Set
集合Set是一個(gè)java集合不包含任何重復(fù)的元素。更正式地說，Set不包含任何元素對e1和e2，使得e1.equals（e2），和至多一個(gè)空元素。所以被添加到一組對象必須實(shí)現(xiàn)equals()和hashCode()方法，使Java可以判斷任何兩個(gè)元素/對象是否是相同的。
集被映射到與映射表中<set>元素，并在java.util.HashSet中初始化。可以使用Set集合在類時(shí)，有一個(gè)集合中不需要重復(fù)的元素。
定義RDBMS表：
考慮一個(gè)情況下，我們需要我們的員工記錄存儲在EMPLOYEE表，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(gè)員工都可以有一個(gè)或多個(gè)與他/她相關(guān)的證書。因此，我們將存儲證書的相關(guān)信息在一個(gè)單獨(dú)的表，該表具有以下結(jié)構(gòu)：
create table CERTIFICATE ( id INT NOT NULL auto_increment, certificate_name VARCHAR(30) default NULL, employee_id INT default NULL, PRIMARY KEY (id));
將有一個(gè)對多EMPLOYEE和證書對象之間的關(guān)系：
定義POJO類：
讓我們實(shí)現(xiàn)我們的POJO類員工將被用于保存與EMPLOYEE表中的對象和有憑證的設(shè)置變量的集合。
import java.util.*;public class Employee { private int id; private String firstName; private String lastName; private int salary; private Set 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 Set getCertificates() { return certificates; } public void setCertificates( Set certificates ) { this.certificates = certificates; }}現(xiàn)在讓我們定義另一個(gè)POJO類對應(yīng)的表的證書，這樣的證書對象可以存儲和檢索到的CERTIFICATE表。這個(gè)類還應(yīng)該同時(shí)實(shí)現(xiàn)了equals（）和hashCode（）方法，使Java可以判斷任意兩個(gè)元素/對象是否相同。
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; } public boolean equals(Object obj) { if (obj == null) return false; if (!this.getClass().equals(obj.getClass())) return false; Certificate obj2 = (Certificate)obj; if((this.id == obj2.getId()) && (this.name.equals(obj2.getName()))) { return true; } return false; } public int hashCode() { int tmp = 0; tmp = ( id + name ).hashCode(); return tmp; }}定義Hibernate映射文件：
讓我們開發(fā)指定Hibernate如何定義的類映射到數(shù)據(jù)庫表的映射文件。<SET>元素將被用來定義用于設(shè)置集合的規(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> <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。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)，映射文件中的所有元素：
映射文檔是具有<hibernate-mapping>為對應(yīng)于每個(gè)類包含2個(gè)<class>元素的根元素的XML文檔。
在<class>元素被用于定義數(shù)據(jù)庫表從一個(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)置為原始的讓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。
<SET>元素是新的，已被引入到設(shè)置CERTIFICATE 和Employee類之間的關(guān)系。我們使用cascade屬性中的<set>元素來告訴Hibernate來保存CERTIFICATE 對象，同時(shí)為Employee對象。name屬性被設(shè)置為在父類中的定義設(shè)置變量，在我們的情況下，它是CERTIFICATE 。對于每一組變量，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象，即在certificate表中的列。表EMPLOYEE。
<one-to-many>元素表示一個(gè)Employee對象涉及到很多證書的對象，并因此，證書對象必須有與父Employee有關(guān)。可以根據(jù)需要使用任何和<one-to-one>，<many-to-one>進(jìn)行或<many-to-many>這個(gè)元素。
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)省一些員工的記錄地連同憑證，然后我們將申請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 */ 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：
下面是步驟來編譯并運(yùn)行上述應(yīng)用程序。請確保已在進(jìn)行的編譯和執(zhí)行之前，適當(dāng)?shù)卦O(shè)置PATH和CLASSPATH。
會在屏幕上獲得以下結(jié)果，并同時(shí)記錄會在員工和證書表被創(chuàng)建。
$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........First Name: Manoj Last Name: Kumar Salary: 4000Certificate: MBACertificate: PMPCertificate: MCAFirst Name: Dilip Last Name: Kumar Salary: 3000Certificate: BCACertificate: BAFirst Name: Manoj Last Name: Kumar Salary: 5000Certificate: MBACertificate: PMPCertificate: 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)
mysql>
SortedSet
SortedSet是java集合，該集合不包含任何重復(fù)的元素和元素使用提供他們的自然順序，或由一個(gè)比較器進(jìn)行排序。
一個(gè)SortedSet映射，在映射表<set>元素和java.util.TreeSet中初始化。排序?qū)傩钥梢员辉O(shè)置為一個(gè)比較器或自然順序。如果使用自然順序，那么它的迭代器遍歷的集合元素升序排列。
我們依然使用上面示例中所定義的RDBMS表，將有一個(gè)對多EMPLOYEE和證書對象之間的關(guān)系，POJO類也是如上定義。
讓我們實(shí)現(xiàn)POJO類員工將被用于保存EMPLOYEE表中的對象和有證書的SortedSet變量的集合。
現(xiàn)在讓我們定義另一個(gè)POJO類對應(yīng)的表的證書，這樣的證書對象可以存儲和檢索到的證書表。這個(gè)類還應(yīng)該實(shí)現(xiàn)Comparable接口和compareTo方法將被用來設(shè)置sort="natural" 在映射文件（見下面的映射文件）的情況下，元素進(jìn)行排序。
public class Certificate implements Comparable <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; } public int compareTo(Certificate that){ final int BEFORE = -1; final int AFTER = 1; if (that == null) { return BEFORE; } Comparable thisCertificate = this.getName(); Comparable thatCertificate = that.getName(); if(thisCertificate == null) { return AFTER; } else if(thatCertificate == null) { return BEFORE; } else { return thisCertificate.compareTo(thatCertificate); } }}定義Hibernate映射文件：
讓我們開發(fā)指定Hibernate如何定義的類映射到數(shù)據(jù)庫表的映射文件。該<set>元素將被用來定義所使用的SortedSet集合的規(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> <set name="certificates" cascade="all" sort="MyClass"> <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>
映射文件中的格式<classname>.hbm.xml，保存映射文件中的文件Employee.hbm.xml。前面已經(jīng)熟悉了大部分的映射細(xì)節(jié)，但讓我們看到了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(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)置為原生讓Hibernate拿起無論是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。
<set>元素用于設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性中的<set>元素來告訴Hibernate來保存證書的對象，同時(shí)為Employee對象。 name屬性被設(shè)置為在父類中的定義的SortedSet的變量，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(shí)現(xiàn)為java.util.Comparator。我們已經(jīng)使用了一個(gè)類MyClass，它實(shí)現(xiàn)為java.util.Comparator扭轉(zhuǎn)證書類實(shí)現(xiàn)的排序順序。
<key>元素是包含外鍵的父對象，即在證書表中的列。表EMPLOYEE。
該<one-to-many>元素表示一個(gè)Employee對象涉及到很多證書的對象，并因此，證書對象必須有與父Employee有關(guān)。可以根據(jù)需要使用任何和<one-to-one>，<many-to-one>進(jìn)行或<many-to-many>這個(gè)元素。
如果使用sort="natural"的設(shè)置，并不需要創(chuàng)建一個(gè)單獨(dú)的類，因?yàn)樽C書類已經(jīng)實(shí)現(xiàn)了Comparable接口和hibernate會使用compareTo（在證書類定義為比較證書名稱）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，所以必須創(chuàng)建這個(gè)類的基礎(chǔ)上實(shí)現(xiàn)排序算法。降序時(shí)使用這個(gè)類在這個(gè)類排序。
import java.util.Comparator;public class MyClass implements Comparator<Certificate>{ public int compare(Certificate o1, Certificate 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.getName(); Comparable thatCertificate = o2.getName(); if(thisCertificate == null) { return AFTER * 1; } else if(thatCertificate == null) { return BEFORE * -1; } else { return thisCertificate.compareTo(thatCertificate) * -1; } }}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)省一些員工的記錄地連同證書，然后我們將申請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 */ TreeSet set1 = new TreeSet(); 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 */ TreeSet set2 = new TreeSet(); 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, SortedSet 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()); SortedSet 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)行后，會在屏幕上獲得以下結(jié)果，并同時(shí)記錄會在員工和證書表被創(chuàng)建。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: 4000Certificate: PMPCertificate: MCACertificate: MBAFirst Name: Dilip Last Name: Kumar Salary: 3000Certificate: BCACertificate: BAFirst Name: Manoj Last Name: Kumar Salary: 5000Certificate: PMPCertificate: MCACertificate: 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 |+----+------------+-----------+--------+| 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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