组件映射（Component Mappings）

优质

小牛编辑

134浏览

2023-12-01

Component映射是具有对另一个类的引用作为成员变量的类的映射。我们已经看到这样的映射，同时有两个表并在映射文件中使用“set”元素。现在我们将在映射文件中使用“component”元素，并使用单个表来保持类变量中包含的属性。

定义RDBMS表 (Define RDBMS Tables)

考虑一种情况，我们需要将员工记录存储在EMPLOYEE表中，该表具有以下结构 -

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，假设每个员工都有一个地址，所以让我们在同一个表中添加地址特定字段，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)
);

Define POJO Classes

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。

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;
   }
}

我们需要定义与具有地址相关字段的ADDRESS实体相对应的另一个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 (Define Hibernate Mapping File)

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。元素将用于为与ADDRESS表关联的所有字段定义规则。

<?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/>
      </id>
      <component name = "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/>
      </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 .hbm.xml的文件中。我们将映射文档保存在Employee.hbm.xml文件中。您已经熟悉了大部分映射细节，但让我们再次看到映射文件的所有元素 -

映射文档是具有《hibernate-mapping》作为根元素的XML文档，其包含对应于每个类的两个“类”元素。
《class》元素用于定义从Java类到数据库表的特定映射。使用class元素的name属性指定Java类名，并使用table属性指定数据库表名。
《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属性包含hibernate映射类型，这种映射类型将从Java转换为SQL数据类型。
id元素中的《generator》元素用于自动生成主键值。生成器元素的class属性设置为native ，让hibernate选择identity, sequence或hilo算法来创建主键，具体取决于底层数据库的功能。
《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属性包含hibernate映射类型，这种映射类型将从Java转换为SQL数据类型。
《component》元素设置Employee类中Address类的不同属性的存在。

创建应用程序类 (Create Application Class)

最后，我们将使用main（）方法创建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少量员工的记录及其证书，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 (Compilation and Execution)

以下是编译和运行上述应用程序的步骤。确保在进行编译和执行之前已适当地设置了PATH和CLASSPATH。

按配置章节中的说明创建hibernate.cfg.xml配置文件。
创建Employee.hbm.xml映射文件，如上所示。
如上所示创建Employee.java源文件并进行编译。
创建ManageEmployee.java源文件，如上所示并编译它。
执行ManageEmployee二进制文件以运行该程序。

您将在屏幕上看到以下结果，并且将在EMPLOYEE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: 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表，它应该有以下记录 -

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>