4.7 error handling mechanism

4.7.1. Default error handling mechanism of SpringBoot

1. Default effect:

1) Return to an error page

Request headers sent by browser:

2) If it is another client, it will respond to a json data by default.

{
    "timestamp":"2020-03-14T15:56:48.897+0000",
    "status":404,
    "error":"Not Found",
    "message":"No message available",
    "path":"/kldf"
}

2. Principle:

Refer to ErrorMvCAutoConfiguration; automatic configuration of error handling;

The following components were added to the container

1),DefaultErrorAttributes:

//Help us share information on the page
@Override
public Map<String, Object> getErrorAttributes(WebRequest webRequest, boolean includeStackTrace) {
	Map<String, Object> errorAttributes = new LinkedHashMap<>();
	errorAttributes.put("timestamp", new Date());
	addStatus(errorAttributes, webRequest);
	addErrorDetails(errorAttributes, webRequest, includeStackTrace);
	addPath(errorAttributes, webRequest);
	return errorAttributes;
}

2) . BasicErrorController: process default / error request

@Controller
@RequestMapping("${server.error.path:${error.path:/error}}")
public class BasicErrorController extends AbstractErrorController {
    
    @RequestMapping(produces = MediaType.TEXT_HTML_VALUE) 
    //Generate html type data; requests sent by browsers come to this processing method
	public ModelAndView errorHtml(HttpServletRequest request, HttpServletResponse response) {
		HttpStatus status = getStatus(request);
		Map<String, Object> model = Collections
				.unmodifiableMap(getErrorAttributes(request, isIncludeStackTrace(request, MediaType.TEXT_HTML)));
		response.setStatus(status.value());
        //Error page view name found
		ModelAndView modelAndView = resolveErrorView(request, response, status, model);
		return (modelAndView != null) ? modelAndView : new ModelAndView("error", model);
	}
    
	@RequestMapping
    //Generate json data, and other clients come to this method for processing;
	public ResponseEntity<Map<String, Object>> error(HttpServletRequest request) {
		HttpStatus status = getStatus(request);
		if (status == HttpStatus.NO_CONTENT) {
			return new ResponseEntity<>(status);
		}
		Map<String, Object> body = getErrorAttributes(request, isIncludeStackTrace(request, MediaType.ALL));
		return new ResponseEntity<>(body, status);
	}

    

3),ErrorProperties

public class ErrorProperties {

	/**
	 * Path of the error controller.
	 */
	@Value("${error.path:/error}")
	private String path = "/error";
    //After an error occurs in the system, it comes to the error request for processing;

4),DefaultErrorViewResolver

	@Override
	public ModelAndView resolveErrorView(HttpServletRequest request, HttpStatus status, Map<String, Object> model) {
		ModelAndView modelAndView = resolve(String.valueOf(status.value()), model);
		if (modelAndView == null && SERIES_VIEWS.containsKey(status.series())) {
			modelAndView = resolve(SERIES_VIEWS.get(status.series()), model);
		}
		return modelAndView;
	}


	private ModelAndView resolve(String viewName, Map<String, Object> model) {
		//By default, SpringBoot can find a page; error/404
        String errorViewName = "error/" + viewName;
        //The template engine can resolve this page address, and then it can use the template engine to resolve it
		TemplateAvailabilityProvider provider = this.templateAvailabilityProviders.getProvider(errorViewName,
				this.applicationContext);
		if (provider != null) {
            //Return to the view address specified by errorViewName when the template engine is available
			return new ModelAndView(errorViewName, model);
		}
        // If the template engine is not available, find the page corresponding to errorViewName under the static resource folder error/404.html
		return resolveResource(errorViewName, model);
	}

3, steps

Once an error such as 4xx or 5xx occurs in the system, the ErrorPageCustomizer will take effect (customizing the error response rules); the / error request will come and be processed by the BasicErrorController;

Response page; which page to go to is parsed by defaultrorviewresolver;

	protected ModelAndView resolveErrorView(HttpServletRequest request, HttpServletResponse response, HttpStatus status,
			Map<String, Object> model) {
        //All errorviewresolvers wait until ModelAndView
		for (ErrorViewResolver resolver : this.errorViewResolvers) {
			ModelAndView modelAndView = resolver.resolveErrorView(request, status, model);
			if (modelAndView != null) {
				return modelAndView;
			}
		}
		return null;
	}

4.7.2. If customized error response

1. How to customize error page

1) If there is a template engine; error / status code; [name the error page as error status code. html and put it in the error folder in the template engine folder], the error of this status code will come to the corresponding page;

We can use 4xx and 5xx as the file names of error pages to match all errors of this type, precision first (precise status code. html is preferred);

Information available on the page:

Timestamp: timestamp

Status: status code

Error: error prompt

Exception: exception object

Errors: JSR303 data validation errors are all here

2) . there is no template engine (the template engine cannot find the error page), which can be found under the static resource folder;

3) , none of the above error pages, i.e. the default error prompt page of SpringBoot;

2. How to customize wrong json data

1) . custom exception handling & return custom json data;

@ControllerAdvice
public class MyExceptionHandler {

    //1. The browser client returns json
    @ResponseBody
    @ExceptionHandler(UserNotExistException.class)
    public Map<String,Object> handleException(Exception e){
        Map<String,Object> map = new HashMap<>();
        map.put("code","user.notexist");
        map.put("message",e.getMessage());
        return map;
    }
}
//No adaptive effect

2) , forward to / error for adaptive response effect processing

    @ExceptionHandler(UserNotExistException.class)
    public String handleException(Exception e, HttpServletRequest request){
        Map<String,Object> map = new HashMap<>();
        //Pass in our own error status code 4xx 5xx
        /**
         * Integer statusCode = (Integer) request
         .getAttribute("javax.servlet.error.status_code");
         */
        request.setAttribute("javax.servlet.error.status_code",500);
        map.put("code","user.notexist");
        map.put("message","User error");

        request.setAttribute("ext",map);
        //Forward to / error
        return "forward:/error";
    }

3. Take our customized data out

After an error is detected, the / error request will come and be processed by the BasicErrorController. The data that can be obtained after the response is obtained by getErrorAttributes (the method specified by AbstractErrorController);

1) , write an implementation class of ErrorController (or a subclass of AbstractErrorController) completely and put it in the container;

2) , the data that can be used on the page, or the data that can be used returned by json, are obtained through errorAttributes.getErrorAttributes;

In the container, DefaultErrorAttributes.getErrorAttributes() is used for data processing by default;

Custom ErrorAttributes

//Add our own defined ErrorAttributes to the container
@Component
public class MyErrorAttributes extends DefaultErrorAttributes {

    //The map of the return value is all the fields that the page and json can get
    @Override
    public Map<String, Object> getErrorAttributes(WebRequest webRequest, boolean includeStackTrace) {
        Map<String, Object> map = super.getErrorAttributes(webRequest, includeStackTrace);
        map.put("company","atguigu");

        //Data carried by our exception handler
        Map<String,Object> ext = (Map<String, Object>) webRequest.getAttribute("ext", 0);
        map.put("ext",ext);
        return map;
    }
}

The final effect: the response is adaptive, and you can change what you need to return by customizing getErrorAttributes

{
	"timestamp": "2020-03-15T12:05:06.783+0000",
	"status": 404,
	"error": "Not Found",
	"message": "No message available",
	"path": "/aad",
	"company": "atguigu",
	"ext": null
}