File upload and exception handling for Spring Boot development

1 file upload

1 Front-end form

<form method="post" action="/upload" enctype="multipart/form-data">
    <input type="file" name="file"><br>
    <input type="submit" value="提交">
</form>

2 background receiving

    /**
     * MultipartFile 自动封装上传过来的文件
     * @param email
     * @param username
     * @param headerImg
     * @param photos
     * @return
     */
    @PostMapping("/upload")
    public String upload(@RequestParam("email") String email,
                         @RequestParam("username") String username,
                         @RequestPart("headerImg") MultipartFile headerImg,
                         @RequestPart("photos") MultipartFile[] photos) throws IOException {
    
    

        log.info("上传的信息：email={}，username={}，headerImg={}，photos={}",
                email,username,headerImg.getSize(),photos.length);

        if(!headerImg.isEmpty()){
    
    
            // 保存到文件服务器，OSS服务器
            String originalFilename = headerImg.getOriginalFilename();
            headerImg.transferTo(new File("D:\\cache\\"+originalFilename));
        }

        if(photos.length > 0){
    
    
            for (MultipartFile photo : photos) {
    
    
                if(!photo.isEmpty()){
    
    
                    String originalFilename = photo.getOriginalFilename();
                    photo.transferTo(new File("D:\\cache\\"+originalFilename));
                }
            }
        }

        return "main";
    }

3 Configuration principles

**File upload automatic configuration class-MultipartAutoConfiguration-**MultipartProperties

• Automatically configured StandardServletMultipartResolver [file upload parser]

  • 1 Request comes in. Use the file upload parser to judge ( isMultipart ) and encapsulate ( resolveMultipart, return MultipartHttpServletRequest ) file upload request

  • 2 parameter parser to parse the file content in the request and encapsulate it into MultipartFile

  • 3 Encapsulate the file information in the request into a Map

2 exception handling

1 Error handling

1 default rule

By default, Spring Boot provides /errora mapping that handles all errors

For machine clients, it will generate a JSON response with details of the error, HTTP status and exception message. For browser clients, respond with a "whitelabel" error view, rendering the same data in HTML

To customize it, add Viewthe parser aserror

• To completely replace the default behavior, you can implement ErrorController and register a bean definition of the type, or add ErrorAttributes类型的组件to use the existing mechanism but replace its contents
• 4xx and 5xx pages under error/ will be automatically parsed

2 Custom error handling logic

Custom error pages:

• error/404.html error/5xx.html; if there is an accurate error status code, the page will match accurately, if not, then find 4xx.html; if there is no page, it will trigger a white page

@ControllerAdvice+@ExceptionHandler handles global exceptions; the bottom layer is supported by ExceptionHandlerExceptionResolver

@ResponseStatus+ custom exception; the bottom layer is ResponseStatusExceptionResolver, and the bottom layer calls response.sendError(statusCode, resolvedReason) for the information annotated by responsestatus; /error sent by tomcat

Spring underlying exceptions, such as parameter type conversion exceptions

• DefaultHandlerExceptionResolver handles exceptions at the bottom of the framework

Custom implementation of HandlerExceptionResolver to handle exceptions; it can be used as the default global exception handling rules

ErrorViewResolver implements custom exception handling

• response. sendError . The error request will be transferred to the controller
• Exceptions are not handled by anyone. tomcat underlying response.sendError. The error request will be transferred to the controller
• The page address that basicErrorController is going to is ErrorViewResolver

2 Principle of exception handling automatic configuration

ErrorMvcAutoConfiguration automatically configures exception handling rules

• Components in container: type: DefaultErrorAttributes -> id: errorAttributes
  • DefaultErrorAttributes define what data can be included in the error page
• **Components in the container: type: **BasicErrorController --> id: basicErrorController (json+white page adaptation response)
  • Handle requests for the default /error path; the page responds with new ModelAndView( “error” , model);
  • There is a component View in the container -> id is error ; (response to the default error page)
  • Put the component BeanNameViewResolver (view resolver) in the container; use the returned view name as the component id to find the View object in the container
• **Components in container: **Type: **DefaultErrorViewResolver -> id: **conventionErrorViewResolver
  • If an error occurs, the HTTP status code will be used as the view page address (viewName) to find the real page
  • error/404、5xx.html

3 exception handling process

1 Execute the target method, any exceptions during the running of the target method will be caught, and mark the end of the current request; and use dispatchException

2 Enter the view parsing process (page rendering)

processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);

3 mv = processHandlerException ; Handle the exception that occurred in the handler, and return to ModelAndView after the processing is completed

• Traverse all **handlerExceptionResolvers to see who can handle the current exception [**HandlerExceptionResolver processor exception resolver]
• System default exception parser
  • DefaultErrorAttributes handle exceptions first. Save the exception information to the rrequest domain and return null;
  • By default no one can handle the exception, so the exception will be thrown
    • No one can handle the final bottom layer will send /error request. Will be handled by the underlying BasicErrorController
    • Parse the error view; traverse all ErrorViewResolver to see who can resolve it
    • The default DefaultErrorViewResolver is used to take the response status code as the address of the error page, error/500.html
    • The template engine finally responds to this page error/500.html