Many to many json request

A Contract has several AbsenceType, and an AbsentType can be in several different Contracts. So I made a manyToMany relation in both classes.

Entity
@Table(name = "contract")
@JsonIdentityInfo(  
    generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "id")

@EntityListeners(AuditingEntityListener.class)
public class Contract {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private int id;

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

    @Column(name = "employee")
    private int employee;


    @ManyToMany(mappedBy = "contracts")
    private List<AbsenceType> absence_types;

    // ... getteur setteur contructor

@Entity
@Table(name = "absence_type")
@EntityListeners(AuditingEntityListener.class)
@JsonIdentityInfo(  
    generator = ObjectIdGenerators.PropertyGenerator.class,
    property = "id")
public class AbsenceType {

    @Id
    @GeneratedValue
    @Column(name = "id")
    private int id;

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

    // hexa : #FFFFFF
    @Column(name = "color")
    private String color;

    
    @Column(name = "is_paid")
    private boolean is_paid;

    @ManyToMany
    @Column(name = "contracts")
    private List<Contract> contracts;
 // ... getteur setteur contructor

I want to be able to create empty Absence types and then when I create a contract, give in my json, the previously created absenceType and not create a new AbsenceType.

In the idea I do this:

{
    "id": 1 # Exemple, in real in don't had id on json
    "name": "Congé sans solde",
    "color": "ff4040",
    "is_paid": false,
    "contracts": []
}

and after ->

{
    "name": "Contract4",
    "employee": 3,
    "absence_types" : [
        {
            "id":1
        }
    ]
}

But the response when i get all contracts is:

    {
        "id": 621,
        "name": "Contract4",
        "employee": 3,
        "absenceTypes": []
    }

But i want :

    {
        "id": 621,
        "name": "Contract4",
        "employee": 3,
        "absenceTypes": [
        {
             "id": 1,
             "name": "Congé sans solde",
             "color": "ff4040",
             "contracts" : [ dudo, i don't think about infinite recursion problem for the moment haha]
        }
    }

For all of my DAO i have a generic class, how look like this

public void save(T obj) {
        Session session = openSession();
        Transaction transaction = session.beginTransaction();
        session.saveOrUpdate(obj);
        transaction.commit();
        session.close();
    }

and on my Contract controller

@PostMapping("")
   public ResponseEntity<String> create(@RequestBody Contract contract) {
       // Error 422 if the input role variable is null
       if (contract == null)
           new ResponseEntity<String>("Can't create null absenceType", HttpStatus.UNPROCESSABLE_ENTITY);
       contractDAO.save(contract);
       return new ResponseEntity<>("absenceType saved !", HttpStatus.OK);
   }