該系列文件是本人在學習 Spring MVC 的原始碼過程中總結下來的，可能對讀者不太友好，請結合我的原始碼註釋 Spring MVC 原始碼分析 GitHub 地址 進行閱讀
Spring 版本：5.2.4.RELEASE
該系列其他文件請檢視：《精盡 Spring MVC 原始碼分析 - 文章導讀》
MultipartResolver 元件，內容型別( Content-Type )為 multipart/* 的請求的解析器，主要解析檔案上傳的請求。例如，MultipartResolver 會將 HttpServletRequest 封裝成 MultipartHttpServletRequest 物件，便於獲取引數資訊以及上傳的檔案
使用方式，可以參考《MyBatis 使用手冊》中的 整合 Spring 模組下的 spring-mvc.xml 檔案中配置 MultipartResolver 為 CommonsMultipartResolver 實現類，然後在方法入參中用 MultipartFile 型別接收
關於在 SpringBoot 中如何使用檔案上傳可參考 Spring 官方文件
回顧
先來回顧一下在 DispatcherServlet 中處理請求的過程中哪裡使用到 MultipartResolver 元件，可以回到《一個請求的旅行過程》中的 DispatcherServlet 的 doDispatch 方法中看看，如下：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
// ... 省略相關程式碼
// <2> 檢測請求是否為上傳請求，如果是則通過 multipartResolver 將其封裝成 MultipartHttpServletRequest 物件
processedRequest = checkMultipart(request);
// ... 省略相關程式碼
}
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
// 如果該請求是一個涉及到 multipart （檔案）的請求
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 {
// 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 物件，解析請求裡面的引數以及檔案
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;
}
<2> 處，如果該請求是一個涉及到 multipart （檔案）的請求，則通過 multipartResolver 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 物件，解析請求裡面的引數以及檔案
MultipartResolver介面
org.springframework.web.multipart.MultipartResolver 介面，內容型別( Content-Type )為 multipart/* 的請求的解析器介面，程式碼如下：
public interface MultipartResolver {
/**
* 是否為 multipart 請求
*/
boolean isMultipart(HttpServletRequest request);
/**
* 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 物件
*/
MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;
/**
* 清理處理 multipart 產生的資源，例如臨時檔案
*/
void cleanupMultipart(MultipartHttpServletRequest request);
}
MultipartResolver 介面體系的結構如下：
一共有兩塊：
- 上半部分，MultipartRequest 介面及其實現類
- 下半部分，MultipartResolver 介面以及其實現類
初始化過程
在 DispatcherServlet 的 initMultipartResolver(ApplicationContext context) 方法，初始化 MultipartResolver 元件，方法如下：
private void initMultipartResolver(ApplicationContext context) {
try {
// 從 Spring 上下文中獲取名稱為 "multipartResolver" ，型別為 MultipartResolver 的 Bean
this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
if (logger.isTraceEnabled()) {
logger.trace("Detected " + this.multipartResolver);
}
else if (logger.isDebugEnabled()) {
logger.debug("Detected " + this.multipartResolver.getClass().getSimpleName());
}
}
catch (NoSuchBeanDefinitionException ex) {
// Default is no multipart resolver.
this.multipartResolver = null;
if (logger.isTraceEnabled()) {
logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
}
}
}
-
在 Spring MVC 中，
multipartResolver預設為null【注意】，需要自己配置，例如《MyBatis 使用手冊》中的 整合 Spring 模組下的spring-mvc.xml檔案中配置 MultipartResolver 為CommonsMultipartResolver實現類，也可以配置為StandardServletMultipartResolver實現類 -
在 Spring Boot 中，
multipartResolver預設為StandardServletMultipartResolver實現類
目前 Spring 只提供上面兩種實現類，接下來依次進行分析
StandardServletMultipartResolver
org.springframework.web.multipart.support.StandardServletMultipartResolver，實現 MultipartResolver 介面，基於 Servlet 3.0 標準的上傳檔案 API 的 MultipartResolver 實現類，程式碼如下：
public class StandardServletMultipartResolver implements MultipartResolver {
/**
* 是否延遲解析
*/
private boolean resolveLazily = false;
public void setResolveLazily(boolean resolveLazily) {
this.resolveLazily = resolveLazily;
}
@Override
public boolean isMultipart(HttpServletRequest request) {
// 請求的 Content-type 必須 multipart/ 開頭
return StringUtils.startsWithIgnoreCase(request.getContentType(), "multipart/");
}
@Override
public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
return new StandardMultipartHttpServletRequest(request, this.resolveLazily);
}
@Override
public void cleanupMultipart(MultipartHttpServletRequest request) {
if (!(request instanceof AbstractMultipartHttpServletRequest) || ((AbstractMultipartHttpServletRequest) request).isResolved()) {
// To be on the safe side: explicitly delete the parts,
// but only actual file parts (for Resin compatibility)
try {
// 刪除臨時的 Part
for (Part part : request.getParts()) {
if (request.getFile(part.getName()) != null) {
part.delete();
}
}
}
catch (Throwable ex) {
LogFactory.getLog(getClass()).warn("Failed to perform cleanup of multipart items", ex);
}
}
}
}
-
isMultipart(HttpServletRequest request)方法，請求的 Content-type 是否以multipart/開頭 -
resolveMultipart(HttpServletRequest request)方法，直接將 HttpServletRequest 轉換成StandardMultipartHttpServletRequest物件 -
cleanupMultipart(MultipartHttpServletRequest request)方法，清理資源，刪除臨時的javax.servlet.http.Part們
StandardMultipartHttpServletRequest
org.springframework.web.multipart.support.StandardMultipartHttpServletRequest，繼承 AbstractMultipartHttpServletRequest 抽象類，基於 Servlet 3.0 的 Multipart HttpServletRequest 實現類，包含了一個 javax.servlet.http.HttpServletRequest 物件和它的 javax.servlet.http.Part 物件們，其中 Part 物件會被封裝成 StandardMultipartFile 物件
構造方法
public class StandardMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
/**
* 普通引數名的集合
*/
@Nullable
private Set<String> multipartParameterNames;
public StandardMultipartHttpServletRequest(HttpServletRequest request) throws MultipartException {
this(request, false);
}
public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing) throws MultipartException {
super(request);
// 如果不需要延遲解析
if (!lazyParsing) {
// 解析請求
parseRequest(request);
}
}
}
multipartParameterNames：普通引數名的集合，非上傳檔案的引數名- 如果不需要延遲解析，則呼叫
parseRequest(HttpServletRequest request)方法，直接解析請求
parseRequest
parseRequest(HttpServletRequest request) 方法，解析請求，解析 HttpServletRequest 中的 Part 物件，如果是檔案，則封裝成 StandardMultipartFile 物件，否則就是普通引數，獲取其名稱，如下：
private void parseRequest(HttpServletRequest request) {
try {
// <1> 從 HttpServletRequest 中獲取 Part 們
Collection<Part> parts = request.getParts();
this.multipartParameterNames = new LinkedHashSet<>(parts.size());
MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
// <2> 遍歷 parts 陣列
for (Part part : parts) {
// <2.1> 獲得請求頭中的 Content-Disposition 資訊，MIME 協議的擴充套件
String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
// <2.2> 對 Content-Disposition 資訊進行解析，生成 ContentDisposition 物件
// 包含請求引數資訊，以面向“物件”的形式進行訪問
ContentDisposition disposition = ContentDisposition.parse(headerValue);
// <2.3> 獲得檔名
String filename = disposition.getFilename();
// <2.4> 情況一，檔名非空，說明是檔案引數，則建立 StandardMultipartFile 物件
if (filename != null) {
if (filename.startsWith("=?") && filename.endsWith("?=")) {
filename = MimeDelegate.decode(filename);
}
files.add(part.getName(), new StandardMultipartFile(part, filename));
}
// <2.5> 情況二，檔名為空，說明是普通引數，則儲存引數名稱
else {
this.multipartParameterNames.add(part.getName());
}
}
// <3> 將上面生成的 StandardMultipartFile 檔案物件們，設定到父類的 multipartFiles 屬性中
setMultipartFiles(files);
}
catch (Throwable ex) {
handleParseFailure(ex);
}
}
-
從 HttpServletRequest 中獲取 Part 們
-
遍歷
parts陣列- 從 Part 物件中獲得請求頭中的
Content-Disposition資訊，MIME 協議的擴充套件 - 對 Content-Disposition 資訊進行解析，生成
ContentDisposition物件，包含請求引數資訊，以面向“物件”的形式進行訪問 - 從
ContentDisposition物件中獲得檔名 - 情況一，檔名非空，說明是檔案引數，則建立
StandardMultipartFile物件 - 情況二，檔名為空，說明是普通引數，則儲存引數名稱
- 從 Part 物件中獲得請求頭中的
-
將上面生成的
StandardMultipartFile檔案物件們，設定到父類的multipartFiles屬性中 -
如果發生異常則丟擲
其他方法
/** 初始化請求 */
@Override
protected void initializeMultipart() {
parseRequest(getRequest());
}
/** 獲取請求中的引數名稱 */
@Override
public Enumeration<String> getParameterNames() {
if (this.multipartParameterNames == null) {
initializeMultipart();
}
if (this.multipartParameterNames.isEmpty()) {
return super.getParameterNames();
}
// Servlet 3.0 getParameterNames() not guaranteed to include multipart form items
// (e.g. on WebLogic 12) -> need to merge them here to be on the safe side
Set<String> paramNames = new LinkedHashSet<>();
Enumeration<String> paramEnum = super.getParameterNames();
while (paramEnum.hasMoreElements()) {
paramNames.add(paramEnum.nextElement());
}
paramNames.addAll(this.multipartParameterNames);
return Collections.enumeration(paramNames);
}
/** 獲取請求中的引數，引數名和引數值的對映 */
@Override
public Map<String, String[]> getParameterMap() {
if (this.multipartParameterNames == null) {
initializeMultipart();
}
if (this.multipartParameterNames.isEmpty()) {
return super.getParameterMap();
}
// Servlet 3.0 getParameterMap() not guaranteed to include multipart form items
// (e.g. on WebLogic 12) -> need to merge them here to be on the safe side
Map<String, String[]> paramMap = new LinkedHashMap<>(super.getParameterMap());
for (String paramName : this.multipartParameterNames) {
if (!paramMap.containsKey(paramName)) {
paramMap.put(paramName, getParameterValues(paramName));
}
}
return paramMap;
}
/** 獲取請求的 Content-Type 內容型別 */
@Override
public String getMultipartContentType(String paramOrFileName) {
try {
Part part = getPart(paramOrFileName);
return (part != null ? part.getContentType() : null);
}
catch (Throwable ex) {
throw new MultipartException("Could not access multipart servlet request", ex);
}
}
/** 獲取請求頭資訊 */
@Override
public HttpHeaders getMultipartHeaders(String paramOrFileName) {
try {
Part part = getPart(paramOrFileName);
if (part != null) {
HttpHeaders headers = new HttpHeaders();
for (String headerName : part.getHeaderNames()) {
headers.put(headerName, new ArrayList<>(part.getHeaders(headerName)));
}
return headers;
}
else {
return null;
}
}
catch (Throwable ex) {
throw new MultipartException("Could not access multipart servlet request", ex);
}
}
StandardMultipartFile
org.springframework.web.multipart.support.StandardMultipartHttpServletRequest 的私有內部靜態類，實現了 MultipartFile 介面和 Serializable 介面，內部封裝了 javax.servlet.http.Part 物件和檔名稱，程式碼如下：
private static class StandardMultipartFile implements MultipartFile, Serializable {
private final Part part;
private final String filename;
public StandardMultipartFile(Part part, String filename) {
this.part = part;
this.filename = filename;
}
@Override
public String getName() {
return this.part.getName();
}
@Override
public String getOriginalFilename() {
return this.filename;
}
@Override
public String getContentType() {
return this.part.getContentType();
}
@Override
public boolean isEmpty() {
return (this.part.getSize() == 0);
}
@Override
public long getSize() {
return this.part.getSize();
}
@Override
public byte[] getBytes() throws IOException {
return FileCopyUtils.copyToByteArray(this.part.getInputStream());
}
@Override
public InputStream getInputStream() throws IOException {
return this.part.getInputStream();
}
@Override
public void transferTo(File dest) throws IOException, IllegalStateException {
this.part.write(dest.getPath());
if (dest.isAbsolute() && !dest.exists()) {
// Servlet 3.0 Part.write is not guaranteed to support absolute file paths:
// may translate the given path to a relative location within a temp dir
// (e.g. on Jetty whereas Tomcat and Undertow detect absolute paths).
// At least we offloaded the file from memory storage; it'll get deleted
// from the temp dir eventually in any case. And for our user's purposes,
// we can manually copy it to the requested location as a fallback.
FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest.toPath()));
}
}
@Override
public void transferTo(Path dest) throws IOException, IllegalStateException {
FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest));
}
}
這個類封裝了 Servlet 3.0 的 Part 物件，也就是我們常用到的 MultipartFile 物件，支援對檔案的操作，內部其實都是呼叫 javax.servlet.http.Part 的方法
AbstractMultipartHttpServletRequest
org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest 抽象類，繼承了 HttpServletRequestWrapper 類，實現了 MultipartHttpServletRequest介面
該類是 StandardMultipartHttpServletRequest 和 DefaultMultipartHttpServletRequest 的父類，實現了一些公共的方法，程式碼如下：
public abstract class AbstractMultipartHttpServletRequest extends HttpServletRequestWrapper implements MultipartHttpServletRequest {
/**
* 請求中的檔案資訊
*/
@Nullable
private MultiValueMap<String, MultipartFile> multipartFiles;
protected AbstractMultipartHttpServletRequest(HttpServletRequest request) {
super(request);
}
@Override
public HttpServletRequest getRequest() {
return (HttpServletRequest) super.getRequest();
}
@Override
public HttpMethod getRequestMethod() {
return HttpMethod.resolve(getRequest().getMethod());
}
/** 獲取請求頭資訊 */
@Override
public HttpHeaders getRequestHeaders() {
HttpHeaders headers = new HttpHeaders();
Enumeration<String> headerNames = getHeaderNames();
while (headerNames.hasMoreElements()) {
String headerName = headerNames.nextElement();
headers.put(headerName, Collections.list(getHeaders(headerName)));
}
return headers;
}
/** 獲取檔名稱列表 */
@Override
public Iterator<String> getFileNames() {
return getMultipartFiles().keySet().iterator();
}
/** 獲取指定檔名的單個檔案 */
@Override
public MultipartFile getFile(String name) {
return getMultipartFiles().getFirst(name);
}
/** 獲取指定檔名的多個檔案 */
@Override
public List<MultipartFile> getFiles(String name) {
List<MultipartFile> multipartFiles = getMultipartFiles().get(name);
if (multipartFiles != null) {
return multipartFiles;
}
else {
return Collections.emptyList();
}
}
@Override
public Map<String, MultipartFile> getFileMap() {
return getMultipartFiles().toSingleValueMap();
}
@Override
public MultiValueMap<String, MultipartFile> getMultiFileMap() {
return getMultipartFiles();
}
public boolean isResolved() {
return (this.multipartFiles != null);
}
protected final void setMultipartFiles(MultiValueMap<String, MultipartFile> multipartFiles) {
this.multipartFiles = new LinkedMultiValueMap<>(Collections.unmodifiableMap(multipartFiles));
}
protected MultiValueMap<String, MultipartFile> getMultipartFiles() {
if (this.multipartFiles == null) {
initializeMultipart();
}
return this.multipartFiles;
}
/** 交由子類實現 */
protected void initializeMultipart() {
throw new IllegalStateException("Multipart request not initialized");
}
}
上面的方法都比較簡單，用於獲取請求中的檔案物件
MultiValueMap<String, MultipartFile> multipartFiles屬性，儲存由子類解析出請求中的 Part 物件所封裝成的 MultipartFile 物件
CommonsMultipartResolver
org.springframework.web.multipart.commons.CommonsMultipartResolver，實現 MultipartResolver、ServletContextAware 介面，繼承 CommonsFileUploadSupport 抽象類，基於 Apache Commons FileUpload 的 MultipartResolver 實現類
如果需要使用這個 MultipartResolver 實現類，需要引入 commons-fileupload、commons-io 和 commons-codec 元件，例如：
<dependencies>
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.4</version>
</dependency>
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.8.0</version>
</dependency>
<dependency>
<groupId>commons-codec</groupId>
<artifactId>commons-codec</artifactId>
<version>1.15</version>
</dependency>
</dependencies>
注意，如果 Spring Boot 專案中需要使用 CommonsMultipartResolver，需要在 application.yml 中新增如下配置，排除其預設的配置，如下：
spring:
autoconfigure:
exclude: org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration
構造方法
public class CommonsMultipartResolver extends CommonsFileUploadSupport implements MultipartResolver, ServletContextAware {
/**
* 是否延遲解析
*/
private boolean resolveLazily = false;
public CommonsMultipartResolver() {
super();
}
public CommonsMultipartResolver(ServletContext servletContext) {
this();
setServletContext(servletContext);
}
}
isMultipart
@Override
public boolean isMultipart(HttpServletRequest request) {
// 必須是 POST 請求，且 Content-Type 為 multipart/ 開頭
return ServletFileUpload.isMultipartContent(request);
}
判斷是否為 multipart 請求，必須是 POST 請求，且 Content-Type 為 multipart/ 開頭
resolveMultipart
@Override
public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {
Assert.notNull(request, "Request must not be null");
if (this.resolveLazily) {
return new DefaultMultipartHttpServletRequest(request) {
@Override
protected void initializeMultipart() {
// 解析請求，獲取檔案、引數資訊
MultipartParsingResult parsingResult = parseRequest(request);
setMultipartFiles(parsingResult.getMultipartFiles());
setMultipartParameters(parsingResult.getMultipartParameters());
setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());
}
};
}
else {
// 解析請求，獲取檔案、引數資訊
MultipartParsingResult parsingResult = parseRequest(request);
return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
}
}
將 HttpServletRequest 轉換成 DefaultMultipartHttpServletRequest 物件
如果開啟了延遲解析，則重寫該物件的 initializeMultipart() 方法，用於解析請求
否則直接呼叫 parseRequest(HttpServletRequest request) 方法解析請求，返回 MultipartParsingResult 物件，包含 MultipartFile 物件和普通引數資訊
parseRequest
parseRequest(HttpServletRequest request)方法，用於解析請求，返回 MultipartParsingResult 物件，包含 MultipartFile 物件、普通引數資訊以及引數的 Content-Type 資訊，方法如下：
protected MultipartParsingResult parseRequest(HttpServletRequest request) throws MultipartException {
// <1> 獲取請求中的編碼
String encoding = determineEncoding(request);
// <2> 獲取 ServletFileUpload 物件
FileUpload fileUpload = prepareFileUpload(encoding);
try {
// <3> 獲取請求中的流資料
List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
// <4> 將這些流資料轉換成 MultipartParsingResult，包含 CommonsMultipartFile、引數資訊、Content-type
return parseFileItems(fileItems, encoding);
}
catch (FileUploadBase.SizeLimitExceededException ex) {
throw new MaxUploadSizeExceededException(fileUpload.getSizeMax(), ex);
}
catch (FileUploadBase.FileSizeLimitExceededException ex) {
throw new MaxUploadSizeExceededException(fileUpload.getFileSizeMax(), ex);
}
catch (FileUploadException ex) {
throw new MultipartException("Failed to parse multipart servlet request", ex);
}
}
-
獲取請求中的編碼
-
根據編碼獲取到 ServletFileUpload 物件（
commons-fileupload中的類），在newFileUpload(FileItemFactory fileItemFactory)方法中返回的就是 ServletFileUpload 物件，可以看到父類 CommonsFileUploadSupport 的構造方法，如下：// org.springframework.web.multipart.commons.CommonsFileUploadSupport.java public CommonsFileUploadSupport() { this.fileItemFactory = newFileItemFactory(); // 由子類實現 this.fileUpload = newFileUpload(getFileItemFactory()); }具體細節就不講述了
-
通過 ServletFileUpload 物件解析請求，返回流資料
List<FileItem> fileItems -
呼叫父類 CommonsFileUploadSupport 的
parseFileItems(List<FileItem> fileItems, String encoding)方法，將這些流資料轉換成 MultipartParsingResult 物件// org.springframework.web.multipart.commons.CommonsFileUploadSupport.java protected MultipartParsingResult parseFileItems(List<FileItem> fileItems, String encoding) { MultiValueMap<String, MultipartFile> multipartFiles = new LinkedMultiValueMap<>(); Map<String, String[]> multipartParameters = new HashMap<>(); Map<String, String> multipartParameterContentTypes = new HashMap<>(); // Extract multipart files and multipart parameters. for (FileItem fileItem : fileItems) { if (fileItem.isFormField()) { String value; String partEncoding = determineEncoding(fileItem.getContentType(), encoding); try { value = fileItem.getString(partEncoding); } catch (UnsupportedEncodingException ex) { if (logger.isWarnEnabled()) { logger.warn("Could not decode multipart item '" + fileItem.getFieldName() + "' with encoding '" + partEncoding + "': using platform default"); } value = fileItem.getString(); } String[] curParam = multipartParameters.get(fileItem.getFieldName()); if (curParam == null) { // simple form field multipartParameters.put(fileItem.getFieldName(), new String[] {value}); } else { // array of simple form fields String[] newParam = StringUtils.addStringToArray(curParam, value); multipartParameters.put(fileItem.getFieldName(), newParam); } multipartParameterContentTypes.put(fileItem.getFieldName(), fileItem.getContentType()); } else { // multipart file field CommonsMultipartFile file = createMultipartFile(fileItem); multipartFiles.add(file.getName(), file); LogFormatUtils.traceDebug(logger, traceOn -> "Part '" + file.getName() + "', size " + file.getSize() + " bytes, filename='" + file.getOriginalFilename() + "'" + (traceOn ? ", storage=" + file.getStorageDescription() : "") ); } } return new MultipartParsingResult(multipartFiles, multipartParameters, multipartParameterContentTypes); }大致就是遍歷
fileItems集合，如果是一個簡單的表單欄位，那麼就是一個普通的引數，將引數名和值儲存起來否則就是檔案，將其封裝成
CommonsMultipartFile儲存起來
cleanupMultipart
cleanupMultipart(MultipartHttpServletRequest request)方法，清理檔案產生的臨時資源，如下：
// CommonsMultipartResolver.java
@Override
public void cleanupMultipart(MultipartHttpServletRequest request) {
if (!(request instanceof AbstractMultipartHttpServletRequest) ||
((AbstractMultipartHttpServletRequest) request).isResolved()) {
try {
cleanupFileItems(request.getMultiFileMap());
}
catch (Throwable ex) {
logger.warn("Failed to perform multipart cleanup for servlet request", ex);
}
}
}
// CommonsFileUploadSupport.java
protected void cleanupFileItems(MultiValueMap<String, MultipartFile> multipartFiles) {
for (List<MultipartFile> files : multipartFiles.values()) {
for (MultipartFile file : files) {
if (file instanceof CommonsMultipartFile) {
CommonsMultipartFile cmf = (CommonsMultipartFile) file;
cmf.getFileItem().delete();
LogFormatUtils.traceDebug(logger, traceOn -> "Cleaning up part '..."));
}
}
}
}
DefaultMultipartHttpServletRequest
org.springframework.web.multipart.support.DefaultMultipartHttpServletRequest，繼承 AbstractMultipartHttpServletRequest 抽象類，MultipartHttpServletRequest 的預設實現類，程式碼如下：
public class DefaultMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
private static final String CONTENT_TYPE = "Content-Type";
@Nullable
private Map<String, String[]> multipartParameters;
@Nullable
private Map<String, String> multipartParameterContentTypes;
public DefaultMultipartHttpServletRequest(HttpServletRequest request, MultiValueMap<String, MultipartFile> mpFiles,
Map<String, String[]> mpParams, Map<String, String> mpParamContentTypes) {
super(request);
setMultipartFiles(mpFiles);
setMultipartParameters(mpParams);
setMultipartParameterContentTypes(mpParamContentTypes);
}
public DefaultMultipartHttpServletRequest(HttpServletRequest request) {
super(request);
}
@Override
@Nullable
public String getParameter(String name) {
String[] values = getMultipartParameters().get(name);
if (values != null) {
return (values.length > 0 ? values[0] : null);
}
return super.getParameter(name);
}
@Override
public String[] getParameterValues(String name) {
String[] parameterValues = super.getParameterValues(name);
String[] mpValues = getMultipartParameters().get(name);
if (mpValues == null) {
return parameterValues;
}
if (parameterValues == null || getQueryString() == null) {
return mpValues;
}
else {
String[] result = new String[mpValues.length + parameterValues.length];
System.arraycopy(mpValues, 0, result, 0, mpValues.length);
System.arraycopy(parameterValues, 0, result, mpValues.length, parameterValues.length);
return result;
}
}
@Override
public Enumeration<String> getParameterNames() {
Map<String, String[]> multipartParameters = getMultipartParameters();
if (multipartParameters.isEmpty()) {
return super.getParameterNames();
}
Set<String> paramNames = new LinkedHashSet<>();
paramNames.addAll(Collections.list(super.getParameterNames()));
paramNames.addAll(multipartParameters.keySet());
return Collections.enumeration(paramNames);
}
@Override
public Map<String, String[]> getParameterMap() {
Map<String, String[]> result = new LinkedHashMap<>();
Enumeration<String> names = getParameterNames();
while (names.hasMoreElements()) {
String name = names.nextElement();
result.put(name, getParameterValues(name));
}
return result;
}
@Override
public String getMultipartContentType(String paramOrFileName) {
MultipartFile file = getFile(paramOrFileName);
if (file != null) {
return file.getContentType();
}
else {
return getMultipartParameterContentTypes().get(paramOrFileName);
}
}
@Override
public HttpHeaders getMultipartHeaders(String paramOrFileName) {
String contentType = getMultipartContentType(paramOrFileName);
if (contentType != null) {
HttpHeaders headers = new HttpHeaders();
headers.add(CONTENT_TYPE, contentType);
return headers;
}
else {
return null;
}
}
protected final void setMultipartParameters(Map<String, String[]> multipartParameters) {
this.multipartParameters = multipartParameters;
}
protected Map<String, String[]> getMultipartParameters() {
if (this.multipartParameters == null) {
initializeMultipart();
}
return this.multipartParameters;
}
protected final void setMultipartParameterContentTypes(Map<String, String> multipartParameterContentTypes) {
this.multipartParameterContentTypes = multipartParameterContentTypes;
}
protected Map<String, String> getMultipartParameterContentTypes() {
if (this.multipartParameterContentTypes == null) {
initializeMultipart();
}
return this.multipartParameterContentTypes;
}
}
程式碼並不複雜，稍微閱讀一下就理解了?
總結
本文對 Spring MVC 處理請求的過程中使用到的 MultipartResolver 元件進行了分析，如果請求的 Content-Type 為 multipart/*，涉及到檔案上傳，所以處理請求的第一步需要通過 MultipartResolver 元件對請求進行轉換處理。會將 HttpServletRequest 請求物件封裝成 MultipartHttpServletRequest 物件，便於獲取引數資訊和操作上傳的檔案（MultipartFile 物件）。
MultipartResolver 元件的實現類有兩種：
org.springframework.web.multipart.support.StandardServletMultipartResolver：實現 MultipartResolver 介面，基於 Servlet 3.0 標準的上傳檔案 API 的 MultipartResolver 實現類org.springframework.web.multipart.commons.CommonsMultipartResolver：實現 MultipartResolver 介面，基於 Apache Commons FileUpload 的 MultipartResolver 實現類
兩者的區別：
-
StandardServletMultipartResolver 會將 HttpServletRequest 封裝成
StandardMultipartHttpServletRequest物件，由 Servlet 3.0 提供 API 獲取請求中的javax.servlet.http.Part物件，然後進行解析，檔案會封裝成StandardMultipartFile物件 -
CommonsMultipartResolver 會將 HttpServletRequest 封裝成
DefaultMultipartHttpServletRequest物件，由 Apache 的 Commons FileUpload 元件來實現，通過org.apache.commons.fileupload.servlet.ServletFileUpload物件獲取請求中的org.apache.commons.fileupload.FileItem物件，然後進行解析，檔案會封裝成CommonsMultipartFile物件，如何使用可以參考上面的 CommonsMultipartResolver 小節
注意事項：
- 在 Spring MVC 中，
multipartResolver預設為null，需要自己配置，例如《MyBatis 使用手冊》中的 整合 Spring 模組下的spring-mvc.xml檔案中配置 MultipartResolver 為CommonsMultipartResolver實現類，也可以配置為StandardServletMultipartResolver實現類 - 在 Spring Boot 中，
multipartResolver預設為StandardServletMultipartResolver實現類
參考文章：芋道原始碼《精盡 Spring MVC 原始碼分析》