举例讲解Java的Hibernate框架中的多对一和一对多映射

举例讲解Java的Hibernate框架中的多对一和一对多映射

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。

"-//Hibernate/Hibernate Mapping DTD//EN"

"http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">

This class contains the employee detail.

class="Address" not-null="true"/>

This class contains the address detail.

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(" 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();

}

}

}

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)

一对多（One-to-Many）映射

一对多的映射可以使用一组Java集合不包含任何重复的元素来实现。我们已经看到了如何设置映射集合在Hibernate中，所以如果你已经学会了集合（Set）映射，那么所有设置可用于一对多的映射。

集合被映射到与映射表中元素，并java.util.HashSet中初始化。您可以使用Set集合在类中，有一个集合中不需要重复的元素。

RDBMS表与POJO类我们依然采用上面例子中定义好的，

定义Hibernate映射文件：

让我们指示Hibernate如何定义的类映射到数据库表的映射文件。

"-//Hibernate/Hibernate Mapping DTD//EN"

"http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">

This class contains the employee detail.

This class contains the certificate records.

应该保存的映射文件中的格式.hbm.xml。我们保存映射文件中的文件Employee.hbm.xml。你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元素内的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identity，sequence或者hilo中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。

元素来告诉Hibernate来保存证书的对象，同时为Employee对象。 name属性被设置为在父类中定义的变量集，在我们的例子是证书。对于每一组变量，我们需要定义在映射文件中单独的一组元素。

元素是包含外键的父对象，即在证书表中的列。表EMPLOYEE。

元素表示一个Employee对象涉及到很多证书的对象。

创建应用程序类：

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http://，以节省一些员工连同记录证书，然后我们将应用上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 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();

}

}

}

编译和执行：

$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

如果检查员工和证书表，就应该记录下了：

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)

class="Address" not-null="true"/>

This class contains the address detail.

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(" 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();

}

}

}

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)

一对多（One-to-Many）映射

一对多的映射可以使用一组Java集合不包含任何重复的元素来实现。我们已经看到了如何设置映射集合在Hibernate中，所以如果你已经学会了集合（Set）映射，那么所有设置可用于一对多的映射。

集合被映射到与映射表中元素，并java.util.HashSet中初始化。您可以使用Set集合在类中，有一个集合中不需要重复的元素。

RDBMS表与POJO类我们依然采用上面例子中定义好的，

定义Hibernate映射文件：

让我们指示Hibernate如何定义的类映射到数据库表的映射文件。

"-//Hibernate/Hibernate Mapping DTD//EN"

"http://hibernate.org/dtd/hibernate-mapping-3.0.dtd">

This class contains the employee detail.

This class contains the certificate records.

应该保存的映射文件中的格式.hbm.xml。我们保存映射文件中的文件Employee.hbm.xml。你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元素内的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identity，sequence或者hilo中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。

元素来告诉Hibernate来保存证书的对象，同时为Employee对象。 name属性被设置为在父类中定义的变量集，在我们的例子是证书。对于每一组变量，我们需要定义在映射文件中单独的一组元素。

元素是包含外键的父对象，即在证书表中的列。表EMPLOYEE。

元素表示一个Employee对象涉及到很多证书的对象。

创建应用程序类：

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http://，以节省一些员工连同记录证书，然后我们将应用上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 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();

}

}

}

编译和执行：

$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

如果检查员工和证书表，就应该记录下了：

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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