public abstract class Converter<A, B>  {protected abstract B doForward(A a);protected abstract A doBackward(B b);
}

import lombok.Data;
import org.bson.types.ObjectId;
import org.springframework.data.annotation.Id;
import org.springframework.data.mongodb.core.mapping.Document;@Data
@Document(collection = "user")
public class User {@Idprivate ObjectId id;private String username;private String password;
}

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;
import org.springframework.beans.BeanUtils;
import study.Converter;import javax.validation.constraints.NotNull;@NoArgsConstructor
@AllArgsConstructor(staticName = "of")
@Data
public class UserDTO {/*** AllArgsConstructor注解和NotNull注解配合使用，参数不为null*/@NotNull(message = "username is null")private String username;@NotNull(message = "password is null")private String password;public User convertToUser(){UserDTOConvert userDTOConvert = new UserDTOConvert();User convert = userDTOConvert.doForward(this);return convert;}public UserDTO convertFor(User user){UserDTOConvert userDTOConvert = new UserDTOConvert();UserDTO convert = userDTOConvert.doBackward(user);return convert;}private static class UserDTOConvert extends Converter<UserDTO, User> {@Overrideprotected User doForward(UserDTO userDTO) {User user = new User();BeanUtils.copyProperties(userDTO,user);return user;}@Overrideprotected UserDTO doBackward(User user) {UserDTO userDTO = new UserDTO();BeanUtils.copyProperties(user,userDTO);return userDTO;}}
}

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;import javax.validation.Valid;@RestController
@RequestMapping(value = "/api/user")
public class UserController {@Autowiredprivate UserService userService;@PostMappingpublic UserDTO addUser(@Valid UserDTO userDTO){User user = userDTO.convertToUser();User addUser = userService.addUser(user);UserDTO result = userDTO.convertFor(addUser);return result;}
}