Map映射
Map映射是一个java集合存储在键 - 值对的元素，并且不允许在列表中重复的元素。 Map接口提供三种collection视图，允许Map内容看作是一组键-值集合，或者设置键 - 值映射关系。
Map被映射到映射表中一个<map>元素和无序的地图可以在java.util.HashMap中被初始化。
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, PRIMARY KEY (id));
此外，假设每个员工都可以有一个或多个与他/她相关的证书。我们将存储证书的相关信息在一个单独的表，该表具有以下结构：
create table CERTIFICATE ( id INT NOT NULL auto_increment, certificate_type VARCHAR(40) default NULL, certificate_name VARCHAR(30) default NULL, employee_id INT default NULL, PRIMARY KEY (id));
有一个对多（one-to-many ）的关系在EMPLOYEE和证书对象之间。
定义POJO类：
让我们实现一个POJO类员工将被用于保存与EMPLOYEE表中的对象和有证书的列表变量的集合。
import java.util.*;public class Employee { private int id; private String firstName; private String lastName; private int salary; private Map certificates; public Employee() {} public Employee(String fname, String lname, int salary) { this.firstName = fname; this.lastName = lname; this.salary = salary; } 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 Map getCertificates() { return certificates; } public void setCertificates( Map certificates ) { this.certificates = certificates; }}我们需要相应的证书表定义另一个POJO类，这样的证书对象可以存储和检索到的证书表。
public class Certificate{ private int id; private String name; public Certificate() {} public Certificate(String name) { this.name = name; } public int getId() { return id; } public void setId( int id ) { this.id = id; } public String getName() { return name; } public void setName( String name ) { this.name = name; }}定义Hibernate映射文件：
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。
<?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 class="native"/> </id> <map name="certificates" cascade="all"> <key column="employee_id"/> <index column="certificate_type" type="string"/> <one-to-many class="Certificate"/> </map> <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>
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。已经熟悉了大部分的映射细节，但让我们再次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属性设置为原产于让Hibernate拿起无论是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。
<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>元素来告诉Hibernate来保存证书的对象，同时为Employee对象。 name属性设置为定义Mapvariable在父类中，在我们的案例中是证书。
<index>的元素用于表示键/值对地图的关键部位。该键将使用一个字符串类型的存储在列certificate_type。
<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，并因此，证书对象必须有与Employee父类有关。可以根据需要使用<one-to-one>，<many-to-one>或<many-to-many>这个元素。
创建应用程序类：
最后，创建应用程序类的main（）方法来运行应用程序。使用这个应用程序来保存员工记录地连同证书列表，然后在应用上执行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 a set of certificates for the first employee */ HashMap set = new HashMap(); set.put("ComputerScience", new Certificate("MCA")); set.put("BusinessManagement", new Certificate("MBA")); set.put("ProjectManagement", new Certificate("PMP")); /* Add employee records in the database */ Integer empID = ME.addEmployee("Manoj", "Kumar", 4000, set); /* List down all the employees */ ME.listEmployees(); /* Update employee's salary records */ ME.updateEmployee(empID, 5000); /* 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, HashMap 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()); Map ec = employee.getCertificates(); System.out.println("Certificate: " + (((Certificate)ec.get("ComputerScience")).getName())); System.out.println("Certificate: " + (((Certificate)ec.get("BusinessManagement")).getName())); System.out.println("Certificate: " + (((Certificate)ec.get("ProjectManagement")).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：
下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前，已经适当地设置PATH和CLASSPATH。
会在屏幕上获得以下结果，并同时创建记录在员工和证书表。
$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........
First Name: Manoj Last Name: Kumar Salary: 4000Certificate: MCACertificate: MBACertificate: PMPFirst Name: Manoj Last Name: Kumar Salary: 5000Certificate: MCACertificate: MBACertificate: PMP
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 |+----+------------+-----------+--------+| 60 | Manoj | Kumar | 5000 |+----+------------+-----------+--------+1 row in set (0.00 sec)
mysql>select * from CERTIFICATE;
+----+--------------------+------------------+-------------+| id | certificate_type | certificate_name | employee_id |+----+--------------------+------------------+-------------+| 16 | ProjectManagement | PMP | 60 || 17 | BusinessManagement | MBA | 60 || 18 | ComputerScience | MCA | 60 |+----+--------------------+------------------+-------------+3 rows in set (0.00 sec)
SortedMap映射
SortedMap是存储在键 - 值对的元素，并提供键的总体排序类似java的集合作为映射。重复的元素未在映射不允许的。该映射是根据其键的自然顺序进行排序，或者通过提供通常在有序映射的创建时间比较。
SortedMap被映射在该映射表中的<map>元素和有序映射可以在java.util.TreeMap中被初始化。
我们依然使用上面定义的RDBMS表和POJO类来讲下面的例子：
定义Hibernate映射文件：
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。
<?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 class="native"/> </id> <map name="certificates" cascade="all" sort="MyClass"> <key column="employee_id"/> <index column="certificate_type" type="string"/> <one-to-many class="Certificate"/> </map> <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>
应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件Employee.hbm.xml。前面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>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属性设置让Hibernate对应于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。
<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>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的定义的SortedMap变量，在我们的情况下，它是证书。排序属性可以设置为自然有自然排序，也可以设置为自定义类实现为java.util.Comparator。我们已经使用了一个类MyClass，它实现为java.util.Comparator扭转证书类实现的排序顺序。
<index>元素用于表示键/值对映射的键部分。该键将使用一个字符串类型的存储在列certificate_type。
<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，并因此，证书对象必须有与Employee父有关联。可以根据需要使用<one-to-one>，<many-to-one>或<many-to-many>这个元素。
如果使用sort="natural"，然后我们并不需要创建一个单独的类，因为证书类已经实现了Comparable接口和hibernate会使用compareTo在证书类定义为比较的SortedMap（）方法。但是，我们使用的是在我们的映射文件自定义的比较器类MyClass，所以我们必须创建这个类的基础上我们的排序算法。让我们做降在映射上可用的按键排序。
import java.util.Comparator;public class MyClass implements Comparator <String>{ public int compare(String o1, String o2) { final int BEFORE = -1; final int AFTER = 1; /* To reverse the sorting order, multiple by -1 */ if (o2 == null) { return BEFORE * -1; } Comparable thisCertificate = o1; Comparable thatCertificate = o2; if(thisCertificate == null) { return AFTER * 1; } else if(thatCertificate == null) { return BEFORE * -1; } else { return thisCertificate.compareTo(thatCertificate) * -1; } }}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，以节省一些员工的记录地连同的证书，然后我们将提交CRUD操作上面的记录。
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 a set of certificates for the first employee */ TreeMap set1 = new TreeMap(); set1.put("ComputerScience", new Certificate("MCA")); set1.put("BusinessManagement", new Certificate("MBA")); set1.put("ProjectManagement", 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 */ TreeMap set2 = new TreeMap(); set2.put("ComputerScience", new Certificate("MCA")); set2.put("BusinessManagement", new Certificate("MBA")); /* 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, TreeMap 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()); SortedMap<String, Certificate> map = employee.getCertificates(); for(Map.Entry<String,Certificate> entry : map.entrySet()){ System.out.print("/tCertificate Type: " + entry.getKey()); System.out.println(", Name: " + (entry.getValue()).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：
可以看到证书已排序顺序相反。可以通过改变映射文件试试，只需设置sort="natural"和执行程序，并比较结果。
$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........First Name: Manoj Last Name: Kumar Salary: 4000 Certificate Type: ProjectManagement, Name: PMP Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBAFirst Name: Dilip Last Name: Kumar Salary: 3000 Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBAFirst Name: Manoj Last Name: Kumar Salary: 5000 Certificate Type: ProjectManagement, Name: PMP Certificate Type: ComputerScience, Name: MCA Certificate Type: BusinessManagement, Name: MBA
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 |+----+------------+-----------+--------+| 74 | Manoj | Kumar | 5000 |+----+------------+-----------+--------+1 row in set (0.00 sec)
mysql> select * from CERTIFICATE;
+----+--------------------+------------------+-------------+| id | certificate_type | certificate_name | employee_id |+----+--------------------+------------------+-------------+| 52 | BusinessManagement | MBA | 74 || 53 | ComputerScience | MCA | 74 || 54 | ProjectManagement | PMP | 74 |+----+--------------------+------------------+-------------+3 rows in set (0.00 sec)
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