java.lang.StackOverflowError: null at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields

Question

I'm trying to create an app that allows to track the couriers, using Sptring boot. But here I got struggled with some problem. When I try to call get_couriers function, it throws me this exception:

java.lang.StackOverflowError: null
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:107) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serialize(CollectionSerializer.java:25) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:728) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:774) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:178) ~[jackson-databind-2.13.3.jar:2.13.3]
    at com.fasterxml.jackson.databind.ser.std.CollectionSerializer.serializeContents(CollectionSerializer.java:145) ~[jackson-databind-2.13.3.jar:2.13.3]
    at 
    at 

I tryied to add @JsonIgnore to entities' fields, but nothing was happened;

Well, here's my code of entities:

UserEntity

@Table(name = "users")
@Entity
public class UserEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String username;
    private String name;
    private String surname;
    private String email;
    private String password;

    @ManyToMany(fetch = FetchType.EAGER)
    @JsonIgnore
    private List<RoleEntity> roles;

    @ManyToOne
    @JoinColumn(name = "user_type_id")
    @JsonIgnore
    private UserTypeEntity userType;

    @ManyToMany(cascade = CascadeType.ALL)
    @JoinTable(
            name = "user_regions",
            joinColumns = @JoinColumn(name = "users_id"),
            inverseJoinColumns = @JoinColumn(name = "regions_id")
    )
    @JsonIgnore
    private List<RegionEntity> regions;

    public UserEntity() {
    }

    public void addRegion(RegionEntity region) {
        regions.add(region);
    }

    public void addRoleToUserRoles(RoleEntity role) {
        roles.add(role);
    }

    public long getId() {
        return id;
    }

    public String getUsername() {
        return username;
    }



    public void setUsername(String username) {
        this.username = username;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
    public UserTypeEntity getUserType() {
        return userType;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSurname() {
        return surname;
    }

    public void setSurname(String surname) {
        this.surname = surname;
    }

    public List<RoleEntity> getRoles() {
        return roles;
    }

    public void setRoles(List<RoleEntity> roles) {
        this.roles = roles;
    }

    public void setUserType(UserTypeEntity userType) {
        this.userType = userType;
    }

    public List<RegionEntity> getRegions() {
        return regions;
    }

    public void setRegions(List<RegionEntity> regions) {
        this.regions = regions;
    }
}

UserTypeEntity

@Table(name = "user_type")
@Entity

public class UserTypeEntity {
    @Id
    @GeneratedValue
    private long id;
    private String type;
    private double capacity;

    @OneToMany(mappedBy = "userType", cascade = CascadeType.ALL)
    @JsonIgnore
    private List<UserEntity> users = new ArrayList<>();

    public UserTypeEntity() {
    }

    public double getCapacity() {
        return capacity;
    }

    public void setCapacity(double capacity) {
        this.capacity = capacity;
    }

    public long getId() {
        return id;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public List<UserEntity> getUsers() {
        return users;
    }

    public void setUsers(List<UserEntity> users) {
        this.users = users;
    }
}

RegionEntiy

@Table(name = "regions")
@Entity
public class RegionEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;
    private String name;
    @ManyToMany(mappedBy = "regions")
    @JsonIgnore
    private List<UserEntity> couriers;

    public RegionEntity() {
    }

    public long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public List<UserEntity> getCouriers() {
        return couriers;
    }

    public void setCouriers(List<UserEntity> couriers) {
        this.couriers = couriers;
    }
}

RoleEntity

@Table(name = "roles")
@Entity
public class RoleEntity {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private long id;
    private String name;

    public RoleEntity() {
    }

    public long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

}

And here's the code that should return all couriers, it stored in CourierService.java and calls in CourierController.java :

public List<CourierGetModel> getCouriers() {
        RoleEntity courier = roleService.getRoleByName(Roles.COURIER);
        List<UserEntity> users = userRepo.findAll().stream()
                .filter(x -> x.getRoles().contains(courier)).toList();
        return generateCourierGetModelList(users);
    }

    private List<CourierGetModel> generateCourierGetModelList(List<UserEntity> couriers) {
        List<CourierGetModel> courierModels = new ArrayList<>();
        for(var courier: couriers) {
            courierModels.add(CourierGetModel.toModel(courier));
        }
        return courierModels;
    }

Here's CourierController.java

@RestController
@RequestMapping("/api/courier")
public class CourierController {
    @Autowired
    private CourierService courierService;

    @PostMapping
    public ResponseEntity<Object> addCourier(@RequestBody DefineCourierModel courierData) {
        courierService.addCourier(courierData);
        return ResponseEntity.ok("user was defined as a courier");
    }
    @GetMapping
    public ResponseEntity<Object> getCouriers() {
        return ResponseEntity.ok(courierService.getCouriers());
    }
}

Also, if you need, here's the CourierGetModel

public class CourierGetModel {
    private long id;
    private String username;
    private String name;
    private String surname;
    private String email;
    private UserTypeEntity type;
    private List<RegionEntity> regions;

    public CourierGetModel() {
    }

    public static CourierGetModel toModel(UserEntity user) {
        CourierGetModel model = new CourierGetModel();
        model.setUsername(user.getUsername());
        model.setId(user.getId());
        model.setType(user.getUserType());
        model.setRegion(user.getRegions());
        return model;
    }

    public long getId() {
        return id;
    }

    public void setId(long id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getSurname() {
        return surname;
    }

    public void setSurname(String surname) {
        this.surname = surname;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public UserTypeEntity getType() {
        return type;
    }

    public void setType(UserTypeEntity type) {
        this.type = type;
    }

    public List<RegionEntity> getRegion() {
        return regions;
    }

    public void setRegion(List<RegionEntity> region) {
        this.regions = region;
    }
}

I'm kinda new to Sptring, so I really hope that you can help me. Thank you in advance!

Answer 1

Serialization frameworks (not just Jackson) can't handle bidirectional many-to-many relations well. When it tries to serialize a UserEntity , it tries to serialize its UserTypeEntity as well. It then tries to serialize all UserEntity instances of each UserTypeEntity , which again leads to the UserTypeEntity being serialized. This repeats until the StackOverflowError occurs.

I think that Jackson has a max depth setting somewhere, that could possibly help you. Other people have used Jackson's annotations like @JsonIgnore . I myself prefer to create DTOs from my entitities, so I can have more fine-grained control over what to serialize. That also allows me to return UserEntity instances for one endpoint with nested UserTypeEntity instances (but not depper), and UserTypeEntity instances from another, with nested UserEntity instances (but not deeper).

Note that your @JsonIgnore doesn't work because Jackson by default prefers to use getters and setters. Your error will probably go away when you move the annotation to the getter, or tell Jackson to serialize fields instead.