请帮忙逐行讲解一下统一异常处理类为什么这么写。以及RequestAttributes是什么？

Question

/**
 * web错误 全局配置
 * Created by 瓦力.
 */
@Controller
public class AppErrorController implements ErrorController {
    private static final String ERROR_PATH = "/error";

    private ErrorAttributes errorAttributes;

    @Override
    public String getErrorPath() {
        return ERROR_PATH;
    }

    @Autowired
    public AppErrorController(ErrorAttributes errorAttributes) {
        this.errorAttributes = errorAttributes;
    }

    /**
     * Web页面错误处理
     */
    @RequestMapping(value = ERROR_PATH, produces = "text/html")
    public String errorPageHandler(HttpServletRequest request, HttpServletResponse response) {
        int status = response.getStatus();
        switch (status) {
            case 403:
                return "403";
            case 404:
                return "404";
            case 500:
                return "500";
        }

        return "index";
    }

    /**
     * 除Web页面外的错误处理，比如Json/XML等
     */
    @RequestMapping(value = ERROR_PATH)
    @ResponseBody
    public ApiResponse errorApiHandler(HttpServletRequest request) {
        RequestAttributes requestAttributes = new ServletRequestAttributes(request);

        Map<String, Object> attr = this.errorAttributes.getErrorAttributes(requestAttributes, false);
        int status = getStatus(request);

        return ApiResponse.ofMessage(status, String.valueOf(attr.getOrDefault("message", "error")));
    }

    private int getStatus(HttpServletRequest request) {
        Integer status = (Integer) request.getAttribute("javax.servlet.error.status_code");
        if (status != null) {
            return status;
        }

        return 500;
    }
}