Many-to-One
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2023-12-01
many-to-one关联是最常见的关联类型，其中Object可以与多个对象相关联。 例如，相同的地址对象可以与多个员工对象相关联。
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,
address INT NOT NULL,
PRIMARY KEY (id)
);
此外，许多员工可以拥有相同的地址，因此可以使用多对一关联来呈现此关联。 我们将地址相关信息存储在具有以下结构的单独表中 -
create table ADDRESS (
id INT NOT NULL auto_increment,
street_name VARCHAR(40) default NULL,
city_name VARCHAR(40) default NULL,
state_name VARCHAR(40) default NULL,
zipcode VARCHAR(10) default NULL,
PRIMARY KEY (id)
);
创建两个RBDMS表并为下一个实现做好准备。
Define POJO Classes
让我们实现一个POJO类Employee,它将用于持久化与EMPLOYEE表相关的对象并具有Address类型的变量。
import java.util.*;
public class Employee{
private int id;
private String firstName;
private String lastName;
private int salary;
private Address address;
public Employee() {}
public Employee(String fname, String lname, int salary, Address address ) {
this.firstName = fname;
this.lastName = lname;
this.salary = salary;
this.address = address;
}
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 Address getAddress() {
return address;
}
public void setAddress( Address address ) {
this.address = address;
}
}
我们需要定义与ADDRESS表对应的另一个POJO类，以便可以将地址对象存储和检索到ADDRESS表中。
import java.util.*;
public class Address{
private int id;
private String street;
private String city;
private String state;
private String zipcode;
public Address() {}
public Address(String street, String city, String state, String zipcode) {
this.street = street;
this.city = city;
this.state = state;
this.zipcode = zipcode;
}
public int getId() {
return id;
}
public void setId( int id ) {
this.id = id;
}
public String getStreet() {
return street;
}
public void setStreet( String street ) {
this.street = street;
}
public String getCity() {
return city;
}
public void setCity( String city ) {
this.city = city;
}
public String getState() {
return state;
}
public void setState( String state ) {
this.state = state;
}
public String getZipcode() {
return zipcode;
}
public void setZipcode( String zipcode ) {
this.zipcode = zipcode;
}
}
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。 元素将用于定义规则，以在EMPLOYEE和ADDRESS实体之间建立多对一关系。
<?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>
<property name = "firstName" column = "first_name" type = "string"/>
<property name = "lastName" column = "last_name" type = "string"/>
<property name = "salary" column = "salary" type = "int"/>
<many-to-one name = "address" column = "address"
not-null="true"/>
</class>
<class name = "Address" table="ADDRESS">
<meta attribute = "class-description">
This class contains the address detail.
</meta>
<id name = "id" type = "int" column = "id">
<generator/>
</id>
<property name = "street" column = "street_name" type = "string"/>
<property name = "city" column = "city_name" type = "string"/>
<property name = "state" column = "state_name" type = "string"/>
<property name = "zipcode" column = "zipcode" 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。
《many-to-one》元素用于设置EMPLOYEE和ADDRESS实体之间的关系。 name属性设置为父类中定义的变量，在我们的例子中它是address 。 column属性用于在父表EMPLOYEE中设置列名。
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来保存很少的员工记录及其地址，然后我们将对这些记录应用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 one address object */
Address address = ME.addAddress("Kondapur","Hyderabad","AP","532");
/* Add employee records in the database */
Integer empID1 = ME.addEmployee("Manoj", "Kumar", 4000, address);
/* Add another employee record in the database */
Integer empID2 = ME.addEmployee("Dilip", "Kumar", 3000, address);
/* 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 address record in the database */
public Address addAddress(String street, String city, String state, String zipcode) {
Session session = factory.openSession();
Transaction tx = null;
Integer addressID = null;
Address address = null;
try {
tx = session.beginTransaction();
address = new Address(street, city, state, zipcode);
addressID = (Integer) session.save(address);
tx.commit();
} catch (HibernateException e) {
if (tx!=null) tx.rollback();
e.printStackTrace();
} finally {
session.close();
}
return address;
}
/* Method to add an employee record in the database */
public Integer addEmployee(String fname, String lname, int salary, Address address){
Session session = factory.openSession();
Transaction tx = null;
Integer employeeID = null;
try {
tx = session.beginTransaction();
Employee employee = new Employee(fname, lname, salary, address);
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 iterator = employees.iterator(); iterator.hasNext();){
Employee employee = (Employee) iterator.next();
System.out.print("First Name: " + employee.getFirstName());
System.out.print(" Last Name: " + employee.getLastName());
System.out.println(" Salary: " + employee.getSalary());
Address add = employee.getAddress();
System.out.println("Address ");
System.out.println("\tStreet: " + add.getStreet());
System.out.println("\tCity: " + add.getCity());
System.out.println("\tState: " + add.getState());
System.out.println("\tZipcode: " + add.getZipcode());
}
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Address.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ADDRESS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
Address
Street: Kondapur
City: Hyderabad
State: AP
Zipcode: 532
First Name: Dilip Last Name: Kumar Salary: 3000
Address
Street: Kondapur
City: Hyderabad
State: AP
Zipcode: 532
First Name: Manoj Last Name: Kumar Salary: 5000
Address
Street: Kondapur
City: Hyderabad
State: AP
Zipcode: 532
如果您检查您的EMPLOYEE和ADDRESS表，它们应该有以下记录 -
mysql> select * from EMPLOYEE;
+----+------------+-----------+--------+---------+
| id | first_name | last_name | salary | address |
+----+------------+-----------+--------+---------+
| 1 | Manoj | Kumar | 5000 | 5 |
+----+------------+-----------+--------+---------+
1 row in set (0.00 sec)
mysql> select * from ADDRESS;
+----+-------------+-----------+------------+---------+
| id | street_name | city_name | state_name | zipcode |
+----+-------------+-----------+------------+---------+
| 1 | Kondapur | Hyderabad | AP | 532 |
+----+-------------+-----------+------------+---------+
1 row in set (0.00 sec)
mysql>