Hibernate 简明教程

Hibernate - Interceptors

您已经了解到，在 Hibernate 中，将创建一个对象并将其持久化。一旦对象发生更改，就必须将其保存回数据库。该过程会持续到下次需要该对象为止，该对象将从持久化储存中加载。

As you have learnt that in Hibernate, an object will be created and persisted. Once the object has been changed, it must be saved back to the database. This process continues until the next time the object is needed, and it will be loaded from the persistent store.

对象在其生命周期中会经过不同的阶段，而 Interceptor Interface 提供可于不同阶段调用以执行一些所需任务的方法。这些方法是会话到应用程序的回调，它允许应用程序在持久对象被保存、更新、删除或加载之前检查和/或操作其属性。以下列出拦截器接口中提供的所有方法 −

Thus an object passes through different stages in its life cycle and Interceptor Interface provides methods, which can be called at different stages to perform some required tasks. These methods are callbacks from the session to the application, allowing the application to inspect and/or manipulate properties of a persistent object before it is saved, updated, deleted or loaded. Following is the list of all the methods available within the Interceptor interface −

Sr.No.	Method & Description
1	findDirty() This method is be called when the flush() method is called on a Session object.
2	instantiate() This method is called when a persisted class is instantiated.
3	isUnsaved() This 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 (i.e. have been changed) during a flush i.e. 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	preFlush() This method is called before a flush.

Hibernate Interceptor 让我们全面控制对象在应用程序和数据库中的呈现方式。

Hibernate Interceptor gives us total control over how an object will look to both the application and the database.

How to Use Interceptors?

要构建拦截器，你可以直接实现 Interceptor 类或扩展 EmptyInterceptor 类。以下是使用 Hibernate Interceptor 功能的简单步骤。

To build an interceptor, you can either implement Interceptor class directly or extend EmptyInterceptor class. Following will be the simple steps to use Hibernate Interceptor functionality.

Create Interceptors

在我们的示例中，我们将扩展 EmptyInterceptor，在 Employee 对象被创建和更新时，Interceptor 的方法将自动调用。你可以根据要求实现更多方法。

We will extend EmptyInterceptor in our example where Interceptor’s method will be called automatically when Employee object is created and updated. You can implement more methods as per your requirements.

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 Classes

现在，让我们稍微修改第一个示例，在示例中我们将使用 EMPLOYEE 表和 Employee 类 −

Now, let us modify a little bit our first example where we used EMPLOYEE table and Employee class to play with −

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 Tables

第二步是在你的数据库中创建表。将有一个表对应一个对象，你愿意提供持久性。考虑上面解释的对象，需要存储并检索到以下 RDBMS 表中 −

Second step would be creating tables in your database. There would be one table corresponding to each object, you are willing to provide persistence. Consider the objects explained above, need to be stored and retrieved into the following RDBMS table −

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 Mapping Configuration File

此步骤是创建映射文件，以指导 Hibernate — 如何将定义的类或类映射到数据库表。

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

<?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 Application Class

最后，我们将创建具有 main() 方法的应用程序类来运行应用程序。此处应该注意的是，在创建会话对象时，我们使用了拦截器类作为参数。

Finally, we will create our application class with the main() method to run the application. Here, it should be noted that while creating session object, we used our Interceptor class as an argument.

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();
      }
   }
}

Compilation and Execution

以下是编译和运行上述应用程序的步骤。请确保在继续编译和执行之前，已适当地设置 PATH 和 CLASSPATH。

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 Employee.hbm.xml mapping file as shown above.
Create Employee.java source file as shown above and compile it.
Create MyInterceptor.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.

您将获得以下结果，并且将在 EMPLOYEE 表中创建记录。

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

$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

如果检查您的 EMPLOYEE 表，它应包含以下记录 −

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

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>