Question

我试图在Hibernate中使用FetchMode.JOIN来理解但面临某些问题。我有3个班级，员工和员工主类。部门与员工有一个关系。

以下是代码部门类： -

@Entity
@Table(name="DEPARTMENT")
public class Department {

@Id
@GeneratedValue
@Column(name="DEPARTMENT_ID")
private Long departmentId;

@Column(name="DEPT_NAME")
private String departmentName;

@OneToMany(mappedBy="department")
@Fetch(FetchMode.JOIN)
private List<Employee> employees = new ArrayList<>();

public Long getDepartmentId() {
    return departmentId;
}

public void setDepartmentId(Long departmentId) {
    this.departmentId = departmentId;
}

public String getDepartmentName() {
    return departmentName;
}

public void setDepartmentName(String departmentName) {
    this.departmentName = departmentName;
}

public List<Employee> getEmployees() {
    return employees;
}

public void setEmployees(List<Employee> employees) {
    this.employees = employees;
}

}

员工类： -

@Entity
@Table(name="EMPLOYEE")
public class Employee {
@Id
@GeneratedValue
@Column(name="employee_id")
private Long employeeId;

@Column(name="firstname")
private String firstname;

@Column(name="lastname")
private String lastname;

@Column(name="birth_date")
private Date birthDate;

@Column(name="cell_phone")
private String cellphone;

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

 public Employee() {

    }

    public Department getDepartment() {
    return department;
}

public void setDepartment(Department department) {
    this.department = department;
}

    public Employee(String firstname, String lastname, String phone) {
        this.firstname = firstname;
        this.lastname = lastname;
        this.birthDate = new Date(System.currentTimeMillis());
        this.cellphone = phone;
    }


public Long getEmployeeId() {
    return employeeId;
}

public void setEmployeeId(Long employeeId) {
    this.employeeId = employeeId;
}

public String getFirstname() {
    return firstname;
}

public void setFirstname(String firstname) {
    this.firstname = firstname;
}

public String getLastname() {
    return lastname;
}

public void setLastname(String lastname) {
    this.lastname = lastname;
}

public Date getBirthDate() {
    return birthDate;
}

public void setBirthDate(Date birthDate) {
    this.birthDate = birthDate;
}

public String getCellphone() {
    return cellphone;
}

public void setCellphone(String cellphone) {
    this.cellphone = cellphone;
}

}

主类： -

public class EmployeeMain {
public static void main(String[] args) {
    SessionFactory sessionFactory = new     Configuration().configure("hibernate.xml").buildSessionFactory();
    Session session = sessionFactory.openSession();
    session.beginTransaction();
    Department department1 = new Department();
    department1.setDepartmentName("Sales");
    session.save(department1);
    Department department2 = new Department();
    department2.setDepartmentName("Operations");
    session.save(department2);


    Employee emp1 = new Employee("Nina", "Mayers", "111");
    Employee emp2 = new Employee("Tony", "Almeida", "222");

    Employee emp3 = new Employee("Rina", "Coyeet", "333");
    Employee emp4 = new Employee("Mary", "Land", "444");

    emp1.setDepartment(department1);
    emp2.setDepartment(department1);

    emp3.setDepartment(department2);
    emp4.setDepartment(department2);

    department1.setEmployees(new ArrayList<Employee>());
    department1.getEmployees().add(emp1);
    department1.getEmployees().add(emp2);

    department2.getEmployees().add(emp3);
    department2.getEmployees().add(emp4);

    session.save(emp1);
    session.save(emp2);

    session.save(emp3);
    session.save(emp4);
    session.getTransaction().commit();
    session.close();

    session = sessionFactory.openSession();
    session.beginTransaction();

    List<Department> departmentList = session.createQuery("from Department").list();
    for(Department department : departmentList){
        department.getEmployees();
    }

}

}

我在每个部门下插入了2个部门和2个员工。尽管在Department类中我已经为employees对象提到了@Fetch（FetchMode.JOIN），但是对于每个部门，Employees的查询仍然运行两次。基本上如果我正确提到的是N + 1选择问题。但据我说，如果FetchMode是Join，那么单个查询就会被解雇加入Employee和department，并且应该已经获取了数据。但是以下查询被解雇了。查询1

Hibernate: select department0_.DEPARTMENT_ID as DEPARTME1_0_, department0_.DEPT_NAME as DEPT_NAM2_0_ from DEPARTMENT department0_

查询2

Hibernate: select employees0_.department_id as departme6_0_0_, employees0_.employee_id as employee1_1_0_, employees0_.employee_id as employee1_1_1_, employees0_.birth_date as birth_da2_1_1_, employees0_.cell_phone as cell_pho3_1_1_, employees0_.department_id as departme6_1_1_, employees0_.firstname as firstnam4_1_1_, employees0_.lastname as lastname5_1_1_ from EMPLOYEE employees0_ where employees0_.department_id=?

查询3

Hibernate: select employees0_.department_id as departme6_0_0_, employees0_.employee_id as employee1_1_0_, employees0_.employee_id as employee1_1_1_, employees0_.birth_date as birth_da2_1_1_, employees0_.cell_phone as cell_pho3_1_1_, employees0_.department_id as departme6_1_1_, employees0_.firstname as firstnam4_1_1_, employees0_.lastname as lastname5_1_1_ from EMPLOYEE employees0_ where employees0_.department_id=?

Answer 1

HQL查询不会根据注释获取关联。要实现此目标，您需要在HQL查询中指定获取策略：

from Department d inner join fetch d.employees

Hibernate Fetch Mode Join不工作

1 个答案: