java - Hibernate 应用程序中 OneToMany 关联集合为 null

Question

我正在开发一个简单的 Hibernate 应用程序来测试 OneToMany 关联。我使用的实体是Employee和Department，其中有许多Employees:

@Entity
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long departmentId;

    @OneToMany(cascade=CascadeType.ALL, fetch = FetchType.EAGER, mappedBy="department")
    private Set<Employee> employees;

...
getters/setters

}

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long employeeId;

    @ManyToOne
    @JoinColumn(name="employee_fk")
    private Department department;

...
getters/setters

}

我创建一些记录:

tx.begin();

        Department department = new Department();
        department.setDepartmentName("Sales");
        session.persist(department);

        Employee emp1 = new Employee("Ar", "Mu", "111");
        Employee emp2 = new Employee("Tony", "Almeida", "222");
        Employee emp3 = new Employee("Va", "Ka", "333");

        emp1.setDepartment(department);
        emp2.setDepartment(department);
        emp3.setDepartment(department);


        session.persist(emp1);
        session.persist(emp2);
        session.persist(emp3);

        Set<Employee> emps = department.getEmployees();
        emps.remove(emp2);

但是在最后一行: emps.remove(emp2); 我收到一个 NullPointerException，emps 集合即 空。我尝试通过以下方式更改关联的所有者:

 @Entity
    public class Department {

        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long departmentId;

        @OneToMany(cascade=CascadeType.ALL, fetch = FetchType.EAGER)
        @JoinColumn(name="department_fk")
        private Set<Employee> employees;

    ...
    getters/setters

    }

    @Entity
    public class Employee {

        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private Long employeeId;

        @ManyToOne
        @JoinColumn(name="department_fk", insertable=false, updatable=false)
        private Department department;

    ...
    getters/setters

    }

但是结果是一样的。为什么Employees的Set没有创建。为了使其发挥作用，必须改变什么？

Answer 1

自己初始化集合，可以在构造函数中或通过指定字段的初始值。

事实上，当您从数据库中获取对象时，Hibernate 会将集合初始化为空集合。但是，当您自己创建对象并持久化它时，Hibernate 不会真正接触您创建的对象，而只会持久化它，因此在这种情况下它不会为您初始化该字段。

如果你想确保该字段永远不为空(我同意这是可取的)，只需自己确保它，甚至在坚持之前。