Spring为什么hibernate用FetchType.LAZY获取子实体？

Question

我有一个简单的用例，其中我创建了两个实体Account & AccountDetail。

这些实体具有一对一的关系，如下所示：表关系

Account实体：

@Entity
@Builder
@AllArgsConstructor
@NoArgsConstructor
@Setter
@Getter
@EqualsAndHashCode
public class Account {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String username;

    private String password;

    @OneToOne(mappedBy = "account", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    private AccountDetail accountDetail;

}

AccountDetail实体：

@Entity
@Builder
@AllArgsConstructor
@NoArgsConstructor
@Setter
@Getter
@EqualsAndHashCode
public class AccountDetail {
    @Id
    private Long id;

    @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL) @MapsId
    private Account account;

    private String firstName;

    private String lastName;
}

我使用Spring (v2.3.0)和以下依赖项：- Spring :使用Hibernate - Rest存储库持久化实体:在REST端点上公开存储库

问题是Hibernate正在获取AccountDetail实体，即使我指定了fetch = FetchType.LAZY。

您可以在日志中看到2个sql语句：hibernate日志

我已经尝试过的：

使用投影的：

@Projection(types = {Account.class})
public interface AccountProjection {
    String getUsername();
}

使用下面的GET请求http://localhost:8080/accounts/1?projection=accountProjection，我得到：

   {
      "username": "warrior24",
      "_links": {
        "self": {
          "href": "http://localhost:8080/accounts/1"
        },
        "account": {
          "href": "http://localhost:8080/accounts/1{?projection}",
          "templated": true
        },
        "accountDetail": {
          "href": "http://localhost:8080/accounts/1/accountDetail"
        }
      }
    }

不幸的是，使用此解决方案，它仍然会发出2个sql请求。

使用Hibernate字节码增强的@LazyToOne(LazyToOneOption.NO_PROXY)

@OneToOne(mappedBy = "account", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
@LazyToOne(LazyToOneOption.NO_PROXY)
private AccountDetail accountDetail;

即使这个解决方案也不能解决这个问题。

结论

我不知道为什么hibernate要抓走孩子。目前，这并不是一个大问题，但是如果数据库记录的数量越来越多，它可能会影响性能。

有没有人有类似的问题，也许知道如何解决？

更新:包括GET请求的调试日志

2020-05-27 15:23:58.414 INFO 2119 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat-3].[localhost].[/] : Initializing Spring DispatcherServlet 'dispatcherServlet' 2020-05-27 15:23:58.414 INFO 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Initializing Servlet 'dispatcherServlet' 2020-05-27 15:23:58.414 DEBUG 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Detected StandardServletMultipartResolver 2020-05-27 15:23:58.415 DEBUG 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : enableLoggingRequestDetails='false': request parameters and headers will be masked to prevent unsafe logging of potentially sensitive data 2020-05-27 15:23:58.415 INFO 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Completed initialization in 1 ms 2020-05-27 15:23:58.415 DEBUG 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : GET "/accounts/1", parameters={} 2020-05-27 15:23:58.416 DEBUG 2119 --- [nio-8080-exec-1] o.s.d.r.w.RepositoryRestHandlerMapping : Mapped to org.springframework.data.rest.webmvc.RepositoryEntityController#getItemResource(RootResourceInformation, Serializable, PersistentEntityResourceAssembler, HttpHeaders) 2020-05-27 15:23:58.416 DEBUG 2119 --- [nio-8080-exec-1] o.j.s.OpenEntityManagerInViewInterceptor : Opening JPA EntityManager in OpenEntityManagerInViewInterceptor 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] stomAnnotationTransactionAttributeSource : Adding transactional method 'findById' with attribute: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Found thread-bound EntityManager [SessionImpl(1137877934<open>)] for JPA transaction 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Creating new transaction with name [org.springframework.data.jpa.repository.support.SimpleJpaRepository.findById]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT,readOnly 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] o.s.jdbc.datasource.DataSourceUtils : Setting JDBC Connection [HikariProxyConnection@646683728 wrapping conn193: url=jdbc:h2:mem:f0df200a-97ea-4e5b-9f80-050ff9a550f7 user=SA] read-only 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Exposing JPA transaction as JDBC [org.springframework.orm.jpa.vendor.HibernateJpaDialect$HibernateConnectionHandle@3a65c273] 2020-05-27 15:23:58.417 DEBUG 2119 --- [nio-8080-exec-1] org.hibernate.SQL : select account0_.id as id1_0_0_, account0_.password as password2_0_0_, account0_.username as username3_0_0_ from account account0_ where account0_.id=? 2020-05-27 15:23:58.417 TRACE 2119 --- [nio-8080-exec-1] o.h.type.descriptor.sql.BasicBinder : binding parameter [1] as [BIGINT] - [1] 2020-05-27 15:23:58.418 DEBUG 2119 --- [nio-8080-exec-1] org.hibernate.SQL : select accountdet0_.account_id as account_3_1_0_, accountdet0_.first_name as first_na1_1_0_, accountdet0_.last_name as last_nam2_1_0_ from account_detail accountdet0_ where accountdet0_.account_id=? 2020-05-27 15:23:58.418 TRACE 2119 --- [nio-8080-exec-1] o.h.type.descriptor.sql.BasicBinder : binding parameter [1] as [BIGINT] - [1] 2020-05-27 15:23:58.418 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Initiating transaction commit 2020-05-27 15:23:58.418 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Committing JPA transaction on EntityManager [SessionImpl(1137877934<open>)] 2020-05-27 15:23:58.418 DEBUG 2119 --- [nio-8080-exec-1] o.s.jdbc.datasource.DataSourceUtils : Resetting read-only flag of JDBC Connection [HikariProxyConnection@646683728 wrapping conn193: url=jdbc:h2:mem:f0df200a-97ea-4e5b-9f80-050ff9a550f7 user=SA] 2020-05-27 15:23:58.418 DEBUG 2119 --- [nio-8080-exec-1] o.s.orm.jpa.JpaTransactionManager : Not closing pre-bound JPA EntityManager after transaction 2020-05-27 15:23:58.421 DEBUG 2119 --- [nio-8080-exec-1] o.s.w.s.m.m.a.HttpEntityMethodProcessor : Using 'application/hal+json', given [*/*] and supported [application/hal+json] 2020-05-27 15:23:58.421 DEBUG 2119 --- [nio-8080-exec-1] o.s.w.s.m.m.a.HttpEntityMethodProcessor : Writing [Resource { content: Account(id=1, username=warrior24, password=1234), links: [<http://localhost:8080 (truncated)...] 2020-05-27 15:23:58.421 DEBUG 2119 --- [nio-8080-exec-1] s.d.r.w.j.PersistentEntityJackson2Module : Serializing PersistentEntity org.springframework.data.jpa.mapping.JpaPersistentEntityImpl@45b7be72. 2020-05-27 15:23:58.423 DEBUG 2119 --- [nio-8080-exec-1] .s.ReloadableResourceBundleMessageSource : No properties file found for [classpath:rest-messages] - neither plain properties nor XML 2020-05-27 15:23:58.424 DEBUG 2119 --- [nio-8080-exec-1] .s.ReloadableResourceBundleMessageSource : No properties file found for [classpath:rest-messages_en] - neither plain properties nor XML 2020-05-27 15:23:58.424 DEBUG 2119 --- [nio-8080-exec-1] .s.ReloadableResourceBundleMessageSource : No properties file found for [classpath:rest-messages_en_GB] - neither plain properties nor XML 2020-05-27 15:23:58.426 DEBUG 2119 --- [nio-8080-exec-1] o.j.s.OpenEntityManagerInViewInterceptor : Closing JPA EntityManager in OpenEntityManagerInViewInterceptor 2020-05-27 15:23:58.426 DEBUG 2119 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet : Completed 200 OK

Answer 1

解决方案(某种)

好的，我想我找到了解决方案，但不幸的是，它与Spring数据Rest不兼容。

通过添加optional = false，您可以告诉hibernate，您将确保子引用永远不会为空(在我的例子中是AccountDetail)。

因此，为了检查是否必须创建代理或分配NULL，它将不需要获取子对象。

@OneToOne(mappedBy = "account", fetch = FetchType.LAZY, cascade = CascadeType.ALL, optional = false)
@LazyToOne(LazyToOneOption.NO_PROXY)
private AccountDetail accountDetail;

此解决方案的问题是，您不能使用Account注释提供的POST端点创建新的@RepositoryRestResource资源。因为现在它希望AccountDetail不是null，所以您需要在JSON中提供这个资源的链接(这个链接还不存在)。而且您不能创建AccountDetail资源，因为它使用帐户表中的外键作为主键。

Answer 2

尝试使用@JsonIgnore注释，该注释指示基于内省的序列化和反序列化功能将忽略带注释的方法或字段。

@JsonIgnore
@OneToOne(mappedBy = "account", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    private AccountDetail accountDetail;

Answer 3

尝试将@EqualsAndHashCode.Exclude注释从equals和hashCode方法中排除出来

@EqualsAndHashCode.Exclude    
 @OneToOne(mappedBy = "account", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
                private AccountDetail accountDetail;