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

程序员文章站 2024-03-08 16:29:23
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...

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)