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?

• dambros: 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.

Answer 1

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.