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JPA实体关系

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本章将指导完成学习实体间的关系。一般的关系数据库中的表之间的更有效。这里的实体类都被视为关系表(JPA的概念)，因此是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

多对一实体之间存在的关系，其中一个实体（列或组列）的引用与另一个实体（列或组列）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/主键应用。

让我们考虑Employee和Department 实体之间的关系的一个例子。在单向方式，即从员工到部门，多到一的关系是可行的。这意味着员工每个记录包含一个部门ID，它应该是在部门表的主键。在这里，在Employee表，Department 的ID是外键。

下图显示了两个表之间的多对一关系。

@ManyToOne Relation

在Eclipse中创建一个IDE JPA项目命名为JPA_Eclipselink_MTO。这个项目的所有模块下面讨论。

创建实体

遵循用于创建实体上面给出的图，在src下创建“com.tutorialspoin.eclipselink.entity'包。创建一个名为Department.java的类。类系实体如下：

package com.yiibai.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;
    }
}

建立在这种关系中的第二个实体 - 名为Employee.java在“com.yiibai.eclipselink.entity'包下的Employee实体类。 Employee实体类如下所示：

package com.yiibai.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文件需要配置数据库和实体类的注册。

Persitence.xml由Eclipse IDE在创建时由JPA项目创建。详细配置信息是用户的规范。 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.yiibai.eclipselink.entity.Employee</class>
   <class>com.yiibai.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，它实现了使用属性初始化关系的一部分。在src下创建一个名为“com.yiibai.eclipselink.service'包。创建一个名为ManyToOne.java的DAO类。在DAO类如下所示：

package com.yiibaieclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.yiibai.eclipselink.entity.Department;
import com.yiibai.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，Eclipse IDE控制台面板上会显示的通知。对于输出，检查MySQL工作台。在此示例中，将创建两个表。

通过在MySQL管理界面中执行下面的查询，Department表的结果显示如下：

Select * from department
ID Name
101 Development

通过在MySQL界面执行下面的查询，Employee表显示如下的结果。

Select * from employee
Eid Deg Ename Salary Department_Id
102 Technical Writer Satish 45000 101
103 Technical Writer Krishna 45000 101
104 Technical Writer Masthanwali 50000 101

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

@OneToMany 关系

在这种关系中，一个实体中的每一行被引用到其它实体的许多子记录。最重要的是，子记录不能有多个父。在表A和表B之间的1对多的关系，在表A中的每一行都可以在表B中被链接到一个或多个行

让我们考虑上面的例子。假设Employee 和Department在上述例子中的表连接在一个相反的单向方式，那么关系成为一个一对多的关系。在Eclipse中创建一个IDE JPA项目命名JPA_Eclipselink_OTM。这个项目的所有模块下面讨论。

创建实体

遵循用于创建实体上面给出的图，在src下创建“com.tutorialspoin.eclipselink.entity'包。创建一个名为Department.java的类。类系实体如下：

package com.yiibai.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;
    }
}

创建第二个实体类关系-Employee实体类，命名为Employee.java在“com.yiibai.eclipselink.entity'包下。 Employee实体类如下所示：

package com.yiibai.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文件如下：

<?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.yiibai.eclipselink.entity.Employee</class>
   <class>com.yiibai.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，它实现了使用属性初始化关系的一部分。在src下创建一个名为“com.yiibai.eclipselink.service'包。并在包下创建一个名为OneToMany.java的DAO类。在DAO类如下所示：

package com.yiibaieclipselink.service;

import java.util.List;
import java.util.ArrayList;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.yiibai.eclipselink.entity.Department;
import com.yiibai.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();
   }
}

编译和执行上述程序后，在Eclipse IDE的控制台面板中得到通知。对于输出检查MySQL工作台如下。

在这个项目中创建三个表。通过在MySQL界面查询，department_employee表的结果显示如下：

Select * from department_Id;
Department_ID Employee_Eid
254 251
254 252
254 253

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

通过在MySQL界面下面查询，department表的结果将显示如下表格格式。

Select * from department;
ID Name
254 Development

通过在MySQL界面下面查询，employee表的结果显示如下：

Select * from employee;
Eid Deg Ename Salary
251 Technical Writer Satish 45000
252 Technical Writer Krishna 45000
253 Technical Writer Masthanwali 50000

@OneToOne 关系

在一对一关系，一个项可以链接到只能另一个项。这意味着一个实体中的每一行被称为一个且仅一个行对另一个实体。

让我们考虑上面的例子。Employee和Department在反向单向的方式，关系是一对一的关系。这意味着每个员工只能属于一个部门。
在Eclipse中创建一个IDE JPA项目命名JPA_Eclipselink_OTO。这个项目的所有模块下面讨论。

创建实体

遵循用于创建实体上面给出的图。在src下创建一个名为“com.tutorialspoin.eclipselink.entity'的包。在这个包下创建类名为：Department.java。类系实体被示为如下：

package com.yiibai.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;
    }
}

创建第二个实体类关系-Employee实体类，命名为Employee.java在“com.yiibai.eclipselink.entity'包下。 Employee实体类如下所示：

package com.yiibai.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.yiibai.eclipselink.entity.Employee</class>
   <class>com.yiibai.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

在src下创建一个名为“com.yiibai.eclipselink.service'的包。在这个包下创建一个名为OneToOne.java的DAO类。在DAO类如下所示：

package com.yiibaieclipselink.service;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.yiibai.eclipselink.entity.Department;
import com.yiibai.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();
   }
}

编译和执行上述程序后，在Eclipse IDE控制台面板的显示通知。对于输出，检查MySQL工作台如下。

在上面的例子中，将创建两个表。通过在MySQL的界面下面的查询，department表的结果显示如下：

Select * from department
ID Name
301 Development

通过在MySQL界面下面查询，employee表的结果显示如下：

Select * from employee
Eid Deg Ename Salary Department_id
302 Technical Writer Satish 45000 301

@ManyToMany 关系

多对多的关系，就是从一个实体中的一个或多个行与其他实体的多个行相关联。

让我们考虑两个实体之间的关系的一个例子：班级和教师。以双向的方式，既班级和教师有多对一的关系。这意味着类中的每个记录由教师组（教师ID），这应该是在教师表中的主键和存储在Teacher_Class表，反之亦然简称。在这里，Teachers_Class表包含外键字段。在Eclipse中创建一个IDE JPA项目命名JPA_Eclipselink_MTM。这个项目的所有模块下面讨论。

@ManyToOne Relation

创建实体

按照上面的图中所示的方案中创建的实体。在src下创建一个名为“com.tutorialspoin.eclipselink.entity'的包。创建一个名为Class.java的类在这个包下。类系实体显示如下：

package com.yiibai.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;
   }	  
}

创建第二个实体这种关系的Employee实体类，命名为Teacher.java 在“com.yiibai.eclipselink.entity'包下。 Employee实体类如下所示：

package com.yiibai.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 文件的内容如下：

<?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.yiibai.eclipselink.entity.Employee</class>
   <class>com.yiibai.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

在src下创建一个名为“com.yiibai.eclipselink.service'的包，在这个包下创建一个名为ManyToMany.java的DAO类。在DAO类如下所示：

package com.yiibai.eclipselink.service;

import java.util.HashSet;
import java.util.Set;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import com.yiibai.eclipselink.entity.Clas;
import com.yiibai.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( );
   }
}

在这个例子中工程，将创建三个表。通过在MySQL界面执行下面的查询，teacher_clas表将显示如下的结果：

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是从eacher表的外键，classet_cid是class表的外键。因此不同的老师被分配到不同的班级。

通过在MySQL的界面下面的查询，teacher表的结果显示如下：

Select * from teacher
Tid Subject Tname
354 Java Satish
355 Adv Java Krishna
356 DB2 Masthanvali

通过在MySQL的界面执行下面的查询, clas表将显示如下的结果：

Select * from clas
Cid Cname
351 1st
352 2nd
353 3rd



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