SpringMVC解析文件参数过程

　　我们知道SpringMVC 接收文件的时候直接用一个MultipartFile 接收即可，但是SpringMVC是如何解析以及如何绑定到参数的不清楚。

1. SpringMVC接收文件的接口如下

    @RequestMapping("/upload")
    @ResponseBody
    public Map<String, Object> uploadPicture(MultipartFile imgFile) {
        Map<String, Object> result = new HashMap<String, Object>();
        result.put("error", 1);

        if (imgFile == null) {
            result.put("message", "文件没接到");
            return result;
        }
        logger.debug("file -> {},viewId ->{}", imgFile.getOriginalFilename());

        String fileOriName = imgFile.getOriginalFilename();// 获取原名称
        String fileNowName = UUIDUtils.getUUID2() + "." + FilenameUtils.getExtension(fileOriName);// 生成唯一的名字
        try {
            FileHandleUtil.uploadSpringMVCFile(imgFile, fileNowName);

        } catch (Exception e) {
            logger.error("uploadPicture error", e);
            return result;
        }

        String id = UUIDUtils.getUUID();
        Document document = new Document();
        document.setCreatetime(new Date());
        document.setPath(fileNowName);
        document.setId(id);
        document.setOriginName(fileOriName);
        document.setUploaderUsername(MySystemUtils.getLoginUsername());

        documentService.insert(document);

        // 回传JSON结果
        result.put("error", 0);
        result.put("url", "/document/getDocument/" + id);
        return result;
    }

　　我们用类似的接口即可实现文件上传。接下来研究SpringMVC 是如何绑定参数的。

2. 绑定参数解读

SpringMVC 处理入口: org.springframework.web.servlet.DispatcherServlet#doDispatch：

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

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

            try {
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

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

                // Actually invoke the handler.
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

1. org.springframework.web.servlet.DispatcherServlet#checkMultipart  检查是否是上传文件的接口，如果是上传文件的接口就重置processedRequest 的引用

    protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +
                        "this typically results from an additional MultipartFilter in web.xml");
            }
            else if (hasMultipartException(request) ) {
                logger.debug("Multipart resolution failed for current request before - " +
                        "skipping re-resolution for undisturbed error rendering");
            }
            else {
                try {
                    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;
    }

1. multipartResolver 是 org.springframework.web.multipart.support.StandardServletMultipartResolver。org.springframework.web.multipart.support.StandardServletMultipartResolver#isMultipart： 判断是否是文件上传的请求。

    public boolean isMultipart(HttpServletRequest request) {
        // Same check as in Commons FileUpload...
        if (!"post".equals(request.getMethod().toLowerCase())) {
            return false;
        }
        String contentType = request.getContentType();
        return (contentType != null && contentType.toLowerCase().startsWith("multipart/"));
    }

2. 最后调用org.springframework.web.multipart.support.StandardServletMultipartResolver#resolveMultipart 解析Multipart

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

可以看出是创建了一个StandardMultipartHttpServletRequest 对象并且返回去替换掉原来request的引用。org.springframework.web.multipart.support.StandardMultipartHttpServletRequest#StandardMultipartHttpServletRequest(javax.servlet.http.HttpServletRequest, boolean) 构造如下

    public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing) throws MultipartException {
        super(request);
        if (!lazyParsing) {
            parseRequest(request);
        }
    }

然后调用： org.springframework.web.multipart.support.StandardMultipartHttpServletRequest#parseRequest

    private void parseRequest(HttpServletRequest request) {
        try {
            // 拿请求接收的文件， 也就是tomcat 临时接收到的文件。
            Collection<Part> parts = request.getParts();
            this.multipartParameterNames = new LinkedHashSet<String>(parts.size());
            MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<String, MultipartFile>(parts.size());
            for (Part part : parts) {
                // 拿到的信息样式： form-data; name="imgFile"; filename="RegexUtils.java"    
                String disposition = part.getHeader(CONTENT_DISPOSITION);
                String filename = extractFilename(disposition);
                if (filename == null) {
                    filename = extractFilenameWithCharset(disposition);
                }
                if (filename != null) {
                    files.add(part.getName(), new StandardMultipartFile(part, filename));
                }
                else {
                    this.multipartParameterNames.add(part.getName());
                }
            }
            setMultipartFiles(files);
        }
        catch (Throwable ex) {
            throw new MultipartException("Could not parse multipart servlet request", ex);
        }
    }

　　(1) 从request.getParts() 获取tomcat 接收到的临时文件

　　(2) files.add(part.getName(), new StandardMultipartFile(part, filename)); 创建一个StandardMultipartFile 

　　(3) org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest#setMultipartFiles 缓存起来。方法如下：

    protected final void setMultipartFiles(MultiValueMap<String, MultipartFile> multipartFiles) {
        this.multipartFiles =
                new LinkedMultiValueMap<String, MultipartFile>(Collections.unmodifiableMap(multipartFiles));
    }

经过上面过程之后的processedRequest 如下：(对上传文件的请求提取了文件信息并且记录起来)

 2. ha.handle(processedRequest, response, mappedHandler.getHandler()); 处理以及装配文件参数到controller

1. 会调用到：org.springframework.web.method.support.InvocableHandlerMethod#invokeForRequest

    public Object invokeForRequest(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
        if (logger.isTraceEnabled()) {
            logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "' with arguments " + Arrays.toString(args));
        }
        Object returnValue = doInvoke(args);
        if (logger.isTraceEnabled()) {
            logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
                    "] returned [" + returnValue + "]");
        }
        return returnValue;
    }

2. org.springframework.web.method.support.InvocableHandlerMethod#getMethodArgumentValues 解析参数

    private Object[] getMethodArgumentValues(NativeWebRequest request, ModelAndViewContainer mavContainer,
            Object... providedArgs) throws Exception {

        MethodParameter[] parameters = getMethodParameters();
        Object[] args = new Object[parameters.length];
        for (int i = 0; i < parameters.length; i++) {
            MethodParameter parameter = parameters[i];
            parameter.initParameterNameDiscovery(this.parameterNameDiscoverer);
            args[i] = resolveProvidedArgument(parameter, providedArgs);
            if (args[i] != null) {
                continue;
            }
            if (this.argumentResolvers.supportsParameter(parameter)) {
                try {
                    args[i] = this.argumentResolvers.resolveArgument(
                            parameter, mavContainer, request, this.dataBinderFactory);
                    continue;
                }
                catch (Exception ex) {
                    if (logger.isDebugEnabled()) {
                        logger.debug(getArgumentResolutionErrorMessage("Failed to resolve", i), ex);
                    }
                    throw ex;
                }
            }
            if (args[i] == null) {
                throw new IllegalStateException("Could not resolve method parameter at index " +
                        parameter.getParameterIndex() + " in " + parameter.getMethod().toGenericString() +
                        ": " + getArgumentResolutionErrorMessage("No suitable resolver for", i));
            }
        }
        return args;
    }

3. 最后调用到org.springframework.web.method.support.HandlerMethodArgumentResolverComposite#resolveArgument 解析参数

    public Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        HandlerMethodArgumentResolver resolver = getArgumentResolver(parameter);
        if (resolver == null) {
            throw new IllegalArgumentException("Unknown parameter type [" + parameter.getParameterType().getName() + "]");
        }
        return resolver.resolveArgument(parameter, mavContainer, webRequest, binderFactory);
    }

(1) 首先调用 org.springframework.web.method.support.HandlerMethodArgumentResolverComposite#getArgumentResolver 获取参数解析器

    private HandlerMethodArgumentResolver getArgumentResolver(MethodParameter parameter) {
        HandlerMethodArgumentResolver result = this.argumentResolverCache.get(parameter);
        if (result == null) {
            for (HandlerMethodArgumentResolver methodArgumentResolver : this.argumentResolvers) {
                if (logger.isTraceEnabled()) {
                    logger.trace("Testing if argument resolver [" + methodArgumentResolver + "] supports [" +
                            parameter.getGenericParameterType() + "]");
                }
                if (methodArgumentResolver.supportsParameter(parameter)) {
                    result = methodArgumentResolver;
                    this.argumentResolverCache.put(parameter, result);
                    break;
                }
            }
        }
        return result;
    }

　　这里获取到的参数解析器是: org.springframework.web.method.annotation.RequestParamMethodArgumentResolver

(2) 然后调用org.springframework.web.method.annotation.AbstractNamedValueMethodArgumentResolver#resolveArgument 解析参数

    public final Object resolveArgument(MethodParameter parameter, ModelAndViewContainer mavContainer,
            NativeWebRequest webRequest, WebDataBinderFactory binderFactory) throws Exception {

        NamedValueInfo namedValueInfo = getNamedValueInfo(parameter);
        MethodParameter nestedParameter = parameter.nestedIfOptional();

        Object resolvedName = resolveStringValue(namedValueInfo.name);
        if (resolvedName == null) {
            throw new IllegalArgumentException(
                    "Specified name must not resolve to null: [" + namedValueInfo.name + "]");
        }

        Object arg = resolveName(resolvedName.toString(), nestedParameter, webRequest);
        if (arg == null) {
            if (namedValueInfo.defaultValue != null) {
                arg = resolveStringValue(namedValueInfo.defaultValue);
            }
            else if (namedValueInfo.required && !nestedParameter.isOptional()) {
                handleMissingValue(namedValueInfo.name, nestedParameter, webRequest);
            }
            arg = handleNullValue(namedValueInfo.name, arg, nestedParameter.getNestedParameterType());
        }
        else if ("".equals(arg) && namedValueInfo.defaultValue != null) {
            arg = resolveStringValue(namedValueInfo.defaultValue);
        }

        if (binderFactory != null) {
            WebDataBinder binder = binderFactory.createBinder(webRequest, null, namedValueInfo.name);
            try {
                arg = binder.convertIfNecessary(arg, parameter.getParameterType(), parameter);
            }
            catch (ConversionNotSupportedException ex) {
                throw new MethodArgumentConversionNotSupportedException(arg, ex.getRequiredType(),
                        namedValueInfo.name, parameter, ex.getCause());
            }
            catch (TypeMismatchException ex) {
                throw new MethodArgumentTypeMismatchException(arg, ex.getRequiredType(),
                        namedValueInfo.name, parameter, ex.getCause());

            }
        }

        handleResolvedValue(arg, namedValueInfo.name, parameter, mavContainer, webRequest);

        return arg;
    }

1》  resolveStringValue(namedValueInfo.name); 获取到参数的名称， 这里获取到的值是：  imgFile

2》 调用org.springframework.web.method.annotation.RequestParamMethodArgumentResolver#resolveName 获取参数的值

    protected Object resolveName(String name, MethodParameter parameter, NativeWebRequest request) throws Exception {
        HttpServletRequest servletRequest = request.getNativeRequest(HttpServletRequest.class);
        MultipartHttpServletRequest multipartRequest =
                WebUtils.getNativeRequest(servletRequest, MultipartHttpServletRequest.class);

        Object mpArg = MultipartResolutionDelegate.resolveMultipartArgument(name, parameter, servletRequest);
        if (mpArg != MultipartResolutionDelegate.UNRESOLVABLE) {
            return mpArg;
        }

        Object arg = null;
        if (multipartRequest != null) {
            List<MultipartFile> files = multipartRequest.getFiles(name);
            if (!files.isEmpty()) {
                arg = (files.size() == 1 ? files.get(0) : files);
            }
        }
        if (arg == null) {
            String[] paramValues = request.getParameterValues(name);
            if (paramValues != null) {
                arg = (paramValues.length == 1 ? paramValues[0] : paramValues);
            }
        }
        return arg;
    }

上面的代码会走return mpArg; 然后结束方法。所以解析的过程是在：org.springframework.web.multipart.support.MultipartResolutionDelegate#resolveMultipartArgument

    public static Object resolveMultipartArgument(String name, MethodParameter parameter, HttpServletRequest request)
            throws Exception {

        MultipartHttpServletRequest multipartRequest =
                WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class);
        boolean isMultipart = (multipartRequest != null || isMultipartContent(request));

        if (MultipartFile.class == parameter.getNestedParameterType()) {
            if (multipartRequest == null && isMultipart) {
                multipartRequest = adaptToMultipartHttpServletRequest(request);
            }
            return (multipartRequest != null ? multipartRequest.getFile(name) : null);
        }
        else if (isMultipartFileCollection(parameter)) {
            if (multipartRequest == null && isMultipart) {
                multipartRequest = adaptToMultipartHttpServletRequest(request);
            }
            return (multipartRequest != null ? multipartRequest.getFiles(name) : null);
        }
        else if (isMultipartFileArray(parameter)) {
            if (multipartRequest == null && isMultipart) {
                multipartRequest = adaptToMultipartHttpServletRequest(request);
            }
            if (multipartRequest != null) {
                List<MultipartFile> multipartFiles = multipartRequest.getFiles(name);
                return multipartFiles.toArray(new MultipartFile[multipartFiles.size()]);
            }
            else {
                return null;
            }
        }
        else if (servletPartClass != null) {
            if (servletPartClass == parameter.getNestedParameterType()) {
                return (isMultipart ? RequestPartResolver.resolvePart(request, name) : null);
            }
            else if (isPartCollection(parameter)) {
                return (isMultipart ? RequestPartResolver.resolvePartList(request, name) : null);
            }
            else if (isPartArray(parameter)) {
                return (isMultipart ? RequestPartResolver.resolvePartArray(request, name) : null);
            }
        }
        return UNRESOLVABLE;
    }

上面代码会走 multipartRequest.getFile(name) 获取MultipartFile， 最后交给：org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest#getFile  (实际上也就是拿取上面checkMultipart 过程中最后一步缓存起来的信息。获取到的实际类型是org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.StandardMultipartFile)

    public MultipartFile getFile(String name) {
        return getMultipartFiles().getFirst(name);
    }

    protected MultiValueMap<String, MultipartFile> getMultipartFiles() {
        if (this.multipartFiles == null) {
            initializeMultipart();
        }
        return this.multipartFiles;
    }

最后映射到参数上的MultipartFile 如下：

3. multipartFile.transferTo(new File(fileDir + fileName));// 保存文件 

实际走的方法是：org.springframework.web.multipart.support.StandardMultipartHttpServletRequest.StandardMultipartFile#transferTo

        public void transferTo(File dest) throws IOException, IllegalStateException {
            this.part.write(dest.getPath());
        }

接着调用： org.apache.catalina.core.ApplicationPart#write

    public void write(String fileName) throws IOException {
        File file = new File(fileName);
        if (!file.isAbsolute()) {
            file = new File(location, fileName);
        }
        try {
            fileItem.write(file);
        } catch (Exception e) {
            throw new IOException(e);
        }
    }

接着调用： org.apache.tomcat.util.http.fileupload.disk.DiskFileItem#write

    public void write(File file) throws Exception {
        if (isInMemory()) {
            FileOutputStream fout = null;
            try {
                fout = new FileOutputStream(file);
                fout.write(get());
                fout.close();
            } finally {
                IOUtils.closeQuietly(fout);
            }
        } else {
            File outputFile = getStoreLocation();
            if (outputFile != null) {
                // Save the length of the file
                size = outputFile.length();
                /*
                 * The uploaded file is being stored on disk
                 * in a temporary location so move it to the
                 * desired file.
                 */
                if (!outputFile.renameTo(file)) {
                    BufferedInputStream in = null;
                    BufferedOutputStream out = null;
                    try {
                        in = new BufferedInputStream(
                            new FileInputStream(outputFile));
                        out = new BufferedOutputStream(
                                new FileOutputStream(file));
                        IOUtils.copy(in, out);
                        out.close();
                    } finally {
                        IOUtils.closeQuietly(in);
                        IOUtils.closeQuietly(out);
                    }
                }
            } else {
                /*
                 * For whatever reason we cannot write the
                 * file to disk.
                 */
                throw new FileUploadException(
                    "Cannot write uploaded file to disk!");
            }
        }
    }

　　getStoreLocation() 获取到文件， 然后进行拷贝操作，拷贝到指定的目录。