java - @OneToMany(EAGER) 与@LazyCollection(FALSE)&OneToMany(LAZY)

Question

代码之间有什么区别？

@LazyCollection(LazyCollectionOption.FALSE)
@OneToMany(mappedBy = "siteesTypeSite", cascade = CascadeType.ALL,fetch = FetchType.LAZY)
public Set<Sites> getSitees() {
    return sitees;
}

和

@OneToMany(mappedBy = "siteesTypeSite", cascade = CascadeType.ALL,fetch = FetchType.EAGER)
public Set<Sites> getSitees() {
    return sitees;
}

对我来说，他们两个都得到了相似的结果，但第二种情况更清晰。如果我记错了请指正。

Answer 1

注释之间的主要区别在于 @OneToMany 是纯 JPA 注释。而 @LazyCollection 是特定于 Hibernate 的。

因此，如果您希望您的代码可以跨各种 JPA 提供程序移植，您应该使用 JPA 注释。

更新

为了解释这两个注释之间的关系，请考虑 Department -> Employee 之间的 OneToMany 关系

案例 1:

@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval = true)
@JoinColumn(name = "DEPARTMENT_ID")
private List<Employee> employees = new ArrayList<>();

如果您使用以下方法从数据库中获取一个 Department 对象:

entityManager.find(Department.class, 1L);

触发以下查询以获取数据

SELECT department0_.DEPARTMENT_ID AS DEPARTMENT_ID1_0_0_,
  department0_.DEPARTMENT_NAME    AS DEPARTMENT_NAME2_0_0_,
  department0_.LOCATION           AS LOCATION3_0_0_,
  employees1_.DEPARTMENT_ID       AS DEPARTMENT_ID3_1_1_,
  employees1_.EMPLOYEE_ID         AS EMPLOYEE_ID1_1_1_,
  employees1_.EMPLOYEE_ID         AS EMPLOYEE_ID1_1_2_,
  employees1_.DEPARTMENT_ID       AS DEPARTMENT_ID3_1_2_,
  employees1_.EMPLOYEE_NAME       AS EMPLOYEE_NAME2_1_2_
FROM DEPARTMENT department0_
LEFT OUTER JOIN EMPLOYEE employees1_
ON department0_.DEPARTMENT_ID   =employees1_.DEPARTMENT_ID
WHERE department0_.DEPARTMENT_ID=?

所以这意味着它将一次获取单个查询中的所有数据。

案例 2:

@LazyCollection(LazyCollectionOption.FALSE)
@OneToMany(cascade = CascadeType.ALL,fetch = FetchType.LAZY, orphanRemoval = true)
@JoinColumn(name = "DEPARTMENT_ID")
private List<Employee> employees = new ArrayList<>();

如果您使用以下方法从数据库中获取 Department 对象，则类似:

entityManager.find(Department.class, 1L);

以下查询被触发以获取数据:

SELECT department0_.DEPARTMENT_ID AS DEPARTMENT_ID1_0_0_,
  department0_.DEPARTMENT_NAME    AS DEPARTMENT_NAME2_0_0_,
  department0_.LOCATION           AS LOCATION3_0_0_
FROM DEPARTMENT department0_
WHERE department0_.DEPARTMENT_ID=?

SELECT employees0_.DEPARTMENT_ID AS DEPARTMENT_ID3_1_0_,
  employees0_.EMPLOYEE_ID        AS EMPLOYEE_ID1_1_0_,
  employees0_.EMPLOYEE_ID        AS EMPLOYEE_ID1_1_1_,
  employees0_.DEPARTMENT_ID      AS DEPARTMENT_ID3_1_1_,
  employees0_.EMPLOYEE_NAME      AS EMPLOYEE_NAME2_1_1_
FROM EMPLOYEE employees0_
WHERE employees0_.DEPARTMENT_ID=?

总结一下，在第一种情况下，因为 FetchType 是 EAGER Employees 与 Department 一起被急切地获取在单个 JOIN 查询中。

还有，

在第二种情况下，Employees 是通过 Department 获取的，但是由于 FetchType 是 LAZY，因此单独的查询将被解雇以获取 Employees。如果你删除 @LazyCollection(LazyCollectionOption.FALSE) Employees 根本不会被获取，直到你访问 Department 上的 Employees > 实例。