Hibernate组件映射

组件映射是一类具有参考到另一个类的成员变量的映射。我们已经看到，同时有两个表，并使用在映射文件中的<set>元素这样的映射。现在我们将使用<component>元素在映射文件和一个单一的表将用于保持包含的类变量里面的属性。

定义RDBMS表：

考虑一个情况下，需要员工记录存储在EMPLOYEE表，表有以下结构：

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，假设每个雇工将有一个地址，所以在同一个表中添加特定的地址字段，如下所示：

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,
   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)
);

定义POJO类：

实现POJO员工类将被用于保存与EMPLOYEE表的对象。

import java.util.*;

public class Employee implements java.io.Serializable {
   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;
   }
}

需要定义对应地址有实体地址相关字段到另一个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：

编写指示Hibernate如何定义的类映射到数据库表的映射文件。<component>元素将被用来定义规则，对所有与地址表关联的字段。

<?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>
      <component name="address" class="Address">
         <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"/>
      </component>
      <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>

保存映射文件名的格式为：<classname>.hbm.xml。保存映射文件Employee.hbm.xml。在前面已经熟悉了大部分的映射细节，但再次看看映射文件中的所有元素：

映射文档是具有<hibernate-mapping>为对应于每个类对应的2个<class>元素的根元素的XML文档。
<class>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。
<meta>元素是可选元素，可以用来创建类的描述。
<id>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元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置为原生让Hibernate拾取无论是identity，sequence还是hilo的算法来创建主键根据底层数据库的支持能力。
<property>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。
<component>元素设置地址类的内部Employee类的不同属性的存在。

创建应用程序类：

最后，创建应用程序类的main（）方法来运行应用程序。使用这个应用程序，以节省一些员工的记录关联证书，然后将应用CRUD操作记录。

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 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("	Street: " +  add.getStreet());
            System.out.println("	City: " + add.getCity());
            System.out.println("	State: " + add.getState());
            System.out.println("	Zipcode: " + 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：

下面是步骤来编译并运行上述应用程序。请确保您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。
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EMPLOYEE表中创建。

$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: 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

如果检查EMPLOYEE表，它应该已记录下：

mysql> select id, first_name,salary, street_name, state_name from EMPLOYEE;
+----+------------+--------+-------------+------------+
| id | first_name | salary | street_name | state_name |
+----+------------+--------+-------------+------------+
|  1 | Manoj      |   5000 | Kondapur    | AP         |
|  2 | Dilip      |   3000 | Saharanpur  | UP         |
+----+------------+--------+-------------+------------+
2 rows in set (0.00 sec)

mysql>