Specifying whether or not to lazily load with Spring Data

Question

I have a lazy fetch type collection in an entity. And I am using Spring Data (JpaRepository) for accessing the entities.

@Entity
public class Parent{
@Id
private Long id;

    @OneToMany(mappedBy = "parentId", fetch = FetchType.LAZY)
    private Set<Child> children;
}

I want two functions in service class and current implementation are as following:

1. "children" should be null when fetching parent

   public Parent getParent(Long parentId){
       return repo.findOne(parentId);
   }
   

2. "children" should be filled when fetching parent:

    public Parent getParentWithChildren(Long parentId){
        Parent p = repo.findOne(parentId);
        Hibernate.initialize(p.children);
        return p;
   }
   

When returning "Parent" entity from a RestController, following exception is thrown:

@RequestMapping("/parent/{parentId}")
public Parent getParent(@PathVariable("parentId") Long id)
{
    Parent p= parentService.getParent(id);//ok till here
    return p;//error thrown when converting to JSON
}

org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: failed to lazily initialize a collection of role: com.entity.Parent.children, could not initialize proxy - no Session (through reference chain: com.entity.Parent["children"]); nested exception is com.fasterxml.jackson.databind.JsonMappingException: failed to lazily initialize a collection of role: com.entity.Parent.children, could not initialize proxy - no Session (through reference chain: com.entity.Parent["children"])

Answer 1

If you are looking to allow for different JSON representations of the same domain model depending on use case, then you can look at the following which will allow you to do so without requiring DTOs:

https://spring.io/blog/2014/12/02/latest-jackson-integration-improvements-in-spring

Alternatively, see also the 'Projections in Spring Data REST' section in the following

https://spring.io/blog/2014/05/21/what-s-new-in-spring-data-dijkstra#projections-in-spring-data-rest