Hibernate - 拦截器( Interceptors)

正如您已经了解到的那样，在Hibernate中，将创建并保留一个对象。对象更改后，必须将其保存回数据库。此过程一直持续到下一次需要该对象时，它将从持久性存储中加载。

因此，对象在其生命周期中经历不同的阶段，并且Interceptor Interface提供方法，可以在不同阶段调用这些方法来执行一些所需的任务。这些方法是从会话到应用程序的回调，允许应用程序在保存，更新，删除或加载之前检查和/或操纵持久对象的属性。以下是Interceptor接口中可用的所有方法的列表 -

Sr.No.	方法和描述
1	findDirty() 在Session对象上调用flush()方法时，将调用此方法。
2	instantiate() 在实例化持久化类时调用此方法。
3	isUnsaved() 将对象传递给saveOrUpdate()方法/时调用此方法
4	onDelete() 在删除对象之前调用此方法。
5	onFlushDirty() 当Hibernate在刷新即更新操作期间检测到对象脏（即已更改）时，将调用此方法。
6	onLoad() 在初始化对象之前调用此方法。
7	onSave() 在保存对象之前调用此方法。
8	postFlush() 在发生刷新并且内存中已更新对象后调用此方法。
9	preFlush() 在刷新之前调用此方法。

Hibernate Interceptor使我们可以完全控制对象对应用程序和数据库的外观。

如何使用拦截器？

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

创建拦截器

我们将在我们的示例中扩展EmptyInterceptor，其中在创建和更新Employee对象时将自动调用Interceptor的方法。您可以根据自己的要求实施更多方法。

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");
   }
}

创建POJO类

现在，让我们稍微修改一下我们使用EMPLOYEE表和Employee类来玩的第一个例子 -

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;
   }
}

创建数据库表

第二步是在数据库中创建表。每个对象都会有一个表，您愿意提供持久性。考虑上面解释的对象，需要存储和检索到以下RDBMS表中 -

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)
);

创建映射配置文件

这一步是创建一个指示Hibernate的映射文件 - 如何将定义的类映射到数据库表。

<?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>

创建应用程序类 (Create Application Class)

最后，我们将使用main（）方法创建应用程序类来运行应用程序。在这里，应该注意的是，在创建会话对象时，我们使用Interceptor类作为参数。

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(); 
      }
   }
}

编译和执行 (Compilation and Execution)

以下是编译和运行上述应用程序的步骤。确保在进行编译和执行之前已适当地设置了PATH和CLASSPATH。

按配置章节中的说明创建hibernate.cfg.xml配置文件。
创建Employee.hbm.xml映射文件，如上所示。
如上所示创建Employee.java源文件并进行编译。
如上所示创建MyInterceptor.java源文件并进行编译。
创建ManageEmployee.java源文件，如上所示并编译它。
执行ManageEmployee二进制文件以运行该程序。

您将获得以下结果，并且将在EMPLOYEE表中创建记录。

$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

如果您检查您的EMPLOYEE表，它应该有以下记录 -

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>

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Hibernate - 教程

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Hibernate - 概述

Hibernate - 架构( Architecture)

Hibernate - 环境

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休眠 - 配置( Configuration)

Hibernate - 持久化类( Persistent Class)

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Hibernate - Native SQL( Native SQL)

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