Attribute Lazy Loading And Jackson Serialization
Description: By default, the attributes of an entity are loaded eager (all at once). We can load them lazy 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 avatar and age lazy. But, returning entities (as JSON) that contains un-fetched lazy loaded attributes from a rest controller (@RestController) will cause lazy initialization exceptions because Jackson tries to "force" the fetching of these attributes outside a session.
Key points:
- in pom.xml, activate Hibernate bytecode instrumentation (e.g. use Maven bytecode enhancement plugin)
- annotate the columns that should be loaded lazy with @Basic(fetch = FetchType.LAZY)
- annotate the Author entity with @JsonFilter("AuthorId")
- create and configure this filter to be used by default via SimpleBeanPropertyFilter.serializeAll()
- at controller level (in the needed endpoint) replace the default filter with one as SimpleBeanPropertyFilter.filterOutAllExcept("id", "name", "genre") and return MappingJacksonValue
Run the following requests (via BookstoreController):
- create several authors: localhost:8080/create
- fetch an author by id: localhost:8080/author/{id}
- fetch authors by age greater than or equal to the given age without age and avatar: localhost:8080/authors/{age}
- fetch authors by age greater than or equal to the given age with age and avatar (but, don't do this, notice the N+1 issue caused by looping the list of authors and triggering SQLs for fetching age and avatar of each author): localhost:8080/authors/details/{age}
Check as well:
- Attribute Lazy Loading (basic)
- Conditionally Loadind Lazy Attributes
