java.util.Map

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2023-12-01

Map是一个java集合，它以键值对的形式存储元素，并且不允许列表中的重复元素。 Map接口提供三个集合视图，允许将地图的内容视为一组键，值集合或键值映射集。

Map与映射表中的元素映射，并且可以使用java.util.HashMap初始化无序映射。

定义RDBMS表 (Define RDBMS Tables)

考虑一种情况，我们需要将员工记录存储在EMPLOYEE表中，该表具有以下结构 -

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，假设每个员工可以拥有一个或多个与他/她相关的证书。我们将证书相关信息存储在一个具有以下结构的单独表中 -

create table CERTIFICATE (
   id INT NOT NULL auto_increment,
   certificate_type VARCHAR(40) default NULL,
   certificate_name VARCHAR(30) default NULL,
   employee_id INT default NULL,
   PRIMARY KEY (id)
);

EMPLOYEE和CERTIFICATE对象之间将存在one-to-many关系。

Define POJO Classes

让我们实现一个POJO类Employee,它将用于持久化与EMPLOYEE表相关的对象，并在List变量中包含一组证书。

import java.util.*;
public class Employee {
   private int id;
   private String firstName; 
   private String lastName;   
   private int salary;
   private Map 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 Map getCertificates() {
      return certificates;
   }
   public void setCertificates( Map certificates ) {
      this.certificates = certificates;
   }
}

我们需要定义与CERTIFICATE表对应的另一个POJO类，以便可以将证书对象存储并检索到CERTIFICATE表中。

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 (Define Hibernate Mapping File)

让我们开发我们的映射文件，它指示Hibernate如何将定义的类映射到数据库表。元素将用于定义所用Map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/>
      </id>
      <map name = "certificates" cascade="all">
         <key column = "employee_id"/>
         <index column = "certificate_type" type = "string"/>
         <one-to-many/>
      </map>
      <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/>
      </id>
      <property name = "name" column = "certificate_name" type = "string"/>
   </class>
</hibernate-mapping>

您应该将映射文档保存在格式为 .hbm.xml的文件中。我们将映射文档保存在Employee.hbm.xml文件中。您已经熟悉了大部分映射细节，但让我们再次看到映射文件的所有元素 -

映射文档是具有《hibernate-mapping》作为根元素的XML文档，其包含对应于每个类的两个“类”元素。
《class》元素用于定义从Java类到数据库表的特定映射。使用class元素的name属性指定Java类名，并使用table属性指定数据库表名。
《meta》元素是可选元素，可用于创建类描述。
《id》元素将类中的唯一ID属性映射到数据库表的主键。 id元素的name属性引用类中的属性， column属性引用数据库表中的列。 type属性包含hibernate映射类型，这种映射类型将从Java转换为SQL数据类型。
id元素中的《generator》元素用于自动生成主键值。生成器元素的class属性设置为native ，让hibernate选择identity, sequence或hilo算法来创建主键，具体取决于底层数据库的功能。
《property》元素用于将Java类属性映射到数据库表中的列。元素的name属性引用类中的属性， column属性引用数据库表中的列。 type属性包含hibernate映射类型，这种映射类型将从Java转换为SQL数据类型。
《map》元素用于设置Certificate和Employee类之间的关系。我们使用“map”元素中的cascade属性告诉Hibernate在Employee对象的同时持久保存Certificate对象。 name属性设置为父类中定义的Map变量，在我们的示例中它是certificates 。
《index》元素用于表示键/值映射对的关键部分。密钥将使用一种字符串存储在certificate_type列中。
《key》元素是CERTIFICATE表中的列，用于保存父对象的外键，即。表EMPLOYEE。
《one-to-many》元素表示一个Employee对象与许多Certificate对象相关，因此，Certificate对象必须具有与之关联的Employee父对象。您可以根据需要使用《one-to-one》 ， 《many-to-one》或《many-to-many》元素。

创建应用程序类 (Create Application Class)

最后，我们将使用main（）方法创建应用程序类来运行应用程序。我们将使用此应用程序保存Employee记录以及证书列表，然后我们将对该记录应用CRUD操作。

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  */
      HashMap set = new HashMap();
      set.put("ComputerScience", new Certificate("MCA"));
      set.put("BusinessManagement", new Certificate("MBA"));
      set.put("ProjectManagement", new Certificate("PMP"));
      /* Add employee records in the database */
      Integer empID = ME.addEmployee("Manoj", "Kumar", 4000, set);
      /* List down all the employees */
      ME.listEmployees();
      /* Update employee's salary records */
      ME.updateEmployee(empID, 5000);
      /* 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, HashMap 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());
            Map ec = employee.getCertificates();
            System.out.println("Certificate: " + 
              (((Certificate)ec.get("ComputerScience")).getName()));
            System.out.println("Certificate: " + 
              (((Certificate)ec.get("BusinessManagement")).getName()));
            System.out.println("Certificate: " + 
              (((Certificate)ec.get("ProjectManagement")).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 (Compilation and Execution)

以下是编译和运行上述应用程序的步骤。在继续编译和执行之前，请确保已正确设置PATH和CLASSPATH。

按配置章节中的说明创建hibernate.cfg.xml配置文件。
创建Employee.hbm.xml映射文件，如上所示。
如上所示创建Employee.java源文件并进行编译。
如上所示创建Certificate.java源文件并进行编译。
创建ManageEmployee.java源文件，如上所示并编译它。
执行ManageEmployee二进制文件以运行该程序。

您将在屏幕上看到以下结果，并且将在EMPLOYEE和CERTIFICATE表中创建相同的时间记录。

$java ManageEmployee
.......VARIOUS LOG MESSAGES WILL DISPLAY HERE........
First Name: Manoj  Last Name: Kumar  Salary: 4000
Certificate: MCA
Certificate: MBA
Certificate: PMP
First Name: Manoj  Last Name: Kumar  Salary: 5000
Certificate: MCA
Certificate: MBA
Certificate: PMP

如果您检查您的EMPLOYEE和CERTIFICATE表，他们应该有以下记录 -

mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+
| id | first_name | last_name | salary |
+----+------------+-----------+--------+
| 60 | Manoj      | Kumar     |   5000 |
+----+------------+-----------+--------+
1 row in set (0.00 sec)
mysql>select * from CERTIFICATE;
+----+--------------------+------------------+-------------+
| id | certificate_type   | certificate_name | employee_id |
+----+--------------------+------------------+-------------+
| 16 | ProjectManagement  | PMP              |          60 |
| 17 | BusinessManagement | MBA              |          60 |
| 18 | ComputerScience    | MCA              |          60 |
+----+--------------------+------------------+-------------+
3 rows in set (0.00 sec)
mysql>