Hibernate - 排序集合對映

一個SortedSet是Java集合，它不包含任何重複元素，並且元素使用其自然順序或提供的比較器進行排序。

SortedSet 在對映表中使用 <set> 元素進行對映，並使用 java.util.TreeSet 初始化。sort 屬性可以設定為比較器或自然排序。如果我們使用自然排序，則其迭代器將按升序元素順序遍歷集合。

定義RDBMS表

考慮一種情況，我們需要將員工記錄儲存在 EMPLOYEE 表中，該表將具有以下結構：

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，假設每個員工可以擁有一個或多個與其相關的證書。因此，我們將證書相關資訊儲存在具有以下結構的單獨表中：

create table CERTIFICATE (
   id INT NOT NULL auto_increment,
   certificate_name VARCHAR(30) default NULL,
   employee_id INT default NULL,
   PRIMARY KEY (id)
);

EMPLOYEE 和 CERTIFICATE 物件之間將存在一對多關係：

定義POJO類

讓我們實現我們的POJO類Employee，它將用於持久化與EMPLOYEE表相關的物件，並在SortedSet變數中包含一系列證書。

import java.util.*;

public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private SortedSet 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 SortedSet getCertificates() {
      return certificates;
   }
   
   public void setCertificates( SortedSet certificates ) {
      this.certificates = certificates;
   }
}

現在讓我們定義另一個與CERTIFICATE表對應的POJO類，以便可以將證書物件儲存和檢索到CERTIFICATE表中。此類還應實現Comparable介面和compareTo方法，這將在您在對映檔案中設定sort="natural"時用於排序元素（請參見下面的對映檔案）：

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對映檔案

讓我們開發我們的對映檔案，該檔案指示Hibernate如何將定義的類對映到資料庫表。<set> 元素將用於定義所使用的 SortedSet 集合的規則。

<?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>

您應該將對映文件儲存到格式為 <classname>.hbm.xml 的檔案中。我們將對映文件儲存在 Employee.hbm.xml 檔案中。您已經熟悉大部分對映細節，但讓我們再次檢視對映檔案的所有元素：

對映文件是一個XML文件，其根元素為<hibernate-mapping>，其中包含兩個與每個類對應的<class>元素。
<class> 元素用於定義從Java類到資料庫表的特定對映。Java類名使用class元素的name屬性指定，資料庫表名使用table屬性指定。
<meta> 元素是可選元素，可用於建立類描述。
<id> 元素將類中唯一的ID屬性對映到資料庫表的主鍵。id元素的name屬性引用類中的屬性，column屬性引用資料庫表中的列。type 屬性包含Hibernate對映型別，此對映型別將從Java轉換為SQL資料型別。
<generator> 元素位於id元素內，用於自動生成主鍵值。generator元素的class屬性設定為native，讓Hibernate根據底層資料庫的功能選擇identity、sequence或hilo演算法來建立主鍵。
<property> 元素用於將Java類屬性對映到資料庫表中的列。元素的name屬性引用類中的屬性，column屬性引用資料庫表中的列。type 屬性包含Hibernate對映型別，此對映型別將從Java轉換為SQL資料型別。
<set> 元素用於設定Certificate和Employee類之間的關係。我們在<set>元素中使用了cascade屬性來告訴Hibernate同時持久化Certificate物件和Employee物件。name屬性設定為父類中定義的SortedSet變數，在本例中為certificates。sort 屬性可以設定為natural以進行自然排序，也可以設定為實現java.util.Comparator的自定義類。我們使用了實現 java.util.Comparator 的類MyClass來反轉在Certificate類中實現的排序順序。
<key> 元素是CERTIFICATE表中儲存對父物件（即EMPLOYEE表）的外部索引鍵的列。
<one-to-many> 元素表示一個Employee物件與多個Certificate物件相關，因此Certificate物件必須與其關聯的Employee父物件相關聯。您可以根據需要使用<one-to-one>、<many-to-one>或<many-to-many>元素。

如果我們使用sort="natural"設定，則不需要建立單獨的類，因為Certificate類已經實現了Comparable介面，Hibernate將使用Certificate類中定義的compareTo()方法來比較證書名稱。但是我們在對映檔案中使用自定義比較器類MyClass，因此我們必須根據我們的排序演算法建立此類。讓我們在這個類中使用這個類進行降序排序。

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;
      }
   }
}

建立應用程式類

最後，我們將建立包含main()方法的應用程式類來執行應用程式。我們將使用此應用程式儲存一些員工記錄及其證書，然後我們將對這些記錄應用CRUD操作。

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(); 
      }
   }
}

編譯和執行

以下是編譯和執行上述應用程式的步驟。在進行編譯和執行之前，請確保已正確設定PATH和CLASSPATH。

建立如配置章節中所述的hibernate.cfg.xml配置檔案。
建立如上所示的Employee.hbm.xml對映檔案。
建立如上所示的Employee.java原始檔並編譯它。
建立如上所示的Certificate.java原始檔並編譯它。
建立如上所示的MyClass.java原始檔並編譯它。
建立如上所示的ManageEmployee.java原始檔並編譯它。
執行ManageEmployee二進位制檔案以執行程式。

您將在螢幕上看到以下結果，同時會在EMPLOYEE和CERTIFICATE表中建立記錄。您可以看到證書已按反序排序。您可以嘗試更改對映檔案，只需設定sort="natural"並執行您的程式並比較結果。

$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

First Name: Manoj  Last Name: Kumar  Salary: 4000
Certificate: PMP
Certificate: MCA
Certificate: MBA
First Name: Dilip  Last Name: Kumar  Salary: 3000
Certificate: BCA
Certificate: BA
First Name: Manoj  Last Name: Kumar  Salary: 5000
Certificate: PMP
Certificate: MCA
Certificate: MBA

如果您檢查EMPLOYEE和CERTIFICATE表，它們應該包含以下記錄：

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>

hibernate_or_mappings.htm

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