Hibernate - 多对多映射

可以使用Set Java 集合来实现多对多映射，该集合不包含任何重复元素。我们已经了解了如何在Hibernate中映射Set集合，因此，如果您已经学习了Set映射，那么您就可以开始进行多对多映射了。

Set在映射表中使用<set>元素映射，并使用java.util.HashSet初始化。当集合中不需要重复元素时，可以在类中使用Set集合。

定义RDBMS表

考虑一种情况，我们需要将员工记录存储在EMPLOYEE表中，该表具有以下结构：

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，假设每个员工可以拥有一个或多个与其相关的证书，并且一个证书可以与多个员工相关联。我们将把证书相关信息存储在单独的表中，该表具有以下结构：

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

现在，为了在EMPLOYEE和CERTIFICATE对象之间实现多对多关系，我们必须引入另一个中间表，其中包含员工ID和证书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)
);

定义POJO类

让我们实现我们的POJO类Employee，它将用于持久化与EMPLOYEE表相关的对象，并在Set变量中包含一系列证书。

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;
   }
}

现在让我们定义另一个与CERTIFICATE表对应的POJO类，以便可以将证书对象存储到CERTIFICATE表中并从中检索。此类还应该实现equals()和hashCode()方法，以便Java可以确定任何两个元素/对象是否相同。

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;
   }
}

定义Hibernate映射文件

让我们开发我们的映射文件，该文件指示Hibernate如何将定义的类映射到数据库表。<set>元素将用于定义多对多关系的规则。

<?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>

您应该将映射文档保存到格式为<classname>.hbm.xml的文件中。我们将映射文档保存在Employee.hbm.xml文件中。您已经熟悉大部分映射细节，但让我们再次查看映射文件的所有元素：

映射文档是一个XML文档，其根元素为<hibernate-mapping>，其中包含两个<class>元素，分别对应于每个类。
<class>元素用于定义从Java类到数据库表的特定映射。Java类名使用class元素的name属性指定，数据库表名使用table属性指定。
<meta>元素是可选元素，可用于创建类描述。
<id>元素将类中唯一的ID属性映射到数据库表的主键。id元素的name属性引用类中的属性，column属性引用数据库表中的列。type属性包含Hibernate映射类型，此映射类型将从Java转换为SQL数据类型。
id元素中的<generator>元素用于自动生成主键值。generator元素的class属性设置为native，以便Hibernate根据底层数据库的功能选择identity、sequence或hilo算法来创建主键。
<property>元素用于将Java类属性映射到数据库表中的列。元素的name属性引用类中的属性，column属性引用数据库表中的列。type属性包含Hibernate映射类型，此映射类型将从Java转换为SQL数据类型。
<set>元素设置Certificate和Employee类之间的关系。我们将cascade属性设置为save-update，以告诉Hibernate在同时执行Employee对象的SAVE（即CREATE和UPDATE）操作时持久化Certificate对象。name属性设置为父类中定义的Set变量，在我们的例子中是certificates。对于每个set变量，我们需要在映射文件中定义一个单独的set元素。这里我们使用name属性将中间表名称设置为EMP_CERT。
<key>元素是EMP_CERT表中保存指向父对象（即表EMPLOYEE）的外键的列，并链接到CERTIFICATE表中的certification_id。
<many-to-many>元素指示一个Employee对象与多个Certificate对象相关，并且column属性用于链接中间表EMP_CERT。

创建应用程序类

最后，我们将创建包含main()方法的应用程序类来运行应用程序。我们将使用此应用程序来保存一些员工记录及其证书，然后我们将对这些记录应用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(); 
      }
   }
}

编译和执行

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，确保已正确设置PATH和CLASSPATH。

创建hibernate.cfg.xml配置文件，如配置章节中所述。
创建Employee.hbm.xml映射文件，如上所示。
创建Employee.java源文件，如上所示，并进行编译。
创建Certificate.java源文件，如上所示，并进行编译。
创建ManageEmployee.java源文件，如上所示，并进行编译。
执行ManageEmployee二进制文件以运行程序。

您将在屏幕上获得以下结果，同时会在EMPLOYEE、EMP_CERT和CERTIFICATE表中创建记录。

$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

如果您检查EMPLOYEE、EMP_CERT和CERTIFICATE表，它们应该具有以下记录：

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

打印页面