Attribute Lazy Loading And Jackson Serialization

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Description: By default, the attributes of an entity are loaded eagerly (all at once). But, we can load them lazily as well. This is useful for column types that store large amounts of data: CLOB, BLOB, VARBINARY, etc or details that should be loaded on demand. In this application, we have an entity named Author. Its properties are: id, name, genre, avatar and age. And, we want to load the avatar lazy. But, returning entities (as JSON) that contains un-fetched lazy attributes from a REST controller (@RestController) will cause lazy initialization exceptions because Jackson tries to force the fetching of these attributes outside a Hibernate session.

Key points:

• for Maven, in pom.xml, activate Hibernate bytecode enhancement (e.g. use Maven bytecode enhancement plugin)
• in entity, annotate the attributes that should be loaded lazy with @Basic(fetch = FetchType.LAZY)
• in application.properties, disable Open Session in View
• annotate the Author entity with @JsonFilter("AuthorId")
• create and configure the AuthorId filter to be used by default via SimpleBeanPropertyFilter.serializeAll()
• at controller level (in the needed endpoint) override the AuthorId filter with one as SimpleBeanPropertyFilter.filterOutAllExcept("id", "name", "age", "genre") and return MappingJacksonValue
• a controller endpoint that need to use the overriden version of AuthorId filter should rely on MappingJacksonValue

Run the following requests (via BookstoreController):

• lazy fetch the avatar of an author by id: localhost:8080/author/avatar/{id}
• fetch authors by age greater than or equal to the given age without avatar: localhost:8080/authors/{age}
• fetch authors by age greater than or equal to the given age with and avatar (but, don't do this, notice the N+1 issue caused by looping the list of authors and triggering SQLs for fetching the avatar of each author): localhost:8080/authors/details/{age}

Check as well:

• Attribute Lazy Loading (basic)
• Default Values For Lazy Loaded Attributes

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