@Nullable
// 文件上传解析器，只能有一个
private MultipartResolver multipartResolver;
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
    // 1.利用文件上传解析器来判断是否是文件上传请求
    if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
        // 如果之前被MultipartFilter包装过了，就不做处理
        if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
            if (DispatcherType.REQUEST.equals(request.getDispatcherType())) {
                logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
            }
        }
        // 是否有异常
        else if (hasMultipartException(request)) {
            logger.debug("Multipart resolution previously failed for current request - " +
                         "skipping re-resolution for undisturbed error rendering");
        }
        else {
            try {
                // 2、利用文件上传解析器来解析文件上传的请求
                return this.multipartResolver.resolveMultipart(request);
            }
            catch (MultipartException ex) {
                if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                    logger.debug("Multipart resolution failed for error dispatch", ex);
                    // Keep processing error dispatch with regular request handle below
                }
                else {
                    throw ex;
                }
            }
        }
    }
    // If not returned before: return original request.
    return request;
}

public interface MultipartResolver {
    // 判断请求是否是文件上传的请求
    boolean isMultipart(HttpServletRequest request);
    // 将请求包装为 StandardMultipartHttpServletRequest
    MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;
    // 清除资源
    void cleanupMultipart(MultipartHttpServletRequest request);
}

// 是否延迟解析
private boolean resolveLazily = false;
// 判断
public boolean isMultipart(HttpServletRequest request) {
    return StringUtils.startsWithIgnoreCase(request.getContentType(),
                                            (this.strictServletCompliance ? MediaType.MULTIPART_FORM_DATA_VALUE : "multipart/"));
}
// 包装请求
public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
    return new StandardMultipartHttpServletRequest(request, this.resolveLazily);
}

// 创建文件上传请求
public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing)
    throws MultipartException {
    super(request);
    // 是否延迟解析，默认false
    if (!lazyParsing) {
        // 解析请求
        parseRequest(request);
    }
}
private void parseRequest(HttpServletRequest request) {
    try {
        // 从 HttpServletRequest 中获取上传的文件
        Collection<Part> parts = request.getParts();
        this.multipartParameterNames = new LinkedHashSet<>(parts.size());
        // 存储封装好的文件对象
        MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
        // 遍历所有的文件
        for (Part part : parts) {
            String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
            ContentDisposition disposition = ContentDisposition.parse(headerValue);
            // 获取文件名字
            String filename = disposition.getFilename();
            if (filename != null) {
                // 添加到集合中
                files.add(part.getName(), new StandardMultipartFile(part, filename));
            }
            else {
                // 没有文件名，就是普通参数了
                this.multipartParameterNames.add(part.getName());
            }
        }
        // 将上面所有生成的 StandardMultipartFile 文件对象设置到父类的 multipartFiles属性中
        // 以便后面封装参数时使用
        setMultipartFiles(files);
    }
    catch (Throwable ex) {
        handleParseFailure(ex);
    }
}
protected final void setMultipartFiles(MultiValueMap<String, MultipartFile> multipartFiles) {
    this.multipartFiles =
        new LinkedMultiValueMap<>(Collections.unmodifiableMap(multipartFiles));
}

@AutoConfiguration
@ConditionalOnClass({ Servlet.class, StandardServletMultipartResolver.class, MultipartConfigElement.class })
@ConditionalOnProperty(prefix = "spring.servlet.multipart", name = "enabled", matchIfMissing = true)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(MultipartProperties.class)
public class MultipartAutoConfiguration {
    private final MultipartProperties multipartProperties;
    public MultipartAutoConfiguration(MultipartProperties multipartProperties) {
        this.multipartProperties = multipartProperties;
    }
    @Bean
    @ConditionalOnMissingBean(MultipartConfigElement.class)
    public MultipartConfigElement multipartConfigElement() {
        return this.multipartProperties.createMultipartConfig();
    }
    @Bean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
    @ConditionalOnMissingBean(MultipartResolver.class)
    public StandardServletMultipartResolver multipartResolver() {
        StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
        multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
        return multipartResolver;
    }
}