詳解Java的Hibernate框架中的List映射表與Bag映射

2019-11-26 14:44:58

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List映射表
List列表是一個java集合存儲在序列中的元素，并允許重復的元素。此接口的用戶可以精確地控制，其中列表中的每個元素插入。用戶可以通過他們的整數索引訪問元素，并搜索列表中的元素。更正式地說，列表通常允許對元素e1和e2，使得e1.equals（e2），它們通常允許多個null元素，如果他們允許的null元素。

List列表被映射在該映射表中的<list>元素，并將java.util.ArrayList中初始化。

定義RDBMS表：
考慮一個情況，需要員工記錄存儲在EMPLOYEE表，將有以下結構：

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，假設每個員工都可以有一個或多個與他/她相關的證書。List集合映射需要在一個集合表的索引列。索引列定義集合中的元素的位置。因此，我們將存儲證書的相關信息在一個單獨的表，該表具有以下結構：

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));

將有一個對多( one-to-many)的EMPLOYEE和證書對象之間的關系。

定義POJO類：
讓我們實現一個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 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;  }}

我們需要相應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：
讓我們開發指定Hibernate如何定義的類映射到數據庫表的映射文件。<list>元素將被用來定義使用List集合的規則。表的索引是整數類型總是和使用<list-index>元素定義映射。

<?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>

應該保存的映射文件中的格式<classname>.hbm.xml。我們保存我們的映射文件中的文件Employee.hbm.xml。你已經熟悉了大部分的映射細節，映射文件中的所有元素：

映射文檔是具有<hibernate-mapping>為對應于每一個類包含2個<class>元素的根元素的XML文檔。

<class>元素被用于定義數據庫表從一個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據庫表名指定。

<id>元素映射在類中的唯一ID屬性到數據庫表的主鍵。 id元素的name屬性是指屬性的類和column屬性是指在數據庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉換為SQL數據類型。

id元素內的<generator>元素被用來自動生成的主鍵值。將生成元素的class屬性設置為原生讓Hibernate拿起無論是identity，sequence或者hilo中的算法來創建主鍵根據底層數據庫的支持能力。

<property>元素用于一個Java類的屬性映射到數據庫表中的列。元素的name屬性是指屬性的類和column屬性是指在數據庫表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會從Java轉換為SQL數據類型。

<list>元素用于設置證書和Employee類之間的關系。我們使用cascade屬性中<list>元素來告訴Hibernate來保存證書的對象，同時為Employee對象。 name屬性被設置為在父類中定義的Listvariable，在我們的情況下，它是證書。

<key>元素是包含外鍵的父對象，即在證書表中的列。即，表EMPLOYEE。

<list-index>元素定義用于保持元件的位置，并與在該集合表的索引列映射。持久性列表的索引從零開始。你可以改變這一點，例如，在你的映射<list-index base="1".../>。

<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，證書對象必須有與之勞動者家長有關。您可以根據您的需要使用任何和<one-to-one>，<many-to-one>進行或<many-to-many>這個元素。如果我們改變了這個例子使用一個many-to-many的關系，我們需要一個關聯表到父和子對象之間的映射。

創建應用程序類：
最后，我們將創建應用程序類的main（）方法來運行應用程序。我們將使用這個應用程序，以節省一些員工的記錄連同的證書，然后我們將申請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 */   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();    }  }}

編譯和執行：
下面是步驟來編譯并運行上述應用程序。請確保您已在進行的編譯和執行之前，適當地設置PATH和CLASSPATH。

創建hibernate.cfg.xml配置文件中配置章節解釋。
創建Employee.hbm.xml映射文件，如上圖所示。
創建Employee.java源文件，如上圖所示，并編譯它.
創建Certificate.java源文件，如上圖所示，并編譯它。
創建ManageEmployee.java源文件，如上圖所示，并編譯它。
執行ManageEmployee二進制文件來運行程序。

會在屏幕上獲得以下結果，并同時記錄會在員工和證書表被創建。可以看到證書已排序順序相反。可以通過改變映射文件試試，只需設置sort="natural"和執行程序，并比較結果。

$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

如果檢查員工和證書表，就應該記錄下了：

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數組，而不是一個列表。一個數組的映射幾乎是相同的前面的例子，除了與不同的元素和屬性名稱（<array>和<array-index>）。然而，原因如前所述，Hibernate應用程序很少使用數組。

Bag映射
Bag是一個java集合存儲元素無需關心順序，但允許列表中的重復元素。Bag是在列表中的對象的隨機分組。

Collection集合被映射在該映射表中的<bag>元件和與java.util.ArrayList中初始化。
下面的示例中RDBMS表和POJO類依然使用上面定義好的。
定義Hibernate映射文件：
讓我們開發指示Hibernate如何定義的類映射到數據庫表的映射文件。<bag>元素將被用來定義所使用的集合規則。

<?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>   <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>

應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。前面已經熟悉了大部分的映射細節，但讓我們看到了映射文件中的所有元素再次：

映射文檔是具有<hibernate-mapping>為對應于每一個類包含2個<class>元素的根元素的XML文檔。

<class>元素被用于定義數據庫表從一個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據庫表名指定。

<bag>元素用于設置證書和Employee類之間的關系。我們使用cascade屬性的<bag>高的元素要告訴Hibernate來保存證書的對象，同時為Employee對象。 name屬性被設置為在父類中的definedCollection變量，在我們的情況下，它是證書。

<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父類有關聯。可以根據需要使用<one-to-one>，<many-to-one>或<many-to-many>這個元素。

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();    }  }}

編譯和執行：

$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

如果檢查員工和證書表，就應該記錄下了：

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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