Tag: JSON

Good morning all! I am new using Spring Boot and I have encountered a problem in building one-to-many relationships. I've made an example to show you what's going on.

I have two entities (person and pet). A person can have multiple pets and a pet can only be one person.

@Entity
@Table(name = "persona")
public class Persona {


    @Id
    @Column(name = "cedula")
    private Integer cedula;

    @Column(name = "nombre")
    private String nombre;


    @OneToMany(mappedBy = "dueno", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    private Set mascotas = new HashSet();
}

@Entity
@Table(name = "mascota")
public class Mascota {


    @Id
    @Column(name = "serial")
    private Integer serial;

    @Column(name = "nombre")
    private String nombre;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumns (foreignKey = @ForeignKey(name = "fk_persona"), 
    value = { @JoinColumn(name = "dueno", referencedColumnName = "cedula")})
    @JsonBackReference
    private Persona dueno;
}

I'm using FetchType.LAZY. There is a search strategy that tells the JPA provider not to search for the associated association. @OneToMany comes default with this search strategy while @ManyToOne with FetchType.EAGER. Running my program in this way returns me the error:

Could not write JSON: No serializer found for class org.hibernate.proxy.pojo.javassist.JavassistLazyInitializer

To solve this problem, I tried to use the following items:

1- @JsonIdentityInfo
2- @JsonManagedReference, @JsonBackReference
3- @JsonIgnore

2 and 3 being the ones that worked for me to handle serialization in a better way. Although then he gave me another mistake:

JsonMappingException: could not initialize proxy - no Session

I looked tired of solving this problem and the only thing that worked for me was to create an entity graph, but that did not solve the problem of infinite recursion created. I kept looking and at the end, I used the entity graph with a bidirectional relationship manager. The code was as follows:

@Entity
@Table(name = "persona")
@NamedEntityGraph(name = "persona.mascota", attributeNodes = {
        @NamedAttributeNode(value = "mascotas")
})
public class Persona {


    @Id
    @Column(name = "cedula")
    private Integer cedula;

    @Column(name = "nombre")
    private String nombre;


    @OneToMany(mappedBy = "dueno", cascade = CascadeType.ALL, fetch = FetchType.LAZY)
    @JsonManagedReference
    private Set mascotas = new HashSet();

/// Getters and Setters
}

@Entity
@Table(name = "mascota")
public class Mascota {


    @Id
    @Column(name = "serial")
    private Integer serial;

    @Column(name = "nombre")
    private String nombre;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumns (foreignKey = @ForeignKey(name = "fk_persona"), 
    value = { @JoinColumn(name = "dueno", referencedColumnName = "cedula")})
    @JsonBackReference
    private Persona dueno;

/// Getters and Setters
}

It is the deposit of Persona

@Transactional
@RepositoryRestResource(path="/personaRepo")
public interface PersonaRepository extends CrudRepository{

    @EntityGraph(value = "persona.mascota", type = EntityGraphType.FETCH)
    Optional findById(Integer id);

    @EntityGraph(value = "persona.mascota", type = EntityGraphType.FETCH)
    List findAll();
}

These are the results:

The people:

If anyone knows how to solve this problem more effectively, I would be very grateful if you could help me.