詳解Java的Hibernate框架中的List映射表與Bag映射

更新時(shí)間：2015年12月18日 08:54:06 投稿：goldensun

這篇文章主要介紹了Java的Hibernate框架中的List映射表與Bag映射,Hibernate是Java的SSH三大web開發(fā)框架之一,需要的朋友可以參考下

List映射表
List列表是一個(gè)java集合存儲(chǔ)在序列中的元素，并允許重復(fù)的元素。此接口的用戶可以精確地控制，其中列表中的每個(gè)元素插入。用戶可以通過(guò)他們的整數(shù)索引訪問(wèn)元素，并搜索列表中的元素。更正式地說(shuō)，列表通常允許對(duì)元素e1和e2，使得e1.equals（e2），它們通常允許多個(gè)null元素，如果他們?cè)试S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(gè)情況，需要員工記錄存儲(chǔ)在EMPLOYEE表，將有以下結(jié)構(gòu)：

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(shè)每個(gè)員工都可以有一個(gè)或多個(gè)與他/她相關(guān)的證書。List集合映射需要在一個(gè)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(chǔ)證書的相關(guān)信息在一個(gè)單獨(dú)的表，該表具有以下結(jié)構(gòu)：

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(gè)對(duì)多( one-to-many)的EMPLOYEE和證書對(duì)象之間的關(guān)系。

定義POJO類：
讓我們實(shí)現(xiàn)一個(gè)POJO類員工將被用于保存與EMPLOYEE表中的對(duì)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(yīng)的證書表定義另一個(gè)POJO類，這樣的證書對(duì)象可以存儲(chǔ)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(fā)指定Hibernate如何定義的類映射到數(shù)據(jù)庫(kù)表的映射文件。<list>元素將被用來(lái)定義使用List集合的規(guī)則。表的索引是整數(shù)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(yīng)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(jīng)熟悉了大部分的映射細(xì)節(jié)，映射文件中的所有元素：

映射文檔是具有<hibernate-mapping>為對(duì)應(yīng)于每一個(gè)類包含2個(gè)<class>元素的根元素的XML文檔。

<class>元素被用于定義數(shù)據(jù)庫(kù)表從一個(gè)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(shù)據(jù)庫(kù)表名指定。

<id>元素映射在類中的唯一ID屬性到數(shù)據(jù)庫(kù)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(shù)據(jù)庫(kù)表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會(huì)從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。

id元素內(nèi)的<generator>元素被用來(lái)自動(dòng)生成的主鍵值。將生成元素的class屬性設(shè)置為原生讓Hibernate拿起無(wú)論是identity，sequence或者h(yuǎn)ilo中的算法來(lái)創(chuàng)建主鍵根據(jù)底層數(shù)據(jù)庫(kù)的支持能力。

<property>元素用于一個(gè)Java類的屬性映射到數(shù)據(jù)庫(kù)表中的列。元素的name屬性是指屬性的類和column屬性是指在數(shù)據(jù)庫(kù)表中的列。 type屬性保存了Hibernate映射類型，這種類型的映射將會(huì)從Java轉(zhuǎn)換為SQL數(shù)據(jù)類型。

<list>元素用于設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性中<list>元素來(lái)告訴Hibernate來(lái)保存證書的對(duì)象，同時(shí)為Employee對(duì)象。 name屬性被設(shè)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(duì)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(diǎn)，例如，在你的映射<list-index base="1".../>。

<one-to-many>元素表示一個(gè)Employee對(duì)象涉及到很多證書的對(duì)象，并因此，證書對(duì)象必須有與之勞動(dòng)者家長(zhǎng)有關(guān)。您可以根據(jù)您的需要使用任何和<one-to-one>，<many-to-one>進(jìn)行或<many-to-many>這個(gè)元素。如果我們改變了這個(gè)例子使用一個(gè)many-to-many的關(guān)系，我們需要一個(gè)關(guān)聯(lián)表到父和子對(duì)象之間的映射。

創(chuàng)建應(yīng)用程序類：
最后，我們將創(chuàng)建應(yīng)用程序類的main（）方法來(lái)運(yùn)行應(yīng)用程序。我們將使用這個(gè)應(yīng)用程序，以節(jié)省一些員工的記錄連同的證書，然后我們將申請(qǐng)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(zhí)行：
下面是步驟來(lái)編譯并運(yùn)行上述應(yīng)用程序。請(qǐng)確保您已在進(jìn)行的編譯和執(zhí)行之前，適當(dāng)?shù)卦O(shè)置PATH和CLASSPATH。

創(chuàng)建hibernate.cfg.xml配置文件中配置章節(jié)解釋。
創(chuàng)建Employee.hbm.xml映射文件，如上圖所示。
創(chuàng)建Employee.java源文件，如上圖所示，并編譯它.
創(chuàng)建Certificate.java源文件，如上圖所示，并編譯它。
創(chuàng)建ManageEmployee.java源文件，如上圖所示，并編譯它。
執(zhí)行ManageEmployee二進(jìn)制文件來(lái)運(yùn)行程序。

會(huì)在屏幕上獲得以下結(jié)果，并同時(shí)記錄會(huì)在員工和證書表被創(chuàng)建。可以看到證書已排序順序相反。可以通過(guò)改變映射文件試試，只需設(shè)置sort="natural"和執(zhí)行程序，并比較結(jié)果。

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

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

如果檢查員工和證書表，就應(yīng)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(shù)組，而不是一個(gè)列表。一個(gè)數(shù)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(yīng)用程序很少使用數(shù)組。

Bag映射
Bag是一個(gè)java集合存儲(chǔ)元素?zé)o需關(guān)心順序，但允許列表中的重復(fù)元素。Bag是在列表中的對(duì)象的隨機(jī)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(fā)指示Hibernate如何定義的類映射到數(shù)據(jù)庫(kù)表的映射文件。<bag>元素將被用來(lái)定義所使用的集合規(guī)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(yīng)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(jīng)熟悉了大部分的映射細(xì)節(jié)，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(duì)應(yīng)于每一個(gè)類包含2個(gè)<class>元素的根元素的XML文檔。

<class>元素被用于定義數(shù)據(jù)庫(kù)表從一個(gè)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(shù)據(jù)庫(kù)表名指定。

<bag>元素用于設(shè)置證書和Employee類之間的關(guān)系。我們使用cascade屬性的<bag>高的元素要告訴Hibernate來(lái)保存證書的對(duì)象，同時(shí)為Employee對(duì)象。 name屬性被設(shè)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(duì)象，即在證書表中的列。表EMPLOYEE。

<one-to-many>元素表示一個(gè)Employee對(duì)象涉及到很多證書的對(duì)象，并因此，證書對(duì)象必須有與Employee父類有關(guān)聯(lián)。可以根據(jù)需要使用<one-to-one>，<many-to-one>或<many-to-many>這個(gè)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(zhí)行：

會(huì)在屏幕上獲得以下結(jié)果，并同時(shí)記錄會(huì)在員工和證書表被創(chuàng)建?？梢钥吹阶C書已排序順序相反?？梢酝ㄟ^(guò)改變映射文件試試，只需設(shè)置sort="natural"和執(zhí)行程序，并比較結(jié)果。

$java ManageEmployee

.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........

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

如果檢查員工和證書表，就應(yīng)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)

您可能感興趣的文章: