MultipartResolver 组件，内容类型( Content-Type )为 multipart/* 的请求的解析器，主要解析文件上传的请求。例如，MultipartResolver 会将 HttpServletRequest 封装成 MultipartHttpServletRequest 对象，便于获取参数信息以及上传的文件
使用方式，可以参考《MyBatis 使用手册》中的 集成 Spring 模块下的 spring-mvc.xml 文件中配置 MultipartResolver 为 CommonsMultipartResolver 实现类，然后在方法入参中用 MultipartFile 类型接收
关于在 SpringBoot 中如何使用文件上传可参考 Spring 官方文档
回顾
先来回顾一下在 DispatcherServlet 中处理请求的过程中哪里使用到 MultipartResolver 组件，可以回到 《一个请求的旅行过程》 中的 DispatcherServlet 的 doDispatch 方法中看看，如下：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
// ... 省略相关代码
// <2> 检测请求是否为上传请求，如果是则通过 multipartResolver 将其封装成 MultipartHttpServletRequest 对象
processedRequest = checkMultipart(request);
// ... 省略相关代码
}
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
// 如果该请求是一个涉及到 multipart （文件）的请求
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 {
// 将 HttpServletRequest 请求封装成 MultipartHttpServletRequest 对象，解析请求里面的参数以及文件
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> 处，如果该请求是一个涉及到 multipart （文件）的请求，则通过 multipartResolver 将 HttpServletRequest 请求封装成 MultipartHttpServletRequest 对象，解析请求里面的参数以及文件
MultipartResolver接口
org.springframework.web.multipart.MultipartResolver 接口，内容类型( Content-Type )为 multipart/* 的请求的解析器接口，代码如下：
public interface MultipartResolver {
/**
* 是否为 multipart 请求
*/
boolean isMultipart(HttpServletRequest request);
/**
* 将 HttpServletRequest 请求封装成 MultipartHttpServletRequest 对象
*/
MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;
/**
* 清理处理 multipart 产生的资源，例如临时文件
*/
void cleanupMultipart(MultipartHttpServletRequest request);
}
MultipartResolver 接口体系的结构如下：
一共有两块：
- 上半部分，MultipartRequest 接口及其实现类
- 下半部分，MultipartResolver 接口以及其实现类
初始化过程
在 DispatcherServlet 的 initMultipartResolver(ApplicationContext context) 方法，初始化 MultipartResolver 组件，方法如下：
private void initMultipartResolver(ApplicationContext context) {
try {
// 从 Spring 上下文中获取名称为 "multipartResolver" ，类型为 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默认为null【注意】 ，需要自己配置，例如《MyBatis 使用手册》中的 集成 Spring 模块下的spring-mvc.xml文件中配置 MultipartResolver 为CommonsMultipartResolver实现类，也可以配置为StandardServletMultipartResolver实现类 - 在 Spring Boot 中，
multipartResolver默认为StandardServletMultipartResolver实现类
目前 Spring 只提供上面两种实现类，接下来依次进行分析
StandardServletMultipartResolver
org.springframework.web.multipart.support.StandardServletMultipartResolver，实现 MultipartResolver 接口，基于 Servlet 3.0 标准的上传文件 API 的 MultipartResolver 实现类，代码如下：
public class StandardServletMultipartResolver implements MultipartResolver {
/**
* 是否延迟解析
*/
private boolean resolveLazily = false;
public void setResolveLazily(boolean resolveLazily) {
this.resolveLazily = resolveLazily;
}
@Override
public boolean isMultipart(HttpServletRequest request) {
// 请求的 Content-type 必须 multipart/ 开头
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 {
// 删除临时的 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，请求的 Content-type 是否以multipart/开头resolveMultipart(HttpServletRequest request)方法，直接将 HttpServletRequest 转换成StandardMultipartHttpServletRequest对象cleanupMultipart(MultipartHttpServletRequest request)方法，清理资源，删除临时的javax.servlet.http.Part们
StandardMultipartHttpServletRequest
org.springframework.web.multipart.support.StandardMultipartHttpServletRequest，继承 AbstractMultipartHttpServletRequest 抽象类，基于 Servlet 3.0 的 Multipart HttpServletRequest 实现类，包含了一个 javax.servlet.http.HttpServletRequest 对象和它的 javax.servlet.http.Part 对象们，其中 Part 对象会被封装成 StandardMultipartFile 对象
构造方法
public class StandardMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
/**
* 普通参数名的集合
*/
@Nullable
private Set<String> multipartParameterNames;
public StandardMultipartHttpServletRequest(HttpServletRequest request) throws MultipartException {
this(request, false);
}
public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing) throws MultipartException {
super(request);
// 如果不需要延迟解析
if (!lazyParsing) {
// 解析请求
parseRequest(request);
}
}
}
multipartParameterNames：普通参数名的集合，非上传文件的参数名- 如果不需要延迟解析，则调用
parseRequest(HttpServletRequest request)方法，直接解析请求
parseRequest
parseRequest(HttpServletRequest request) 方法，解析请求，解析 HttpServletRequest 中的 Part 对象，如果是文件，则封装成 StandardMultipartFile 对象，否则就是普通参数，获取其名称，如下：
private void parseRequest(HttpServletRequest request) {
try {
// <1> 从 HttpServletRequest 中获取 Part 们
Collection<Part> parts = request.getParts();
this.multipartParameterNames = new LinkedHashSet<>(parts.size());
MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
// <2> 遍历 parts 数组
for (Part part : parts) {
// <2.1> 获得请求头中的 Content-Disposition 信息，MIME 协议的扩展
String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
// <2.2> 对 Content-Disposition 信息进行解析，生成 ContentDisposition 对象
// 包含请求参数信息，以面向“对象”的形式进行访问
ContentDisposition disposition = ContentDisposition.parse(headerValue);
// <2.3> 获得文件名
String filename = disposition.getFilename();
// <2.4> 情况一，文件名非空，说明是文件参数，则创建 StandardMultipartFile 对象
if (filename != null) {
if (filename.startsWith("=?") && filename.endsWith("?=")) {
filename = MimeDelegate.decode(filename);
}
files.add(part.getName(), new StandardMultipartFile(part, filename));
}
// <2.5> 情况二，文件名为空，说明是普通参数，则保存参数名称
else {
this.multipartParameterNames.add(part.getName());
}
}
// <3> 将上面生成的 StandardMultipartFile 文件对象们，设置到父类的 multipartFiles 属性中
setMultipartFiles(files);
}
catch (Throwable ex) {
handleParseFailure(ex);
}
}
-
从 HttpServletRequest 中获取 Part 们
-
遍历
parts数组- 从 Part 对象中获得请求头中的
Content-Disposition信息，MIME 协议的扩展 - 对 Content-Disposition 信息进行解析，生成
ContentDisposition对象，包含请求参数信息，以面向“对象”的形式进行访问 - 从
ContentDisposition对象中获得文件名 - 情况一，文件名非空，说明是文件参数，则创建
StandardMultipartFile对象 - 情况二，文件名为空，说明是普通参数，则保存参数名称
- 从 Part 对象中获得请求头中的
-
将上面生成的
StandardMultipartFile文件对象们，设置到父类的multipartFiles属性中 -
如果发生异常则抛出
其他方法
/** 初始化请求 */
@Override
protected void initializeMultipart() {
parseRequest(getRequest());
}
/** 获取请求中的参数名称 */
@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);
}
/** 获取请求中的参数，参数名和参数值的映射 */
@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;
}
/** 获取请求的 Content-Type 内容类型 */
@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);
}
}
/** 获取请求头信息 */
@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 的私有内部静态类，实现了 MultipartFile 接口和 Serializable 接口，内部封装了 javax.servlet.http.Part 对象和文件名称，代码如下：
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));
}
}
这个类封装了 Servlet 3.0 的 Part 对象，也就是我们常用到的 MultipartFile 对象，支持对文件的操作，内部其实都是调用 javax.servlet.http.Part 的方法
AbstractMultipartHttpServletRequest
org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest 抽象类，继承了 HttpServletRequestWrapper 类，实现了 MultipartHttpServletRequest接口
该类是 StandardMultipartHttpServletRequest 和 DefaultMultipartHttpServletRequest 的父类，实现了一些公共的方法，代码如下：
public abstract class AbstractMultipartHttpServletRequest extends HttpServletRequestWrapper implements MultipartHttpServletRequest {
/**
* 请求中的文件信息
*/
@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());
}
/** 获取请求头信息 */
@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;
}
/** 获取文件名称列表 */
@Override
public Iterator<String> getFileNames() {
return getMultipartFiles().keySet().iterator();
}
/** 获取指定文件名的单个文件 */
@Override
public MultipartFile getFile(String name) {
return getMultipartFiles().getFirst(name);
}
/** 获取指定文件名的多个文件 */
@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;
}
/** 交由子类实现 */
protected void initializeMultipart() {
throw new IllegalStateException("Multipart request not initialized");
}
}
上面的方法都比较简单，用于获取请求中的文件对象
MultiValueMap<String, MultipartFile> multipartFiles属性，保存由子类解析出请求中的 Part 对象所封装成的 MultipartFile 对象
CommonsMultipartResolver
org.springframework.web.multipart.commons.CommonsMultipartResolver，实现 MultipartResolver、ServletContextAware 接口，继承 CommonsFileUploadSupport 抽象类，基于 Apache Commons FileUpload 的 MultipartResolver 实现类
如果需要使用这个 MultipartResolver 实现类，需要引入 commons-fileupload、commons-io 和 commons-codec 组件，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 项目中需要使用 CommonsMultipartResolver，需要在 application.yml 中添加如下配置，排除其默认的配置，如下：
spring:
autoconfigure:
exclude: org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration
构造方法
public class CommonsMultipartResolver extends CommonsFileUploadSupport implements MultipartResolver, ServletContextAware {
/**
* 是否延迟解析
*/
private boolean resolveLazily = false;
public CommonsMultipartResolver() {
super();
}
public CommonsMultipartResolver(ServletContext servletContext) {
this();
setServletContext(servletContext);
}
}
isMultipart
@Override
public boolean isMultipart(HttpServletRequest request) {
// 必须是 POST 请求，且 Content-Type 为 multipart/ 开头
return ServletFileUpload.isMultipartContent(request);
}
判断是否为 multipart 请求，必须是 POST 请求，且 Content-Type 为 multipart/ 开头
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() {
// 解析请求，获取文件、参数信息
MultipartParsingResult parsingResult = parseRequest(request);
setMultipartFiles(parsingResult.getMultipartFiles());
setMultipartParameters(parsingResult.getMultipartParameters());
setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());
}
};
}
else {
// 解析请求，获取文件、参数信息
MultipartParsingResult parsingResult = parseRequest(request);
return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
}
}
将 HttpServletRequest 转换成 DefaultMultipartHttpServletRequest 对象
如果开启了延迟解析，则重写该对象的 initializeMultipart() 方法，用于解析请求
否则直接调用 parseRequest(HttpServletRequest request) 方法解析请求，返回 MultipartParsingResult 对象，包含 MultipartFile 对象和普通参数信息
parseRequest
parseRequest(HttpServletRequest request)方法，用于解析请求，返回 MultipartParsingResult 对象，包含 MultipartFile 对象、普通参数信息以及参数的 Content-Type 信息，方法如下：
protected MultipartParsingResult parseRequest(HttpServletRequest request) throws MultipartException {
// <1> 获取请求中的编码
String encoding = determineEncoding(request);
// <2> 获取 ServletFileUpload 对象
FileUpload fileUpload = prepareFileUpload(encoding);
try {
// <3> 获取请求中的流数据
List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
// <4> 将这些流数据转换成 MultipartParsingResult，包含 CommonsMultipartFile、参数信息、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);
}
}
- 获取请求中的编码
- 根据编码获取到 ServletFileUpload 对象（
commons-fileupload中的类），在newFileUpload(FileItemFactory fileItemFactory)方法中返回的就是 ServletFileUpload 对象，可以看到父类 CommonsFileUploadSupport 的构造方法，如下：
// org.springframework.web.multipart.commons.CommonsFileUploadSupport.java
public CommonsFileUploadSupport() {
this.fileItemFactory = newFileItemFactory();
// 由子类实现
this.fileUpload = newFileUpload(getFileItemFactory());
}
具体细节就不讲述了
- 通过 ServletFileUpload 对象解析请求，返回流数据
List<FileItem> fileItems - 调用父类 CommonsFileUploadSupport 的
parseFileItems(List<FileItem> fileItems, String encoding)方法，将这些流数据转换成 MultipartParsingResult 对象
// 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);
}
大致就是遍历 `fileItems` 集合，如果是一个简单的表单字段，那么就是一个普通的参数，将参数名和值保存起来
否则就是文件，将其封装成 `CommonsMultipartFile` 保存起来
cleanupMultipart
cleanupMultipart(MultipartHttpServletRequest request)方法，清理文件产生的临时资源，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，继承 AbstractMultipartHttpServletRequest 抽象类，MultipartHttpServletRequest 的默认实现类，代码如下：
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;
}
}
代码并不复杂，稍微阅读一下就理解了
总结
本文对 Spring MVC 处理请求的过程中使用到的 MultipartResolver 组件进行了分析，如果请求的 Content-Type 为 multipart/*，涉及到文件上传，所以处理请求的第一步需要通过 MultipartResolver 组件对请求进行转换处理。会将 HttpServletRequest 请求对象封装成 MultipartHttpServletRequest 对象，便于获取参数信息和操作上传的文件（MultipartFile 对象）。
MultipartResolver 组件的实现类有两种：
org.springframework.web.multipart.support.StandardServletMultipartResolver：实现 MultipartResolver 接口，基于 Servlet 3.0 标准的上传文件 API 的 MultipartResolver 实现类org.springframework.web.multipart.commons.CommonsMultipartResolver：实现 MultipartResolver 接口，基于 Apache Commons FileUpload 的 MultipartResolver 实现类
两者的区别：
- StandardServletMultipartResolver 会将 HttpServletRequest 封装成
StandardMultipartHttpServletRequest对象，由 Servlet 3.0 提供 API 获取请求中的javax.servlet.http.Part对象，然后进行解析，文件会封装成StandardMultipartFile对象 - CommonsMultipartResolver 会将 HttpServletRequest 封装成
DefaultMultipartHttpServletRequest对象，由 Apache 的 Commons FileUpload 组件来实现，通过org.apache.commons.fileupload.servlet.ServletFileUpload对象获取请求中的org.apache.commons.fileupload.FileItem对象，然后进行解析，文件会封装成CommonsMultipartFile对象，如何使用可以参考上面的 CommonsMultipartResolver 小节
注意事项：
- 在 Spring MVC 中，
multipartResolver默认为null，需要自己配置，例如《MyBatis 使用手册》中的 集成 Spring 模块下的spring-mvc.xml文件中配置 MultipartResolver 为CommonsMultipartResolver实现类，也可以配置为StandardServletMultipartResolver实现类 - 在 Spring Boot 中，
multipartResolver默认为StandardServletMultipartResolver实现类
