Issue accessing property via hateoas link when lazy loading is active.

[saschakr]

I have following class Note.java

@Entity
public class Note {
    //.. id
    @ManyToOne(fetch = FetchType.LAZY)
    private Author author;
}

When accessing a element via GET /notes/1 I receive follwing response.

{
  "title" : "Note#1",
  "text" : "Content#1",
  "_links" : {
    "self" : {
      "href" : "http://localhost:8080/notes/1"
    },
    "note" : {
      "href" : "http://localhost:8080/notes/1"
    },
    "author" : {
      "href" : "http://localhost:8080/notes/1/author"
    }
  }
}

But when doing GET http://localhost:8080/notes/1/author I get following exception:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class org.hibernate.proxy.pojo.bytebuddy.ByteBuddyInterceptor and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: org.springframework.data.rest.webmvc.json.PersistentEntityJackson2Module$PersistentEntityResourceSerializer$1["content"]->github.skreutz.model.Author$HibernateProxy$XZxIw8Hs["hibernateLazyInitializer"])
	at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77) ~[jackson-databind-2.14.1.jar:2.14.1]

When changing @ManyToOne(fetch = FetchType.LAZY) to @ManyToOne(fetch = FetchType.EAGER) this endpoints works fine.

I created a sample project: https://github.com/saschakr/spring-boot-data-rest-example

[odrotbohm]

Do you use the Jackson Hibernate integration? This should actually take care of this problem.

As a side node: @ManyTo… mappings are a smell as the logically don't have a place in an aggregate model. Either Author is an aggregate root, too. In that case, rather model the property as an identifier, optionally using @Reference(…) to explicitly declare the target aggregate type. The latter shouldn't be needed if you use dedicated identifier types (i.e. we can find out that e.g. AuthorId is the identifier type of Author. If Author is not an aggregate root but a simple entity, then the many-to-relationship doesn't make sense as aggregates own their entities to be able to control invariants.

[jacko9et]

Do you use the Jackson Hibernate integration? This should actually take care of this problem.

As a side node: @ManyTo… mappings are a smell as the logically don't have a place in an aggregate model. Either Author is an aggregate root, too. In that case, rather model the property as an identifier, optionally using @Reference(…) to explicitly declare the target aggregate type. The latter shouldn't be needed if you use dedicated identifier types (i.e. we can find out that e.g. AuthorId is the identifier type of Author. If Author is not an aggregate root but a simple entity, then the many-to-relationship doesn't make sense as aggregates own their entities to be able to control invariants.

In the springboot3.x version, the many-to-one relationship does not have lazy loading. Printing SQL found that a secondary query was triggered, so I tried to explicitly declare lazy loading, but I also encountered this problem. I tried @odrotbohm solution, but currently jackson-datatype-hibernate6 is not released, but jackson-datatype-hibernate5-jakarta seems to work too, this is the solution I found.

        Hibernate5JakartaModule hibernate5JakartaModule = new Hibernate5JakartaModule();
        hibernate5JakartaModule.disable(Hibernate5JakartaModule.Feature.USE_TRANSIENT_ANNOTATION);
        hibernate5JakartaModule.enable(Hibernate5JakartaModule.Feature.FORCE_LAZY_LOADING);

        objectMapper.registerModule(hibernate5JakartaModule);

[odrotbohm]

We actually already register this module with a slightly different configuration, but the lazy loading forcibly enabled in Jackson2DatatypeHelper. So, enabling this should be a matter of adding the Jackson Hibernate integration library on the classpath.

[saschakr]

@jacko9et jackson-datatype-hibernate6 was released with version 2.15.0 (23-Apr-2023)

Thanks for the answers @odrotbohm and @jacko9et

Following works for me:

Add jackson-datatype-hibernate6

<dependency>
    <groupId>com.fasterxml.jackson.datatype</groupId>
    <artifactId>jackson-datatype-hibernate6</artifactId>
    <version>2.15.0</version>
</dependency> 

Register hibernate 6 module

@Autowired
void configureObjectMapper(final ObjectMapper mapper) {
    Hibernate6Module hibernate6Module = new Hibernate6Module();
    hibernate6Module.disable(Hibernate6Module.Feature.USE_TRANSIENT_ANNOTATION);
    hibernate6Module.enable(Hibernate6Module.Feature.FORCE_LAZY_LOADING);
    mapper.registerModule(hibernate6Module);
}

After this the provided test works as expected see commit: saschakr/spring-boot-data-rest-example@91794fe

[odrotbohm]

Oh, that's nice to know. Let me take this ticket to upgrade to the H6 integration as we require that version for Spring Data JPA anyway.