1.convention目录下创建exception目录，并创建AbstractException类，
ClientException类，ServiceException类，RemoteException类

/**
 * 抽象项目中三类异常体系，客户端异常、服务端异常以及远程服务调用异常
 *
 * @see ClientException
 * @see ServiceException
 * @see RemoteException
 */
@Getter
public abstract class AbstractException extends RuntimeException {
    public final String errorCode;
    public final String errorMessage;

    public AbstractException(String message, Throwable throwable, IErrorCode errorCode) {
        super(message, throwable);
        this.errorCode = errorCode.code();
        this.errorMessage = Optional.ofNullable(StringUtils.hasLength(message) ? message : null)
                .orElse(errorCode.message());
    }
}

/**
 * 客户端异常
 */
public class ClientException extends AbstractException {
    public ClientException(IErrorCode errorCode) {
        this(null, null, errorCode);
    }

    public ClientException(String message) {
        this(message, null, BaseErrorCode.CLIENT_ERROR);
    }

    public ClientException(String message, IErrorCode errorCode) {
        this(message, null, errorCode);
    }

    public ClientException(String message, Throwable throwable, IErrorCode errorCode) {
        super(message, throwable, errorCode);
    }

    @Override
    public String toString() {
        return "ClientException{" +
                "code='" + errorCode + "'," +
                "message='" + errorMessage + "'" +
                '}';
    }
}

/**
 * 远程服务调用异常
 */
public class RemoteException extends AbstractException {
    public RemoteException(String message) {
        this(message, null, BaseErrorCode.REMOTE_ERROR);
    }

    public RemoteException(String message, IErrorCode errorCode) {
        this(message, null, errorCode);
    }

    public RemoteException(String message, Throwable throwable, IErrorCode errorCode) {
        super(message, throwable, errorCode);
    }

    @Override
    public String toString() {
        return "RemoteException{" +
                "code='" + errorCode + "'," +
                "message='" + errorMessage + "'" +
                '}';
    }
}

/**
 * 服务端异常
 */
public class ServiceException extends AbstractException {
    public ServiceException(String message) {
        this(message, null, BaseErrorCode.SERVICE_ERROR);
    }

    public ServiceException(IErrorCode errorCode) {
        this(null, errorCode);
    }

    public ServiceException(String message, IErrorCode errorCode) {
        this(message, null, errorCode);
    }

    public ServiceException(String message, Throwable throwable, IErrorCode errorCode) {
        super(Optional.ofNullable(message).orElse(errorCode.message()), throwable, errorCode);
    }

    @Override
    public String toString() {
        return "ServiceException{" +
                "code='" + errorCode + "'," +
                "message='" + errorMessage + "'" +
                '}';
    }
}

2.UserServiceImpl就可以修改为

代码片段实现了UserService接口的一个方法getUserByUsername，该方法用于根据用户名获取用户信息，并返回一个UserRespDTO对象。在方法中有一个判断if (userDO == null)来处理当用户不存在时的情况（userDO对象如果为null，代表数据库中没有匹配的记录）。
如果去掉if (userDO == null)这个判断，当数据库中没有找到匹配的UserDO记录时，userDO变量会被赋值为null。随后的BeanUtils.copyProperties(userDO, result);调用将会失败，因为BeanUtils.copyProperties方法会尝试访问userDO的属性来复制到result对象，如果userDO是null，那么就会引发NullPointerException。

3.result目录下创建Results类，即可简化UserController

4.common目录下创建web目录，再创建GlobalExceptionHandler
全局异常拦截器，需要拦截器的原因在于，我们不可能在代码的逻辑上拦截所有异常，比如如果没写return null就会有空指针异常，这时拦截器可以拦截这样的错误，并报出异常

/**
 * 全局异常处理器
 *
 */
@Component
@Slf4j
@RestControllerAdvice
public class GlobalExceptionHandler {

    /**
     * 拦截参数验证异常
     */
    @SneakyThrows
    @ExceptionHandler(value = MethodArgumentNotValidException.class)
    public Result validExceptionHandler(HttpServletRequest request, MethodArgumentNotValidException ex) {
        BindingResult bindingResult = ex.getBindingResult();
        FieldError firstFieldError = CollectionUtil.getFirst(bindingResult.getFieldErrors());
        String exceptionStr = Optional.ofNullable(firstFieldError)
                .map(FieldError::getDefaultMessage)
                .orElse(StrUtil.EMPTY);
        log.error("[{}] {} [ex] {}", request.getMethod(), getUrl(request), exceptionStr);
        return Results.failure(BaseErrorCode.CLIENT_ERROR.code(), exceptionStr);
    }

    /**
     * 拦截应用内抛出的异常
     */
    @ExceptionHandler(value = { AbstractException.class })
    public Result abstractException(HttpServletRequest request, AbstractException ex) {
        if (ex.getCause() != null) {
            log.error("[{}] {} [ex] {}", request.getMethod(), request.getRequestURL().toString(), ex.toString(),
                    ex.getCause());
            return Results.failure(ex);
        }
        log.error("[{}] {} [ex] {}", request.getMethod(), request.getRequestURL().toString(), ex.toString());
        return Results.failure(ex);
    }

    /**
     * 拦截未捕获异常
     */
    @ExceptionHandler(value = Throwable.class)
    public Result defaultErrorHandler(HttpServletRequest request, Throwable throwable) {
        log.error("[{}] {} ", request.getMethod(), getUrl(request), throwable);
        return Results.failure();
    }

    private String getUrl(HttpServletRequest request) {
        if (StringUtils.isEmpty(request.getQueryString())) {
            return request.getRequestURL().toString();
        }
        return request.getRequestURL().toString() + "?" + request.getQueryString();
    }
}