詳解Java的Hibernat框架中的Map映射與SortedMap映射

更新時間：2015年12月19日 09:29:45 投稿：goldensun

這篇文章主要介紹了Java的Hibernat框架中的Map映射與SortedMap映射,Hibernat是Java的SSH三大web開發(fā)框架之一,需要的朋友可以參考下

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(xì)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。

<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(jìn)行的編譯和執(zhí)行之前，已經(jīng)適當(dāng)?shù)卦O(shè)置PATH和CLASSPATH。

創(chuàng)建hibernate.cfg.xml配置文件,在配置章節(jié)已經(jīng)解釋和學(xué)習(xí)。
創(chuàng)建Employee.hbm.xml映射文件，如上圖所示。
創(chuàng)建Employee.java源文件，如上圖所示，并編譯它。
創(chuàng)建Certificate.java源文件，如上圖所示，并編譯它。
創(chuàng)建ManageEmployee.java源文件，如上圖所示，并編譯它。
執(zhí)行ManageEmployee二進(jìn)制文件來運行程序。

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: 4000
Certificate: MCA
Certificate: MBA
Certificate: PMP
First Name: Manoj Last Name: Kumar Salary: 5000
Certificate: MCA
Certificate: MBA
Certificate: 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(jìn)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(xì)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。

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。

<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?？梢酝ㄟ^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: 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(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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