【SpringBoot】文件上传原理分析

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对文件上传功能的配置都在 MultipartAutoConfiguration.class 中

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一、请求进入，使用文件上传解析器判断并封装

1. 文件上传解析器：只能接收标准的 Servlet 方式上传的文件

@ConditionalOnMissingBean({MultipartResolver.class}) 判断容器中无文件上传解析器，若无自动创建

    @ConditionalOnMissingBean({MultipartResolver.class}) // 判断注解
    public StandardServletMultipartResolver multipartResolver() {
        StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
        multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
        return multipartResolver;
    }

2. DispatcherServlet.class —— doDispatch（）方法

与文件上传相关功能有关的语句，请见下面的注释：

    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            try {
                ModelAndView mv = null;
                Exception dispatchException = null;

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request; // 判断是否是文件上传请求（封装好的文件上传请求与原请求不相等，则判断是文件上传请求）
                    mappedHandler = this.getHandler(processedRequest); // 找谁能处理文件上传请求
                    if (mappedHandler == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }

                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                    String method = request.getMethod();
                    boolean isGet = "GET".equals(method);
                    if (isGet || "HEAD".equals(method)) {
                        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                        if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                            return;
                        }
                    }

                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }

                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(processedRequest, mv);
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }

                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }
    }

（1）processedRequest = this.checkMultipart(request) —— 用于判断是否是文件上传请求
—— Step Into 查看：发现使用 multipartResolver 判断是否是文件上传请求

   protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) { // 判断是否是文件上传请求
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
                    this.logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
                }
            } else if (this.hasMultipartException(request)) {
                this.logger.debug("Multipart resolution previously failed for current request - skipping re-resolution for undisturbed error rendering");
            } else {
                try {
                    return this.multipartResolver.resolveMultipart(request); // 解析文件上传请求
                } catch (MultipartException var3) {
                    if (request.getAttribute("javax.servlet.error.exception") == null) {
                        throw var3;
                    }
                }

                this.logger.debug("Multipart resolution failed for error dispatch", var3);
            }
        }

        return request;
    }

StepInto——
multipartResolver 判断是否是文件上传请求方式 —— 用 String 工具类判断是否以 multipart/ 开头（这也解释了为什么我们在写前端表单接收文件时，必须使用multipart）

    public boolean isMultipart(HttpServletRequest request) {
        return StringUtils.startsWithIgnoreCase(request.getContentType(), "multipart/");
    }

（2）this.multipartResolver.resolveMultipart(request) 解析文件上传请求 ——

其将文件上传请求封装为 MultipartHttpServletRequest 类返回

    public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
        return new StandardMultipartHttpServletRequest(request, this.resolveLazily);
    }

二、参数解析器解析请求中的文件内容封装成 MultipartFile

文件请求参数解析器：

(1) InvocableHandlerMethod.class 找到参数解析器，执行文件上传代理

  public Object resolveArgument(MethodParameter parameter, @Nullable ModelAndViewContainer mavContainer, NativeWebRequest request, @Nullable WebDataBinderFactory binderFactory) throws Exception {
        HttpServletRequest servletRequest = (HttpServletRequest)request.getNativeRequest(HttpServletRequest.class);
        Assert.state(servletRequest != null, "No HttpServletRequest");
        RequestPart requestPart = (RequestPart)parameter.getParameterAnnotation(RequestPart.class);
        boolean isRequired = (requestPart == null || requestPart.required()) && !parameter.isOptional();
        String name = this.getPartName(parameter, requestPart);
        parameter = parameter.nestedIfOptional();
        Object arg = null;
        Object mpArg = MultipartResolutionDelegate.resolveMultipartArgument(name, parameter, servletRequest); // 文件上传代理
        if (mpArg != MultipartResolutionDelegate.UNRESOLVABLE) {
            arg = mpArg;
        } else {
            try {
                HttpInputMessage inputMessage = new RequestPartServletServerHttpRequest(servletRequest, name);
                arg = this.readWithMessageConverters(inputMessage, parameter, parameter.getNestedGenericParameterType());
                if (binderFactory != null) {
                    WebDataBinder binder = binderFactory.createBinder(request, arg, name);
                    if (arg != null) {
                        this.validateIfApplicable(binder, parameter);
                        if (binder.getBindingResult().hasErrors() && this.isBindExceptionRequired(binder, parameter)) {
                            throw new MethodArgumentNotValidException(parameter, binder.getBindingResult());
                        }
                    }

                    if (mavContainer != null) {
                        mavContainer.addAttribute(BindingResult.MODEL_KEY_PREFIX + name, binder.getBindingResult());
                    }
                }
            } catch (MultipartException | MissingServletRequestPartException var13) {
                if (isRequired) {
                    throw var13;
                }
            }
        }
  }

（2）确定每个参数的值

       Object[] args = this.getMethodArgumentValues(request, mavContainer, providedArgs);

（3）解析所有参数

           for(int i = 0; i < parameters.length; ++i) {
                MethodParameter parameter = parameters[i];
                parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
                args[i] = findProvidedArgument(parameter, providedArgs);
                if (args[i] == null) {
                    if (!this.resolvers.supportsParameter(parameter)) {
                        throw new IllegalStateException(formatArgumentError(parameter, "No suitable resolver"));
                    }

                    try {
                        args[i] = this.resolvers.resolveArgument(parameter, mavContainer, request, this.dataBinderFactory);
                    } catch (Exception var10) {
                        if (logger.isDebugEnabled()) {
                            String exMsg = var10.getMessage();
                            if (exMsg != null && !exMsg.contains(parameter.getExecutable().toGenericString())) {
                                logger.debug(formatArgumentError(parameter, exMsg));
                            }
                        }

                        throw var10;
                    }
                }
            }

三、将 request 文件封装为一个 Map (Map)

AbstractMultipartHttpServletRequest.class

    public List<MultipartFile> getFiles(String name) {
        List<MultipartFile> multipartFiles = (List)this.getMultipartFiles().get(name);
        return multipartFiles != null ? multipartFiles : Collections.emptyList();
    }

public Map<String, MultipartFile> getFileMap() {
        return this.getMultipartFiles().toSingleValueMap();
    }

总结：

1. 请求进入，使用文件上传解析器判断（isMultipart）并封装（resolveMultipart，返回MultipartHttpServletRequest）文件上传请求
2. 参数解析器解析请求中的文件内容封装成 MultipartFile
3. 将 request 文件封装为一个 Map (Map)

通过观察源码，可以得到许多 SpringBoot 为我们封装好的文件工具类，如 FileCopyUtils 实现文件流的拷贝