Hibernate Interceptors

Reprinted: https://www.w3cschool.cn/hibernate/npyd1iek.html

interceptor

You have learned that in Hibernate, an object is created and persisted. Once the object has been modified, it must be saved to the database. This process continues until the next time the object is needed, it will be loaded from persistent storage.

So an object goes through different stages in its life cycle, and the  Interceptor interface provides methods that can be called at different stages to perform some required tasks. These methods are callback functions from the session to the application that allow the application to inspect or manipulate the properties of a persistent object before it is saved, updated, deleted or uploaded. The following is a list of all methods available in the Interceptor interface.

S.N.	method and description
1	The findDirty() method is called when the  flush()  method is called on a Session object.
2	instantiate() This method is called when a persistent class is instantiated.
3	The isUnsaved( ) method is called when an object is passed to the  saveOrUpdate()  method.
4	onDelete() This method is called before an object is deleted.
5	onFlushDirty() This method is called when Hibernate detects that an object is dirty (eg, modified) during a flush (eg, an update operation).
6	onLoad() This method is called before an object is initialized.
7	onSave() This method is called before an object is saved.
8	postFlush() This method is called after a flush has occurred and an object has been updated in memory.
9	The preFlush() method is called before a flush.

Hibernate interceptors give us total control over how an object is applied to the application and database.

How to use interceptor?

To create an interceptor you can directly implement the  Interceptor  class or  extend the EmptyInterceptor  class. Following are the simple steps to use the Hibernate interceptor feature.

Create an interceptor

We will inherit EmptyInterceptor in our example,   the methods of the interceptor will be called automatically when the Employee object is created and updated. You can implement more methods according to your needs.

import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;

import org.hibernate.EmptyInterceptor;
import org.hibernate.Transaction;
import org.hibernate.type.Type;

public class MyInterceptor extends EmptyInterceptor {
   private int updates;
   private int creates;
   private int loads;

   public void onDelete(Object entity,
                     Serializable id,
                     Object[] state,
                     String[] propertyNames,
                     Type[] types) {
       // do nothing
   }

   // This method is called when Employee object gets updated.
   public boolean onFlushDirty(Object entity,
                     Serializable id,
                     Object[] currentState,
                     Object[] previousState,
                     String[] propertyNames,
                     Type[] types) {
       if ( entity instanceof Employee ) {
          System.out.println("Update Operation");
          return true; 
       }
       return false;
   }
   public boolean onLoad(Object entity,
                    Serializable id,
                    Object[] state,
                    String[] propertyNames,
                    Type[] types) {
       // do nothing
       return true;
   }
   // This method is called when Employee object gets created.
   public boolean onSave(Object entity,
                    Serializable id,
                    Object[] state,
                    String[] propertyNames,
                    Type[] types) {
       if ( entity instanceof Employee ) {
          System.out.println("Create Operation");
          return true; 
       }
       return false;
   }
   //called before commit into database
   public void preFlush(Iterator iterator) {
      System.out.println("preFlush");
   }
   //called after committed into database
   public void postFlush(Iterator iterator) {
      System.out.println("postFlush");
   }
}

Create POJO class

Now let's modify our first example slightly, we use the EMPLOYEE form and the Employee class:

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

   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;
   }
}

Create database table

The second step will be to create the table in your database. A table for each object you provide persistence. Consider the above objects that need to be stored and retrieved into the following RDBM tables:

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)
);

Create a Mapping configuration file

This step is to create a mapping file that instructs Hibernate how to map the defined class or classes to the database form.

<?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>
      <property name="firstName" column="first_name" type="string"/>
      <property name="lastName" column="last_name" type="string"/>
      <property name="salary" column="salary" type="int"/>
   </class>
</hibernate-mapping>

Create the Application class

Finally, we will create the application class with main() to run the application. It should be noted here that we use the Interceptor class as a parameter when creating the session object.

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.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();

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

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

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

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

      /* List down new list of the employees */
      ME.listEmployees();
   }
   /* Method to CREATE an employee in the database */
   public Integer addEmployee(String fname, String lname, int salary){
      Session session = factory.openSession( new MyInterceptor() );
      Transaction tx = null;
      Integer employeeID = null;
      try{
         tx = session.beginTransaction();
         Employee employee = new Employee(fname, lname, 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 */
   public void listEmployees( ){
      Session session = factory.openSession( new MyInterceptor() );
      Transaction tx = null;
      try{
         tx = session.beginTransaction();
         List employees = session.createQuery("FROM Employee").list(); 
         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 */
   public void updateEmployee(Integer EmployeeID, int salary ){
      Session session = factory.openSession( new MyInterceptor() );
      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( new MyInterceptor() );
      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(); 
      }
   }
}

　compile and execute

Here are the steps to compile and run the application mentioned above. Make sure you have set the PATH and CLASSPATH correctly before processing compilation and execution.

• Create the hibernate.cfg.xml configuration file explained in the configuration chapter.
• Create the Employee.hbm.xml mapping file shown above.
• 创建如上所示的 Employee.java 源文件并编译。
• 创建如上所示的 MyInterceptor.java 源文件并编译。
• 创建如上所示的 ManageEmployee.java 源文件并编译。
• 执行 ManageEmployee 来运行程序。

你将得到以下结果，而且记录将在 EMPLOYEE 表单中被创建。

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

Create Operation
preFlush
postFlush
Create Operation
preFlush
postFlush
Create Operation
preFlush
postFlush
First Name: Zara  Last Name: Ali  Salary: 1000
First Name: Daisy  Last Name: Das  Salary: 5000
First Name: John  Last Name: Paul  Salary: 10000
preFlush
postFlush
preFlush
Update Operation
postFlush
preFlush
postFlush
First Name: Zara  Last Name: Ali  Salary: 5000
First Name: John  Last Name: Paul  Salary: 10000
preFlush
postFlush

如果你检查你的 EMPLOYEE 表单，它应该有如下结果：

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