Hibernate - 攔截器

正如您所瞭解的，在 Hibernate 中，將建立一個物件並將其持久化。一旦物件發生更改，就必須將其儲存回資料庫。此過程將持續到下次需要該物件時，它將從持久化儲存中載入。

因此，物件在其生命週期中會經歷不同的階段，而攔截器介面提供可在不同階段呼叫的方法，以執行某些所需的任務。這些方法是從會話到應用程式的回撥，允許應用程式在持久化物件儲存、更新、刪除或載入之前檢查和/或操作其屬性。以下是攔截器介面中所有可用方法的列表：

Hibernate 攔截器使我們能夠完全控制物件對應用程式和資料庫的外觀。

如何使用攔截器？

要構建攔截器，您可以直接實現Interceptor類或擴充套件EmptyInterceptor類。以下是使用 Hibernate 攔截器功能的簡單步驟：

建立攔截器

在我們的示例中，我們將擴充套件 EmptyInterceptor，其中攔截器的方法將在建立和更新Employee物件時自動呼叫。您可以根據需要實現更多方法。

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");
   }
}

建立 POJO 類

現在，讓我們稍微修改一下我們的第一個示例，在該示例中，我們使用了 EMPLOYEE 表和 Employee 類進行操作：

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;
   }
}

建立資料庫表

第二步是在資料庫中建立表。將有一個表對應於每個物件，您希望為其提供永續性。考慮上面解釋的物件，需要儲存和檢索到以下 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)
);

建立對映配置檔案

此步驟是建立一個對映檔案，該檔案指示 Hibernate 如何將定義的類或類對映到資料庫表。

<?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>

建立應用程式類

最後，我們將建立具有 main() 方法的應用程式類來執行應用程式。這裡應該注意的是，在建立會話物件時，我們使用了我們的攔截器類作為引數。

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(); 
      }
   }
}

編譯和執行

以下是編譯和執行上述應用程式的步驟。在進行編譯和執行之前，請確保已正確設定 PATH 和 CLASSPATH。

• 建立 hibernate.cfg.xml 配置檔案，如配置章節中所述。

• 建立 Employee.hbm.xml 對映檔案，如上所示。

• 建立 Employee.java 原始檔，如上所示，並編譯它。

• 建立 MyInterceptor.java 原始檔，如上所示，並編譯它。

• 建立 ManageEmployee.java 原始檔，如上所示，並編譯它。

• 執行 ManageEmployee 二進位制檔案以執行程式。

您將獲得以下結果，並且記錄將被建立到 EMPLOYEE 表中。

$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

如果檢查您的 EMPLOYEE 表，它應該包含以下記錄：

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>