验证期间的NotReadablePropertyException (Spring Boot)

import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;

public class User {
    @NotNull
    private String username;

    @Size(min = 6, max = 20)
    private String password;

    // getter and setter methods
}

@RestController
public class UserController {
    @PostMapping("/users")
    public ResponseEntity<String> createUser(@Valid @RequestBody User user) {
        // 处理用户创建逻辑
        return ResponseEntity.ok("User created successfully");
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<String> handleValidationException(MethodArgumentNotValidException ex) {
        BindingResult result = ex.getBindingResult();
        List<FieldError> fieldErrors = result.getFieldErrors();
        StringBuilder errorMessage = new StringBuilder();

        for (FieldError error : fieldErrors) {
            errorMessage.append(error.getField()).append(": ").append(error.getDefaultMessage()).append("; ");
        }

        return ResponseEntity.badRequest().body(errorMessage.toString());
    }
}