Map映射
Map映射是一個java集合存儲在鍵 - 值對的元素，并且不允許在列表中重復(fù)的元素。 Map接口提供三種collection視圖，允許Map內(nèi)容看作是一組鍵-值集合，或者設(shè)置鍵 - 值映射關(guān)系。
Map被映射到映射表中一個<map>元素和無序的地圖可以在java.util.HashMap中被初始化。
定義RDBMS表：
考慮一個情況，我們需要員工記錄存儲在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, PRIMARY KEY (id));
此外，假設(shè)每個員工都可以有一個或多個與他/她相關(guān)的證書。我們將存儲證書的相關(guān)信息在一個單獨的表，該表具有以下結(jié)構(gòu)：
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));
有一個對多（one-to-many ）的關(guān)系在EMPLOYEE和證書對象之間。
定義POJO類：
讓我們實現(xiàn)一個POJO類員工將被用于保存與EMPLOYEE表中的對象和有證書的列表變量的集合。
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; }}我們需要相應(yīng)的證書表定義另一個POJO類，這樣的證書對象可以存儲和檢索到的證書表。
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; }}定義Hibernate映射文件：
讓我們開發(fā)指示Hibernate如何定義的類映射到數(shù)據(jù)庫表的映射文件。<map>元素將被用于定義所使用的映射的規(guī)則。
<?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>
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。已經(jīng)熟悉了大部分的映射細節(jié)，但讓我們再次看映射文件中的所有元素：
映射文檔是具有<hibernate-mapping>為對應(yīng)于每一個類包含2個<class>元素的根元素的XML文檔。
<class>元素被用于定義數(shù)據(jù)庫表從一個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)庫表名指定。
<meta>元素是可選元素，可以用來創(chuàng)建類的描述。
<id>元素映射在類中的唯一ID屬性到數(shù)據(jù)庫表的主鍵。 id元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。
id元素內(nèi)的<generator>元素被用來自動生成的主鍵值。將生成元素的class屬性設(shè)置為原產(chǎn)于讓Hibernate拿起無論是identity，sequence或者hilo中的算法來創(chuàng)建主鍵根據(jù)底層數(shù)據(jù)庫的支持能力。
<property>元素用于一個Java類的屬性映射到數(shù)據(jù)庫表中的列。元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。
<map>元素用于設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性的<map>元素來告訴Hibernate來保存證書的對象，同時為Employee對象。 name屬性設(shè)置為定義Mapvariable在父類中，在我們的案例中是證書。
<index>的元素用于表示鍵/值對地圖的關(guān)鍵部位。該鍵將使用一個字符串類型的存儲在列certificate_type。
<key>元素是包含外鍵的父對象，即在證書表中的列。表EMPLOYEE。
<one-to-many>元素表示一個Employee對象涉及到很多證書的對象，并因此，證書對象必須有與Employee父類有關(guān)。可以根據(jù)需要使用<one-to-one>，<many-to-one>或<many-to-many>這個元素。
創(chuàng)建應(yīng)用程序類：
最后，創(chuàng)建應(yīng)用程序類的main（）方法來運行應(yīng)用程序。使用這個應(yīng)用程序來保存員工記錄地連同證書列表，然后在應(yīng)用上執(zhí)行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 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(); } }}編譯和執(zhí)行：
下面是步驟來編譯并運行上述應(yīng)用程序。請確保您已在進行的編譯和執(zhí)行之前，已經(jīng)適當(dāng)?shù)卦O(shè)置PATH和CLASSPATH。
會在屏幕上獲得以下結(jié)果，并同時創(chuàng)建記錄在員工和證書表。
$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 |+----+------------+-----------+--------+| 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是存儲在鍵 - 值對的元素，并提供鍵的總體排序類似java的集合作為映射。重復(fù)的元素未在映射不允許的。該映射是根據(jù)其鍵的自然順序進行排序，或者通過提供通常在有序映射的創(chuàng)建時間比較。
SortedMap被映射在該映射表中的<map>元素和有序映射可以在java.util.TreeMap中被初始化。
我們依然使用上面定義的RDBMS表和POJO類來講下面的例子：
定義Hibernate映射文件：
讓我們開發(fā)指示Hibernate如何定義的類映射到數(shù)據(jù)庫表的映射文件。<map>元素將被用于定義所使用的映射的規(guī)則。
<?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>
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Employee.hbm.xml。前面已經(jīng)熟悉了大部分的映射細節(jié)，但讓我們再次看看映射文件中的所有元素：
映射文檔是具有<hibernate-mapping>對應(yīng)于每一個類包含2個<class>元素的根元素的XML文檔。
<class>元素被用于定義數(shù)據(jù)庫表從一個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)庫表名指定。
<meta>元素是可選元素，可以用來創(chuàng)建類的描述。
<id>元素映射在類中的唯一ID屬性到數(shù)據(jù)庫表的主鍵。 id元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。
id元素內(nèi)的<generator>元素被用來自動生成的主鍵值。將生成元素的class屬性設(shè)置讓Hibernate對應(yīng)于identity，sequence或者hilo中的算法來創(chuàng)建主鍵根據(jù)底層數(shù)據(jù)庫的支持能力。
<property>元素用于一個Java類的屬性映射到數(shù)據(jù)庫表中的列。元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。
<map>元素用于設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性的<map>元素來告訴Hibernate來保存證書的對象，同時為Employee對象。 name屬性被設(shè)置為在父類中的定義的SortedMap變量，在我們的情況下，它是證書。排序?qū)傩钥梢栽O(shè)置為自然有自然排序，也可以設(shè)置為自定義類實現(xiàn)為java.util.Comparator。我們已經(jīng)使用了一個類MyClass，它實現(xiàn)為java.util.Comparator扭轉(zhuǎn)證書類實現(xiàn)的排序順序。
<index>元素用于表示鍵/值對映射的鍵部分。該鍵將使用一個字符串類型的存儲在列certificate_type。
<key>元素是包含外鍵的父對象，即在證書表中的列。表EMPLOYEE。
<one-to-many>元素表示一個Employee對象涉及到很多證書的對象，并因此，證書對象必須有與Employee父有關(guān)聯(lián)。可以根據(jù)需要使用<one-to-one>，<many-to-one>或<many-to-many>這個元素。
如果使用sort="natural"，然后我們并不需要創(chuàng)建一個單獨的類，因為證書類已經(jīng)實現(xiàn)了Comparable接口和hibernate會使用compareTo在證書類定義為比較的SortedMap（）方法。但是，我們使用的是在我們的映射文件自定義的比較器類MyClass，所以我們必須創(chuàng)建這個類的基礎(chǔ)上我們的排序算法。讓我們做降在映射上可用的按鍵排序。
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，我們將創(chuàng)建應(yīng)用程序類的main（）方法來運行應(yīng)用程序。我們將使用這個應(yīng)用程序，以節(jié)省一些員工的記錄地連同的證書，然后我們將提交CRUD操作上面的記錄。
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 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(); } }}編譯和執(zhí)行：
可以看到證書已排序順序相反。可以通過改變映射文件試試，只需設(shè)置sort="natural"和執(zhí)行程序，并比較結(jié)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 |+----+------------+-----------+--------+| 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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