MultipartConfigElement

MultipartConfigElement 是 javax.servlet 包中的是 JavaEE Servlet 规范定义的标准包。

该类定义了Http服务上传文件存储位置、最大文件大小、最大请求的长度

javax.servlet.MultipartConfigElement

public class MultipartConfigElement {

    private final String location;// = "";
    private final long maxFileSize;// = -1;
    private final long maxRequestSize;// = -1;
    private final int fileSizeThreshold;// = 0;

  。。。。
}

以Springboot Tomcat为例，Tomcat 的 Request 请求会被传递到 SpringMVC 的 DispatcherServlet 中，在 doDispartch() 方法会 Request 的 getParts() 方法被调用， 会去获取 MultipartConfigElement 对象，从中获取存储位置将上传的文件存储在临时存储位置，这些文件以List<Part>的形式存在于 StandardMuiltpartHttpSevrletRequest 中返回给 DispatcherServlet 继续处理。在整个请求处理完成后，DispatcherServlet 会调用 StandardServletMultipartResolver 的 cleanupMultipart() 的方法，清理掉所有的缓存文件。

org.springframework.web.servlet.DispatcherServlet 

	/**
	 * Process the actual dispatching to the handler.
	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
	 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
	 * to find the first that supports the handler class.
	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
	 * themselves to decide which methods are acceptable.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @throws Exception in case of any kind of processing failure
	 */
	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);                    // 将请求转化为 multipart request 让多媒体处理器可生效
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == 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 (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); // 请求处理完成后清除多媒体文件
				}
			}
		}
	}


	/** 将请求转化为 multipart request 让多媒体处理器可生效
	 * Convert the request into a multipart request, and make multipart resolver available.
	 * <p>If no multipart resolver is set, simply use the existing request.
	 * @param request current HTTP request
	 * @return the processed request (multipart wrapper if necessary)
	 * @see MultipartResolver#resolveMultipart
	 */
	protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
		if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
			if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
				if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
					logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
				}
			}
			else if (hasMultipartException(request)) {
				logger.debug("Multipart resolution previously failed for current request - " +
						"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;
	}

org.springframework.web.multipart.support.StandardServletMultipartResolver 

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

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest

	/**
	 * Create a new StandardMultipartHttpServletRequest wrapper for the given request.
	 * @param request the servlet request to wrap
	 * @param lazyParsing whether multipart parsing should be triggered lazily on
	 * first access of multipart files or parameters
	 * @throws MultipartException if an immediate parsing attempt failed
	 * @since 3.2.9
	 */
	public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing)
			throws MultipartException {

		super(request);
		if (!lazyParsing) {
			parseRequest(request);
		}
	}


	private void parseRequest(HttpServletRequest request) {
		try {
			Collection<Part> parts = request.getParts();
			this.multipartParameterNames = new LinkedHashSet<>(parts.size());
			MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
			for (Part part : parts) {
				String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
				ContentDisposition disposition = ContentDisposition.parse(headerValue);
				String filename = disposition.getFilename();
				if (filename != null) {
					if (filename.startsWith("=?") && filename.endsWith("?=")) {
						filename = MimeDelegate.decode(filename);
					}
					files.add(part.getName(), new StandardMultipartFile(part, filename));
				}
				else {
					this.multipartParameterNames.add(part.getName());
				}
			}
			setMultipartFiles(files);
		}
		catch (Throwable ex) {
			handleParseFailure(ex);
		}
	}

	protected void handleParseFailure(Throwable ex) {
		String msg = ex.getMessage();
		if (msg != null && msg.contains("size") && msg.contains("exceed")) {
			throw new MaxUploadSizeExceededException(-1, ex);
		}
		throw new MultipartException("Failed to parse multipart servlet request", ex);
	}

 org.apache.catalina.connector.Request 

/**
 * {@inheritDoc}
 */
@Override
public Collection<Part> getParts() throws IOException, IllegalStateException,
        ServletException {

    parseParts(true);

    if (partsParseException != null) {
        if (partsParseException instanceof IOException) {
            throw (IOException) partsParseException;
        } else if (partsParseException instanceof IllegalStateException) {
            throw (IllegalStateException) partsParseException;
        } else if (partsParseException instanceof ServletException) {
            throw (ServletException) partsParseException;
        }
    }

    return parts;
}

private void parseParts(boolean explicit) {

    // Return immediately if the parts have already been parsed
    if (parts != null || partsParseException != null) {
        return;
    }

    Context context = getContext();
    MultipartConfigElement mce = getWrapper().getMultipartConfigElement();

    if (mce == null) {
        if(context.getAllowCasualMultipartParsing()) {
            mce = new MultipartConfigElement(null, connector.getMaxPostSize(),
                    connector.getMaxPostSize(), connector.getMaxPostSize());
        } else {
            if (explicit) {
                partsParseException = new IllegalStateException(
                        sm.getString("coyoteRequest.noMultipartConfig"));
                return;
            } else {
                parts = Collections.emptyList();
                return;
            }
        }
    }

    Parameters parameters = coyoteRequest.getParameters();
    parameters.setLimit(getConnector().getMaxParameterCount());

    boolean success = false;
    try {
        File location;
        String locationStr = mce.getLocation();
        if (locationStr == null || locationStr.length() == 0) {
            location = ((File) context.getServletContext().getAttribute(
                    ServletContext.TEMPDIR));
        } else {
            // If relative, it is relative to TEMPDIR
            location = new File(locationStr);
            if (!location.isAbsolute()) {
                location = new File(
                        (File) context.getServletContext().getAttribute(ServletContext.TEMPDIR),
                        locationStr).getAbsoluteFile();
            }
        }

        if (!location.exists() && context.getCreateUploadTargets()) {
            log.warn(sm.getString("coyoteRequest.uploadCreate",
                    location.getAbsolutePath(), getMappingData().wrapper.getName()));
            if (!location.mkdirs()) {
                log.warn(sm.getString("coyoteRequest.uploadCreateFail",
                        location.getAbsolutePath()));
            }
        }

        if (!location.isDirectory()) {
            parameters.setParseFailedReason(FailReason.MULTIPART_CONFIG_INVALID);
            partsParseException = new IOException(
                    sm.getString("coyoteRequest.uploadLocationInvalid",
                            location));
            return;
        }


        // Create a new file upload handler
        DiskFileItemFactory factory = new DiskFileItemFactory();
        try {
            factory.setRepository(location.getCanonicalFile());
        } catch (IOException ioe) {
            parameters.setParseFailedReason(FailReason.IO_ERROR);
            partsParseException = ioe;
            return;
        }
        factory.setSizeThreshold(mce.getFileSizeThreshold());

        ServletFileUpload upload = new ServletFileUpload();
        upload.setFileItemFactory(factory);
        upload.setFileSizeMax(mce.getMaxFileSize());
        upload.setSizeMax(mce.getMaxRequestSize());

        parts = new ArrayList<>();
        try {
            List<FileItem> items =
                    upload.parseRequest(new ServletRequestContext(this));
            int maxPostSize = getConnector().getMaxPostSize();
            int postSize = 0;
            Charset charset = getCharset();
            for (FileItem item : items) {
                ApplicationPart part = new ApplicationPart(item, location);
                parts.add(part);
                if (part.getSubmittedFileName() == null) {
                    String name = part.getName();
                    String value = null;
                    try {
                        value = part.getString(charset.name());
                    } catch (UnsupportedEncodingException uee) {
                        // Not possible
                    }
                    if (maxPostSize >= 0) {
                        // Have to calculate equivalent size. Not completely
                        // accurate but close enough.
                        postSize += name.getBytes(charset).length;
                        if (value != null) {
                            // Equals sign
                            postSize++;
                            // Value length
                            postSize += part.getSize();
                        }
                        // Value separator
                        postSize++;
                        if (postSize > maxPostSize) {
                            parameters.setParseFailedReason(FailReason.POST_TOO_LARGE);
                            throw new IllegalStateException(sm.getString(
                                    "coyoteRequest.maxPostSizeExceeded"));
                        }
                    }
                    parameters.addParameter(name, value);
                }
            }

            success = true;
        } catch (InvalidContentTypeException e) {
            parameters.setParseFailedReason(FailReason.INVALID_CONTENT_TYPE);
            partsParseException = new ServletException(e);
        } catch (SizeException e) {
            parameters.setParseFailedReason(FailReason.POST_TOO_LARGE);
            checkSwallowInput();
            partsParseException = new IllegalStateException(e);
        } catch (FileUploadException e) {
            parameters.setParseFailedReason(FailReason.IO_ERROR);
            partsParseException = new IOException(e);
        } catch (IllegalStateException e) {
            // addParameters() will set parseFailedReason
            checkSwallowInput();
            partsParseException = e;
        }
    } finally {
        // This might look odd but is correct. setParseFailedReason() only
        // sets the failure reason if none is currently set. This code could
        // be more efficient but it is written this way to be robust with
        // respect to changes in the remainder of the method.
        if (partsParseException != null || !success) {
            parameters.setParseFailedReason(FailReason.UNKNOWN);
        }
    }
}

org.apache.tomcat.util.http.fileupload.FileUploadBase 

    /**
     * Processes an <a href="http://www.ietf.org/rfc/rfc1867.txt">RFC 1867</a>
     * compliant <code>multipart/form-data</code> stream.
     *
     * @param ctx The context for the request to be parsed.
     *
     * @return A list of <code>FileItem</code> instances parsed from the
     *         request, in the order that they were transmitted.
     *
     * @throws FileUploadException if there are problems reading/parsing
     *                             the request or storing files.
     */
    public List<FileItem> parseRequest(RequestContext ctx)
            throws FileUploadException {
        List<FileItem> items = new ArrayList<>();
        boolean successful = false;
        try {
            FileItemIterator iter = getItemIterator(ctx);
            FileItemFactory fac = getFileItemFactory();
            final byte[] buffer = new byte[Streams.DEFAULT_BUFFER_SIZE];
            if (fac == null) {
                throw new NullPointerException("No FileItemFactory has been set.");
            }
            while (iter.hasNext()) {
                final FileItemStream item = iter.next();
                // Don't use getName() here to prevent an InvalidFileNameException.
                final String fileName = ((FileItemStreamImpl) item).getName();
                FileItem fileItem = fac.createItem(item.getFieldName(), item.getContentType(),
                                                   item.isFormField(), fileName);
                items.add(fileItem);
                try {
                    Streams.copy(item.openStream(), fileItem.getOutputStream(), true, buffer);        // 文件最终上传到temp目录到地方
                } catch (FileUploadIOException e) {
                    throw (FileUploadException) e.getCause();
                } catch (IOException e) {
                    throw new IOFileUploadException(String.format("Processing of %s request failed. %s",
                                                           MULTIPART_FORM_DATA, e.getMessage()), e);
                }
                final FileItemHeaders fih = item.getHeaders();
                fileItem.setHeaders(fih);
            }
            successful = true;
            return items;
        } catch (FileUploadIOException e) {
            throw (FileUploadException) e.getCause();
        } catch (IOException e) {
            throw new FileUploadException(e.getMessage(), e);
        } finally {
            if (!successful) {
                for (FileItem fileItem : items) {
                    try {
                        fileItem.delete();
                    } catch (Exception ignored) {
                        // ignored TODO perhaps add to tracker delete failure list somehow?
                    }
                }
            }
        }
    }