Hibernate - 排序集合映射

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一个SortedSet是Java集合，它不包含任何重复元素，并且元素使用其自然顺序或提供的比较器进行排序。

SortedSet 在映射表中使用 <set> 元素进行映射，并使用 java.util.TreeSet 初始化。sort 属性可以设置为比较器或自然排序。如果我们使用自然排序，则其迭代器将按升序元素顺序遍历集合。

定义RDBMS表

考虑一种情况，我们需要将员工记录存储在 EMPLOYEE 表中，该表将具有以下结构：

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，假设每个员工可以拥有一个或多个与其相关的证书。因此，我们将证书相关信息存储在具有以下结构的单独表中：

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 对象之间将存在一对多关系：

定义POJO类

让我们实现我们的POJO类Employee，它将用于持久化与EMPLOYEE表相关的对象，并在SortedSet变量中包含一系列证书。

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;
   }
}

现在让我们定义另一个与CERTIFICATE表对应的POJO类，以便可以将证书对象存储和检索到CERTIFICATE表中。此类还应实现Comparable接口和compareTo方法，这将在您在映射文件中设置sort="natural"时用于排序元素（请参见下面的映射文件）：

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);
      }
   }
}

定义Hibernate映射文件

让我们开发我们的映射文件，该文件指示Hibernate如何将定义的类映射到数据库表。<set> 元素将用于定义所使用的 SortedSet 集合的规则。

<?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>
      <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>

您应该将映射文档保存到格式为 <classname>.hbm.xml 的文件中。我们将映射文档保存在 Employee.hbm.xml 文件中。您已经熟悉大部分映射细节，但让我们再次查看映射文件的所有元素：

  • 映射文档是一个XML文档，其根元素为<hibernate-mapping>，其中包含两个与每个类对应的<class>元素。

  • <class> 元素用于定义从Java类到数据库表的特定映射。Java类名使用class元素的name属性指定，数据库表名使用table属性指定。

  • <meta> 元素是可选元素，可用于创建类描述。

  • <id> 元素将类中唯一的ID属性映射到数据库表的主键。id元素的name属性引用类中的属性，column属性引用数据库表中的列。type 属性包含Hibernate映射类型，此映射类型将从Java转换为SQL数据类型。

  • <generator> 元素位于id元素内，用于自动生成主键值。generator元素的class属性设置为native，让Hibernate根据底层数据库的功能选择identity、sequence或hilo算法来创建主键。

  • <property> 元素用于将Java类属性映射到数据库表中的列。元素的name属性引用类中的属性，column属性引用数据库表中的列。type 属性包含Hibernate映射类型，此映射类型将从Java转换为SQL数据类型。

  • <set> 元素用于设置Certificate和Employee类之间的关系。我们在<set>元素中使用了cascade属性来告诉Hibernate同时持久化Certificate对象和Employee对象。name属性设置为父类中定义的SortedSet变量，在本例中为certificates。sort 属性可以设置为natural以进行自然排序，也可以设置为实现java.util.Comparator的自定义类。我们使用了实现 java.util.Comparator 的类MyClass来反转在Certificate类中实现的排序顺序。

  • <key> 元素是CERTIFICATE表中保存对父对象（即EMPLOYEE表）的外键的列。

  • <one-to-many> 元素表示一个Employee对象与多个Certificate对象相关，因此Certificate对象必须与其关联的Employee父对象相关联。您可以根据需要使用<one-to-one>、<many-to-one>或<many-to-many>元素。

如果我们使用sort="natural"设置，则不需要创建单独的类，因为Certificate类已经实现了Comparable接口，Hibernate将使用Certificate类中定义的compareTo()方法来比较证书名称。但是我们在映射文件中使用自定义比较器类MyClass，因此我们必须根据我们的排序算法创建此类。让我们在这个类中使用这个类进行降序排序。

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;
      }
   }
}

创建应用程序类

最后，我们将创建包含main()方法的应用程序类来运行应用程序。我们将使用此应用程序保存一些员工记录及其证书，然后我们将对这些记录应用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(); 
      }
   }
}

编译和执行

以下是编译和运行上述应用程序的步骤。在进行编译和执行之前，请确保已正确设置PATH和CLASSPATH。

  • 创建如配置章节中所述的hibernate.cfg.xml配置文件。

  • 创建如上所示的Employee.hbm.xml映射文件。

  • 创建如上所示的Employee.java源文件并编译它。

  • 创建如上所示的Certificate.java源文件并编译它。

  • 创建如上所示的MyClass.java源文件并编译它。

  • 创建如上所示的ManageEmployee.java源文件并编译它。

  • 执行ManageEmployee二进制文件以运行程序。

您将在屏幕上看到以下结果，同时会在EMPLOYEE和CERTIFICATE表中创建记录。您可以看到证书已按反序排序。您可以尝试更改映射文件，只需设置sort="natural"并执行您的程序并比较结果。

$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

如果您检查EMPLOYEE和CERTIFICATE表，它们应该包含以下记录：

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>
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