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。

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 −

Sr.No.	Method & Description
1	findDirty() 当对会话对象调用 flush() 方法时，将调用此方法。
2	instantiate() 当实例化持久化类时，将调用此方法。
3	isUnsaved() 当将对象传递给 saveOrUpdate() 方法时，将调用此方法。
4	onDelete() 当删除对象之前，将调用此方法。
5	onFlushDirty() 当 Hibernate 检测到对象在刷新期间（即更新操作）变为脏（即发生更改）时，将调用此方法。
6	onLoad() 当初始化对象之前，将调用此方法。
7	onSave() 当保存对象之前，将调用此方法。
8	postFlush() 当刷新已经发生并且对象已经在内存中更新后，将调用此方法。
9	preFlush() 当刷新之前，将调用此方法。

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。

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。

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。

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 −

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 −

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。

<?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。

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。

如配置章节中所述，创建 hibernate.cfg.xml 配置文件。
如上所示创建 Employee.hbm.xml 映射文件。
创建 Employee.java 源文件（如上所示），并进行编译。
创建 MyInterceptor.java 源文件（如上所示）并编译它。
按上方代码创建一个 ManageEmployee.java 源文件并进行编译。
执行 ManageEmployee 二进制文件来运行该程序。

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

如果检查您的 EMPLOYEE 表，它应包含以下记录 −

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>