对象映射关系分为单向关系和双向关系,单向关系只在一方对象上存在对方对象,双向关系是在双方对象上存在彼此对象。
一、单向关系
public class Department implements Serializable { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer departmentId; private String departmentCode; @OneToMany(cascade = {CascadeType.ALL}) @JoinColumn private List<Employee> employeeList;}
在没有@JoinColumn时,将多增加一个中间关系表,由此表来维护两个对象关系,增加之后只有两个表,由Employee表维护关系。默认情况下jpa会使用主键来做关联,并在子表中增加外键约束。
二、单向关系使用code关联
在设计表结构时,主键一般会使用自增ID,但在做子表关联时由于分布式结构原因不想使用自增ID来做关系维护,则可自定义字段code来维护关系,如下:
public class Department implements Serializable { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer departmentId; private String departmentCode; @OneToMany(cascade = {CascadeType.ALL}) @JoinColumn(name = "departmentCode", referencedColumnName = "departmentCode") private List<Employee> employeeList;}
在employee表中会增加一个department_code字段来维护关联关系。注意:从add操作中的SQL可以看出,employee是先insert之后,再去update关系字段的,多一步update。
Hibernate: insert into department (department_code, department_name) values (?, ?)Hibernate: insert into employee (employee_code, employee_name) values (?, ?)Hibernate: insert into employee (employee_code, employee_name) values (?, ?)Hibernate: update employee set department_code=? where employee_id=?Hibernate: update employee set department_code=? where employee_id=?
三、双向关系
public class Department implements Serializable { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer departmentId; private String departmentCode; @OneToMany(cascade = {CascadeType.ALL}, mappedBy = "department") private List<Employee> employeeList;}public class Employee { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Integer employeeId; private String employeeCode; @ManyToOne private Department department;}
使用mappedBy将去掉中间关系表,由employee维护department的关系。
注意事项:1.在add操作中子对象employee必须设置主对象,否则数据库层面是没有维护外键关系的,如:
@Testpublic void add(){ Department department = Department.builder() .departmentCode("D001") .departmentName("部门1") .build(); Employee employee = Employee.builder() .employeeCode("E001") .employeeName("员工1") .build(); // 如无此操作,将无关联关系 employee.setDepartment(department); List<Employee> employeeList = new ArrayList<>(); employeeList.add(employee); department.setEmployeeList(employeeList); departmentRepository.save(department);}
2.此种设置是不需要额外update关系的
Hibernate: insert into department (department_code, department_name) values (?, ?)Hibernate: insert into employee (department_department_id, employee_code, employee_name) values (?, ?, ?)Hibernate: insert into employee (department_department_id, employee_code, employee_name) values (?, ?, ?)
3.关闭子对象中的父对象toString,避免无限循环调用
四、双向关系使用code关联
同单向关系类似,且有额外的update操作code关联,使用JoinColumn时不可同时使用mappedBy
五、级联操作设置
- CascadeType.PERSIST:级联保存,在保存department的同时保存employee对象
- CascadeType.MERGE:级联更新,将department和employee视为一个整体,任何一个对象有变化,都会更新
- CascadeType.REMOVE:级联删除当没有设置时,delete主对象时,子对象只是去掉关系;remove子对象时也只是去掉关系,如果增加orphanRemoval = true则会删除remove的子对象当有设置时,delete主对象时,子对象同样会被删除
- CascadeType.REFRESH:级联刷新(较少使用),在并发的场景下避免脏数据
- CascadeType.DETACH:级联脱管(较少使用)
- CascadeType.ALL:以上全部,需要根据实际情况谨慎设置,以免产生混乱
面试资料可私聊我,Java,资料,面试