JPA - 实体关系

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本章将引导您了解实体之间的关系。通常，数据库中表之间的关系更有效。这里实体类被视为关系表（JPA 的概念），因此实体类之间的关系如下：

  • @ManyToOne 关系
  • @OneToMany 关系
  • @OneToOne 关系
  • @ManyToMany 关系

@ManyToOne 关系

实体之间的多对一关系：其中一个实体（列或列集）被引用到另一个实体（列或列集），该实体包含唯一值。在关系数据库中，这些关系是通过使用表之间的外键/主键来实现的。

让我们考虑一下员工和部门实体之间关系的示例。以单向方式，即从员工到部门，适用多对一关系。这意味着每个员工记录都包含一个部门 ID，该 ID 应该是部门表中的主键。此处，在员工表中，部门 ID 是外键。

该图解释了多对一关系如下：

@ManyToOne Relation

在 Eclipse IDE 中创建一个名为 **JPA_Eclipselink_MTO** 的 JPA 项目。该项目的所有模块如下所示：

创建实体

按照上面给出的图创建实体。在 **src** 包下创建一个名为 **‘com.tutorialspoin.eclipselink.entity’** 的包。在给定的包下创建一个名为 **Department.java** 的类。Department 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Department {

   @Id 
   @GeneratedValue( strategy=GenerationType.AUTO )

   private int id;
   private String name;

   public int getId() {
      return id;
   }

   public void setId(int id) {
      this.id = id;
   }

   public String getName( ){
      return name;
   }

   public void setName( String deptName ){
      this.name = deptName;
   }
}

在此关系中创建第二个实体 - 在 **‘com.tutorialspoint.eclipselink.entity’** 包下创建一个名为 **Employee.java** 的 Employee 实体类。Employee 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Employee{

   @Id
   @GeneratedValue( strategy= GenerationType.AUTO ) 	
   
   private int eid;
   private String ename;
   private double salary;
   private String deg;
   
   @ManyToOne
   private Department department;

   public Employee(int eid, String ename, double salary, String deg) {
      super( );
      this.eid = eid;
      this.ename = ename;
      this.salary = salary;
      this.deg = deg;
   }

   public Employee( ) {
      super();
   }

   public int getEid( ) {
      return eid;
   }
   
   public void setEid(int eid)  {
      this.eid = eid;
   }

   public String getEname( ) {
      return ename;
   }
   
   public void setEname(String ename) {
      this.ename = ename;
   }

   public double getSalary( ) {
      return salary;
   }
   
   public void setSalary(double salary) {
      this.salary = salary;
   }

   public String getDeg( ) {
      return deg;
   }
   
   public void setDeg(String deg) {
      this.deg = deg;
   }

   public Department getDepartment() {
      return department;
   }

   public void setDepartment(Department department) {
      this.department = department;
   }
}

persistence.xml

persistence.xml 文件是配置数据库和注册实体类所必需的。

在创建 JPA 项目时，persistence.xml 文件将由 Eclipse IDE 创建。配置细节是用户指定的。persistence.xml 文件如下所示：

<?xml version="1.0" encoding = "UTF-8"?>

<persistence version = "2.0" 
   xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   
   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.tutorialspoint.eclipselink.entity.Employee</class>
      <class>com.tutorialspoint.eclipselink.entity.Department</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://127.0.0.1:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value="root"/>
         <property name = "javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" value = "create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>

服务类

此模块包含服务类，这些类使用属性初始化实现关系部分。在 **src** 包下创建一个名为 **‘com.tutorialspoint.eclipselink.service’** 的包。在给定的包下创建一个名为 **ManyToOne.java** 的 DAO 类。DAO 类如下所示：

package com.tutorialspointeclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.tutorialspoint.eclipselink.entity.Department;
import com.tutorialspoint.eclipselink.entity.Employee;

public class ManyToOne {
   public static void main( String[ ] args ) {
   
   EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
   EntityManager entitymanager = emfactory.createEntityManager( );
   entitymanager.getTransaction( ).begin( );

   //Create Department Entity
   Department department = new Department();
   department.setName("Development");
   
   //Store Department
   entitymanager.persist(department);

   //Create Employee1 Entity
   Employee employee1 = new Employee();
   employee1.setEname("Satish");
   employee1.setSalary(45000.0);
   employee1.setDeg("Technical Writer");
   employee1.setDepartment(department);

   //Create Employee2 Entity
   Employee employee2 = new Employee();
   employee2.setEname("Krishna");
   employee2.setSalary(45000.0);
   employee2.setDeg("Technical Writer");
   employee2.setDepartment(department);

   //Create Employee3 Entity
   Employee employee3 = new Employee();
   employee3.setEname("Masthanvali");
   employee3.setSalary(50000.0);
   employee3.setDeg("Technical Writer");
   employee3.setDepartment(department);

   //Store Employees
   entitymanager.persist(employee1);
   entitymanager.persist(employee2);
   entitymanager.persist(employee3);

   entitymanager.getTransaction().commit();
   entitymanager.close();
   emfactory.close();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。对于输出，请检查 MySQL Workbench。在此示例中，创建了两个表。

在 MySQL 接口中传递以下查询，部门表的表格格式结果如下所示：

Select * from department;

Id	Name
101	Development

在 MySQL 接口中传递以下查询，员工表的表格格式结果如下所示：

Select * from employee;

Eid Deg                 Ename	        Salary	Department_Id
102 Technical Writer	Satish	        45000	101
103 Technical Writer	Krishna	        45000	101
104 Technical Writer	Masthan Wali	50000	101

在上表中，Deparment_Id 是来自部门表的外键（引用字段）。

@OneToMany 关系

在此关系中，一个实体的每一行都引用另一个实体中的许多子记录。重要的是，子记录不能有多个父记录。在一个表 A 和表 B 之间的一对多关系中，表 A 中的每一行都链接到表 B 中的 0、1 或多行。

让我们考虑上面的例子。如果 **Employee** 和 **Department** 以反向单向方式存在，则关系为多对一关系。在 Eclipse IDE 中创建一个名为 **JPA_Eclipselink_OTM** 的 JPA 项目。该项目的所有模块如下所示：

创建实体

按照上面给出的图创建实体。在 **src** 包下创建一个名为 **‘com.tutorialspoin.eclipselink.entity’** 的包。在给定的包下创建一个名为 **Department.java** 的类。Department 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import java.util.List;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;

@Entity
public class Department {

    @Id 
    @GeneratedValue( strategy=GenerationType.AUTO )
    
    private int id;
    private String name;
    
    @OneToMany( targetEntity=Employee.class )
    private List employeelist;

    public int getId() {
    	return id;
    }
    
    public void setId(int id) {
    	this.id = id;
    }
    
    public String getName( ) {
    	return name;
    }
    
    public void setName( String deptName ) {
    	this.name = deptName;
    }

    public List getEmployeelist() {
      return employeelist;
    }

   public void setEmployeelist(List employeelist) {
      this.employeelist = employeelist;
   }
}

在此关系中创建第二个实体 - 在 **‘com.tutorialspoint.eclipselink.entity’** 包下创建一个名为 **Employee.java** 的 Employee 实体类。Employee 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Employee {

   @Id
   @GeneratedValue( strategy= GenerationType.AUTO ) 	

   private int eid;
   private String ename;
   private double salary;
   private String deg;

   public Employee(int eid, String ename, double salary, String deg) {
      super( );
      this.eid = eid;
      this.ename = ename;
      this.salary = salary;
      this.deg = deg;
   }

   public Employee( ) {
      super();
   }

   public int getEid( ) {
      return eid;
   }
   
   public void setEid(int eid) {
      this.eid = eid;
   }

   public String getEname( ) {
      return ename;
   }
   
   public void setEname(String ename) {
      this.ename = ename;
   }

   public double getSalary( ) {
      return salary;
   }
   
   public void setSalary(double salary) {
      this.salary = salary;
   }

   public String getDeg( ) {
      return deg;
   }
   
   public void setDeg(String deg) {
      this.deg = deg;
   }	
}

persistence.xml

在创建 JPA 项目时，persistence.xml 文件将由 Eclipse IDE 创建。配置细节是用户指定的。persistence.xml 文件如下所示：

<?xml version = "1.0" encoding = "UTF-8"?>

<persistence version = "2.0" xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">

   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.tutorialspoint.eclipselink.entity.Employee</class>
      <class>com.tutorialspoint.eclipselink.entity.Department</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://127.0.0.1:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value = "root"/>
         <property name = "javax.persistence.jdbc.driver" value = "com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" value = "create-tables"/>
      </properties>
      
   </persistence-unit>
</persistence>

服务类

此模块包含服务类，这些类使用属性初始化实现关系部分。在 **src** 包下创建一个名为 **‘com.tutorialspoint.eclipselink.service’** 的包。在给定的包下创建一个名为 **OneToMany.java** 的 DAO 类。DAO 类如下所示：

package com.tutorialspointeclipselink.service;

import java.util.List;
import java.util.ArrayList;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.tutorialspoint.eclipselink.entity.Department;
import com.tutorialspoint.eclipselink.entity.Employee;

public class OneToMany {
   public static void main(String[] args) {
   
   EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
   EntityManager entitymanager = emfactory.createEntityManager( );
   entitymanager.getTransaction( ).begin( );

   //Create Employee1 Entity
   Employee employee1 = new Employee();
   employee1.setEname("Satish");
   employee1.setSalary(45000.0);
   employee1.setDeg("Technical Writer");

   //Create Employee2 Entity
   Employee employee2 = new Employee();
   employee2.setEname("Krishna");
   employee2.setSalary(45000.0);
   employee2.setDeg("Technical Writer");

   //Create Employee3 Entity
   Employee employee3 = new Employee();
   employee3.setEname("Masthanvali");
   employee3.setSalary(50000.0);
   employee3.setDeg("Technical Writer");

   //Store Employee
   entitymanager.persist(employee1);
   entitymanager.persist(employee2);
   entitymanager.persist(employee3);

   //Create Employeelist
   List<Employee> emplist = new ArrayList();
   emplist.add(employee1);
   emplist.add(employee2);
   emplist.add(employee3);

   //Create Department Entity
   Department department = new Department();
   department.setName("Development");
   department.setEmployeelist(emplist);

   //Store Department
   entitymanager.persist(department);

   entitymanager.getTransaction().commit();
   entitymanager.close();
   emfactory.close();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。对于输出，请检查 MySQL Workbench，如下所示。在此项目中创建了三个表。

在 MySQL 接口中传递以下查询，department_employee 表的表格格式结果如下所示：

Select * from department_Id;

Department_Id	Employee_Eid
254	        251
254	        252
254	        253

在上表中，deparment_id 和 employee_id 字段是来自部门表和员工表的外键（引用字段）。

在 MySQL 接口中传递以下查询，部门表的表格格式结果如下所示：

Select * from department;

Id	Name
254	Development

在 MySQL 接口中传递以下查询，员工表的表格格式结果如下所示：

Select * from employee;

Eid	Deg	                Ename	       Salary
251	Technical Writer	Satish	       45000
252	Technical Writer	Krishna	       45000
253	Technical Writer	Masthanvali    50000

@OneToOne 关系

在一对一关系中，一个项目只能属于另一个项目。这意味着一个实体的每一行都只引用另一个实体的一行。

让我们考虑上面的例子。**Employee** 和 **Department** 以反向单向方式存在，则关系为一对一关系。这意味着每个员工只属于一个部门。在 Eclipse IDE 中创建一个名为 **JPA_Eclipselink_OTO** 的 JPA 项目。该项目的所有模块如下所示：

创建实体

按照上面给出的图创建实体。在 **src** 包下创建一个名为 **‘com.tutorialspoin.eclipselink.entity’** 的包。在给定的包下创建一个名为 **Department.java** 的类。Department 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;

@Entity
public class Department {

   @Id 
   @GeneratedValue( strategy=GenerationType.AUTO )
   private int id;
   private String name;

   public int getId() {
      return id;
   }

   public void setId(int id) {
      this.id = id;
   }

   public String getName( ) {
      return name;
   }

   public void setName( String deptName ) {
      this.name = deptName;
   }
}

在此关系中创建第二个实体 - 在 **‘com.tutorialspoint.eclipselink.entity’** 包下创建一个名为 **Employee.java** 的 Employee 实体类。Employee 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToOne;

@Entity
public class Employee {

   @Id
   @GeneratedValue( strategy= GenerationType.AUTO ) 	
   private int eid;
   private String ename;
   private double salary;
   private String deg;

   @OneToOne
   private Department department;

   public Employee(int eid, String ename, double salary, String deg) {
      super( );
      this.eid = eid;
      this.ename = ename;
      this.salary = salary;
      this.deg = deg;
   }

   public Employee( ) {
      super();
   }

   public int getEid( ) {
      return eid;
   }
   
   public void setEid(int eid) {
      this.eid = eid;
   }

   public String getEname( ) {
      return ename;
   }
   
   public void setEname(String ename) {
      this.ename = ename;
   }

   public double getSalary( ) {
      return salary;
   }
   
   public void setSalary(double salary) {
      this.salary = salary;
   }

   public String getDeg( ) {
      return deg;
   }
   
   public void setDeg(String deg) {
      this.deg = deg;
   }

   public Department getDepartment() {
      return department;
   }

   public void setDepartment(Department department) {
      this.department = department;
   }	
}

persistence.xml

在创建 JPA 项目时，persistence.xml 文件将由 Eclipse IDE 创建。配置细节是用户指定的。persistence.xml 文件如下所示：

<?xml version = "1.0" encoding = "UTF-8"?>
<persistence version = "2.0" xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   
   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.tutorialspoint.eclipselink.entity.Employee</class>
      <class>com.tutorialspoint.eclipselink.entity.Department</class>
      
      <properties>
         <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://127.0.0.1:3306/jpadb"/>
         <property name = "javax.persistence.jdbc.user" value = "root"/>
         <property name = "javax.persistence.jdbc.password" value = "root"/>
         <property name = "javax.persistence.jdbc.driver" value = "com.mysql.jdbc.Driver"/>
         <property name = "eclipselink.logging.level" value = "FINE"/>
         <property name = "eclipselink.ddl-generation" value = "create-tables"/>
      </properties>
   
   </persistence-unit>
</persistence>

服务类

此模块包含服务类，这些类使用属性初始化实现关系部分。在 **src** 包下创建一个名为 **‘com.tutorialspoint.eclipselink.service’** 的包。在给定的包下创建一个名为 **OneToOne.java** 的 DAO 类。DAO 类如下所示：

package com.tutorialspointeclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.tutorialspoint.eclipselink.entity.Department;
import com.tutorialspoint.eclipselink.entity.Employee;

public class OneToOne {
   public static void main(String[] args) {
   
   EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
   EntityManager entitymanager = emfactory.createEntityManager( );
   entitymanager.getTransaction( ).begin( );

   //Create Department Entity
   Department department = new Department();
   department.setName("Development");

   //Store Department
   entitymanager.persist(department);

   //Create Employee Entity
   Employee employee = new Employee();
   employee.setEname("Satish");
   employee.setSalary(45000.0);
   employee.setDeg("Technical Writer");
   employee.setDepartment(department);

   //Store Employee
   entitymanager.persist(employee);

   entitymanager.getTransaction().commit();
   entitymanager.close();
   emfactory.close();
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。对于输出，请检查 MySQL Workbench，如下所示。在上面的示例中，创建了两个表。

在 MySQL 接口中传递以下查询，department 表的表格格式结果如下所示：

Select * from department

Id	Name
301	Development

在 MySQL 接口中传递以下查询，employee 表的表格格式结果如下所示：

Select * from employee

Eid	Deg	                Ename	Salary	Department_id
302	Technical Writer	Satish	45000	301

@ManyToMany 关系

多对多关系是指一个实体的一行或多行与另一个实体的多行相关联。

让我们考虑一下班级和教师实体之间关系的示例。在双向方式下，班级和教师都具有多对一关系。这意味着每个班级的记录都被教师集（教师 ID）引用，这些 ID 应该是教师表中的主键，并存储在 Teacher_Class 表中，反之亦然。此处，Teachers_Class 表包含两个外键字段。在 Eclipse IDE 中创建一个名为 **JPA_Eclipselink_MTM** 的 JPA 项目。该项目的所有模块如下所示：

@ManyToOne Relation

创建实体

按照上面给出的图创建实体。在 **src** 包下创建一个名为 **‘com.tutorialspoin.eclipselink.entity’** 的包。在给定的包下创建一个名为 **Clas.java** 的类。Department 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Clas {

   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )
   
   private int cid;
   private String cname;

   @ManyToMany(targetEntity=Teacher.class)
   private Set teacherSet;

   public Clas(){
      super();
   }
   
   public Clas(int cid, String cname, Set teacherSet) {
      super();
      this.cid = cid;
      this.cname = cname;
      this.teacherSet = teacherSet;
   }
   
   public int getCid(){
      return cid;
   }
   
   public void setCid(int cid) {
      this.cid = cid;
   }
   
   public String getCname() {
      return cname;
   }
   
   public void setCname(String cname) {
      this.cname = cname;
   }
   
   public Set getTeacherSet() {
      return teacherSet;
   }
   
   public void setTeacherSet(Set teacherSet) {
      this.teacherSet = teacherSet;
   }	  
}

在此关系中创建第二个实体 - 在 **‘com.tutorialspoint.eclipselink.entity’** 包下创建一个名为 **Teacher.java** 的 Employee 实体类。Employee 实体类如下所示：

package com.tutorialspoint.eclipselink.entity;

import java.util.Set;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;

@Entity
public class Teacher {

   @Id
   @GeneratedValue( strategy = GenerationType.AUTO )
   private int tid;
   private String tname;
   private String subject;

   @ManyToMany(targetEntity = Clas.class)
   private Set clasSet;

   public Teacher(){
      super();
   }
   
   public Teacher(int tid, String tname, String subject, Set clasSet) {
      super();
      this.tid = tid;
      this.tname = tname;
      this.subject = subject;
      this.clasSet = clasSet;
   }
   
   public int getTid() {
      return tid;
   }
   
   public void setTid(int tid) {
      this.tid = tid;
   }
   
   public String getTname() {
      return tname;
   }
   
   public void setTname(String tname) {
      this.tname = tname;
   }
   
   public String getSubject() {
      return subject;
   }
   
   public void setSubject(String subject) {
      this.subject = subject;
   }
   
   public Set getClasSet() {
      return clasSet;
   }
   
   public void setClasSet(Set clasSet) {
      this.clasSet = clasSet;
   }
}

persistence.xml

在创建 JPA 项目时，persistence.xml 文件将由 Eclipse IDE 创建。配置细节是用户指定的。persistence.xml 文件如下所示：

<?xml version = "1.0" encoding = "UTF-8"?>
<persistence version = "2.0" xmlns = "http://java.sun.com/xml/ns/persistence" 
   xmlns:xsi = "http://www.w3.org/2001/XMLSchema-instance" 
   xsi:schemaLocation = "http://java.sun.com/xml/ns/persistence 
   http://java.sun.com/xml/ns/persistence/persistence_2_0.xsd">
   
   <persistence-unit name = "Eclipselink_JPA" transaction-type = "RESOURCE_LOCAL">
      <class>com.tutorialspoint.eclipselink.entity.Employee</class>
      <class>com.tutorialspoint.eclipselink.entity.Department</class>
      
      <properties>
      <property name = "javax.persistence.jdbc.url" value = "jdbc:mysql://127.0.0.1:3306/jpadb"/>
      <property name = "javax.persistence.jdbc.user" value = "root"/>
      <property name = "javax.persistence.jdbc.password" value = "root"/>
      <property name = "javax.persistence.jdbc.driver" value = "com.mysql.jdbc.Driver"/>
      <property name = "eclipselink.logging.level" value = "FINE"/>
      <property name = "eclipselink.ddl-generation" value = "create-tables"/>
      </properties>
   
   </persistence-unit>
</persistence>

服务类

此模块包含服务类，这些类使用属性初始化实现关系部分。在 **src** 包下创建一个名为 **‘com.tutorialspoint.eclipselink.service’** 的包。在给定的包下创建一个名为 **ManyToMany.java** 的 DAO 类。DAO 类如下所示：

package com.tutorialspoint.eclipselink.service;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import com.tutorialspoint.eclipselink.entity.Clas;
import com.tutorialspoint.eclipselink.entity.Teacher;

public class ManyToMany {
   public static void main(String[] args) {
   
   EntityManagerFactory emfactory = Persistence.createEntityManagerFactory( "Eclipselink_JPA" );
   EntityManager entitymanager = emfactory.createEntityManager( );
   entitymanager.getTransaction( ).begin( );

   //Create Clas Entity
   Clas clas1 = new Clas(0, "1st", null);
   Clas clas2 = new Clas(0, "2nd", null);
   Clas clas3 = new Clas(0, "3rd", null);

   //Store Clas
   entitymanager.persist(clas1);
   entitymanager.persist(clas2);
   entitymanager.persist(clas3);

   //Create Clas Set1
   Set<Clas> classSet1 = new HashSet();
   classSet1.add(clas1);
   classSet1.add(clas2);
   classSet1.add(clas3);

   //Create Clas Set2
   Set<Clas> classSet2 = new HashSet();
   classSet2.add(clas3);
   classSet2.add(clas1);
   classSet2.add(clas2);

   //Create Clas Set3
   Set<Clas> classSet3 = new HashSet();
   classSet3.add(clas2);
   classSet3.add(clas3);
   classSet3.add(clas1);

   //Create Teacher Entity
   Teacher teacher1 = new Teacher(0, "Satish","Java",classSet1);
   Teacher teacher2 = new Teacher(0, "Krishna","Adv Java",classSet2);
   Teacher teacher3 = new Teacher(0, "Masthanvali","DB2",classSet3);

   //Store Teacher
   entitymanager.persist(teacher1);
   entitymanager.persist(teacher2);
   entitymanager.persist(teacher3);


   entitymanager.getTransaction( ).commit( );
   entitymanager.close( );
   emfactory.close( );
   }
}

编译并执行上述程序后，您将在 Eclipse IDE 的控制台面板中收到通知。对于输出，请检查 MySQL Workbench，如下所示。在此示例项目中，创建了三个表。

在 MySQL 接口中传递以下查询，teacher_clas 表的表格格式结果如下所示。

Select * form teacher_clas;

Teacher _tid	Classet_cid
354	        351
355	        351
356	        351
354	        352
355	        352
356	        352
354	        353
355	        353
356	        353

在上表中，teacher_tid 是来自教师表的外键，classet_cid 是来自班级表的外键。因此，不同的教师被分配到不同的班级。

在 MySQL 接口中传递以下查询，教师表的表格格式结果如下所示：

Select * from teacher;

Tid	Subject	    Tname
354	Java	    Satish
355	Adv Java    Krishna
356	DB2         Masthanvali

在 MySQL 接口中传递以下查询，clas 表的表格格式结果如下所示：

Select * from clas;

cid	Cname
351	1st
352	2nd
353	3rd
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