Many-to-Many

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2023-12-01

可以使用不包含任何重复元素的Set java集合来实现Many-to-Many映射。我们已经看过如何在hibernate中映射Set集合，所以如果你已经学习了Set映射，那么你就可以使用多对多映射了。

Set与映射表中的元素映射，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 (Define RDBMS Tables)

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，假设每个雇员可以拥有与他/她相关联的一个或多个证书，并且类似的证书可以与多个雇员相关联。我们将证书相关信息存储在一个单独的表中，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)
);

现在要实现EMPLOYEE和CERTIFICATE对象之间many-to-many关系，我们必须再引入一个具有员工ID和证书ID的中间表，如下所示 -

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

Define POJO Classes

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变量中包含一组证书。

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;
   }
}

现在让我们定义与CERTIFICATE表对应的另一个POJO类，以便可以将证书对象存储并检索到CERTIFICATE表中。此类还应实现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 (Define Hibernate Mapping File)

让我们开发我们的映射文件，它指示Hibernate - 如何将定义的类映射到数据库表。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/>
      </id>
      <set name = "certificates" cascade="save-update" table="EMP_CERT">
         <key column = "employee_id"/>
         <many-to-many column = "certificate_id"/>
      </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/>
      </id>
      <property name = "name" column = "certificate_name" type = "string"/>
   </class>
</hibernate-mapping>

您应该将映射文档保存在格式为 .hbm.xml的文件中。我们将映射文档保存在Employee.hbm.xml文件中。您已经熟悉了大部分映射细节，但让我们再次看到映射文件的所有元素 -

映射文档是具有《hibernate-mapping》作为根元素的XML文档，其包含对应于每个类的两个“类”元素。
《class》元素用于定义从Java类到数据库表的特定映射。使用class元素的name属性指定Java类名，并使用table属性指定数据库表名。
《meta》元素是可选元素，可用于创建类描述。
《id》元素将类中的唯一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，这种映射类型将从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。生成器元素的class属性设置为native ，让hibernate选择identity, sequence或hilo算法来创建主键，具体取决于底层数据库的功能。
《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，这种映射类型将从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以告知Hibernate持久保存证书对象以进行SAVE，即与Employee对象同时进行CREATE和UPDATE操作。 name属性设置为父类中定义的Set变量，在我们的示例中它是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属性将中间表名设置为EMP_CERT。
《key》元素是EMP_CERT表中的列，用于保存父对象的外键，即。表EMPLOYEE和指向CERTIFICATE表中的certification_id的链接。
《many-to-many》元素表示一个Employee对象与许多Certificate对象相关，而列属性用于链接中间EMP_CERT。

创建应用程序类 (Create Application Class)

最后，我们将使用main（）方法创建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少量员工的记录及其证书，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 (Compilation and Execution)

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，请确保已正确设置PATH和CLASSPATH。

按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源文件并进行编译。
如上所示创建Certificate.java源文件并进行编译。
创建ManageEmployee.java源文件，如上所示并编译它。
执行ManageEmployee二进制文件以运行该程序。

您将在屏幕上看到以下结果，并且将在EMPLOYEE，EMP_CERT和CERTIFICATE表中创建相同的时间记录。

$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

如果您检查您的EMPLOYEE，EMP_CERT和CERTIFICATE表，他们应该有以下记录 -

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>