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详解Java的Hibernate框架中的set映射集与SortedSet映射

Set
集合Set是一个java集合不包含任何重复的元素。更正式地说,Set不包含任何元素对e1和e2,使得e1.equals(e2),和至多一个空元素。所以被添加到一组对象必须实现equals()和hashCode()方法,使Java可以判断任何两个元素/对象是否是相同的。

集被映射到与映射表中<set>元素,并在java.util.HashSet中初始化。可以使用Set集合在类时,有一个集合中不需要重复的元素。

定义RDBMS表:
考虑一个情况下,我们需要我们的员工记录存储在EMPLOYEE表,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。因此,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_name VARCHAR(30) default NULL,
 employee_id INT default NULL,
 PRIMARY KEY (id)
);

将有一个对多EMPLOYEE和证书对象之间的关系:

定义POJO类:
让我们实现我们的POJO类员工将被用于保存与EMPLOYEE表中的对象和有凭证的设置变量的集合。

import java.util.*;

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

现在让我们定义另一个POJO类对应的表的证书,这样的证书对象可以存储和检索到的CERTIFICATE表。这个类还应该同时实现了equals()和hashCode()方法,使Java可以判断任意两个元素/对象是否相同。

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;
 }
 public boolean equals(Object obj) {
  if (obj == null) return false;
  if (!this.getClass().equals(obj.getClass())) return false;

  Certificate obj2 = (Certificate)obj;
  if((this.id == obj2.getId()) && (this.name.equals(obj2.getName())))
  {
   return true;
  }
  return false;
 }
 public int hashCode() {
  int tmp = 0;
  tmp = ( id + name ).hashCode();
  return tmp;
 }
}

定义Hibernate映射文件:
让我们开发指定Hibernate如何定义的类映射到数据库表的映射文件。<SET>元素将被用来定义用于设置集合的规则。

<?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">
   <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>

应该保存的映射文件中的格式<classname>.hbm.xml。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,映射文件中的所有元素:

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

在<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。

<meta>元素是可选元素,可以用来创建类的描述。

<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属性设置为原始的让Hibernate拾取,无论是identity,sequence或者hilo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。

<SET>元素是新的,已被引入到设置CERTIFICATE 和Employee类之间的关系。我们使用cascade属性中的<set>元素来告诉Hibernate来保存CERTIFICATE 对象,同时为Employee对象。name属性被设置为在父类中的定义设置变量,在我们的情况下,它是CERTIFICATE 。对于每一组变量,我们需要定义在映射文件中单独的一组元素。

<key>元素是包含外键的父对象,即在certificate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,证书对象必须有与父Employee有关。可以根据需要使用任何和<one-to-one>,<many-to-one>进行或<many-to-many>这个元素。

创建应用程序类:
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凭证,然后我们将申请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 */
  HashSet set1 = new HashSet();
  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 */
  HashSet set2 = new HashSet();
  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, Set 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());
   Set 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:
下面是步骤来编译并运行上述应用程序。请确保已在进行的编译和执行之前,适当地设置PATH和CLASSPATH。

  • 创建hibernate.cfg.xml配置文件中配置章节解释。
  • 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,并同时记录会在员工和证书表被创建。

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

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

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 |
+----+------------+-----------+--------+
| 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>

SortedSet
SortedSet是java集合,该集合不包含任何重复的元素和元素使用提供他们的自然顺序,或由一个比较器进行排序。

一个SortedSet映射,在映射表<set>元素和java.util.TreeSet中初始化。排序属性可以被设置为一个比较器或自然顺序。如果使用自然顺序,那么它的迭代器遍历的集合元素升序排列。

我们依然使用上面示例中所定义的RDBMS表,将有一个对多EMPLOYEE和证书对象之间的关系,POJO类也是如上定义。

让我们实现POJO类员工将被用于保存EMPLOYEE表中的对象和有证书的SortedSet变量的集合。

现在让我们定义另一个POJO类对应的表的证书,这样的证书对象可以存储和检索到的证书表。这个类还应该实现Comparable接口和compareTo方法将被用来设置sort=”natural” 在映射文件(见下面的映射文件)的情况下,元素进行排序。

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:
让我们开发指定Hibernate如何定义的类映射到数据库表的映射文件。该<set>元素将被用来定义所使用的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>

映射文件中的格式<classname>.hbm.xml,保存映射文件中的文件Employee.hbm.xml。前面已经熟悉了大部分的映射细节,但让我们看到了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。

<meta>元素是可选元素,可以用来创建类的描述。

<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属性设置为原生让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。

<set>元素用于设置证书和Employee类之间的关系。我们使用cascade属性中的<set>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SortedSet的变量,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。

<key>元素是包含外键的父对象,即在证书表中的列。表EMPLOYEE。

该<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,证书对象必须有与父Employee有关。可以根据需要使用任何和<one-to-one>,<many-to-one>进行或<many-to-many>这个元素。

如果使用sort=”natural”的设置,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证书名称)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,所以必须创建这个类的基础上实现排序算法。降序时使用这个类在这个类排序。

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证书,然后我们将申请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 */
  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(); 
  }
 }
}

编译和执行后,会在屏幕上获得以下结果,并同时记录会在员工和证书表被创建。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: 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 |
+----+------------+-----------+--------+
| 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)

本文地址:H5W3 » 详解Java的Hibernate框架中的set映射集与SortedSet映射

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