Hibernate - 拦截器

正如您所知，在 Hibernate 中，将创建并持久保存一个对象。一旦对象发生更改，就必须将其保存回数据库。此过程将持续到下一次需要该对象时为止，并且将从持久存储中加载该对象。

因此，一个对象在其生命周期中会经历不同的阶段，Interceptor Interface提供了方法，可以在不同的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、删除或加载持久对象之前检查和/或操作持久对象的属性。以下是拦截器接口中所有可用方法的列表 -

先生。	方法及说明
1	查找脏() 当对 Session 对象调用lush()方法时，将调用此方法。
2	实例化() 当实例化持久化类时调用此方法。
3	未保存() 当对象传递给saveOrUpdate()方法时调用此方法/
4	删除() 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	负载（）该方法在对象初始化之前调用。
7	onSave() 在保存对象之前调用此方法。
8	后刷新（）在发生刷新并且内存中的对象已更新后调用此方法。
9	预冲洗() 该方法在刷新之前调用。

Hibernate Interceptor 使我们能够完全控制对象在应用程序和数据库中的外观。

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

我们将在示例中扩展 EmptyInterceptor，其中当创建和更新Employee对象时，将自动调用 Interceptor 的方法。您可以根据您的要求实现更多方法。

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 类的第一个示例 -

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。每个对象都会对应一个表，你愿意提供持久化。考虑上面解释的对象，需要将其存储和检索到以下 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

这一步是创建一个映射文件，指示 Hibernate 如何将定义的一个或多个类映射到数据库表。

<?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

最后，我们将使用 main() 方法创建应用程序类来运行应用程序。这里，应该注意的是，在创建会话对象时，我们使用了 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(); 
      }
   }
}

编译与执行

以下是编译和运行上述应用程序的步骤。在继续编译和执行之前，请确保您已正确设置 PATH 和 CLASSPATH。

按照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 源文件并编译它。
如上所示创建 MyInterceptor.java 源文件并编译它。
如上所示创建 ManageEmployee.java 源文件并编译它。
执行 ManageEmployee 二进制文件来运行该程序。

您将得到以下结果，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

如果你检查你的 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>