Hibernate - 多對多對映

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可以使用Set Java 集合來實現多對多對映，該集合不包含任何重複元素。我們已經瞭解瞭如何在Hibernate中對映Set集合，因此，如果您已經學習了Set對映，那麼您就可以開始進行多對多映射了。

Set在對映表中使用<set>元素對映，並使用java.util.HashSet初始化。當集合中不需要重複元素時，可以在類中使用Set集合。

定義RDBMS表

考慮一種情況，我們需要將員工記錄儲存在EMPLOYEE表中，該表具有以下結構：

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)
);

此外，假設每個員工可以擁有一個或多個與其相關的證書，並且一個證書可以與多個員工相關聯。我們將把證書相關資訊儲存在單獨的表中，該表具有以下結構：

create table CERTIFICATE (
   id INT NOT NULL auto_increment,
   certificate_name VARCHAR(30) default NULL,
   PRIMARY KEY (id)
);

現在，為了在EMPLOYEE和CERTIFICATE物件之間實現多對多關係，我們必須引入另一箇中間表，其中包含員工ID和證書ID，如下所示：

create table EMP_CERT (
   employee_id INT NOT NULL,
   certificate_id INT NOT NULL,
   PRIMARY KEY (employee_id,certificate_id)
);

定義POJO類

讓我們實現我們的POJO類Employee，它將用於持久化與EMPLOYEE表相關的物件，並在Set變數中包含一系列證書。

import java.util.*;

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private Set certificates;

   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;
   }

   public Set getCertificates() {
      return certificates;
   }
   
   public void setCertificates( Set certificates ) {
      this.certificates = certificates;
   }
}

現在讓我們定義另一個與CERTIFICATE表對應的POJO類，以便可以將證書物件儲存到CERTIFICATE表中並從中檢索。此類還應該實現equals()和hashCode()方法，以便Java可以確定任何兩個元素/物件是否相同。

public class Certificate {
   private int id;
   private String name; 

   public Certificate() {}
   
   public Certificate(String name) {
      this.name = name;
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getName() {
      return name;
   }
   public void setName( String name ) {
      this.name = name;
   }
   
   public boolean equals(Object obj) {
      if (obj == null) return false;
      if (!this.getClass().equals(obj.getClass())) return false;

      Certificate obj2 = (Certificate)obj;
      if((this.id == obj2.getId()) && (this.name.equals(obj2.getName()))) {
         return true;
      }
      return false;
   }
   
   public int hashCode() {
      int tmp = 0;
      tmp = ( id + name ).hashCode();
      return tmp;
   }
}

定義Hibernate對映檔案

讓我們開發我們的對映檔案，該檔案指示Hibernate如何將定義的類對映到資料庫表。<set>元素將用於定義多對多關係的規則。

<?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>
      
      <set name = "certificates" cascade="save-update" table="EMP_CERT">
         <key column = "employee_id"/>
         <many-to-many column = "certificate_id" class="Certificate"/>
      </set>
      
      <property name = "firstName" column = "first_name" type = "string"/>
      <property name = "lastName" column = "last_name" type = "string"/>
      <property name = "salary" column = "salary" type = "int"/>
      
   </class>

   <class name = "Certificate" table = "CERTIFICATE">
      
      <meta attribute = "class-description">
         This class contains the certificate records. 
      </meta>
      
      <id name = "id" type = "int" column = "id">
         <generator class="native"/>
      </id>
      
      <property name = "name" column = "certificate_name" type = "string"/>
      
   </class>

</hibernate-mapping>

您應該將對映文件儲存到格式為<classname>.hbm.xml的檔案中。我們將對映文件儲存在Employee.hbm.xml檔案中。您已經熟悉大部分對映細節，但讓我們再次檢視對映檔案的所有元素：

  • 對映文件是一個XML文件，其根元素為<hibernate-mapping>，其中包含兩個<class>元素，分別對應於每個類。

  • <class>元素用於定義從Java類到資料庫表的特定對映。Java類名使用class元素的name屬性指定，資料庫表名使用table屬性指定。

  • <meta>元素是可選元素，可用於建立類描述。

  • <id>元素將類中唯一的ID屬性對映到資料庫表的主鍵。id元素的name屬性引用類中的屬性，column屬性引用資料庫表中的列。type屬性包含Hibernate對映型別，此對映型別將從Java轉換為SQL資料型別。

  • id元素中的<generator>元素用於自動生成主鍵值。generator元素的class屬性設定為native，以便Hibernate根據底層資料庫的功能選擇identity、sequence或hilo演算法來建立主鍵。

  • <property>元素用於將Java類屬性對映到資料庫表中的列。元素的name屬性引用類中的屬性，column屬性引用資料庫表中的列。type屬性包含Hibernate對映型別，此對映型別將從Java轉換為SQL資料型別。

  • <set>元素設定Certificate和Employee類之間的關係。我們將cascade屬性設定為save-update，以告訴Hibernate在同時執行Employee物件的SAVE（即CREATE和UPDATE）操作時持久化Certificate物件。name屬性設定為父類中定義的Set變數，在我們的例子中是certificates。對於每個set變數，我們需要在對映檔案中定義一個單獨的set元素。這裡我們使用name屬性將中間表名稱設定為EMP_CERT。

  • <key>元素是EMP_CERT表中儲存指向父物件（即表EMPLOYEE）的外部索引鍵的列，並連結到CERTIFICATE表中的certification_id。

  • <many-to-many>元素指示一個Employee物件與多個Certificate物件相關，並且column屬性用於連結中間表EMP_CERT。

建立應用程式類

最後，我們將建立包含main()方法的應用程式類來執行應用程式。我們將使用此應用程式來儲存一些員工記錄及其證書，然後我們將對這些記錄應用CRUD操作。

import java.util.*;
 
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();
      /* Let us have a set of certificates for the first employee  */
      HashSet certificates = new HashSet();

      certificates.add(new Certificate("MCA"));
      certificates.add(new Certificate("MBA"));
      certificates.add(new Certificate("PMP"));
     
      /* Add employee records in the database */
      Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, certificates);

      /* Add another employee record in the database */
      Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, certificates);

      /* List down all the employees */
      ME.listEmployees();

      /* Update employee's salary records */
      ME.updateEmployee(empID1, 5000);

      /* Delete an employee from the database */
      ME.deleteEmployee(empID2);

      /* List down all the employees */
      ME.listEmployees();

   }

   /* Method to add an employee record in the database */
   public Integer addEmployee(String fname, String lname, int salary, Set cert){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = new Employee(fname, lname, salary);
         employee.setCertificates(cert);
         employeeID = (Integer) session.save(employee); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return employeeID;
   }

   /* Method to list all the employees detail */
   public void listEmployees( ){
      Session session = factory.openSession();
      Transaction tx = null;
      
      try {
         tx = session.beginTransaction();
         List employees = session.createQuery("FROM Employee").list(); 
         for (Iterator iterator1 = employees.iterator(); iterator1.hasNext();){
            Employee employee = (Employee) iterator1.next(); 
            System.out.print("First Name: " + employee.getFirstName()); 
            System.out.print("  Last Name: " + employee.getLastName()); 
            System.out.println("  Salary: " + employee.getSalary());
            Set certificates = employee.getCertificates();
            for (Iterator iterator2 = certificates.iterator(); iterator2.hasNext();){
               Certificate certName = (Certificate) iterator2.next(); 
               System.out.println("Certificate: " + certName.getName()); 
            }
         }
         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();
      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();
      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。在繼續編譯和執行之前，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，並進行編譯。

  • 建立Certificate.java原始檔，如上所示，並進行編譯。

  • 建立ManageEmployee.java原始檔，如上所示，並進行編譯。

  • 執行ManageEmployee二進位制檔案以執行程式。

您將在螢幕上獲得以下結果，同時會在EMPLOYEE、EMP_CERT和CERTIFICATE表中建立記錄。

$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Manoj  Last Name: Kumar  Salary: 4000
Certificate: MBA
Certificate: PMP
Certificate: MCA
First Name: Dilip  Last Name: Kumar  Salary: 3000
Certificate: MBA
Certificate: PMP
Certificate: MCA
First Name: Manoj  Last Name: Kumar  Salary: 5000
Certificate: MBA
Certificate: PMP
Certificate: MCA

如果您檢查EMPLOYEE、EMP_CERT和CERTIFICATE表，它們應該具有以下記錄：

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 22 | Manoj      | Kumar     |   5000 |
+----+------------+-----------+--------+
1 row in set (0.00 sec)

mysql> select * from CERTIFICATE;
+----+------------------+
| id | certificate_name |
+----+------------------+
|  4 | MBA              |
|  5 | PMP              |
|  6 | MCA              |
+----+------------------+
3 rows in set (0.00 sec)

mysql> select * from EMP_CERT;
+-------------+----------------+
| employee_id | certificate_id |
+-------------+----------------+
|          22 |              4 |
|          22 |              5 |
|          22 |              6 |
+-------------+----------------+
3 rows in set (0.00 sec)

mysql>
hibernate_or_mappings.htm
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