详解Java的Hibernate框架中的List映射表与Bag映射

2019-11-26 14:44:58

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List映射表
List列表是一个java集合存储在序列中的元素，并允许重复的元素。此接口的用户可以精确地控制，其中列表中的每个元素插入。用户可以通过他们的整数索引访问元素，并搜索列表中的元素。更正式地说，列表通常允许对元素e1和e2，使得e1.equals（e2），它们通常允许多个null元素，如果他们允许的null元素。

List列表被映射在该映射表中的<list>元素，并将java.util.ArrayList中初始化。

定义RDBMS表：
考虑一个情况，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。List集合映射需要在一个集合表的索引列。索引列定义集合中的元素的位置。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,  idx INT default NULL,   employee_id INT default NULL,  PRIMARY KEY (id));

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

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 List 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 List getCertificates() {   return certificates;  }  public void setCertificates( List 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：
让我们开发指定Hibernate如何定义的类映射到数据库表的映射文件。<list>元素将被用来定义使用List集合的规则。表的索引是整数类型总是和使用<list-index>元素定义映射。

<?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>   <list name="certificates" cascade="all">     <key column="employee_id"/>     <list-index column="idx"/>     <one-to-many class="Certificate"/>   </list>   <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。我们保存我们的映射文件中的文件Employee.hbm.xml。你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。

<list>元素用于设置证书和Employee类之间的关系。我们使用cascade属性中<list>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定义的Listvariable，在我们的情况下，它是证书。

<key>元素是包含外键的父对象，即在证书表中的列。即，表EMPLOYEE。

<list-index>元素定义用于保持元件的位置，并与在该集合表的索引列映射。持久性列表的索引从零开始。你可以改变这一点，例如，在你的映射<list-index base="1".../>。

<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，证书对象必须有与之劳动者家长有关。您可以根据您的需要使用任何和<one-to-one>，<many-to-one>进行或<many-to-many>这个元素。如果我们改变了这个例子使用一个many-to-many的关系，我们需要一个关联表到父和子对象之间的映射。

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连同的证书，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 */   ArrayList set1 = new ArrayList();   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 */   ArrayList set2 = new ArrayList();   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, ArrayList 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());      List 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。请确保您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。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: 4000Certificate: MCACertificate: MBACertificate: PMPFirst Name: Dilip Last Name: Kumar Salary: 3000Certificate: BCACertificate: BAFirst Name: Manoj Last Name: Kumar Salary: 5000Certificate: MCACertificate: MBACertificate: 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 |+----+------------+-----------+--------+| 51 | Manoj   | Kumar   |  5000 |+----+------------+-----------+--------+1 row in set (0.00 sec)

mysql> select * from CERTIFICATE;

+----+------------------+------+-------------+| id | certificate_name | idx | employee_id |+----+------------------+------+-------------+| 6 | MCA       |  0 |     51 || 7 | MBA       |  1 |     51 || 8 | PMP       |  2 |     51 |+----+------------------+------+-------------+3 rows in set (0.00 sec)

或者，您也可以映射Java数组，而不是一个列表。一个数组的映射几乎是相同的前面的例子，除了与不同的元素和属性名称（<array>和<array-index>）。然而，原因如前所述，Hibernate应用程序很少使用数组。

Bag映射
Bag是一个java集合存储元素无需关心顺序，但允许列表中的重复元素。Bag是在列表中的对象的随机分组。

Collection集合被映射在该映射表中的<bag>元件和与java.util.ArrayList中初始化。
下面的示例中RDBMS表和POJO类依然使用上面定义好的。
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。<bag>元素将被用来定义所使用的集合规则。

<?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>   <bag name="certificates" cascade="all">     <key column="employee_id"/>     <one-to-many class="Certificate"/>   </bag>   <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，但让我们看到了映射文件中的所有元素再次：

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。

<bag>元素用于设置证书和Employee类之间的关系。我们使用cascade属性的<bag>高的元素要告诉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definedCollection变量，在我们的情况下，它是证书。

<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父类有关联。可以根据需要使用<one-to-one>，<many-to-one>或<many-to-many>这个元素。

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 */   ArrayList set1 = new ArrayList();   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 */   ArrayList set2 = new ArrayList();   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, ArrayList 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());      Collection 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：

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........First Name: Manoj Last Name: Kumar Salary: 4000Certificate: MCACertificate: MBACertificate: PMPFirst Name: Dilip Last Name: Kumar Salary: 3000Certificate: BCACertificate: BAFirst Name: Manoj Last Name: Kumar Salary: 5000Certificate: MCACertificate: MBACertificate: 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 |+----+------------+-----------+--------+| 53 | Manoj   | Kumar   |  5000 |+----+------------+-----------+--------+1 row in set (0.00 sec)

mysql> select * from CERTIFICATE;

+----+------------------+-------------+| id | certificate_name | employee_id |+----+------------------+-------------+| 11 | MCA       |     53 || 12 | MBA       |     53 || 13 | PMP       |     53 |+----+------------------+-------------+3 rows in set (0.00 sec)

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