Hibernate - 多对一映射

多对一关联是最常见的关联类型，其中一个对象可以与多个对象关联。例如，同一个地址对象可以与多个员工对象相关联。

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,
   address    INT NOT NULL,
   PRIMARY KEY (id)
);

此外，许多员工可以有相同的地址，因此可以使用多对一关联来呈现该关联。我们将地址相关信息存储在具有以下结构的单独表中 −

create table ADDRESS (
   id INT NOT NULL auto_increment,
   street_name VARCHAR(40) default NULL,
   city_name VARCHAR(40) default NULL,
   state_name VARCHAR(40) default NULL,
   zipcode VARCHAR(10) default NULL,
   PRIMARY KEY (id)
);

创建两个 RBDMS 表并为下一次实施做好准备。

定义 POJO 类

让我们实现一个 POJO 类 Employee，将用于持久化与 EMPLOYEE 表相关的对象，并具有 Address 类型的变量。

import java.util.*;

public class Employee{
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private Address address;

   public Employee() {}
   
   public Employee(String fname, String lname, int salary, Address address ) {
      this.firstName = fname;
      this.lastName = lname;
      this.salary = salary;
      this.address = address;
   }
   
   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 Address getAddress() {
      return address;
   }
   
   public void setAddress( Address address ) {
      this.address = address;
   }
}

我们需要定义另一个与 ADDRESS 表对应的 POJO 类，以便可以将地址对象存储和检索到 ADDRESS 表中。

import java.util.*;

public class Address{
   private int id;
   private String street;     
   private String city;     
   private String state;    
   private String zipcode; 

   public Address() {}
   
   public Address(String street, String city, String state, String zipcode) {
      this.street = street; 
      this.city = city; 
      this.state = state; 
      this.zipcode = zipcode; 
   }
   
   public int getId() {
      return id;
   }
   
   public void setId( int id ) {
      this.id = id;
   }
   
   public String getStreet() {
      return street;
   }
   
   public void setStreet( String street ) {
      this.street = street;
   }
   
   public String getCity() {
      return city;
   }
   
   public void setCity( String city ) {
      this.city = city;
   }
   
   public String getState() {
      return state;
   }
   
   public void setState( String state ) {
      this.state = state;
   }
   
   public String getZipcode() {
      return zipcode;
   }
   
   public void setZipcode( String zipcode ) {
      this.zipcode = zipcode;
   }
   
}

定义 Hibernate 映射文件

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。 <many-to-one> 元素将用于定义在 EMPLOYEE 和 ADDRESS 实体之间建立多对一关系的规则。

<?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>
      
      <property name = "firstName" column = "first_name" type = "string"/>
      <property name = "lastName" column = "last_name" type = "string"/>
      <property name = "salary" column = "salary" type = "int"/>
      <many-to-one name = "address" column = "address" 
         class="Address" not-null="true"/>
         
   </class>

   <class name = "Address" table="ADDRESS">
      
      <meta attribute = "class-description">
         This class contains the address detail. 
      </meta>
      
      <id name = "id" type = "int" column = "id">
         <generator class="native"/>
      </id>
      
      <property name = "street" column = "street_name" type = "string"/>
      <property name = "city" column = "city_name" type = "string"/>
      <property name = "state" column = "state_name" type = "string"/>
      <property name = "zipcode" column = "zipcode" 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文件 Employee.hbm.xml 中。您已经熟悉了大部分映射细节，但让我们再次查看映射文件的所有元素 −

映射文档是以 <hibernate-mapping> 为根元素的XML文档，其中包含对应于每个类的两个 <class> 元素。
<class> 元素用于定义从 Java 类到数据库表的特定映射。Java 类名使用类元素的 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 属性保存hibernate 映射类型，这种映射类型将从Java 转换为SQL 数据类型。
<generator> 元素中的 a1 元素用于自动生成主键值。生成器元素的 class 属性设置为 native 以让 hibernate 选择 identity, sequence 或 hilo 算法来根据底层数据库的功能创建主键。
<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，这种映射类型将从Java 转换为SQL 数据类型。
<many-to-one> 元素用于设置 EMPLOYEE 和 ADDRESS 实体之间的关系。name 属性设置为父类中定义的变量，在我们的例子中是 address。 column 属性用于设置父表 EMPLOYEE 中的列名。

最后，我们将使用 main() 方法创建我们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的记录及其地址，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 one address object */
      Address address = ME.addAddress("Kondapur","Hyderabad","AP","532");

      /* Add employee records in the database */
      Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, address);

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

      /* 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 address record in the database */
   public Address addAddress(String street, String city, String state, String zipcode) {
      Session session = factory.openSession();
      Transaction tx = null;
      Integer addressID = null;
      Address address = null;
      
      try {
         tx = session.beginTransaction();
         address = new Address(street, city, state, zipcode);
         addressID = (Integer) session.save(address); 
         tx.commit();
      } catch (HibernateException e) {
         if (tx!=null) tx.rollback();
         e.printStackTrace(); 
      } finally {
         session.close(); 
      }
      return address;
   }

   /* Method to add an employee record in the database */
   public Integer addEmployee(String fname, String lname, int salary, Address address){
      Session session = factory.openSession();
      Transaction tx = null;
      Integer employeeID = null;
      
      try {
         tx = session.beginTransaction();
         Employee employee = new Employee(fname, lname, salary, address);
         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 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());
            Address add = employee.getAddress();
            System.out.println("Address ");
            System.out.println("\tStreet: " +  add.getStreet());
            System.out.println("\tCity: " + add.getCity());
            System.out.println("\tState: " + add.getState());
            System.out.println("\tZipcode: " + add.getZipcode());
         }
         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(); 
      }
   }
}

编译与执行

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，请确保您已正确设置 PATH 和 CLASSPATH。

按照配置章节中的说明创建 hibernate.cfg.xml 配置文件。
如上所示创建 Employee.hbm.xml 映射文件。
如上所示创建 Employee.java 源文件并编译它。
Create Address.java source file as shown above and compile it.
如上图创建 ManageEmployee.java源文件并编译。
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，并且将在 EMPLOYEE 和 ADDRESS 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
Address
        Street: Kondapur
        City: Hyderabad
        State: AP
        Zipcode: 532
First Name: Dilip  Last Name: Kumar  Salary: 3000
Address
        Street: Kondapur
        City: Hyderabad
        State: AP
        Zipcode: 532
First Name: Manoj  Last Name: Kumar  Salary: 5000
Address
        Street: Kondapur
        City: Hyderabad
        State: AP
        Zipcode: 532

如果您检查您的 EMPLOYEE 和 ADDRESS 表，它们应该有以下记录 −

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+---------+
| id | first_name | last_name | salary | address |
+----+------------+-----------+--------+---------+
|  1 | Manoj      | Kumar     |   5000 | 5       |
+----+------------+-----------+--------+---------+
1 row in set (0.00 sec)

mysql> select * from ADDRESS;
+----+-------------+-----------+------------+---------+
| id | street_name | city_name | state_name | zipcode |
+----+-------------+-----------+------------+---------+
|  1 | Kondapur    | Hyderabad | AP         | 532     |
+----+-------------+-----------+------------+---------+
1 row in set (0.00 sec)

mysql>

❮ Hibernate - O/R 映射

Hibernate 教程

Hibernate 其他

Hibernate - 多对一映射

定义 RDBMS 表

定义 POJO 类

定义 Hibernate 映射文件

编译与执行

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