理解注释和JPA（hibernate）
有一个结构。 我想以这种方式链接这三个实体：公司应该包含id，公司名称和部门列表，每个部门都有一个Worker列表，id和部门名称。 每个工人都有名字，身份证。
+Company -int companyId -String companyName -Set listOfDepartments = new HashSet(); +Department -int departmentId -String departmentName -Set listOfWorkers = new HashSet(); +Worker -int workerId -String workerName 我尝试与一对多和多对一建立连接，但是没有成功。
Сompany
@Entity @Table(name="Company") public class Company { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int idCompany; private String companyName; @OneToMany(mappedBy = "company") private Set listOfDepartments = new HashSet();
部门
@Entity public class Department { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int idDepartment; private String departmentName; @ManyToOne @JoinColumn(name="idCompany") private Company company; @OneToMany(mappedBy = "department") private Set listOfWorkers = new HashSet();
工人
@Entity public class Worker { @Id @GeneratedValue(strategy = GenerationType.AUTO) private int idWorker; private String workerName; @ManyToOne @JoinColumn(name="idDepartment")
我从：
Session session = sessionFactory.openSession(); session.beginTransaction(); Worker worker1 = new Worker("WorkerName1"); Worker worker2 = new Worker("WorkerName2"); Worker worker3 = new Worker("WorkerName3"); Worker worker4 = new Worker("WorkerName4"); Department department = new Department("Technical"); department.getListOfWorkers().add(worker1); department.getListOfWorkers().add(worker2); department.getListOfWorkers().add(worker3); department.getListOfWorkers().add(worker4); company = new Company("MyCompanyName"); company.getListOfDepartments().add(department); session.save(company); session.getTransaction().commit(); session.close();
它填充公司，但没有填充其他表，也没有创建任何连接（映射）错误：
Hibernate: alter table Department drop foreign key FK_l7sg67atqhnn0soqynpvxrtpk Hibernate: alter table Worker drop foreign key FK_s53hyohtyjy93srd2wkksairk Hibernate: drop table if exists Company Hibernate: drop table if exists Department Hibernate: drop table if exists Worker Hibernate: create table Company (idCompany integer not null auto_increment, companyName varchar(255), primary key (idCompany)) Hibernate: create table Department (idDepartment integer not null auto_increment, departmentName varchar(255), idCompany integer, primary key (idDepartment)) Hibernate: create table Worker (idWorker integer not null auto_increment, workerName varchar(255), idDepartment integer, primary key (idWorker)) Hibernate: alter table Department add index FK_l7sg67atqhnn0soqynpvxrtpk (idCompany), add constraint FK_l7sg67atqhnn0soqynpvxrtpk foreign key (idCompany) references Company (idCompany) Hibernate: alter table Worker add index FK_s53hyohtyjy93srd2wkksairk (idDepartment), add constraint FK_s53hyohtyjy93srd2wkksairk foreign key (idDepartment) references Department (idDepartment) ноя 11, 2013 3:10:31 AM org.hibernate.tool.hbm2ddl.SchemaExport execute INFO: HHH000230: Schema export complete Hibernate: insert into Company (companyName) values (?)
除了Glenn Lane的答案中提到的级联之外，您还需要了解双向关联的工作原理。
他们有一个所有者方面，反面。 JPA只关心所有者方来决定实体之间存在哪种关联。 所有者方是没有 mappedBy属性的方。
您的代码会向公司添加分类，向部门添加工作人员，但它只会初始化反面。 您忘记初始化所有者方：
worker1.setDepartment(department); worker2.setDepartment(department); ... department.setCompany(company);
如果你想让JPA自动保留孩子，你需要进一步装饰你的@OneToMany ：
@OneToMany(mappedBy = "department", cascade={CascadeType.PERSIST})
如果您想要其他操作的级联行为，例如删除，合并，刷新，您还需要将它们添加到级联列表中。
您可以使用其他方法：
Unidirectional OneToMany： http ： //en.wikibooks.org/wiki/Java_Persistence/OneToMany#Unidirectional_OneToMany.2C_No_Inverse_ManyToOne.2C_No_Join_Table_.28JPA_2.0_ONLY.29
您仍然需要保留Object，然后将其添加到Collection中。