Getting java.lang.StackOverflowError when two users follow each other

I think your code works well..

I wrote test code

@Service
@RequiredArgsConstructor
public class UserService {

    private final UserRepository userRepository;

    public User test() {
        User user = userRepository.findById(1L).orElseThrow();
        User follower = userRepository.findById(2L).orElseThrow();

        user.getFollowings().add(follower);
        follower.getFollowers().add(user);

        follower.getFollowings().add(user);
        user.getFollowers().add(follower);

        userRepository.save(user);
        return user;
    }
}



@RestController
@AllArgsConstructor
public class TestController {
    private final UserService userService;

    @GetMapping("/test")
    public User test() {
        return userService.test();
    }
}

and i got this response

{
    "id": 1,
    "userName": "userName1",
    "password": "userPw1",
    "followings": [
        {
            "id": 2,
            "userName": "follower",
            "password": "follower"
        }
    ],
    "followers": [
        {
            "id": 2,
            "userName": "follower",
            "password": "follower"
        }
    ]
}

To begin with, an observation.

The error occurs when there are cycles, by users following each other, as the OP noted.

I've written and tested three versions, the last of which correctly lists all followers and following for all users.

The last version appears to be essentially the same as what the OP has written. It has been tested with transitive and direct self-references.

The following two versions both work, without listing followings or followers.

The first uses annotation @JsonIgnore above each field, while the second uses annotation @JsonIgnoreProperties({"followers", "followings"}) above the class declaration.

@Entity
@Table(name = "userentity")
@Builder
@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonInclude(JsonInclude.Include.NON_NULL)
public class UserEntity {

    @Id
    private Long id;
    @Column
    private String userName;
    @Column
    private String password;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "user_followings",
            joinColumns = @JoinColumn(name = "user_id"),
            inverseJoinColumns = @JoinColumn(name = "following_id")
    )
    @Builder.Default
    @EqualsAndHashCode.Exclude
    @JsonIgnore
    private Set<UserEntity> followings = new HashSet<>();

    @ManyToMany(mappedBy = "followings", fetch = FetchType.EAGER)
    @Builder.Default
    @EqualsAndHashCode.Exclude
    @JsonIgnore
    private Set<UserEntity> followers = new HashSet<>();

}

@Entity
@Table(name = "userentity")
@Builder
@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonInclude(JsonInclude.Include.NON_NULL)
@JsonIgnoreProperties({"followers", "followings"})
public class UserEntity {

    @Id
    private Long id;
    @Column
    private String userName;
    @Column
    private String password;

    @ManyToMany(fetch = FetchType.EAGER)
    @JoinTable(
            name = "user_followings",
            joinColumns = @JoinColumn(name = "user_id"),
            inverseJoinColumns = @JoinColumn(name = "following_id")
    )
    @Builder.Default
    @EqualsAndHashCode.Exclude
    private Set<UserEntity> followings = new HashSet<>();

    @ManyToMany(mappedBy = "followings", fetch = FetchType.EAGER)
    @Builder.Default
    @EqualsAndHashCode.Exclude
    private Set<UserEntity> followers = new HashSet<>();

}

The following code works and lists the first level of followings. Note the use of @JsonIgnoreProperties({"followers", "followings"}), as the OP attempted:

@Entity
@Table(name = "userentity")
@Builder
@Data
@AllArgsConstructor
@NoArgsConstructor
@JsonInclude(JsonInclude.Include.NON_NULL)
public class UserEntity {

    @Id
    private Long id;
    @Column
    private String userName;
    @Column
    private String password;

    @ManyToMany(fetch = FetchType.LAZY)
    @JoinTable(
            name = "user_followings",
            joinColumns = @JoinColumn(name = "id"),
            inverseJoinColumns = @JoinColumn(name = "following_id")
    )
    @Builder.Default
    @EqualsAndHashCode.Exclude
    @JsonIgnoreProperties({"followers", "followings"})
    private Set<UserEntity> followings = new HashSet<>();

    @ManyToMany(mappedBy = "followings", fetch = FetchType.LAZY)
    @Builder.Default
    @EqualsAndHashCode.Exclude
    @JsonIgnoreProperties({"followers", "followings"})
    private Set<UserEntity> followers = new HashSet<>();

}

Actual output:

[
    {
        "id": 1,
        "userName": "u1",
        "password": "pass1",
        "followings": [
            {
                "id": 3,
                "userName": "u1",
                "password": "pass"
            },
            {
                "id": 2,
                "userName": "u2",
                "password": "pass2"
            }
        ],
        "followers": [
            {
                "id": 3,
                "userName": "u1",
                "password": "pass"
            },
            {
                "id": 2,
                "userName": "u2",
                "password": "pass2"
            }
        ]
    },
    {
        "id": 2,
        "userName": "u2",
        "password": "pass2",
        "followings": [
            {
                "id": 3,
                "userName": "u1",
                "password": "pass"
            },
            {
                "id": 1,
                "userName": "u1",
                "password": "pass1"
            }
        ],
        "followers": [
            {
                "id": 3,
                "userName": "u1",
                "password": "pass"
            },
            {
                "id": 1,
                "userName": "u1",
                "password": "pass1"
            }
        ]
    },
    {
        "id": 3,
        "userName": "u1",
        "password": "pass",
        "followings": [
            {
                "id": 2,
                "userName": "u2",
                "password": "pass2"
            },
            {
                "id": 1,
                "userName": "u1",
                "password": "pass1"
            }
        ],
        "followers": [
            {
                "id": 2,
                "userName": "u2",
                "password": "pass2"
            },
            {
                "id": 1,
                "userName": "u1",
                "password": "pass1"
            }
        ]
    }
]

Note some other minor fixes to get the code working on my setup. Other issues, such as the eager loading have not been addressed.