Many-to-Many relationship in real world:
Two items are said to be in Many-to-Many relationship if many occurrence of item are belong to the many occurrences of other item and vice versa. E.g. Many student – Many subject.
Many-to-Many relationship in programming:
Two entities are said to be in Many-to-Many relationship if many occurrence of entity belongs to many occurrence of other entity and vice versa.
Example:
Student.java
import java.util.Set; /** * This class represents a persistent class for Student. * @author w3spoint */ public class Student { //data members private int studentId; private String firstName; private String lastName; private String className; private String rollNo; private int age; private Set subjects; //no-argument constructor public Student(){ } //argument constructor public Student(String firstName, String lastName, String className, String rollNo, int age){ this.firstName = firstName; this.lastName = lastName; this.className = className; this.rollNo = rollNo; this.age = age; } //getter and setter methods 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 String getClassName() { return className; } public void setClassName(String className) { this.className = className; } public String getRollNo() { return rollNo; } public void setRollNo(String rollNo) { this.rollNo = rollNo; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public Set getSubjects() { return subjects; } public void setSubjects(Set subjects) { this.subjects = subjects; } } |
Subject.java
/** * This class represents a persistent class for Subject. * @author w3spoint */ public class Subject { //data members private int subjectId; private String subjectName; //no argument constructor public Subject(){ } //argument constructor public Subject(String subjectName){ this.subjectName = subjectName; } //getter and setter methods public int getSubjectId() { return subjectId; } public void setSubjectId(int subjectId) { this.subjectId = subjectId; } public String getSubjectName() { return subjectName; } public void setSubjectName(String subjectName) { this.subjectName = subjectName; } public boolean equals(Object obj) { if (obj == null){ return false; } if (!this.getClass().equals(obj.getClass())){ return false; } Subject obj2 = (Subject)obj; if((this.subjectId == obj2.getSubjectId()) && (this.subjectName.equals(obj2.getSubjectName()))){ return true; } return false; } public int hashCode() { int tmp = 0; tmp = ( subjectId + subjectName).hashCode(); return tmp; } } |
hibernate.cfg.xml
<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE hibernate-configuration SYSTEM "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"> <hibernate-configuration> <session-factory> <property name="dialect"> org.hibernate.dialect.OracleDialect </property> <property name="connection.url"> jdbc:oracle:thin:@localhost:1521:XE </property> <property name="connection.username"> system </property> <property name="connection.password"> oracle </property> <property name="connection.driver_class"> oracle.jdbc.driver.OracleDriver </property> <property name="hbm2ddl.auto"> update </property> <property name="show_sql"> true </property> <mapping resource="student.hbm.xml"/> <mapping resource="subject.hbm.xml"/> </session-factory> </hibernate-configuration> |
student.hbm.xml
<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE hibernate-mapping SYSTEM "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="com.w3spoint.business.Student" table="Student"> <id name="studentId" type="int" column="Student_Id"> <generator class="native"></generator> </id> <property name="firstName" column="First_Name" type="string"/> <property name="lastName" column="Last_Name" type="string"/> <property name="className" column="Class" type="string"/> <property name="rollNo" column="RollNo" type="string"/> <property name="age" column="Age" type="int"/> <set name="subjects" cascade="all"> <key column="Student_Id"/> <many-to-many class="com.w3spoint.business.Subject"/> </set> </class> </hibernate-mapping> |
subject.hbm.xml
<?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE hibernate-mapping SYSTEM "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"> <hibernate-mapping> <class name="com.w3spoint.business.Subject" table="Subject"> <id name="subjectId" type="int" column="Subject_Id"> <generator class="native"></generator> </id> <property name="subjectName" column="Subject_Name" type="string"></property> </class> </hibernate-mapping> |
HibernateUtil.java
import org.hibernate.SessionFactory; import org.hibernate.cfg.Configuration; /** * This is a utility class for getting the hibernate session object. * @author w3spoint */ public class HibernateUtil { private static final SessionFactory sessionFactory = buildSessionFactory(); private static SessionFactory buildSessionFactory() { SessionFactory sessionFactory = null; try { //Create the configuration object. Configuration configuration = new Configuration(); //Initialize the configuration object //with the configuration file data configuration.configure("hibernate.cfg.xml"); // Get the SessionFactory object from configuration. sessionFactory = configuration.buildSessionFactory(); } catch (Exception e) { e.printStackTrace(); } return sessionFactory; } public static SessionFactory getSessionFactory() { return sessionFactory; } } |
HibernateTest.java
import java.util.HashSet; import com.w3spoint.persistence.StudentDBOperations; /** * This class is used for the hibernate operations. * @author w3spoint */ public class HibernateTest { public static void main(String args[]){ HashSet subjectSet1 = new HashSet(); subjectSet1.add(new Subject("Data Structure")); subjectSet1.add(new Subject("Operting System")); HashSet subjectSet2 = new HashSet(); subjectSet2.add(new Subject("Compier")); subjectSet2.add(new Subject("Networking")); subjectSet2.add(new Subject("DBMS")); //Create the student object. Student student1 = new Student("Shveta", "Gogia", "MCA final", "MCA/07/72", 27); Student student2 = new Student("Bharti", "Grover", "MCA final", "MCA/07/73", 32); student1.setSubjects(subjectSet1); student2.setSubjects(subjectSet2); StudentDBOperations obj = new StudentDBOperations(); //insert student object. obj.addStudent(student1); obj.addStudent(student2); //show all student object. obj.showAllStudentDetails(); } } |
StudentDBOperations.java
import java.util.List; import java.util.Set; import org.hibernate.HibernateException; import org.hibernate.Session; import org.hibernate.Transaction; import com.w3spoint.business.Student; import com.w3spoint.business.Subject; /** * This class contains the methods to interact with database. * @author w3spoint */ public class StudentDBOperations { /** * This method is used to insert a new student record. * @param student * @return studentId * @author w3spoint */ public Integer addStudent(Student student){ Transaction tx = null; Integer studentId = null; //Get the session object. Session session = HibernateUtil.getSessionFactory().openSession(); try{ tx = session.beginTransaction(); studentId = (Integer) session.save(student); tx.commit(); }catch (HibernateException e) { if(tx!=null){ tx.rollback(); } e.printStackTrace(); }finally { session.close(); } return studentId; } /** * This method is used retrieve and show the records. * @author w3spoint */ public void showAllStudentDetails(){ Transaction tx = null; //Get the session object. Session session = HibernateUtil.getSessionFactory().openSession(); try{ tx = session.beginTransaction(); List<Student> students = session.createQuery("FROM Student").list(); for(Student student : students){ System.out.println("First Name: " + student.getFirstName()); System.out.println("Last Name: " + student.getLastName()); System.out.println("Class: " + student.getClassName()); System.out.println("RollNo: " + student.getRollNo()); System.out.println("Age: " + student.getAge()); Set<Subject> subjects = student.getSubjects(); for(Subject subject : subjects){ System.out.println("Subject Name:" + subject.getSubjectName()); } } tx.commit(); }catch (HibernateException e) { if(tx!=null){ tx.rollback(); } e.printStackTrace(); }finally { session.close(); } } } |
Output:
Hibernate: select hibernate_sequence.nextval from dual Hibernate: select hibernate_sequence.nextval from dual Hibernate: select hibernate_sequence.nextval from dual Hibernate: insert into Student (First_Name, Last_Name, Class, RollNo, Age, Student_Id) values (?, ?, ?, ?, ?, ?) Hibernate: insert into Subject (Subject_Name, Subject_Id) values (?, ?) Hibernate: insert into Subject (Subject_Name, Subject_Id) values (?, ?) Hibernate: insert into subjects (Student_Id, elt) values (?, ?) Hibernate: insert into subjects (Student_Id, elt) values (?, ?) Hibernate: select hibernate_sequence.nextval from dual Hibernate: select hibernate_sequence.nextval from dual Hibernate: select hibernate_sequence.nextval from dual Hibernate: select hibernate_sequence.nextval from dual Hibernate: insert into Student (First_Name, Last_Name, Class, RollNo, Age, Student_Id) values (?, ?, ?, ?, ?, ?) Hibernate: insert into Subject (Subject_Name, Subject_Id) values (?, ?) Hibernate: insert into Subject (Subject_Name, Subject_Id) values (?, ?) Hibernate: insert into Subject (Subject_Name, Subject_Id) values (?, ?) Hibernate: insert into subjects (Student_Id, elt) values (?, ?) Hibernate: insert into subjects (Student_Id, elt) values (?, ?) Hibernate: insert into subjects (Student_Id, elt) values (?, ?) Hibernate: select student0_.Student_Id as Student1_0_, student0_.First_Name as First2_0_, student0_.Last_Name as Last3_0_, student0_.Class as Class0_, student0_.RollNo as RollNo0_, student0_.Age as Age0_ from Student student0_ First Name: Shveta Last Name: Gogia Class: MCA final RollNo: MCA/07/72 Age: 27 Hibernate: select subjects0_.Student_Id as Student1_1_, subjects0_.elt as elt1_, subject1_.Subject_Id as Subject1_2_0_, subject1_.Subject_Name as Subject2_2_0_ from subjects subjects0_, Subject subject1_ where subjects0_.elt=subject1_.Subject_Id(+) and subjects0_.Student_Id=? Subject Name:Operting System Subject Name:Data Structure First Name: Bharti Last Name: Grover Class: MCA final RollNo: MCA/07/73 Age: 32 Hibernate: select subjects0_.Student_Id as Student1_1_, subjects0_.elt as elt1_, subject1_.Subject_Id as Subject1_2_0_, subject1_.Subject_Name as Subject2_2_0_ from subjects subjects0_, Subject subject1_ where subjects0_.elt=subject1_.Subject_Id(+) and subjects0_.Student_Id=? Subject Name:Compier Subject Name:DBMS Subject Name:Networking |
