@RestController
@RequestMapping("/users")
public class UserController {

    @Autowired
    UserRepository userRepository;
    @Autowired
    BCryptPasswordEncoder bCryptPasswordEncoder;

    @PostMapping("/sign-up")
    public void signUp(@RequestBody User user) {
        user.setPassword(bCryptPasswordEncoder.encode(user.getPassword()));
        userRepository.save(user);
    }
}

@Entity
@Table(name = "req_user")
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;
    private String username;
    @JsonIgnore
    private String password;
    private String email;

    public User() { }

    public User(String username, String password, String email) {
        this.id = null;
        this.username = username;
        this.password = password;
        this.email = email;
    }

    ...

    @JsonIgnore
    public String getPassword() {
        return password;
    }

    @JsonProperty
    public void setPassword(String password) {
        this.password = password;
    }

    ...
}