Jackson 何时需要无参数构造函数进行反序列化？

Question

In my spring boot project, I noticed a strange Jackson behavior. I searched over internet, found out what to do, but haven't found out why .

UserDto:

@Setter
@Getter
@AllArgsConstructor
public class UserDto {

    private String username;

    private String email;

    private String password;

    private String name;

    private String surname;

    private UserStatus status;

    private byte[] avatar;

    private ZonedDateTime created_at;
}

Adding a new user works just fine.

TagDto:

@Setter
@Getter
@AllArgsConstructor
public class TagDto {

    private String tag;
}

Trying to add a new tag ends with an error:

com.fasterxml.jackson.databind.exc.MismatchedInputException: Cannot construct instance of TagDto (although at least one Creator exists): cannot deserialize from Object value (no delegate- or property-based Creator)

The solution to the problem was to add zero-arg constructor to the TagDto class.

Why does Jackson require no-arg constructor for deserialization in TagDto, while working just fine with UserDto?

Used same method for adding both. My Tag and User entities are both annotated with

@Entity
@Setter
@Getter
@NoArgsConstructor

and have all args constructors:

@Entity
@Setter
@Getter
@NoArgsConstructor
public class User extends AbstractModel {

    private String username;

    private String password;

    private String email;

    private String name;

    private String surname;

    private UserStatus status;

    @Lob
    private byte[] avatar;

    @Setter(AccessLevel.NONE)
    private ZonedDateTime created_at;

    public User(final String username, final String password, final String email, final String name, final String surname) {
        this.username = username;
        this.password = password;
        this.email = email;
        this.name = name;
        this.surname = surname;
        this.created_at = ZonedDateTime.now();
    }
}

@Entity
@Setter
@Getter
@NoArgsConstructor
@AllArgsConstructor
public class Tag extends AbstractModel {

    private String tag;
}

@MappedSuperclass
@Getter
public abstract class AbstractModel {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;
}

Entity generation:

    @PostMapping(path = "/add")
    public ResponseEntity<String> add(@Valid @RequestBody final D dto) {
        this.abstractModelService.add(dto);
        return new ResponseEntity<>("Success", HttpStatus.CREATED);
    }
    
    public void add(final D dto) {
    //CRUD repository save method
        this.modelRepositoryInterface.save(this.getModelFromDto(dto));
    }

    @Override
    protected Tag getModelFromDto(final TagDto tagDto) {
        return new Tag(tagDto.getTag());
    }

    @Override
    protected User getModelFromDto(final UserDto userDto) {
        return new User(userDto.getUsername(), userDto.getPassword(), userDto.getEmail(), userDto.getName(), userDto.getSurname());
    }

Error occurs when parsing JSON

{"tag":"example"}

sent via postman localhost:8081/tag/add, returns

{
    "timestamp": "2020-09-26T18:50:39.974+00:00",
    "status": 400,
    "error": "Bad Request",
    "message": "",
    "path": "/tag/add"
}

I am using Lombok v1.18.12 and Spring boot 2.3.3.RELEASE with Jackson v2.11.2.

Answer 1

TL;DR: Solution is at the end.

Jackson supports multiple ways of creating POJOs. The following lists the most common ways, but it likely not a complete list:

1. Create instance using no-arg constructor, then call setter methods to assign property values.

    public class Foo { private int id; public int getId() { return this.id; } @JsonProperty public void setId(int id) { this.id = id; } }

   Specifying @JsonProperty is optional, but can be used to fine-tune the mappings, together with annotations like @JsonIgnore , @JsonAnyGetter , ...

2. Create instance using constructor with arguments.

    public class Foo { private int id; @JsonCreator public Foo(@JsonProperty("id") int id) { this.id = id; } public int getId() { return this.id; } }

   Specifying @JsonCreator for the constructor is optional, but I believe it is required if there is more than one constructor. Specifying @JsonProperty for the parameters is optional, but is required for naming the properties if the parameter names are not included in the class file ( -parameters compiler option).

   The parameters imply that the properties are required. Optional properties can be set using setter methods.

3. Create instance using factory method.

    public class Foo { private int id; @JsonCreator public static Foo create(@JsonProperty("id") int id) { return new Foo(id); } private Foo(int id) { this.id = id; } public int getId() { return this.id; } }

4. Create instance from text value using String constructor.

    public class Foo { private int id; @JsonCreator public Foo(String str) { this.id = Integer.parseInt(id); } public int getId() { return this.id; } @JsonValue public String asJsonValue() { return Integer.toString(this.id); } }

   This is useful when a the POJO has a simply text representation, eg a LocalDate is a POJO with 3 properties ( year , month , dayOfMonth ), but is generally best serialized as a single string ( yyyy-MM-dd format). @JsonValue identifies the method to be used during serialization, and @JsonCreator identifies the constructor/factory-method to be used during deserialization.

   Note: This can also be used for single-value construction using JSON values other than String , but that is very rare.

Ok, that was the background information. What is happening for the examples in the question, it that UserDto works because there is only one constructor (so @JsonCreator is not needed), and many arguments (so @JsonProperty is not needed).

However, for TagDto there is only a single-argument constructor without any annotations, so Jackson classifies that constructor as a type #4 (from my list above), not a type #2.

Which means that it is expecting the POJO to be a value-class, where the JSON for the enclosing object would be { ..., "tag": "value", ... } , not { ..., "tag": {"tag": "example"}, ... } .

To resolve the issue, you need to tell Jackson that the constructor is a property initializing constructor (#2), not a value-type constructor (#4), by specifying @JsonProperty on the constructor argument.

This means that you cannot have Lombok create the constructor for you:

@Setter
@Getter
public class TagDto {

    private String tag;

    public TagDto(@JsonProperty("tag") String tag) {
        this.tag = tag;
    }
}