Hibernate SortedMap映射

SortedMap是存儲在鍵 - 值對的元素，並提供鍵的總體排序類似java的集合作為映射。重複的元素未在映射不允許的。該映射是根據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。

定義RDBMS表：

考慮一個情況，需要將員工記錄存儲在EMPLOYEE表，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，假設每個員工都可以有一個或多個與他/她相關的證書。我們將存儲證書的相關信息在一個單獨的表，該表具有以下結構：

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）的關係在EMPLOYEE和證書對象之間。

定義POJO類：

讓我們實現一個POJO類員工將被用於保存與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 SortedMap 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 SortedMap getCertificates() {
      return certificates;
   }
   public void setCertificates( SortedMap certificates ) {
      this.certificates = certificates;
   }
}

我們需要相應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。這個類還應該實現Comparable接口和compareTo方法將用於設置sort="natural"在映射文件（見下麵的映射文件）中的SortedMap的關鍵元素進行排序的情況。

public class Certificate implements Comparable <String>{
   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(String that){
      final int BEFORE = -1;
      final int AFTER = 1;

      if (that == null) {
         return BEFORE;
      }

      Comparable thisCertificate = this;
      Comparable thatCertificate = that;

      if(thisCertificate == null) {
         return AFTER;
      } else if(thatCertificate == null) {
         return BEFORE;
      } else {
         return thisCertificate.compareTo(thatCertificate);
      }
   }
}

定義Hibernate映射文件：

讓我們開發指示Hibernate如何定義的類映射到數據庫表的映射文件。<map>元素將被用於定義所使用的映射的規則。

<?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>
      <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。前麵已經熟悉了大部分的映射細節，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>對應於每一個類包含2個<class>元素的根元素的XML文檔。
<class>元素被用於定義數據庫表從一個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據庫表名指定。
<meta>元素是可選元素，可以用來創建類的描述。
<id>元素映射在類中的唯一ID屬性到數據庫表的主鍵。 id元素的name屬性是指屬性的類和column屬性是指在數據庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉換為SQL數據類型。
id元素內的<generator>元素被用來自動生成的主鍵值。將生成元素的class屬性設置讓Hibernate對應於identity，sequence或者hilo中的算法來創建主鍵根據底層數據庫的支持能力。
<property>元素用於一個Java類的屬性映射到數據庫表中的列。元素的name屬性是指屬性的類和column屬性是指在數據庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉換為SQL數據類型。
<map>元素用於設置證書和Employee類之間的關係。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。排序屬性可以設置為自然有自然排序，也可以設置為自定義類實現為java.util.Comparator。我們已經使用了一個類MyClass，它實現為java.util.Comparator扭轉證書類實現的排序順序。
<index>元素用於表示鍵/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，並因此，證書對象必須有與Employee父有關聯。可以根據需要使用<one-to-one>，<many-to-one>或<many-to-many>這個元素。

如果使用sort="natural"，然後我們並不需要創建一個單獨的類，因為證書類已經實現了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，所以我們必須創建這個類的基礎上我們的排序算法。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;
      }
   }
}

最後，我們將創建應用程序類的main（）方法來運行應用程序。我們將使用這個應用程序，以節省一些員工的記錄地連同的證書，然後我們將提交CRUD操作上麵的記錄。

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(); 
      }
   }
}

編譯和執行：

下麵是步驟來編譯並運行上述應用程序。請確保已在進行的編譯和執行之前，適當地設置PATH和CLASSPATH。

創建hibernate.cfg.xml配置文件中配置章節解釋。
創建Employee.hbm.xml映射文件，如上圖所示。
創建Employee.java源文件，如上圖所示，並編譯它。
創建Certificate.java源文件，如上圖所示，並編譯它。
創建ManageEmployee.java源文件，如上圖所示，並編譯它。
執行ManageEmployee二進製文件來運行程序。

會在屏幕上獲得以下結果，並同時記錄會在員工和證書表被創建。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，隻需設置sort="natural"和執行程序，並比較結果。

$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: MBA
First Name: Dilip  Last Name: Kumar  Salary: 3000
	Certificate Type: ComputerScience,  Name: MCA
	Certificate Type: BusinessManagement,  Name: MBA
First 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)

mysql>