Hibernate - Annotations

So far you have seen how Hibernate uses XML mapping file for the transformation of data from POJO to database tables and vice versa. Hibernate annotations are the newest way to define mappings without the use of XML file. You can use annotations in addition to or as a replacement of XML mapping metadata.

Hibernate Annotations is the powerful way to provide the metadata for the Object and Relational Table mapping. All the metadata is clubbed into the POJO java file along with the code, this helps the user to understand the table structure and POJO simultaneously during the development.

If you going to make your apppcation portable to other EJB 3 comppant ORM apppcations, you must use annotations to represent the mapping information, but still if you want greater flexibipty, then you should go with XML-based mappings.

Environment Setup for Hibernate Annotation

First of all you would have to make sure that you are using JDK 5.0 otherwise you need to upgrade your JDK to JDK 5.0 to take advantage of the native support for annotations.

Second, you will need to install the Hibernate 3.x annotations distribution package, available from the sourceforge: (Download Hibernate Annotation) and copy hibernate-annotations.jar, pb/hibernate-comons-annotations.jar and pb/ejb3-persistence.jar from the Hibernate Annotations distribution to your CLASSPATH.

Annotated Class Example

As I mentioned above while working with Hibernate Annotation, all the metadata is clubbed into the POJO java file along with the code, this helps the user to understand the table structure and POJO simultaneously during the development.

Consider we are going to use the following EMPLOYEE table to store our objects −

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)
);

Following is the mapping of Employee class with annotations to map objects with the defined EMPLOYEE table −

import javax.persistence.*;

@Entity
@Table(name = "EMPLOYEE")
pubpc class Employee {
   @Id @GeneratedValue
   @Column(name = "id")
   private int id;

   @Column(name = "first_name")
   private String firstName;

   @Column(name = "last_name")
   private String lastName;

   @Column(name = "salary")
   private int salary;  

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

Hibernate detects that the @Id annotation is on a field and assumes that it should access properties of an object directly through fields at runtime. If you placed the @Id annotation on the getId() method, you would enable access to properties through getter and setter methods by default. Hence, all other annotations are also placed on either fields or getter methods, following the selected strategy.

Following section will explain the annotations used in the above class.

@Entity Annotation

The EJB 3 standard annotations are contained in the javax.persistence package, so we import this package as the first step. Second, we used the @Entity annotation to the Employee class, which marks this class as an entity bean, so it must have a no-argument constructor that is visible with at least protected scope.

@Table Annotation

The @Table annotation allows you to specify the details of the table that will be used to persist the entity in the database.

The @Table annotation provides four attributes, allowing you to override the name of the table, its catalogue, and its schema, and enforce unique constraints on columns in the table. For now, we are using just table name, which is EMPLOYEE.

@Id and @GeneratedValue Annotations

Each entity bean will have a primary key, which you annotate on the class with the @Id annotation. The primary key can be a single field or a combination of multiple fields depending on your table structure.

By default, the @Id annotation will automatically determine the most appropriate primary key generation strategy to be used but you can override this by applying the @GeneratedValue annotation, which takes two parameters strategy and generator that I m not going to discuss here, so let us use only the default key generation strategy. Letting Hibernate determine which generator type to use makes your code portable between different databases.

@Column Annotation

The @Column annotation is used to specify the details of the column to which a field or property will be mapped. You can use column annotation with the following most commonly used attributes −

name attribute permits the name of the column to be exppcitly specified.

length attribute permits the size of the column used to map a value particularly for a String value.

nullable attribute permits the column to be marked NOT NULL when the schema is generated.

unique attribute permits the column to be marked as containing only unique values.

Create Apppcation Class

Finally, we will create our apppcation class with the main() method to run the apppcation. We will use this apppcation to save few Employee s records and then we will apply CRUD operations on those records.

import java.util.List; 
import java.util.Date;
import java.util.Iterator; 
 
import org.hibernate.HibernateException; 
import org.hibernate.Session; 
import org.hibernate.Transaction;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

pubpc class ManageEmployee {
   private static SessionFactory factory; 
   pubpc static void main(String[] args) {
      
      try {
         factory = new AnnotationConfiguration().
                   configure().
                   //addPackage("com.xyz") //add package if used.
                   addAnnotatedClass(Employee.class).
                   buildSessionFactory();
      } catch (Throwable ex) { 
         System.err.println("Failed to create sessionFactory object." + ex);
         throw new ExceptionInInitiapzerError(ex); 
      }
      
      ManageEmployee ME = new ManageEmployee();

      /* Add few employee records in database */
      Integer empID1 = ME.addEmployee("Zara", "Ap", 1000);
      Integer empID2 = ME.addEmployee("Daisy", "Das", 5000);
      Integer empID3 = ME.addEmployee("John", "Paul", 10000);

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

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

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

      /* List down new pst of the employees */
      ME.pstEmployees();
   }
   
   /* Method to CREATE an employee in the database */
   pubpc Integer addEmployee(String fname, String lname, int salary){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = new Employee();
         employee.setFirstName(fname);
         employee.setLastName(lname);
         employee.setSalary(salary);
         employeeID = (Integer) session.save(employee); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return employeeID;
   }
   
   /* Method to  READ all the employees */
   pubpc void pstEmployees( ){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         List employees = session.createQuery("FROM Employee").pst(); 
         for (Iterator iterator = employees.iterator(); iterator.hasNext();){
            Employee employee = (Employee) iterator.next(); 
            System.out.print("First Name: " + employee.getFirstName()); 
            System.out.print("  Last Name: " + employee.getLastName()); 
            System.out.println("  Salary: " + employee.getSalary()); 
         }
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
   }
   
   /* Method to UPDATE salary for an employee */
   pubpc 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 */
   pubpc 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(); 
      }
   }
}

Database Configuration

Now let us create hibernate.cfg.xml configuration file to define database related parameters.

<?xml version = "1.0" encoding = "utf-8"?>
<!DOCTYPE hibernate-configuration SYSTEM 
"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
   <session-factory>
   
      <property name = "hibernate.dialect">
         org.hibernate.dialect.MySQLDialect
      </property>
   
      <property name = "hibernate.connection.driver_class">
         com.mysql.jdbc.Driver
      </property>

      <!-- Assume students is the database name -->
   
      <property name = "hibernate.connection.url">
         jdbc:mysql://localhost/test
      </property>
   
      <property name = "hibernate.connection.username">
         root
      </property>
   
      <property name = "hibernate.connection.password">
         cohondob
      </property>

   </session-factory>
</hibernate-configuration>

Compilation and Execution

Here are the steps to compile and run the above mentioned apppcation. Make sure, you have set PATH and CLASSPATH appropriately before proceeding for the compilation and execution.

Delete Employee.hbm.xml mapping file from the path.

Create Employee.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.

You would get the following result, and records would be created in EMPLOYEE table.

$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Zara  Last Name: Ap  Salary: 1000
First Name: Daisy  Last Name: Das  Salary: 5000
First Name: John  Last Name: Paul  Salary: 10000
First Name: Zara  Last Name: Ap  Salary: 5000
First Name: John  Last Name: Paul  Salary: 10000

If you check your EMPLOYEE table, it should have the following records −

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 29 | Zara       | Ap       |   5000 |
| 31 | John       | Paul      |  10000 |
+----+------------+-----------+--------+
2 rows in set (0.00 sec

mysql>