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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;
}

至于我，他们两个都有类似的结果，但第二种情况更清洁。如果我错误地纠正了我。

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，如果您希望代码可以在各种JPA提供程序中移植，则应使用JPA注释。

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要在这两个注释之间进行说明，请考虑OneToMany

之间的Department -> Employee关系

案例1：

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

如果您使用以下命令从数据库中获取Department对象：

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_,
  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=?

所以这意味着它将同时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=?

总而言之，在第一种情况下，FetchType EAGER Employees在Department个查询中与JOIN一起热切地提取。

和

在第二种情况下，使用Employees获取Department，但由于FetchType为LAZY，因此会触发单独的查询以获取Employees。如果您移除@LazyCollection(LazyCollectionOption.FALSE) Employees，则在访问Employees个实例上的Department之前，根本不会提取{。}}。

@OneToMany（EAGER）vs @LazyCollection（FALSE）＆amp; OneToMany（LAZY）

1 个答案:

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