, firstName VARCHAR(20) default NULL, lastName VARCHAR(20) default NULL, grade INT default NULL, streetName VARCHAR(40) default NULL, -- Belongs to Address class cityName VARCHAR(40) default NULL, -- Belongs to Address class stateName VARCHAR(40) default NULL, -- Belongs to Address class zipCode VARCHAR(10) default NULL, -- Belongs to Address class PRIMARY KEY (studentId) );
It is enough if we have only 1 table and it should have all the fields specified in the second class. Here our first class is ‘Student’ and second class is ‘Address’. Let us see the POJO classes
Student.java
Java
public class Student { // It is always good to go with each attribute of MySQL // table private int studentId; private String firstName; private String lastName; private int grade; // Attributes that are available in 'Address' class. By // referring here, we can access them. As Component // Mapping is used, it is possible private Address address; // As address itself is set as an attribute in student // class, // all the address related arguments can be passed // easily via set method public Address getAddress() { return address; } public void setAddress(Address address) { this.address = address; } public int getStudentId() { return studentId; } public void setStudentId(int studentId) { this.studentId = studentId; } 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; } public int getGrade() { return grade; } public void setGrade(int grade) { this.grade = grade; } public Student(String firstName, String lastName, int grade, Address address) { this.firstName = firstName; this.lastName = lastName; this.grade = grade; this.address = address; } public Student() {}} |
Address.java
Java
public class Address extends Student { private String streetName; private String cityName; private String stateName; private String zipCode; public Address() {} // We should have this constructor so that address // parameters // can be set easily and can be accessed in student // class public Address(String street, String city, String state, String zipcode) { this.streetName = street; this.cityName = city; this.stateName = state; this.zipCode = zipcode; } public String getStreetName() { return streetName; } public void setStreetName(String streetName) { this.streetName = streetName; } public String getCityName() { return cityName; } public void setCityName(String cityName) { this.cityName = cityName; } public String getStateName() { return stateName; } public void setStateName(String stateName) { this.stateName = stateName; } public String getZipCode() { return zipCode; } public void setZipCode(String zipCode) { this.zipCode = zipCode; }} |
This project is a maven driven project. Hence let us start with pom.xml
pom.xml
XML
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 <modelVersion>4.0.0</modelVersion> <groupId>HibernateComponentMapping</groupId> <artifactId>HibernateComponentMapping</artifactId> <version>0.0.1-SNAPSHOT</version> <build> <sourceDirectory>src</sourceDirectory> <resources> <resource> <directory>src</directory> <excludes> <exclude>**/*.java</exclude> </excludes> </resource> </resources> <plugins> <plugin> <artifactId>maven-compiler-plugin</artifactId> <version>3.8.1</version> <configuration> <release>9</release> </configuration> </plugin> </plugins> </build> <dependencies> <dependency> <groupId>org.hibernate</groupId> <artifactId>hibernate-core</artifactId> <version>5.4.15.Final</version> </dependency> <!-- As we are connecting with MySQL, this is needed --> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>5.1.34</version> </dependency> </dependencies> <properties> <maven.compiler.source>1.8</maven.compiler.source> <maven.compiler.target>1.8</maven.compiler.target> </properties></project> |
For hibernate, hibernate.cfg.xml is the main configuration file
hibernate.cfg.xml
XML
<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE hibernate-configuration PUBLIC "-//Hibernate/Hibernate Configuration DTD 3.0//EN"<hibernate-configuration> <session-factory> <!-- As we are connecting mysql, those driver classes, database name, username and password are specified Please change the information as per your requirement --> <property name="hbm2ddl.auto">update</property> <property name="connection.driver_class">com.mysql.jdbc.Driver</property> <property name="connection.username">root</property> <property name="connection.password">admin</property> <!-- We are going to connect student.hbm.xml which has the table information about student which is present in mysql --> <mapping resource="student.hbm.xml" /> </session-factory></hibernate-configuration> |
Our mapping resource is student.hbm.xml.
student.hbm.xml
XML
<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 5.3//EN" <hibernate-mapping> <!-- The <class> elements denotes specific mappings from a Java classes to the database tables. The Java class name is given at the name attribute database table name is given using the table attribute.--> <class name="com.gfg.componentmapping.pojo.Student" table="Student"> <!-- Details about the enclosed class --> <meta attribute = "class-description"> This class contains the student detail. </meta> <!-- Unique ID to the Primary Key of the database table --> <id name = "studentId" type = "int" column = "studentId"> <generator class="native"/> </id> <!-- The <component> element can help to bring about different attributes of Address class inside Student classes. --> <component name = "address" class="com.gfg.componentmapping.pojo.Address"> <property name = "streetName" column = "streetName" type = "string"/> <property name = "cityName" column = "cityName" type = "string"/> <property name = "stateName" column = "stateName" type = "string"/> <property name = "zipCode" column = "zipCode" type = "string"/> </component> <property name = "firstName" column = "firstName" type = "string"/> <property name = "lastName" column = "lastName" type = "string"/> <property name = "grade" column = "grade" type = "int"/> </class> </hibernate-mapping> |
Now let us see the main class where we can do a normal CRUD operation. Let us see the steps of addition and updation of a student in this example.
ComponentMappingPatternOfStoringData.java
Java
import com.gfg.componentmapping.pojo.Address;import com.gfg.componentmapping.pojo.Student;import java.util.Iterator;import java.util.List;import org.hibernate.HibernateException;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;public class ComponentMappingPatternOfStoringData { private static SessionFactory factory; public static void main(String[] args) { try { factory = new Configuration() .configure() .buildSessionFactory(); } catch (Throwable ex) { System.err.println( "Failed to create sessionFactory object." + ex); throw new ExceptionInInitializerError(ex); } ComponentMappingPatternOfStoringData componentMappingExample = new ComponentMappingPatternOfStoringData(); // Address1 Address address1 = componentMappingExample.addAddress( "ByepassRoad", "Chennai", "TN", "600028"); // Add student records in the database Integer studentId = componentMappingExample.addStudent( "Ashwin", "Kumar", 10, address1); // Address2 Address address2 = componentMappingExample.addAddress( "Pattinapakkam", "Chennai", "TN", "600028"); // Add another student record in the database Integer student2 = componentMappingExample.addStudent( "Ruchitaa", "Agarwal", 8, address2); // List down all the students componentMappingExample.listStudents(); // Update student's name as a sample componentMappingExample.updateStudent(studentId, "Nikilesh"); // List down all the students. We can see the // updated value for the first student componentMappingExample.listStudents(); } // Method to add an address record in the database public Address addAddress(String street, String city, String state, String zipcode) { Session session = factory.openSession(); Transaction tx = null; Integer addressID = null; Address address = null; try { tx = session.beginTransaction(); address = new Address(street, city, state, zipcode); addressID = (Integer)session.save(address); tx.commit(); } catch (HibernateException e) { if (tx != null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } return address; } // Method to add a student record in the database public Integer addStudent(String fname, String lname, int salary, Address address) { Session session = factory.openSession(); Transaction tx = null; Integer studentID = null; try { tx = session.beginTransaction(); // By using Student constructor, we can see the // fields Student student = new Student(fname, lname, salary, address); studentID = (Integer)session.save(student); tx.commit(); } catch (HibernateException e) { if (tx != null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } return employeeID; } // Method to list all the student detail public void listStudents() { Session session = factory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); // Take the data from Student table List students = session.createQuery("FROM Student") .list(); // Iterate the details and display them for (Iterator iterator = students.iterator(); iterator.hasNext();) { Student student = (Student)iterator.next(); System.out.print("First Name: " + student.getFirstName()); System.out.print(" Last Name: " + student.getLastName()); System.out.println(" Grade: " + student.getGrade()); Address add = student.getAddress(); System.out.println("Address "); System.out.println("\tStreet: " + add.getStreetName()); System.out.println("\tCity: " + add.getCityName()); System.out.println("\tState: " + add.getStateName()); System.out.println("\tZipcode: " + add.getZipCode()); } tx.commit(); } catch (HibernateException e) { if (tx != null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } } // Method to update name for a given student public void updateStudent(Integer studentId, String firstName) { Session session = factory.openSession(); Transaction tx = null; try { tx = session.beginTransaction(); Student student = (Student)session.get( Student.class, studentId); student.setFirstName(firstName); session.update(student); tx.commit(); } catch (HibernateException e) { if (tx != null) tx.rollback(); e.printStackTrace(); } finally { session.close(); } }} |
On running the project, we can see the below output.
Output:
MySQL output also matches the same
Conclusion
By using component mapping, the association of class elements can be done easily and in MySQL, it is enough if we have a single table only to satisfy the component mapping.
