---

JPA - 实体关系

本章将带您了解实体之间的关系。 一般来说，数据库中的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:一个实体(列或列集)被包含唯一值的另一个实体(列或列集)引用。 在关系数据库中，这些关系可以通过在表之间使用外键/主键来应用。

让我们考虑一个员工和部门实体之间关系的示例。 单向方式，即从员工到部门，适用多对一关系。 这意味着员工的每条记录都包含一个部门ID，该ID应该是部门表中的主键。 在 Employee 表中，Department id 是外键。

该图解释了多对一关系如下:

在 Eclipse IDE 中创建一个名为 JPA_Eclipselink_MTO 的 JPA 项目。 本项目各模块如下所示:

创建实体

按照上面给出的图表创建实体。 在 'src' 包下创建一个名为 'com.tutorialspoin.eclipselink.entity' 的包。 在给定包下创建一个名为 Department.java 的类。 类Department实体如下所示:

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

配置数据库和注册实体类需要Persistence.xml文件。

Persitence.xml 将在创建 JPA 项目时由 eclipse IDE 创建。 配置详细信息是用户规范。 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://localhost: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>

Service 类

该模块包含服务类，它使用属性初始化来实现关系部分。 在 'src' 包下创建一个名为 'com.tutorialspoint.eclipselink.service' 的包。 名为 ManyToOne.java 的 DAO 类在给定包下创建。 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 工作台。 在此示例中创建了两个表。

在MySQL接口中传递以下查询，查询中表格格式的Department表的结果如下:

Select * from department;

Id	Name
101	Development

在MySQL接口中传递以下查询，查询中Employee表以表格形式显示的结果如下:

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

上表中的 Deparment_Id 是 Department 表中的外键(参考字段)。

@OneToMany 关系

在此关系中，一个实体的每一行都引用其他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。

让我们考虑一下上面的例子。 如果Employee和Department是反向单向关系，则关系是多对一关系。 在 Eclipse IDE 中创建一个名为 JPA_Eclipselink_OTM 的 JPA 项目。 本项目各模块如下所示:

创建实体

按照上面给出的图表创建实体。 在 'src' 包下创建一个名为 'com.tutorialspoin.eclipselink.entity' 的包。 在给定包下创建一个名为 Department.java 的类。 类Department实体如下所示:

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;
   }
}

在该关系中创建第二个实体——Employee实体类，在'com.tutorialspoint.eclipselink.entity'包下命名为Employee.java。 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

Persistence.xml 将在创建 JPA 项目时由 eclipse IDE 创建。 配置详细信息是用户规范。 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://localhost: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>

Service 类

该模块包含服务类，它使用属性初始化来实现关系部分。 在 'src' 包下创建一个名为 'com.tutorialspoint.eclipselink.service' 的包。 名为 OneToMany.java 的 DAO 类在给定包下创建。 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，请按如下方式检查 MySQL 工作台。 在此项目中创建了三个表。

在MySQL接口中传递以下查询，查询中表格格式的department_employee表的结果如下:

Select * from department_Id;

Department_Id	Employee_Eid
254	        251
254	        252
254	        253

在上表中，deparment_id 和employee_id 字段是部门表和员工表中的外键(引用字段)。

在MySQL接口中传递以下查询，查询中部门表的结果以表格形式显示如下:

Select * from department;

Id	Name
254	Development

在MySQL接口中传递以下查询，查询中employee表的结果以表格形式显示如下:

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，一项只能属于另一项。 这意味着一个实体的每一行都引用另一个实体的一行且仅一行。

让我们考虑一下上面的例子。 Employee和Department是反向单向关系，是一对一的关系。 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。 本项目各模块如下所示:

创建实体

按照上面给出的图表创建实体。 在 'src' 包下创建一个名为 'com.tutorialspoin.eclipselink.entity' 的包。 在给定包下创建一个名为 Department.java 的类。 类Department实体如下所示:

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;
   }
}

在该关系中创建第二个实体——Employee实体类，在'com.tutorialspoint.eclipselink.entity'包下命名为Employee.java。 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

Persistence.xml 将在创建 JPA 项目时由 eclipse IDE 创建。 配置详细信息是用户规范。 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://localhost: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>

Service 类

该模块包含服务类，它使用属性初始化来实现关系部分。 在 'src' 包下创建一个名为 'com.tutorialspoint.eclipselink.service' 的包。 名为 OneToOne.java 的 DAO 类在给定包下创建。 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 工作台，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，查询中department表以表格形式显示的结果如下:

Select * from department

Id	Name
301	Development

在MySQL接口中传递以下查询，查询中表格格式的employee表的结果如下:

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中的一行或多行与另一实体中的多行相关联。

让我们考虑一个班级和教师实体之间关系的示例。 在双向方式中，班级和教师都具有多对一的关系。 这意味着班级的每条记录都由教师集(教师 ID)引用，教师集应该是教师表中的主键并存储在 Teacher_Class 表中，反之亦然。 这里，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。 本项目各模块如下所示:

创建实体

按照上面给出的图表创建实体。 在 'src' 包下创建一个名为 'com.tutorialspoin.eclipselink.entity' 的包。 在给定包下创建一个名为 Clas.java 的类。 类Department实体如下所示:

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;
   }	  
}

在该关系中创建第二个实体-Employee实体类，在'com.tutorialspoint.eclipselink.entity'包下命名为Teacher.java。 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

Persistence.xml 将在创建 JPA 项目时由 eclipse IDE 创建。 配置详细信息是用户规范。 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://localhost: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>

Service 类

该模块包含服务类，它使用属性初始化来实现关系部分。 在 'src' 包下创建一个名为 'com.tutorialspoint.eclipselink.service' 的包。 名为 ManyToMany.java 的 DAO 类在给定包下创建。 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 工作台，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，查询中以表格格式显示teacher_clas表的结果如下。

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

上表中，teacher_tid 是来自teacher 表的外键，classet_cid 是来自class 表的外键。 因此不同的老师被分配到不同的班级。

在MySQL接口中传递以下查询，查询中教师表以表格形式显示的结果如下:

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，查询中以表格形式显示clas表的结果如下:

Select * from clas;

cid	Cname
351	1st
352	2nd
353	3rd

---