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。<many-to-one>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。

<"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> <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" 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/> </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>

应该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<hibernate-mapping>为对应于每一个类包含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<generator>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。

<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。

<many-to-one>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<set>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。

<"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> <set name="certificates" cascade="all"> <key column="employee_id"/> <one-to-many/> </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/> </id> <property name="name" column="certificate_name" type="string"/> </class> </hibernate-mapping>

应该保存的映射文件中的格式<classname>.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<hibernate-mapping>为对应于每一个类包含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。

<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。

<set>元素设置证书和Employee类之间的关系。我们使用cascade属性中<set>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。

<key>元素是包含外键的父对象，即在证书表中的列。表EMPLOYEE。

<one-to-many>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，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)