Hibernat的Map映射与SortedMap映射详解Java的Hibernat框架中的Map映射与SortedMap映射

时间:2021-03-20 人气:0

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Map映射
Map映射是一个java集合存储在键 - 值对的元素，并且不允许在列表中重复的元素。 Map接口提供三种collection视图，允许Map内容看作是一组键-值集合，或者设置键 - 值映射关系。

Map被映射到映射表中一个<map>元素和无序的地图可以在java.util.HashMap中被初始化。

定义RDBMS表：
考虑一个情况，我们需要员工记录存储在EMPLOYEE表，将有以下结构：

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，假设每个员工都可以有一个或多个与他/她相关的证书。我们将存储证书的相关信息在一个单独的表，该表具有以下结构：

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)
);

有一个对多（one-to-many ）的关系在EMPLOYEE和证书对象之间。

定义POJO类：
让我们实现一个POJO类员工将被用于保存与EMPLOYEE表中的对象和有证书的列表变量的集合。

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;
  }
}

我们需要相应的证书表定义另一个POJO类，这样的证书对象可以存储和检索到的证书表。

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;
  }
}

定义Hibernate映射文件：
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。

<?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>
   <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>

应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件中的文件Employee.hbm.xml。已经熟悉了大部分的映射细节，但让我们再次看映射文件中的所有元素：

映射文档是具有<hibernate-mapping>为对应于每一个类包含2个<class>元素的根元素的XML文档。

<class>元素被用于定义数据库表从一个Java类特定的映射。 Java类名指定使用class元素的name属性和使用表属性数据库表名指定。

<id>元素映射在类中的唯一ID属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型，这种类型的映射将会从Java转换为SQL数据类型。

id元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置为原产于让Hibernate拿起无论是identity，sequence或者hilo中的算法来创建主键根据底层数据库的支持能力。

<property>元素用于一个Java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了Hibernate映射类型，这种类型的映射将会从Java转换为SQL数据类型。

<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>元素来告诉Hibernate来保存证书的对象，同时为Employee对象。 name属性设置为定义Mapvariable在父类中，在我们的案例中是证书。

<index>的元素用于表示键/值对地图的关键部位。该键将使用一个字符串类型的存储在列certificate_type。

<key>元素是包含外键的父对象，即在证书表中的列。表EMPLOYEE。

<one-to-many>元素表示一个Employee对象涉及到很多证书的对象，并因此，证书对象必须有与Employee父类有关。可以根据需要使用<one-to-one>，<many-to-one>或<many-to-many>这个元素。

创建应用程序类：
最后，创建应用程序类的main（）方法来运行应用程序。使用这个应用程序来保存员工记录地连同证书列表，然后在应用上执行CRUD操作上的记录。

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(); 
   }
  }
}

编译和执行：
下面是步骤来编译并运行上述应用程序。请确保您已在进行的编译和执行之前，已经适当地设置PATH和CLASSPATH。

创建hibernate.cfg.xml配置文件,在配置章节已经解释和学习。
创建Employee.hbm.xml映射文件，如上图所示。
创建Employee.java源文件，如上图所示，并编译它。
创建Certificate.java源文件，如上图所示，并编译它。
创建ManageEmployee.java源文件，如上图所示，并编译它。
执行ManageEmployee二进制文件来运行程序。

会在屏幕上获得以下结果，并同时创建记录在员工和证书表。

$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

如果检查员工和证书表，就应该记录下了：

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是存储在键 - 值对的元素，并提供键的总体排序类似java的集合作为映射。重复的元素未在映射不允许的。该映射是根据其键的自然顺序进行排序，或者通过提供通常在有序映射的创建时间比较。

SortedMap被映射在该映射表中的<map>元素和有序映射可以在java.util.TreeMap中被初始化。
我们依然使用上面定义的RDBMS表和POJO类来讲下面的例子：
定义Hibernate映射文件：
让我们开发指示Hibernate如何定义的类映射到数据库表的映射文件。<map>元素将被用于定义所使用的映射的规则。

<?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>
   <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>

应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件Employee.hbm.xml。前面已经熟悉了大部分的映射细节，但让我们再次看看映射文件中的所有元素：

映射文档是具有<hibernate-mapping>对应于每一个类包含2个<class>元素的根元素的XML文档。

<class>元素被用于定义数据库表从一个Java类特定的映射。 Java类名指定使用class元素的name属性和使用表属性数据库表名指定。

id元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置让Hibernate对应于identity，sequence或者hilo中的算法来创建主键根据底层数据库的支持能力。

<map>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<map>元素来告诉Hibernate来保存证书的对象，同时为Employee对象。 name属性被设置为在父类中的定义的SortedMap变量，在我们的情况下，它是证书。排序属性可以设置为自然有自然排序，也可以设置为自定义类实现为java.util.Comparator。我们已经使用了一个类MyClass，它实现为java.util.Comparator扭转证书类实现的排序顺序。

<index>元素用于表示键/值对映射的键部分。该键将使用一个字符串类型的存储在列certificate_type。

<key>元素是包含外键的父对象，即在证书表中的列。表EMPLOYEE。

<one-to-many>元素表示一个Employee对象涉及到很多证书的对象，并因此，证书对象必须有与Employee父有关联。可以根据需要使用<one-to-one>，<many-to-one>或<many-to-many>这个元素。

如果使用sort="natural"，然后我们并不需要创建一个单独的类，因为证书类已经实现了Comparable接口和hibernate会使用compareTo在证书类定义为比较的SortedMap（）方法。但是，我们使用的是在我们的映射文件自定义的比较器类MyClass，所以我们必须创建这个类的基础上我们的排序算法。让我们做降在映射上可用的按键排序。

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，我们将创建应用程序类的main（）方法来运行应用程序。我们将使用这个应用程序，以节省一些员工的记录地连同的证书，然后我们将提交CRUD操作上面的记录。

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(); 
   }
  }
}

编译和执行：
可以看到证书已排序顺序相反。可以通过改变映射文件试试，只需设置sort="natural"和执行程序，并比较结果。

$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

如果检查员工和证书表，就应该记录下了：

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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Hibernat的Map映射与SortedMap映射 详解Java的Hibernat框架中的Map映射与SortedMap映射

Hibernat的Map映射与SortedMap映射详解Java的Hibernat框架中的Map映射与SortedMap映射