JPA实体关系

本章将带您了解实体之间的关系。通常，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（列或一组列）被另一个包含唯一值的实体（一列或一组列）引用。在关系数据库中，这些关系通过在表之间使用外键/主键来适用。

让我们考虑员工与部门实体之间关系的一个例子。以单向的方式，即从员工到部门，多对一关系是适用的。这意味着员工的每条记录都包含一个部门ID，该部门ID应该是部门表中的主键。在Employee表中，部门ID是外键。

该图解释了多对一关系，如下所示：

@ManyToOne关系

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

按照上面给出的图来创建实体。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。班级部门实体如下所示：

package com.CodingDict.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;
   }
}

在这个关系中创建第二个实体 - 在 'com.CodingDict.eclipselink.entity' 包下 的 名为 Employee.java的 Employee实体类 。Employee实体类如下所示： **

package com. CodingDict.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中

需要Persistence.xml文件来配置数据库和实体类的注册。

Persitence.xml将在创建JPA项目时由eclipse IDE创建。配置细节是用户规格。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.CodingDict.eclipselink.entity.Employee</class>
      <class>com.CodingDict.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>

服务类

该模块包含服务类，它使用属性初始化实现关系部分。在名为 'com.CodingDict.eclipselink.service'的 'src' 包下创建一个包。名为 ManyToOne.java 的DAO类是在给定的包下创建的。DAO类显示如下： 

package com. CodingDicteclipselink.service;

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

import com.CodingDict.eclipselink.entity.Department;
import com.CodingDict.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。在这个例子中创建了两个表。

在MySQL接口中传递以下查询，查询中以表格格式显示 部门 表的结果如下所示：

Select * from department;

Id  Name
101 Development

在MySQL接口中传递以下查询，查询中显示表格格式的 Employee 表结果如下：

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

在上表中，Deparment_Id是Department表中的外键（引用字段）。

@OneToMany关系

在这种关系中，一个实体的每一行都被引用到其他实体中的许多子记录中。重要的是，孩子记录不能有多个父母。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 员工 和 部门 处于反向单向关系，则关系为多对一关系。在名为 JPA_Eclipselink_OTM的 eclipse IDE中创建一个JPA项目 。该项目的所有模块如下所示：

创建实体

按照上面给出的图来创建实体。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。班级部门实体如下所示：

package com.CodingDict.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 - 雇员 实体类，名为 Employee.java， 位于 'com.CodingDict.eclipselink.eventity' 包下。Employee实体类如下所示：

package com.CodingDict.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中

Persistence.xml将在创建JPA项目时由Eclipse IDE创建。配置细节是用户规格。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.CodingDict.eclipselink.entity.Employee</class>
      <class>com.CodingDict.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>

服务类

该模块包含服务类，它使用属性初始化实现关系部分。在名为 'com.CodingDict.eclipselink.service'的 'src' 包下创建一个包。名为 OneToMany.java 的DAO类是在给定的包下创建的。DAO类显示如下： 

package com. CodingDicteclipselink.service;

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

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

import com.CodingDict.eclipselink.entity.Department;
import com.CodingDict.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。对于输出检查MySQL工作台如下。在这个项目中创建了三个表格。

在MySQL接口中传递以下查询，并且表格格式的 department_employee 表的结果在查询中显示如下：

Select * from department_Id;

Department_Id   Employee_Eid
254         251
254         252
254         253

在上表中，deparment_id和employee_id字段是来自部门和员工表的外键（参考字段）。

在MySQL接口中传递以下查询，查询中以表格形式显示部门表的结果如下所示：

Select * from department;

Id  Name
254 Development

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

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被引用到另一个实体的一行，并且只有一行。

让我们考虑一下上面的例子。 员工 和 部门 以反向单向的方式，关系是一对一关系。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

按照上面给出的图来创建实体。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。班级部门实体如下所示：

package com.CodingDict.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 - 雇员 实体类，名为 Employee.java， 位于 'com.CodingDict.eclipselink.eventity' 包下。Employee实体类如下所示：

package com.CodingDict.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中

Persistence.xml将在创建JPA项目时由Eclipse IDE创建。配置细节是用户规格。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.CodingDict.eclipselink.entity.Employee</class>
      <class>com.CodingDict.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>

服务类

该模块包含服务类，它使用属性初始化实现关系部分。在名为 'com.CodingDict.eclipselink.service'的 'src' 包下创建一个包。名为 OneToOne.java 的DAO类是在给定的包下创建的。DAO类显示如下： **

package com. CodingDicteclipselink.service;

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

import com.CodingDict.eclipselink.entity.Department;
import com.CodingDict.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，请按照以下方式检查MySQL工作台。在上面的例子中创建了两个表。

在MySQL接口中传递以下查询，查询中以表格形式显示 部门 表的结果如下所示：

Select * from department

Id  Name
301 Development

在MySQL接口中传递以下查询，查询中以表格形式显示 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

多对多关系是来自一个实体的一行或多行与其他实体中的多行关联的情况。

让我们考虑一下班级和教师实体之间的关系。以双向的方式，班级和老师都有多对一的关系。这意味着Class的每个记录都由Teacher集（Teacher ID）引用，它应该是Teacher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。该项目的所有模块如下所示：

@ManyToOne关系

创建实体

按照上面给出的图来创建实体。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。班级部门实体如下所示：

package com.CodingDict.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 - 雇员 实体类，名为 Teacher.java， 位于 'com.CodingDict.eclipselink.eventity' 包下。Employee实体类如下所示：

package com.CodingDict.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中

在创建JPA项目的同时，eclipse IDE将创建Persistence.xml。配置细节是用户规格。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.CodingDict.eclipselink.entity.Employee</class>
      <class>com.CodingDict.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>

服务类

该模块包含服务类，它使用属性初始化实现关系部分。在名为 'com.CodingDict.eclipselink.service'的 'src' 包下创建一个包。名为 ManyToMany.java 的DAO类是在给定的包下创建的。DAO类显示如下： 

package com. CodingDict.eclipselink.service;

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

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

import com.CodingDict.eclipselink.entity.Clas;
import com.CodingDict.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，请按照以下方式检查MySQL工作台。在这个示例项目中，创建了三个表。

在MySQL接口中传递以下查询，查询中以表格形式显示 teacher_clas 表的结果如下所示。

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teacher表的外键，而classet_cid是来自class表的外键。因此不同的教师被分配到不同的班级。

在MySQL界面中传递以下查询，查询中以表格形式显示教师表的结果如下：

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，如下所示：

Select * from clas;

cid Cname
351 1st
352 2nd
353 3rd