Hibernate - 一对多映射

一对多映射可以使用Set Java 集合来实现，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，所以如果您已经学习了Set映射，那么您就可以开始进行一对多映射了。

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

定义 RDBMS 表

考虑我们需要将员工记录存储在 EMPLOYEE 表中，该表将具有以下结构：

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之相关的证书。因此，我们将证书相关信息存储在具有以下结构的单独表中：

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

EMPLOYEE 和 CERTIFICATE 对象之间将存在一对多关系：

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变量中包含证书的集合。

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和检索到 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

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。

<?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="all">
         <key column = "employee_id"/>
         <one-to-many 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在格式为 <classname>.hbm.xml 的文件中。我们将映射文档保存在 Employee.hbm.xml 文件中。您已经熟悉大多数映射细节，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，其中<hibernate-mapping>作为根元素，包含两个与每个类对应的 <class> 元素。
<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属性保存 Hibernate 映射类型，这些映射类型将从 Java 转换为 SQL 数据类型。
<generator>元素位于 id 元素内，用于自动生成主键值。generator 元素的class属性设置为 native，让 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属性保存 Hibernate 映射类型，这些映射类型将从 Java 转换为 SQL 数据类型。
<set>元素设置 Certificate 和 Employee 类之间的关系。我们在 <set> 元素中使用了cascade属性来告诉 Hibernate 在同时持久化 Employee 对象时也持久化 Certificate 对象。name属性设置为父类中定义的Set变量，在本例中为certificates。对于每个 set 变量，都需要在映射文件中定义一个单独的 set 元素。
<key>元素是 CERTIFICATE 表中保存对父对象（即 EMPLOYEE 表）的外键的列。
<one-to-many>元素表示一个 Employee 对象与多个 Certificate 对象相关联。

创建应用程序类

最后，我们将创建我们的应用程序类，其中包含 main() 方法来运行应用程序。我们将使用此应用程序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 set1 = new HashSet();
      set1.add(new Certificate("MCA"));
      set1.add(new Certificate("MBA"));
      set1.add(new Certificate("PMP"));
     
      /* Add employee records in the database */
      Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, set1);

      /* Another set of certificates for the second employee  */
      HashSet set2 = new HashSet();
      set2.add(new Certificate("BCA"));
      set2.add(new Certificate("BA"));

      /* Add another employee record in the database */
      Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, set2);

      /* 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。在继续进行编译和执行之前，请确保已正确设置 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，并编译它。
创建 Certificate.java 源文件，如上所示，并编译它。
创建 ManageEmployee.java 源文件，如上所示，并编译它。
执行 ManageEmployee 二进制文件以运行程序。

您将在屏幕上看到以下结果，同时在 EMPLOYEE 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: BCA
Certificate: BA
First Name: Manoj  Last Name: Kumar  Salary: 5000
Certificate: MBA
Certificate: PMP
Certificate: MCA

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

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

mysql> select * from certificate;
+----+------------------+-------------+
| id | certificate_name | employee_id |
+----+------------------+-------------+
| 1  | MBA              |          1  |
| 2  | PMP              |          1  |
| 3  | MCA              |          1  |
+----+------------------+-------------+
3 rows in set (0.00 sec)

mysql>

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