Hibernate - 多對一對映

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多對一關聯是最常見的關聯型別，其中一個物件可以與多個物件關聯。例如，同一個地址物件可以與多個員工物件關聯。

定義 RDBMS 表

考慮這樣一個場景，我們需要將員工記錄儲存在 EMPLOYEE 表中，該表將具有以下結構：

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，許多員工可能具有相同的地址，因此可以使用多對一關聯來表示此關聯。我們將把與地址相關的資訊儲存在具有以下結構的單獨表中：

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)
);

建立這兩個 RBDMS 表，併為下一步的實現做好準備。

定義 POJO 類

讓我們實現一個 POJO 類Employee，它將用於持久化與 EMPLOYEE 表相關的物件，並具有 Address 型別的變數。

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;
   }
}

我們需要定義另一個與 ADDRESS 表對應的 POJO 類，以便可以將地址物件儲存到 ADDRESS 表中並從中檢索。

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;
   }
   
}

定義 Hibernate 對映檔案

讓我們開發我們的對映檔案，它指示 Hibernate 如何將定義的類對映到資料庫表。<many-to-one> 元素將用於定義規則來建立 EMPLOYEE 和 ADDRESS 實體之間的多對一關係。

<?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>

您應該將對映文件儲存在格式為 <classname>.hbm.xml 的檔案中。我們將對映文件儲存在 Employee.hbm.xml 檔案中。您已經熟悉大部分對映細節，但讓我們再次檢視對映檔案的所有元素：

  • 對映文件是一個 XML 文件，其中<hibernate-mapping>作為根元素，包含兩個與每個類對應的<class>元素。

  • <class>元素用於定義從 Java 類到資料庫表的特定對映。Java 類名使用 class 元素的 name 屬性指定，資料庫表名使用 table 屬性指定。

  • <meta>元素是可選元素，可用於建立類描述。

  • <id>元素將類中唯一的 ID 屬性對映到資料庫表的主鍵。id 元素的 name 屬性引用類中的屬性，column 屬性引用資料庫表中的列。type 屬性包含 Hibernate 對映型別，此對映型別將從 Java 轉換為 SQL 資料型別。

  • id 元素內的<generator>元素用於自動生成主鍵值。generator 元素的 class 屬性設定為native，以便讓 Hibernate 根據底層資料庫的功能選擇identity、sequence或hilo演算法來建立主鍵。

  • <property>元素用於將 Java 類屬性對映到資料庫表中的列。元素的 name 屬性引用類中的屬性，column 屬性引用資料庫表中的列。type 屬性包含 Hibernate 對映型別，此對映型別將從 Java 轉換為 SQL 資料型別。

  • <many-to-one>元素用於設定 EMPLOYEE 和 ADDRESS 實體之間的關係。name 屬性設定為父類中定義的變數，在本例中為address。column 屬性用於設定父表 EMPLOYEE 中的列名。

最後，我們將建立包含 main() 方法的應用程式類以執行應用程式。我們將使用此應用程式儲存一些員工記錄及其地址，然後我們將對這些記錄應用 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(); 
      }
   }
}

編譯和執行

以下是編譯和執行上述應用程式的步驟。在繼續編譯和執行之前，請確保已正確設定 PATH 和 CLASSPATH。

  • 建立 hibernate.cfg.xml 配置檔案，如配置章節中所述。

  • 建立如上所示的 Employee.hbm.xml 對映檔案。

  • 建立如上所示的 Employee.java 原始檔並編譯它。

  • 建立如上所示的 Address.java 原始檔並編譯它。

  • 建立如上所示的 ManageEmployee.java 原始檔並編譯它。

  • 執行 ManageEmployee 二進位制檔案以執行程式。

您將在螢幕上看到以下結果，同時會在 EMPLOYEE 和 ADDRESS 表中建立記錄。

$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

如果您檢查 EMPLOYEE 和 ADDRESS 表，它們應該具有以下記錄：

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_or_mappings.htm
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