[FIXED] Why @RequestBody works without setters?

Issue

I am writing a controller with the annotation @RequestBody in order to map to a Java object. The method that uses the annotation is:

@PostMapping("/users")
public ResponseEntity<Object> createUserForProject(@Valid @RequestBody User user) {
        log.info("Creating a user " + user.getEmail());
}

This is the User class:

@Getter
@AllArgsConstructor
@Slf4j
@EqualsAndHashCode
@ToString
public class User {
    @NotEmpty
    @Email
    private String email;

    @NotEmpty
    private String firstName;

    @NotEmpty
    private String lastName;

    @JsonIgnore
    private Optional<LocalDate> lastLogonDate = Optional.empty();

    @JsonIgnore
    private Optional<LocalDate> lastModificationDate = Optional.empty();

    @JsonIgnore
    private Optional<LocalDate> creationDate = Optional.empty();

    @JsonIgnore
    private Optional<LocalDate> mfaWarningDate = Optional.empty();

    @JsonIgnore
    private Optional<LocalDate> auditStartNotificationDate = Optional.empty();

    @JsonIgnore
    private boolean enabled = true;

    public User() {
        log.info("HI");
    }

    (More code without explicit setters)

So when I make a POST call with the body

{
   "email":"[email protected]",
   "firstName":"testName",
   "lastName":"testLastName"
}

Outputs HI and the log with the Creating a user [email protected] message, so the object is created. My point here is... why does this really work? The HttpMessageConverter is calling the no-args constructor and there are no setters to call after create the object with the constructor. How do the object attributes get their values without any setter? What am I missing here?

Solution

Spring boot uses Jackson for Object <-> JSON conversion, and Jackson does not need setters, it sets fields via reflection.

Here is a relevant question about Jackson and why it doesn't need setters How does jackson set private properties without setters?

Answered By - Igor Flakiewicz
Answer Checked By - David Marino (JavaFixing Volunteer)