JPA Hibernate Lazy Fetch Attributes with a Query答案

【问题标题】：JPA Hibernate Lazy Fetch Attributes with one QueryJPA Hibernate Lazy Fetch Attributes with a Query
【发布时间】：2021-06-02 14:20:34
【问题描述】：

考虑以下实体：

用户实体：

@Entity(name = "user")
public class UserEntity {

    @Id
    @Column(name = "id", columnDefinition = "serial")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

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

    @OneToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "user_role", joinColumns = @JoinColumn(name = "user_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id"))
    private List<RoleEntity> roles = new ArrayList<>();

}

角色实体：

@Entity(name = "role")
@NamedEntityGraph(
        name = "role.entity_graph",
        attributeNodes = {
                @NamedAttributeNode(value = "permissions")
        })
public class RoleEntity {

    @Id
    @Column(name = "id", columnDefinition = "serial")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

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


    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(name = "role_permission", joinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "permission_id", referencedColumnName = "id"))
    private List<PermissionEntity> permissions = new ArrayList<>();

}

权限实体：

@Entity(name = "permission")
public class PermissionEntity {

    @Id
    @Column(name = "id", columnDefinition = "serial")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

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


}

UserEntity 中的roles 应该是惰性的。问题是当使用roles（调用getter）时，hibernate会通过查询每个角色和查询每个角色的每个权限来获取它们，从而导致n+1问题。

因此我的问题是：我怎样才能通过一个查询延迟获取用户角色？我可以以某种方式利用RoleEntity 的EntityGraph 吗？

注意：我已经尝试过使用@Fetch：

@Fetch(FetchMode.JOIN)
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "role_permission", joinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "permission_id", referencedColumnName = "id"))
private List<PermissionEntity> permissions = new ArrayList<>();

这不起作用。然而，与@BatchSize 一起使用时，它可以正常工作。

@Fetch(FetchMode.JOIN)
@BatchSize(size = 1000)
@ManyToMany(fetch = FetchType.LAZY)
@JoinTable(name = "role_permission", joinColumns = @JoinColumn(name = "role_id", referencedColumnName = "id"), inverseJoinColumns = @JoinColumn(name = "permission_id", referencedColumnName = "id"))
private List<PermissionEntity> permissions = new ArrayList<>();

不过，我对这个解决方案并不满意。为什么要加@BatchSize？

【问题讨论】：

标签： java performance hibernate jpa spring-data-jpa

【解决方案1】：

我认为使用子图可能会对您有所帮助。看看他网站上Thorben Janssen 的描述：Entitygraph with multiple Subgraphs，引用：

@NamedEntityGraph(
    name = "graph.AuthorBooksPublisherEmployee", 
    attributeNodes = @NamedAttributeNode(value = "books", subgraph = "subgraph.book"), 
    subgraphs = {
        @NamedSubgraph(name = "subgraph.book", 
                       attributeNodes = @NamedAttributeNode(value = "publisher", subgraph = "subgraph.publisher")),
        @NamedSubgraph(name = "subgraph.publisher", 
                       attributeNodes = @NamedAttributeNode(value = "employees")) })

【讨论】：