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JPA實體關係

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本章將指導完成學習實體間的關係。一般的關係數據庫中的表之間的更有效。這裡的實體類都被視為關係表(JPA的概念)，因此是實體類之間的關係如下：

  • @ManyToOne 關係
  • @OneToMany 關係
  • @OneToOne 關係
  • @ManyToMany 關係

@ManyToOne 關係

多對一實體之間存在的關係，其中一個實體（列或組列）的引用與另一個實體（列或組列）包含唯一值。在關係數據庫中，這些關係是通過使用表之間的外鍵/主鍵應用。

讓我們考慮Employee和Department 實體之間的關係的一個例子。在單向方式，即從員工到部門，多到一的關係是可行的。這意味著員工每個記錄包含一個部門ID，它應該是在部門表的主鍵。在這裡，在Employee表，Department 的ID是外鍵。

下圖顯示了兩個表之間的多對一關係。

@ManyToOne Relation

在Eclipse中創建一個IDE JPA項目命名為JPA_Eclipselink_MTO。這個項目的所有模塊下麵討論。

創建實體

遵循用於創建實體上麵給出的圖，在src下創建“com.tutorialspoin.eclipselink.entity'包。創建一個名為Department.java的類。類係實體如下：

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;
    }
}

建立在這種關係中的第二個實體 - 名為Employee.java在“com.yiibai.eclipselink.entity'包下的Employee實體類。 Employee實體類如下所示：

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文件需要配置數據庫和實體類的注冊。

Persitence.xml由Eclipse IDE在創建時由JPA項目創建。詳細配置信息是用戶的規範。 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>

服務類

該模塊包含服務類，它實現了使用屬性初始化關係的一部分。在src下創建一個名為“com.yiibai.eclipselink.service'包。創建一個名為ManyToOne.java的DAO類。在DAO類如下所示：

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();
   }
}

編譯並執行上述程序後，Eclipse IDE控製台麵板上會顯示的通知。對於輸出，檢查MySQL工作台。在此示例中，將創建兩個表。

通過在MySQL管理界麵中執行下麵的查詢，Department表的結果顯示如下：

Select * from department
ID Name
101 Development

通過在MySQL界麵執行下麵的查詢，Employee表顯示如下的結果。

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表的外鍵（參考字段）。

@OneToMany 關係

在這種關係中，一個實體中的每一行被引用到其它實體的許多子記錄。最重要的是，子記錄不能有多個父。在表A和表B之間的1對多的關係，在表A中的每一行都可以在表B中被鏈接到一個或多個行

讓我們考慮上麵的例子。假設Employee 和Department在上述例子中的表連接在一個相反的單向方式，那麼關係成為一個一對多的關係。在Eclipse中創建一個IDE JPA項目命名JPA_Eclipselink_OTM。這個項目的所有模塊下麵討論。

創建實體

遵循用於創建實體上麵給出的圖，在src下創建“com.tutorialspoin.eclipselink.entity'包。創建一個名為Department.java的類。類係實體如下：

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;
    }
}

創建第二個實體類關係-Employee實體類，命名為Employee.java在“com.yiibai.eclipselink.entity'包下。 Employee實體類如下所示：

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>

服務類

該模塊包含服務類，它實現了使用屬性初始化關係的一部分。在src下創建一個名為“com.yiibai.eclipselink.service'包。並在包下創建一個名為OneToMany.java的DAO類。在DAO類如下所示：

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();
   }
}

編譯和執行上述程序後，在Eclipse IDE的控製台麵板中得到通知。對於輸出檢查MySQL工作台如下。

在這個項目中創建三個表。通過在MySQL界麵查詢，department_employee表的結果顯示如下：

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

在上表中，deparment_id和employee_id 是部門和員工表的外鍵（參考字段）。

通過在MySQL界麵下麵查詢，department表的結果將顯示如下表格格式。

Select * from department;
ID Name
254 Development

通過在MySQL界麵下麵查詢，employee表的結果顯示如下：

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

@OneToOne 關係

在一對一關係，一個項可以鏈接到隻能另一個項。這意味著一個實體中的每一行被稱為一個且僅一個行對另一個實體。

讓我們考慮上麵的例子。Employee和Department在反向單向的方式，關係是一對一的關係。這意味著每個員工隻能屬於一個部門。
在Eclipse中創建一個IDE JPA項目命名JPA_Eclipselink_OTO。這個項目的所有模塊下麵討論。

創建實體

遵循用於創建實體上麵給出的圖。在src下創建一個名為“com.tutorialspoin.eclipselink.entity'的包。在這個包下創建類名為：Department.java。類係實體被示為如下：

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;
    }
}

創建第二個實體類關係-Employee實體類，命名為Employee.java在“com.yiibai.eclipselink.entity'包下。 Employee實體類如下所示：

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>

服務類

在src下創建一個名為“com.yiibai.eclipselink.service'的包。在這個包下創建一個名為OneToOne.java的DAO類。在DAO類如下所示：

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();
   }
}

編譯和執行上述程序後，在Eclipse IDE控製台麵板的顯示通知。對於輸出，檢查MySQL工作台如下。

在上麵的例子中，將創建兩個表。通過在MySQL的界麵下麵的查詢，department表的結果顯示如下：

Select * from department
ID Name
301 Development

通過在MySQL界麵下麵查詢，employee表的結果顯示如下：

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

@ManyToMany 關係

多對多的關係，就是從一個實體中的一個或多個行與其他實體的多個行相關聯。

讓我們考慮兩個實體之間的關係的一個例子：班級和教師。以雙向的方式，既班級和教師有多對一的關係。這意味著類中的每個記錄由教師組（教師ID），這應該是在教師表中的主鍵和存儲在Teacher_Class表，反之亦然簡稱。在這裡，Teachers_Class表包含外鍵字段。在Eclipse中創建一個IDE JPA項目命名JPA_Eclipselink_MTM。這個項目的所有模塊下麵討論。

@ManyToOne Relation

創建實體

按照上麵的圖中所示的方案中創建的實體。在src下創建一個名為“com.tutorialspoin.eclipselink.entity'的包。創建一個名為Class.java的類在這個包下。類係實體顯示如下：

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;
   }	  
}

創建第二個實體這種關係的Employee實體類，命名為Teacher.java 在“com.yiibai.eclipselink.entity'包下。 Employee實體類如下所示：

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 文件的內容如下：

<?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>

服務類

在src下創建一個名為“com.yiibai.eclipselink.service'的包，在這個包下創建一個名為ManyToMany.java的DAO類。在DAO類如下所示：

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( );
   }
}

在這個例子中工程，將創建三個表。通過在MySQL界麵執行下麵的查詢，teacher_clas表將顯示如下的結果：

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是從eacher表的外鍵，classet_cid是class表的外鍵。因此不同的老師被分配到不同的班級。

通過在MySQL的界麵下麵的查詢，teacher表的結果顯示如下：

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

通過在MySQL的界麵執行下麵的查詢, clas表將顯示如下的結果：

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



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