retrieving data from database as JSON in spring boot

Question

I have a MySQL database and I want to retrieve some data as JSON.

And I have an entity Offre which has @OneToMany relation with the AssociationCandidatOffre entity.

and I have an API which calls this method in my repository:

offreRepository.findAll();

#Offre entity:

@Entity
public class Offre implements Serializable {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "CODE_OFFRE")
    private Long codeOffre;
    private String titre;

    @OneToMany(mappedBy = "offre")
    private Collection<AssociationCandidatOffre> associationCandidatOffres;
    
    public Collection<AssociationCandidatOffre> getAssociationCandidatOffres() {
        return associationCandidatOffres;
    }

    public void setAssociationCandidatOffres(Collection<AssociationCandidatOffre> associationCandidatOffres) {
        this.associationCandidatOffres = associationCandidatOffres;
 

    }
     //... getters/setters      
    }

#AssociationCandidatOffre entity:

@Entity
public class AssociationCandidatOffre implements Serializable {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long idAssociation;
    private String lettreMotivation;
    private String tarifJournalier;
    private Date dateDisponibilite;

    @ManyToOne
    private Candidat candidat;

    @ManyToOne
    private Offre offre;
    @JsonIgnore
    @XmlTransient
    public Candidat getCandidat() {
        return candidat;
    }
    @JsonSetter
    public void setCandidat(Candidat candidat) {
        this.candidat = candidat;
    }
    @JsonIgnore
    @XmlTransient
    public Offre getOffre() {
        return offre;
    }
    @JsonSetter
    public void setOffre(Offre offre) {
        this.offre = offre;
    }

    //... getters/setters
}

the problem is when I call the API /offres to return me a JSON object I get this error message instead:

    Failed to write HTTP message: org.springframework.http.converter.HttpMessageNotWritableException: Could not write content: could not extract ResultSet (through reference chain: java.util.ArrayList[0]->com.***.Rekrute.entities.Offre["associationCandidatOffres"]); 
nested exception is com.fasterxml.jackson.databind.JsonMappingException: could not extract ResultSet (through reference chain: java.util.ArrayList[0]->com.***.Rekrute.entities.Offre["associationCandidatOffres"])

when I use @JsonIgnore in the getAssocationCandidatOffres I don't get any errors but I want that association in the JSON result as well.

Normally, this shouldn't generate any error since I have @JsonIgnore in the other side of the relation which is getOffre().

How can I solve this problem?

Are you sure the list getAssocationCandidatOffres is populated? Keep in mind that IDEs in debug mode will usually run a query to get any lazy loading list in background when you expand the list. — dambros
– dambros, Commented Mar 31, 2016 at 10:56

Soumitri Pattnaik · Accepted Answer · 2017-06-13 10:10:40Z

2

You can't convert a bidirectional relation of an enitity to JSON. You get an endless loop.

JSON-Parser starts with the entity Offer and reads the associated AssociationCandidatOffre via getAssociationCandidatOffres(). For every AssociationCandidatOffre the JSON-Parser read getOffre() and starts again. The parser don't know when he must end.

edited Jun 13, 2017 at 10:10

Soumitri Pattnaik

3,5966 gold badges28 silver badges47 bronze badges

answered Mar 31, 2016 at 12:54

UKoehler

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