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。

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;@Entitypublic 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;@Entitypublic 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

IDName101Development

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

EidDegEnameSalaryDepartment_Id102Technical WriterSatish45000101103Technical WriterKrishna45000101104Technical WriterMasthanwali50000101

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;@Entitypublic 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;@Entitypublic 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_IDEmployee_Eid254251254252254253

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;

IDName254Development

通过在MySQL界面下面查询，employee表的结果显示如下：

Select * from employee;

EidDegEnameSalary251Technical WriterSatish45000252Technical WriterKrishna45000253Technical WriterMasthanwali50000

@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;@Entitypublic 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;@Entitypublic 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

IDName301Development

通过在MySQL界面下面查询，employee表的结果显示如下：

Select * from employee

EidDegEnameSalaryDepartment_id302Technical WriterSatish45000301

@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。

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;@Entitypublic 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;@Entitypublic 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_tidClasset_cid354351355351356351354352355352356352354353355353356353

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

TidSubjectTname354JavaSatish355Adv JavaKrishna356DB2Masthanvali

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

CidCname3511st3522nd3533rd

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