Hibernate 多对一（Many-to-One）映射

多对一（many-to-one）关联是最常见的关联关系，其中一个对象可以与多个对象相关联。例如，一个相同的地址对象可以与多个雇员的对象相关联。

定义RDBMS表：

考虑一个情况，我们需要员工记录存储在EMPLOYEE表，将有以下结构：

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，许多员工都可以有相同的地址，所以这种关联可以使用许多一对一的关联呈现。我们将存储地址相关的信息在一个单独的表，该表具有以下结构：

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)
);

同时创建RBDMS表，并让他们准备下一个实现。

定义POJO类：

让我们实现一个POJO类员工将被用于保存与EMPLOYEE表的对象和其地址类型的变量。

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;
   }
}

我们需要定义相应的地址表，这样地址对象可以存储和检索到地址表中的另一个POJO类。

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：

开发我们的映射文件，可指示Hibernate如何定义的类映射到数据库表。进行元素将被用来定义规则建立Employee和Address实体之间的多对一关系。

xml version="1.0" encoding="utf-8"?>
 


    name="Employee" table="EMPLOYEE">
       attribute="class-description">
         This class contains the employee detail. 
      
       name="id" type="int" column="id">
          class="native"/>
      
       name="firstName" column="first_name" type="string"/>
       name="lastName" column="last_name" type="string"/>
       name="salary" column="salary" type="int"/>
       name="address" column="address" 
       class="Address" not-null="true"/>
   

    name="Address" table="ADDRESS">
       attribute="class-description">
         This class contains the address detail. 
      
       name="id" type="int" column="id">
          class="native"/>
      
       name="street" column="street_name" type="string"/>
       name="city" column="city_name" type="string"/>
       name="state" column="state_name" type="string"/>
       name="zipcode" column="zipcode" type="string"/>
   

应该保存的映射文件中的格式.hbm.xml。保存映射文件中的文件Employee.hbm.xml。已经熟悉了大部分的映射细节，但让我们再次看看映射文件中的所有元素： 

• 映射文档是具有为对应于每一个类包含2个元素的根元素的XML文档。

• 元素被用于定义数据库表从一个Java类特定的映射。 Java类名指定使用class元素的name属性和使用表属性数据库表名指定。

• 元素是可选元素，可以用来创建类的描述。

• 元素映射在类中的唯一ID属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型，这种类型的映射将会从Java转换为SQL数据类型。

• id元素内元素被用来自动生成的主键值。将生成元素class属性设置为原生让Hibernate拾取无论是identity，sequence或者hilo的算法来创建主键根据底层数据库的支持能力。

• 元素用于一个Java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型，这种类型的映射将会从Java转换为SQL数据类型。

• 进行元素是用来设置EMPLOYEE和地址的实体之间的关系。name属性被设置为在父类中定义的变量，在我们的情况下，它是地址。列属性用于在父表EMPLOYEE集的列名。

最后，我们将创建应用程序类的main（）方法来运行应用程序。我们将使用这个应用程序，以节省一些employee连同的记录他们的地址，然后我们将申请CRUD操作上的记录。

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(); 
      }
   }
}

编译和执行：

下面是步骤来编译并运行上述应用程序。请确保已在进行的编译和执行之前，适当地设置PATH和CLASSPATH。

• 创建hibernate.cfg.xml配置文件中配置章节解释。

• 创建Employee.hbm.xml映射文件，如上图所示。

• 创建Employee.java源文件，如上图所示，并编译它。

• 创建Address.java源文件，如上图所示，并编译它。

• 创建ManageEmployee.java源文件，如上图所示，并编译它。

• 执行ManageEmployee二进制文件来运行程序。

在屏幕上获得以下结果，并同时记录会在员工和地址表创建。

$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

如果检查员工和地址表，就应该记录下了：

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>