Hibernate - 一对一映射

一对一关联类似于多对一关联，区别在于该列将被设置为唯一。例如，一个地址对象可以与一个员工对象关联。

定义 RDBMS 表

考虑这样一个场景：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，假设一个地址只能与一个员工关联，因此可以使用一对一关联来表示此关联。我们将把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)
);

创建这两个 RDBMS 表，并为下一步的实现做好准备。

定义 POJO 类

让我们实现一个 POJO 类**Employee**，它将用于持久化与 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到 ADDRESS 表中并从中检索。

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。元素将用于定义规则以在 EMPLOYEE 和 ADDRESS 实体之间建立一对一关系，但 column 属性将设置为**unique** 约束，其余映射文件将保持与多对一关联的情况相同。

<?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" unique="true" 
         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>

应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，其根元素为，其中包含两个与每个类对应的元素。
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。
元素是可选元素，可用于创建类描述。
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，此映射类型将 Java 类型转换为 SQL 数据类型。
id 元素中的元素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。
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 Java 类型转换为 SQL 数据类型。
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，在本例中为 *address*。column 属性用于设置父表 EMPLOYEE 中的列名，该列名设置为 unique，以便只有一个 Employee 对象可以与一个地址对象关联。

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 one address object */
      Address address1 = ME.addAddress("Kondapur","Hyderabad","AP","532");

      /* Add employee records in the database */
      Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, address1);

      /* Let us have another address object */
      Address address2 = ME.addAddress("Saharanpur","Ambehta","UP","111");
  
      /* Add another employee record in the database */
      Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, address2);

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

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

      /* 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(); 
      }
   }
}

编译和执行

以下是编译和运行上述应用程序的步骤。在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。
创建如上所示的 Employee.hbm.xml 映射文件。
创建如上所示的 Employee.java 源文件并编译它。
创建如上所示的 Address.java 源文件并编译它。
创建如上所示的 ManageEmployee.java 源文件并编译它。
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，同时在 EMPLOYEE 和 ADDRESS 表中创建记录。

$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: Saharanpur
        City: Ambehta
        State: UP
        Zipcode: 111
First Name: Manoj  Last Name: Kumar  Salary: 5000
Address
        Street: Kondapur
        City: Hyderabad
        State: AP
        Zipcode: 532
First Name: Dilip  Last Name: Kumar  Salary: 3000
Address
        Street: Saharanpur
        City: Ambehta
        State: UP
        Zipcode: 111

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

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+---------+
| id | first_name | last_name | salary | address |
+----+------------+-----------+--------+---------+
|  7 | Manoj      | Kumar     |   5000 |       5 |
|  8 | Dilip      | Kumar     |   3000 |       6 |
+----+------------+-----------+--------+---------+
2 rows in set (0.00 sec)

mysql> select * from ADDRESS;
+----+-------------+-----------+------------+---------+
| id | street_name | city_name | state_name | zipcode |
+----+-------------+-----------+------------+---------+
|  5 | Kondapur    | Hyderabad | AP         | 532     |
|  6 | Saharanpur  | Ambehta   | UP         | 111     |
+----+-------------+-----------+------------+---------+
2 rows in set (0.00 sec)

mysql>

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