Hibernate - Many-to-Many Mappings

A Many-to-Many mapping can be implemented using a Set java collection that does not contain any duplicate element. We already have seen how to map Set collection in hibernate, so if you already learned Set mapping, then you are all set to go with manyto-many mapping.
A Set is mapped with a <set> element in the mapping table and initialized with java.util.HashSet. You can use Set collection in your class when there is no duplicate element required in the collection.
Consider a situation where we need to store our employee records in EMPLOYEE table, which will have the following structure −
create table EMPLOYEE (
   id INT NOT NULL auto_increment,
   first_name VARCHAR(20) default NULL,
   last_name  VARCHAR(20) default NULL,
   salary     INT  default NULL,
   PRIMARY KEY (id)
);
Further, assume each employee can have one or more certificate associated with him/her and a similar certificate can be associated with more than one employee. We will store certificate related information in a separate table, which has the following structure −
create table CERTIFICATE (
   id INT NOT NULL auto_increment,
   certificate_name VARCHAR(30) default NULL,
   PRIMARY KEY (id)
);
Now to implement many-to-many relationship between EMPLOYEE and CERTIFICATE objects, we would have to introduce one more intermediate table having Employee ID and Certificate ID as follows −
create table EMP_CERT (
   employee_id INT NOT NULL,
   certificate_id INT NOT NULL,
   PRIMARY KEY (employee_id,certificate_id)
);
Let us implement our POJO class Employee, which will be used to persist the objects related to EMPLOYEE table and having a collection of certificates in Set variable.
import java.util.*;

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private Set certificates;

   public Employee() {}
   
   public Employee(String fname, String lname, int salary) {
      this.firstName = fname;
      this.lastName = lname;
      this.salary = salary;
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getFirstName() {
      return firstName;
   }
   
   public void setFirstName( String first_name ) {
      this.firstName = first_name;
   }
   
   public String getLastName() {
      return lastName;
   }
   
   public void setLastName( String last_name ) {
      this.lastName = last_name;
   }
   
   public int getSalary() {
      return salary;
   }
   
   public void setSalary( int salary ) {
      this.salary = salary;
   }

   public Set getCertificates() {
      return certificates;
   }
   
   public void setCertificates( Set certificates ) {
      this.certificates = certificates;
   }
}
Now let us define another POJO class corresponding to CERTIFICATE table so that certificate objects can be stored and retrieved into the CERTIFICATE table. This class should also implement both the equals() and hashCode() methods so that Java can determine whether any two elements/objects are identical.
public class Certificate {
   private int id;
   private String name; 

   public Certificate() {}
   
   public Certificate(String name) {
      this.name = name;
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getName() {
      return name;
   }
   public void setName( String name ) {
      this.name = name;
   }
   
   public boolean equals(Object obj) {
      if (obj == null) return false;
      if (!this.getClass().equals(obj.getClass())) return false;

      Certificate obj2 = (Certificate)obj;
      if((this.id == obj2.getId()) && (this.name.equals(obj2.getName()))) {
         return true;
      }
      return false;
   }
   
   public int hashCode() {
      int tmp = 0;
      tmp = ( id + name ).hashCode();
      return tmp;
   }
}
Let us develop our mapping file, which instructs Hibernate — how to map the defined classes to the database tables. The <set> element will be used to define the rule for manyto-many relationship.
<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-mapping PUBLIC 
"-//Hibernate/Hibernate Mapping DTD//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> 

<hibernate-mapping>
   <class name = "Employee" table = "EMPLOYEE">
      
      <meta attribute = "class-description">
         This class contains the employee detail. 
      </meta>
      
      <id name = "id" type = "int" column = "id">
         <generator class="native"/>
      </id>
      
      <set name = "certificates" cascade="save-update" table="EMP_CERT">
         <key column = "employee_id"/>
         <many-to-many column = "certificate_id" class="Certificate"/>
      </set>
      
      <property name = "firstName" column = "first_name" type = "string"/>
      <property name = "lastName" column = "last_name" type = "string"/>
      <property name = "salary" column = "salary" type = "int"/>
      
   </class>

   <class name = "Certificate" table = "CERTIFICATE">
      
      <meta attribute = "class-description">
         This class contains the certificate records. 
      </meta>
      
      <id name = "id" type = "int" column = "id">
         <generator class="native"/>
      </id>
      
      <property name = "name" column = "certificate_name" type = "string"/>
      
   </class>

</hibernate-mapping>
You should save the mapping document in a file with the format <classname>.hbm.xml. We saved our mapping document in the file Employee.hbm.xml. You are already familiar with most of the mapping detail, but let us see all the elements of mapping file once again −
The mapping document is an XML document having <hibernate-mapping> as the root element which contains two <class> elements corresponding to each class.
The mapping document is an XML document having <hibernate-mapping> as the root element which contains two <class> elements corresponding to each class.
The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
The <class> elements are used to define specific mappings from a Java classes to the database tables. The Java class name is specified using the name attribute of the class element and the database table name is specified using the table attribute.
The <meta> element is optional element and can be used to create the class description.
The <meta> element is optional element and can be used to create the class description.
The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
The <id> element maps the unique ID attribute in class to the primary key of the database table. The name attribute of the id element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
The <generator> element within the id element is used to generate the primary key values automatically. The class attribute of the generator element is set to native to let hibernate pick up either identity, sequence or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
The <generator> element within the id element is used to generate the primary key values automatically. The class attribute of the generator element is set to native to let hibernate pick up either identity, sequence or hilo algorithm to create primary key depending upon the capabilities of the underlying database.
The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
The <property> element is used to map a Java class property to a column in the database table. The name attribute of the element refers to the property in the class and the column attribute refers to the column in the database table. The type attribute holds the hibernate mapping type, this mapping types will convert from Java to SQL data type.
The <set> element sets the relationship between Certificate and Employee classes. We set cascade attribute to save-update to tell Hibernate to persist the Certificate objects for SAVE i.e. CREATE and UPDATE operations at the same time as the Employee objects. The name attribute is set to the defined Set variable in the parent class, in our case it is certificates. For each set variable, we need to define a separate set element in the mapping file. Here we used name attribute to set the intermediate table name to EMP_CERT.
The <set> element sets the relationship between Certificate and Employee classes. We set cascade attribute to save-update to tell Hibernate to persist the Certificate objects for SAVE i.e. CREATE and UPDATE operations at the same time as the Employee objects. The name attribute is set to the defined Set variable in the parent class, in our case it is certificates. For each set variable, we need to define a separate set element in the mapping file. Here we used name attribute to set the intermediate table name to EMP_CERT.
The <key> element is the column in the EMP_CERT table that holds the foreign key to the parent object ie. table EMPLOYEE and links to the certification_id in the CERTIFICATE table.
The <key> element is the column in the EMP_CERT table that holds the foreign key to the parent object ie. table EMPLOYEE and links to the certification_id in the CERTIFICATE table.
The <many-to-many> element indicates that one Employee object relates to many Certificate objects and column attributes are used to link intermediate EMP_CERT.
The <many-to-many> element indicates that one Employee object relates to many Certificate objects and column attributes are used to link intermediate EMP_CERT.
Finally, we will create our application class with the main() method to run the application. We will use this application to save few Employee's records along with their certificates and then we will apply CRUD operations on those records.
import java.util.*;
 
import org.hibernate.HibernateException; 
import org.hibernate.Session; 
import org.hibernate.Transaction;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class ManageEmployee {
   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); 
      }
      
      ManageEmployee ME = new ManageEmployee();
      /* Let us have a set of certificates for the first employee  */
      HashSet certificates = new HashSet();

      certificates.add(new Certificate("MCA"));
      certificates.add(new Certificate("MBA"));
      certificates.add(new Certificate("PMP"));
     
      /* Add employee records in the database */
      Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, certificates);

      /* Add another employee record in the database */
      Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, certificates);

      /* List down all the employees */
      ME.listEmployees();

      /* Update employee's salary records */
      ME.updateEmployee(empID1, 5000);

      /* Delete an employee from the database */
      ME.deleteEmployee(empID2);

      /* List down all the employees */
      ME.listEmployees();

   }

   /* Method to add an employee record in the database */
   public Integer addEmployee(String fname, String lname, int salary, Set cert){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = new Employee(fname, lname, salary);
         employee.setCertificates(cert);
         employeeID = (Integer) session.save(employee); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return employeeID;
   }

   /* Method to list all the employees detail */
   public void listEmployees( ){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         List employees = session.createQuery("FROM Employee").list(); 
         for (Iterator iterator1 = employees.iterator(); iterator1.hasNext();){
            Employee employee = (Employee) iterator1.next(); 
            System.out.print("First Name: " + employee.getFirstName()); 
            System.out.print("  Last Name: " + employee.getLastName()); 
            System.out.println("  Salary: " + employee.getSalary());
            Set certificates = employee.getCertificates();
            for (Iterator iterator2 = certificates.iterator(); iterator2.hasNext();){
               Certificate certName = (Certificate) iterator2.next(); 
               System.out.println("Certificate: " + certName.getName()); 
            }
         }
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
   
   /* Method to update salary for an employee */
   public void updateEmployee(Integer EmployeeID, int salary ){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = (Employee)session.get(Employee.class, EmployeeID); 
         employee.setSalary( salary );
         session.update(employee);
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
   
   /* Method to delete an employee from the records */
   public void deleteEmployee(Integer EmployeeID){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = (Employee)session.get(Employee.class, EmployeeID); 
         session.delete(employee); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
}
Here are the steps to compile and run the above mentioned application. Make sure you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.
Create hibernate.cfg.xml configuration file as explained in configuration chapter.
Create hibernate.cfg.xml configuration file as explained in configuration chapter.
Create Employee.hbm.xml mapping file as shown above.
Create Employee.hbm.xml mapping file as shown above.
Create Employee.java source file as shown above and compile it.
Create Employee.java source file as shown above and compile it.
Create Certificate.java source file as shown above and compile it.
Create Certificate.java source file as shown above and compile it.
Create ManageEmployee.java source file as shown above and compile it.
Create ManageEmployee.java source file as shown above and compile it.
Execute ManageEmployee binary to run the program.
Execute ManageEmployee binary to run the program.
You would get the following result on the screen, and same time records would be created in EMPLOYEE, EMP_CERT and CERTIFICATE tables.
$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Manoj  Last Name: Kumar  Salary: 4000
Certificate: MBA
Certificate: PMP
Certificate: MCA
First Name: Dilip  Last Name: Kumar  Salary: 3000
Certificate: MBA
Certificate: PMP
Certificate: MCA
First Name: Manoj  Last Name: Kumar  Salary: 5000
Certificate: MBA
Certificate: PMP
Certificate: MCA
If you check your EMPLOYEE, EMP_CERT and CERTIFICATE tables, they should have following records −
mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 22 | Manoj      | Kumar     |   5000 |
+----+------------+-----------+--------+
1 row in set (0.00 sec)

mysql> select * from CERTIFICATE;
+----+------------------+
| id | certificate_name |
+----+------------------+
|  4 | MBA              |
|  5 | PMP              |
|  6 | MCA              |
+----+------------------+
3 rows in set (0.00 sec)

mysql> select * from EMP_CERT;
+-------------+----------------+
| employee_id | certificate_id |
+-------------+----------------+
|          22 |              4 |
|          22 |              5 |
|          22 |              6 |
+-------------+----------------+
3 rows in set (0.00 sec)

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