Hibernate，JPA 对象关系映射之关联关系映射策略

编程语言 2018-05-12 14:59:12 阅读次数: 0

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;@EntitypublicclassDepartment{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint id;privateString name;publicint getId(){return id;}publicvoid setId(int id){this.id = id;}publicString getName(){return name;}publicvoid 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;@EntitypublicclassEmployee{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint eid;privateString ename;privatedouble salary;privateString deg;@ManyToOneprivateDepartment department;publicEmployee(int eid,String ename,double salary,String deg){super();this.eid = eid;this.ename = ename;this.salary = salary;this.deg = deg;}publicEmployee(){super();}publicint getEid(){return eid;}publicvoid setEid(int eid){this.eid = eid;}publicString getEname(){return ename;}publicvoid setEname(String ename){this.ename = ename;}publicdouble getSalary(){return salary;}publicvoid setSalary(double salary){this.salary = salary;}publicString getDeg(){return deg;}publicvoid setDeg(String deg){this.deg = deg;}publicDepartment getDepartment(){return department;}publicvoid 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"?><persistenceversion="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-unitname="Eclipselink_JPA"transaction-type="RESOURCE_LOCAL"><class>com.yiibai.eclipselink.entity.Employee</class><class>com.yiibai.eclipselink.entity.Department</class><properties><propertyname="javax.persistence.jdbc.url"value="jdbc:mysql://localhost:3306/jpadb"/><propertyname="javax.persistence.jdbc.user"value="root"/><propertyname="javax.persistence.jdbc.password"value="root"/><propertyname="javax.persistence.jdbc.driver"value="com.mysql.jdbc.Driver"/><propertyname="eclipselink.logging.level"value="FINE"/><propertyname="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;publicclassManyToOne{publicstaticvoid main(String[] args ){EntityManagerFactory emfactory =Persistence.
   			createEntityManagerFactory("Eclipselink_JPA");EntityManager entitymanager = emfactory.
   			createEntityManager();
   	entitymanager.getTransaction().begin();//Create Department EntityDepartment department =newDepartment();
   	department.setName("Development");//Store Department
   	entitymanager.persist(department);//Create Employee1 EntityEmployee employee1 =newEmployee();
   	employee1.setEname("Satish");
   	employee1.setSalary(45000.0);
   	employee1.setDeg("Technical Writer");
   	employee1.setDepartment(department);//Create Employee2 EntityEmployee employee2 =newEmployee();
   	employee2.setEname("Krishna");
   	employee2.setSalary(45000.0);
   	employee2.setDeg("Technical Writer");
   	employee2.setDepartment(department);//Create Employee3 EntityEmployee employee3 =newEmployee();
   	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;@EntitypublicclassDepartment{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint id;privateString name;@OneToMany( targetEntity=Employee.class)privateList employeelist;publicint getId(){return id;}publicvoid setId(int id){this.id = id;}publicString getName(){return name;}publicvoid setName(String deptName ){this.name = deptName;}publicList getEmployeelist(){return employeelist;}publicvoid 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;@EntitypublicclassEmployee{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint eid;privateString ename;privatedouble salary;privateString deg;publicEmployee(int eid,String ename,double salary,String deg){super();this.eid = eid;this.ename = ename;this.salary = salary;this.deg = deg;}publicEmployee(){super();}publicint getEid(){return eid;}publicvoid setEid(int eid){this.eid = eid;}publicString getEname(){return ename;}publicvoid setEname(String ename){this.ename = ename;}publicdouble getSalary(){return salary;}publicvoid setSalary(double salary){this.salary = salary;}publicString getDeg(){return deg;}publicvoid setDeg(String deg){this.deg = deg;}}

Persistence.xml

persistence.xml文件如下：

<?xml version="1.0" encoding="UTF-8"?><persistenceversion="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-unitname="Eclipselink_JPA"transaction-type="RESOURCE_LOCAL"><class>com.yiibai.eclipselink.entity.Employee</class><class>com.yiibai.eclipselink.entity.Department</class><properties><propertyname="javax.persistence.jdbc.url"value="jdbc:mysql://localhost:3306/jpadb"/><propertyname="javax.persistence.jdbc.user"value="root"/><propertyname="javax.persistence.jdbc.password"value="root"/><propertyname="javax.persistence.jdbc.driver"value="com.mysql.jdbc.Driver"/><propertyname="eclipselink.logging.level"value="FINE"/><propertyname="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;publicclassOneToMany{publicstaticvoid main(String[] args){EntityManagerFactory emfactory =Persistence.
   			createEntityManagerFactory("Eclipselink_JPA");EntityManager entitymanager = emfactory.
   			createEntityManager();
   	entitymanager.getTransaction().begin();//Create Employee1 EntityEmployee employee1 =newEmployee();
   	employee1.setEname("Satish");
   	employee1.setSalary(45000.0);
   	employee1.setDeg("Technical Writer");//Create Employee2 EntityEmployee employee2 =newEmployee();
   	employee2.setEname("Krishna");
   	employee2.setSalary(45000.0);
   	employee2.setDeg("Technical Writer");//Create Employee3 EntityEmployee employee3 =newEmployee();
   	employee3.setEname("Masthanvali");
   	employee3.setSalary(50000.0);
   	employee3.setDeg("Technical Writer");//Store Employee
   	entitymanager.persist(employee1);
   	entitymanager.persist(employee2);
   	entitymanager.persist(employee3);//Create EmployeelistList<Employee> emplist =newArrayList();
   	emplist.add(employee1);
   	emplist.add(employee2);
   	emplist.add(employee3);//Create Department EntityDepartment department=newDepartment();
   	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;@EntitypublicclassDepartment{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint id;privateString name;publicint getId(){return id;}publicvoid setId(int id){this.id = id;}publicString getName(){return name;}publicvoid 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;@EntitypublicclassEmployee{@Id@GeneratedValue( strategy=GenerationType.AUTO )privateint eid;privateString ename;privatedouble salary;privateString deg;@OneToOneprivateDepartment department;publicEmployee(int eid,String ename,double salary,String deg){super();this.eid = eid;this.ename = ename;this.salary = salary;this.deg = deg;}publicEmployee(){super();}publicint getEid(){return eid;}publicvoid setEid(int eid){this.eid = eid;}publicString getEname(){return ename;}publicvoid setEname(String ename){this.ename = ename;}publicdouble getSalary(){return salary;}publicvoid setSalary(double salary){this.salary = salary;}publicString getDeg(){return deg;}publicvoid setDeg(String deg){this.deg = deg;}publicDepartment getDepartment(){return department;}publicvoid setDepartment(Department department){this.department = department;}}

Persistence.xml

persistence.xml文件，如下所示：

<?xml version="1.0" encoding="UTF-8"?><persistenceversion="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-unitname="Eclipselink_JPA"transaction-type="RESOURCE_LOCAL"><class>com.yiibai.eclipselink.entity.Employee</class><class>com.yiibai.eclipselink.entity.Department</class><properties><propertyname="javax.persistence.jdbc.url"value="jdbc:mysql://localhost:3306/jpadb"/><propertyname="javax.persistence.jdbc.user"value="root"/><propertyname="javax.persistence.jdbc.password"value="root"/><propertyname="javax.persistence.jdbc.driver"value="com.mysql.jdbc.Driver"/><propertyname="eclipselink.logging.level"value="FINE"/><propertyname="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;publicclassOneToOne{publicstaticvoid main(String[] args){EntityManagerFactory emfactory =Persistence.
   			createEntityManagerFactory("Eclipselink_JPA");EntityManager entitymanager = emfactory.
   			createEntityManager();
   	entitymanager.getTransaction().begin();//Create Department EntityDepartment department =newDepartment();
   	department.setName("Development");//Store Department
   	entitymanager.persist(department);//Create Employee EntityEmployee employee =newEmployee();
   	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;@EntitypublicclassClas{@Id@GeneratedValue( strategy =GenerationType.AUTO )privateint cid;privateString cname;@ManyToMany(targetEntity=Teacher.class)privateSet teacherSet;publicClas(){super();}publicClas(int cid,String cname,Set teacherSet){super();this.cid = cid;this.cname = cname;this.teacherSet = teacherSet;}publicint getCid(){return cid;}publicvoid setCid(int cid){this.cid = cid;}publicString getCname(){return cname;}publicvoid setCname(String cname){this.cname = cname;}publicSet getTeacherSet(){return teacherSet;}publicvoid 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;@EntitypublicclassTeacher{@Id@GeneratedValue( strategy =GenerationType.AUTO )privateint tid;privateString tname;privateString subject;@ManyToMany(targetEntity=Clas.class)privateSet clasSet;publicTeacher(){super();}publicTeacher(int tid,String tname,String subject,Set clasSet){super();this.tid = tid;this.tname = tname;this.subject = subject;this.clasSet = clasSet;}publicint getTid(){return tid;}publicvoid setTid(int tid){this.tid = tid;}publicString getTname(){return tname;}publicvoid setTname(String tname){this.tname = tname;}publicString getSubject(){return subject;}publicvoid setSubject(String subject){this.subject = subject;}publicSet getClasSet(){return clasSet;}publicvoid setClasSet(Set clasSet){this.clasSet = clasSet;}}

Persistence.xml

Persistence.xml 文件的内容如下：

<?xml version="1.0" encoding="UTF-8"?><persistenceversion="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-unitname="Eclipselink_JPA"transaction-type="RESOURCE_LOCAL"><class>com.yiibai.eclipselink.entity.Employee</class><class>com.yiibai.eclipselink.entity.Department</class><properties><propertyname="javax.persistence.jdbc.url"value="jdbc:mysql://localhost:3306/jpadb"/><propertyname="javax.persistence.jdbc.user"value="root"/><propertyname="javax.persistence.jdbc.password"value="root"/><propertyname="javax.persistence.jdbc.driver"value="com.mysql.jdbc.Driver"/><propertyname="eclipselink.logging.level"value="FINE"/><propertyname="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;publicclassManyToMany{publicstaticvoid main(String[] args){EntityManagerFactory emfactory =Persistence.
   			createEntityManagerFactory("Eclipselink_JPA");EntityManager entitymanager = emfactory.
   			createEntityManager();
   	entitymanager.getTransaction().begin();//Create Clas EntityClas clas1=newClas(0,"1st",null);Clas clas2=newClas(0,"2nd",null);Clas clas3=newClas(0,"3rd",null);//Store Clas
   	entitymanager.persist(clas1);
   	entitymanager.persist(clas2);
   	entitymanager.persist(clas3);//Create Clas Set1Set<Clas> classSet1 =newHashSet();
   	classSet1.add(clas1);
   	classSet1.add(clas2);
   	classSet1.add(clas3);//Create Clas Set2Set<Clas> classSet2 =newHashSet();
   	classSet2.add(clas3);
   	classSet2.add(clas1);
   	classSet2.add(clas2);//Create Clas Set3Set<Clas> classSet3 =newHashSet();
   	classSet3.add(clas2);
   	classSet3.add(clas3);
   	classSet3.add(clas1);//Create Teacher EntityTeacher teacher1 =newTeacher(0,"Satish","Java",classSet1);Teacher teacher2 =newTeacher(0,"Krishna","Adv Java",classSet2);Teacher teacher3 =newTeacher(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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标签：JPA实体关系    实体    JPA    

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