舉例講解Java的Hibernate框架中的多對(duì)一和一對(duì)多映射

更新時(shí)間：2015年12月21日 08:55:43 投稿：goldensun

這篇文章主要介紹了Java的Hibernate框架中的多對(duì)一和一對(duì)多映射,Hibernate是Java的SSH三大web開發(fā)框架之一,需要的朋友可以參考下

多對(duì)一（Many-to-One）映射
多對(duì)一（many-to-one）關(guān)聯(lián)是最常見的關(guān)聯(lián)關(guān)系，其中一個(gè)對(duì)象可以與多個(gè)對(duì)象相關(guān)聯(lián)。例如，一個(gè)相同的地址對(duì)象可以與多個(gè)雇員的對(duì)象相關(guān)聯(lián)。

定義RDBMS表：
考慮一個(gè)情況，我們需要員工記錄存儲(chǔ)在EMPLOYEE表，將有以下結(jié)構(gòu)：

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，許多員工都可以有相同的地址，所以這種關(guān)聯(lián)可以使用許多一對(duì)一的關(guān)聯(lián)呈現(xiàn)。我們將存儲(chǔ)地址相關(guān)的信息在一個(gè)單獨(dú)的表，該表具有以下結(jié)構(gòu)：

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)
);

同時(shí)創(chuàng)建RBDMS表，并讓他們準(zhǔn)備下一個(gè)實(shí)現(xiàn)。

定義POJO類：
讓我們實(shí)現(xiàn)一個(gè)POJO類員工將被用于保存與EMPLOYEE表的對(duì)象和其地址類型的變量。

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;
  }
}

我們需要定義相應(yīng)的地址表，這樣地址對(duì)象可以存儲(chǔ)和檢索到地址表中的另一個(gè)POJO類。

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映射文件：
開發(fā)我們的映射文件，可指示Hibernate如何定義的類映射到數(shù)據(jù)庫(kù)表。<many-to-one>進(jìn)行元素將被用來定義規(guī)則建立Employee和Address實(shí)體之間的多對(duì)一關(guān)系。

<?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>

應(yīng)該保存的映射文件中的格式<classname>.hbm.xml。保存映射文件中的文件Employee.hbm.xml。已經(jīng)熟悉了大部分的映射細(xì)節(jié)，但讓我們?cè)俅慰纯从成湮募械乃性兀?/p>

映射文檔是具有<hibernate-mapping>為對(duì)應(yīng)于每一個(gè)類包含2個(gè)<class>元素的根元素的XML文檔。

<class>元素被用于定義數(shù)據(jù)庫(kù)表從一個(gè)Java類特定的映射。 Java類名指定使用class元素的name屬性和使用表屬性數(shù)據(jù)庫(kù)表名指定。

<id>元素映射在類中的唯一ID屬性到數(shù)據(jù)庫(kù)表的主鍵。 id元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫(kù)表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會(huì)從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。

id元素內(nèi)<generator>元素被用來自動(dòng)生成的主鍵值。將生成元素class屬性設(shè)置為原生讓Hibernate拾取無論是identity，sequence或者h(yuǎn)ilo的算法來創(chuàng)建主鍵根據(jù)底層數(shù)據(jù)庫(kù)的支持能力。

<property>元素用于一個(gè)Java類的屬性映射到數(shù)據(jù)庫(kù)表中的列。元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫(kù)表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會(huì)從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。

<many-to-one>進(jìn)行元素是用來設(shè)置EMPLOYEE和地址的實(shí)體之間的關(guān)系。name屬性被設(shè)置為在父類中定義的變量，在我們的情況下，它是地址。列屬性用于在父表EMPLOYEE集的列名。

最后，我們將創(chuàng)建應(yīng)用程序類的main（）方法來運(yùn)行應(yīng)用程序。我們將使用這個(gè)應(yīng)用程序，以節(jié)省一些employee連同的記錄他們的地址，然后我們將申請(qǐng)CRUD操作上的記錄。

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(" 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(); 
   }
  }
  /* 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(zhí)行：
下面是步驟來編譯并運(yùn)行上述應(yīng)用程序。請(qǐng)確保已在進(jìn)行的編譯和執(zhí)行之前，適當(dāng)?shù)卦O(shè)置PATH和CLASSPATH。

創(chuàng)建hibernate.cfg.xml配置文件中配置章節(jié)解釋。
創(chuàng)建Employee.hbm.xml映射文件，如上圖所示。
創(chuàng)建Employee.java源文件，如上圖所示，并編譯它。
創(chuàng)建Address.java源文件，如上圖所示，并編譯它。
創(chuàng)建ManageEmployee.java源文件，如上圖所示，并編譯它。
執(zhí)行ManageEmployee二進(jìn)制文件來運(yùn)行程序。

在屏幕上獲得以下結(jié)果，并同時(shí)記錄會(huì)在員工和地址表創(chuàng)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

如果檢查員工和地址表，就應(yīng)該記錄下了：

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)

一對(duì)多（One-to-Many）映射
一對(duì)多的映射可以使用一組Java集合不包含任何重復(fù)的元素來實(shí)現(xiàn)。我們已經(jīng)看到了如何設(shè)置映射集合在Hibernate中，所以如果你已經(jīng)學(xué)會(huì)了集合（Set）映射，那么所有設(shè)置可用于一對(duì)多的映射。

集合被映射到與映射表中<set>元素，并java.util.HashSet中初始化。您可以使用Set集合在類中，有一個(gè)集合中不需要重復(fù)的元素。
RDBMS表與POJO類我們依然采用上面例子中定義好的，
定義Hibernate映射文件：
讓我們指示Hibernate如何定義的類映射到數(shù)據(jù)庫(kù)表的映射文件。

<?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>
   <set name="certificates" cascade="all">
     <key column="employee_id"/>
     <one-to-many class="Certificate"/>
   </set>
   <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>

應(yīng)該保存的映射文件中的格式<classname>.hbm.xml。我們保存映射文件中的文件Employee.hbm.xml。你已經(jīng)熟悉了大部分的映射細(xì)節(jié)，但讓我們?cè)俅慰纯从成湮募械乃性兀?/p>

映射文檔是具有<hibernate-mapping>為對(duì)應(yīng)于每一個(gè)類包含2個(gè)<class>元素的根元素的XML文檔。

<class>元素被用于定義數(shù)據(jù)庫(kù)表從一個(gè)Java類特定的映射。 Java類名指定使用class元素的name屬性和使用表屬性數(shù)據(jù)庫(kù)表名指定。

id元素內(nèi)的<generator>元素被用來自動(dòng)生成的主鍵值。將生成元素的class屬性設(shè)置為原生讓Hibernate拾取identity，sequence或者h(yuǎn)ilo中的算法來創(chuàng)建主鍵根據(jù)底層數(shù)據(jù)庫(kù)的支持能力。

<set>元素設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性中<set>元素來告訴Hibernate來保存證書的對(duì)象，同時(shí)為Employee對(duì)象。 name屬性被設(shè)置為在父類中定義的變量集，在我們的例子是證書。對(duì)于每一組變量，我們需要定義在映射文件中單獨(dú)的一組元素。

<key>元素是包含外鍵的父對(duì)象，即在證書表中的列。表EMPLOYEE。

<one-to-many>元素表示一個(gè)Employee對(duì)象涉及到很多證書的對(duì)象。

創(chuàng)建應(yīng)用程序類：
最后，我們將創(chuàng)建應(yīng)用程序類的main（）方法來運(yùn)行應(yīng)用程序。我們將使用這個(gè)應(yīng)用程序，以節(jié)省一些員工連同記錄證書，然后我們將應(yīng)用上CRUD操作記錄。

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 a set of certificates for the first employee */
   HashSet set1 = new HashSet();
   set1.add(new Certificate("MCA"));
   set1.add(new Certificate("MBA"));
   set1.add(new Certificate("PMP"));
   
   /* Add employee records in the database */
   Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, set1);

   /* Another set of certificates for the second employee */
   HashSet set2 = new HashSet();
   set2.add(new Certificate("BCA"));
   set2.add(new Certificate("BA"));

   /* Add another employee record in the database */
   Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, set2);

   /* 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 employee record in the database */
  public Integer addEmployee(String fname, String lname, 
                      int salary, Set cert){
   Session session = factory.openSession();
   Transaction tx = null;
   Integer employeeID = null;
   try{
     tx = session.beginTransaction();
     Employee employee = new Employee(fname, lname, salary);
     employee.setCertificates(cert);
     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 iterator1 = 
              employees.iterator(); iterator1.hasNext();){
      Employee employee = (Employee) iterator1.next(); 
      System.out.print("First Name: " + employee.getFirstName()); 
      System.out.print(" Last Name: " + employee.getLastName()); 
      System.out.println(" Salary: " + employee.getSalary());
      Set certificates = employee.getCertificates();
      for (Iterator iterator2 = 
             certificates.iterator(); iterator2.hasNext();){
         Certificate certName = (Certificate) iterator2.next(); 
         System.out.println("Certificate: " + certName.getName()); 
      }
     }
     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(zhí)行：

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Manoj Last Name: Kumar Salary: 4000
Certificate: MBA
Certificate: PMP
Certificate: MCA
First Name: Dilip Last Name: Kumar Salary: 3000
Certificate: BCA
Certificate: BA
First Name: Manoj Last Name: Kumar Salary: 5000
Certificate: MBA
Certificate: PMP
Certificate: MCA

如果檢查員工和證書表，就應(yīng)該記錄下了：

mysql> select * from employee;

+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 1 | Manoj   | Kumar   |  5000 |
+----+------------+-----------+--------+
1 row in set (0.00 sec)

mysql> select * from certificate;

+----+------------------+-------------+
| id | certificate_name | employee_id |
+----+------------------+-------------+
| 1 | MBA       |     1 |
| 2 | PMP       |     1 |
| 3 | MCA       |     1 |
+----+------------------+-------------+
3 rows in set (0.00 sec)

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