java OneToMany关联集合在Hibernate应用程序中为空

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，它们有许多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

}

我使用以下工具创建一些记录：

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);

然而，在最后一行：emps.remove(emp2);我收到一个NullPointerException，emps集合就是null。我已尝试通过以下方式更改协会的所有者：

 @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

    }

然而，结果是一样的。为什么Employees的Set没有被创建。为了使它工作，必须改变什么

Answer 1

在构造函数中或通过为字段指定初始值，自己初始化集合

实际上，当您从数据库获取对象时，Hibernate会将集合初始化为空集合。但是，当您自己创建对象并将其持久化时，Hibernate不会真正触及您创建的对象，只会持久化它，因此在这种情况下，它不会为您初始化字段

如果你想确保这个字段永远不为空，我同意这是可取的，你只需要自己确保它，甚至在坚持之前

Answer 2

让我们仔细看看你的代码：

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();

您已经创建了Department，三个Employees，为每个Employee设置对该Department的引用，并将它们持久化

Setemps将为空，因为在将员工持久化到数据库后，您没有重新加载department。如果您这样更改代码：

....
session.persist(emp3);
department = session.get(Department.class, department.getDepartmentId());

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

你应该没事的

我想提到的另一点是Department和Employees之间的关系是mappedBy{}，这意味着Department是主实体，它的Employee是“从实体”，或者，如果你愿意，它承担维护映射和引用的所有责任。但是，在我上面评论的代码中，您将这个映射责任放在了Employee实体上。您可能希望以这种方式更改代码：

department.add(emp1);
department.add(emp2);
department.add(emp2);

session.persist(department);

为了让Department完成维护映射和引用的工作