Spring MVC文件请求处理详解：MultipartResolver
org.springframework.web.multipart.MultipartResolver是Spring-Web针对RFC1867实现的多文件上传解决策略。
1 使用场景
前端上传文件时，无论是使用比较传统的表单，还是使用FormData对象，其本质都是发送一个multipart/form-data请求。
例如，前端模拟上传代码如下：
var formdata = new FormData();
formdata.append("key1", "value1");
formdata.append("key2", "value2");
formdata.append("file1", fileInput.files[0], "/d:/Downloads/rfc1867.pdf");
formdata.append("file2", fileInput.files[0], "/d:/Downloads/rfc1314.pdf");
var requestOptions = {
method: 'POST',
body: formdata,
redirect: 'follow'
};
fetch("http://localhost:10001/file/upload", requestOptions)
.then(response => response.text())
.then(result => console.log(result))
.catch(error => console.log('error', error));
实际会发送如下HTTP请求：
POST /file/upload HTTP/1.1
Host: localhost:10001
Content-Length: 536
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="key1"
value1
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="key2"
value2
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file1"; filename="/d:/Downloads/rfc1867.pdf"
Content-Type: application/pdf
(data)
----WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="file2"; filename="/d:/Downloads/rfc1314.pdf"
Content-Type: application/pdf
(data)
----WebKitFormBoundary7MA4YWxkTrZu0gW
在后端可以通过MultipartHttpServletRequest接收文件：
@RestController
@RequestMapping("file")
public class FileUploadController {
@RequestMapping("/upload")
public String upload(MultipartHttpServletRequest request) {
// 获取非文件参数
String value1 = request.getParameter("key1");
System.out.println(value1); // value1
String value2 = request.getParameter("key2");
System.out.println(value2); // value2
// 获取文件
MultipartFile file1 = request.getFile("file1");
System.out.println(file1 != null ? file1.getOriginalFilename() : "null"); // rfc1867.pdf
MultipartFile file2 = request.getFile("file2");
System.out.println(file2 != null ? file2.getOriginalFilename() : "null"); // rfc1314.pdf
return "Hello MultipartResolver!";
}
}
2 MultipartResolver接口
2.1 MultipartResolver的功能
org.springframework.web.multipart.MultipartResolver是Spring-Web根据RFC1867规范实现的多文件上传的策略接口。
同时，MultipartResolver是Spring对文件上传处理流程在接口层次的抽象。
也就是说，当涉及到文件上传时，Spring都会使用MultipartResolver接口进行处理，而不涉及具体实现类。
MultipartResolver接口源码如下：
public interface MultipartResolver {
/**
* 判断当前HttpServletRequest请求是否是文件请求
*/
boolean isMultipart(HttpServletRequest request);
/**
* 将当前HttpServletRequest请求的数据（文件和普通参数）封装成MultipartHttpServletRequest对象
*/
MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;
/**
* 清除文件上传产生的临时资源（如服务器本地临时文件）
*/
void cleanupMultipart(MultipartHttpServletRequest request);
}
2.2 在DispatcherServlet中的使用
DispatcherServlet中持有MultipartResolver成员变量：
public class DispatcherServlet extends FrameworkServlet {
/** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";
/** MultipartResolver used by this servlet. */
@Nullable
private MultipartResolver multipartResolver;
}
DispatcherServlet在初始化时，会从Spring容器中获取名为multipartResolver的对象（该对象是MultipartResolver实现类），作为文件上传解析器：
/**
* Initialize the MultipartResolver used by this class. * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
* no multipart handling is provided. */
private void initMultipartResolver(ApplicationContext context) {
try {
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");
}
}
}
需要注意的是，如果Spring容器中不存在名为multipartResolver的对象，DispatcherServlet并不会额外指定默认的文件解析器。此时，DispatcherServlet不会对文件上传请求进行处理。也就是说，尽管当前请求是文件请求，也不会被处理成MultipartHttpServletRequest，如果我们在控制层进行强制类型转换，会抛异常。
DispatcherServlet在处理业务时，会按照顺序分别调用这些方法进行文件上传处理，相关核心源码如下：
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
boolean multipartRequestParsed = false;
try {
// 判断&封装文件请求
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// 请求处理……
}
finally {
// 清除文件上传产生的临时资源
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
在checkMultipart()方法中，会进行判断、封装文件请求：
/**
* 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 (DispatcherType.REQUEST.equals(request.getDispatcherType())) {
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;
}
总的来说，DispatcherServlet处理文件请求会经过以下步骤：
- 判断当前HttpServletRequest请求是否是文件请求
- 是：将当前
HttpServletRequest请求的数据（文件和普通参数）封装成MultipartHttpServletRequest对象 - 不是：不处理
- 是：将当前
DispatcherServlet对原始HttpServletRequest或MultipartHttpServletRequest对象进行业务处理- 业务处理完成，清除文件上传产生的临时资源
2.3 MultipartResolver实现类&配置方式
Spring提供了两个MultipartResolver实现类：
org.springframework.web.multipart.support.StandardServletMultipartResolver：根据Servlet 3.0+ Part Api实现org.springframework.web.multipart.commons.CommonsMultipartResolver：根据Apache Commons FileUpload实现
在Spring Boot 2.0+中，默认会在org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration中创建StandardServletMultipartResolver作为默认文件解析器：
@AutoConfiguration
@ConditionalOnClass({ Servlet.class, StandardServletMultipartResolver.class, MultipartConfigElement.class })
@ConditionalOnProperty(prefix = "spring.servlet.multipart", name = "enabled", matchIfMissing = true)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(MultipartProperties.class)
public class MultipartAutoConfiguration {
private final MultipartProperties multipartProperties;
public MultipartAutoConfiguration(MultipartProperties multipartProperties) {
this.multipartProperties = multipartProperties;
}
@Bean
@ConditionalOnMissingBean({ MultipartConfigElement.class, CommonsMultipartResolver.class })
public MultipartConfigElement multipartConfigElement() {
return this.multipartProperties.createMultipartConfig();
}
@Bean(name = DispatcherServlet.MULTIPART_RESOLVER_BEAN_NAME)
@ConditionalOnMissingBean(MultipartResolver.class)
public StandardServletMultipartResolver multipartResolver() {
StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
multipartResolver.setResolveLazily(this.multipartProperties.isResolveLazily());
return multipartResolver;
}
}
当需要指定其他文件解析器时，只需要引入相关依赖，然后配置一个名为multipartResolver的bean对象：
@Bean
public MultipartResolver multipartResolver() {
MultipartResolver multipartResolver = ...;
return multipartResolver;
}
接下来，我们分别详细介绍两种实现类的使用和原理。
3 StandardServletMultipartResolver解析器
3.1 StandardServletMultipartResolver#isMultipart
StandardServletMultipartResolver解析器的通过判断请求的Content-Type来判断是否是文件请求：
public boolean isMultipart(HttpServletRequest request) {
return StringUtils.startsWithIgnoreCase(request.getContentType(),
(this.strictServletCompliance ? "multipart/form-data" : "multipart/"));
}
其中，strictServletCompliance是StandardServletMultipartResolver的成员变量，默认false，表示是否严格遵守Servlet 3.0规范。简单来说就是对Content-Type校验的严格程度。如果strictServletCompliance为false，请求头以multipart/开头就满足文件请求条件；如果strictServletCompliance为true，则需要请求头以multipart/form-data开头。
3.2 StandardServletMultipartResolver#resolveMultipart
StandardServletMultipartResolver在解析文件请求时，会将原始请求封装成StandardMultipartHttpServletRequest对象：
public MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException {
return new StandardMultipartHttpServletRequest(request, this.resolveLazily);
}
需要注意的是，这里传入this.resolveLazily成员变量，表示是否延迟解析。我们可以来看对应构造函数源码：
public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing)
throws MultipartException {
super(request);
if (!lazyParsing) {
parseRequest(request);
}
}
如果需要修改resolveLazily成员变量的值，需要在初始化StandardServletMultipartResolver时指定值。
在Spring Boot 2.0+中，默认会在org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration中创建StandardServletMultipartResolver作为默认文件解析器，此时会从MultipartProperties中读取resolveLazily值。因此，如果是使用Spring Boot 2.0+默认配置的文件解析器，可以在properties或.yml文件中指定resolveLazily值：
spring.servlet.multipart.resolve-lazily=true
如果是使用自定义配置的方式配置StandardServletMultipartResolver，则可以在初始化的手动赋值：
@Bean
public MultipartResolver multipartResolver() {
StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();
multipartResolver.setResolveLazily(true);
return multipartResolver;
}
3.3 StandardMultipartHttpServletRequest#parseRequest
当resolveLazily为true时，会马上调用parseRequest()方法会对请求进行实际解析，该方法会完成两件事情：
- 使用Servlet 3.0的
PartAPI，获取Part集合 - 解析
Part对象，封装表单参数和表单文件
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);
}
}
经过parseRequest()方法处理，我们在业务处理时，直接调用StandardMultipartHttpServletRequest接口的getXxx()方法就可以获取表单参数或表单文件信息。
当resolveLazily为false时，在MultipartResolver#resolveMultipart()阶段并不会进行文件请求解析。也就是说，此时StandardMultipartHttpServletRequest对象的成员变量都是空值。那么，resolveLazily为false时文件请求解析是在什么时候完成的呢？
实际上，在调用StandardMultipartHttpServletRequest接口的getXxx()方法时，内部会判断是否已经完成文件请求解析。如果未解析，就会调用partRequest()方法进行解析，例如：
@Override
public Enumeration<String> getParameterNames() {
if (this.multipartParameterNames == null) {
initializeMultipart(); // parseRequest(getRequest());
}
// 业务处理……
}
3.4 HttpServletRequest#getParts
根据StandardMultipartHttpServletRequest#parseRequest源码可以发现，StandardServletMultipartResolver解析文件请求依靠的是HttpServletRequest#getParts方法。
这是StandardServletMultipartResolver是根据标准Servlet 3.0实现的核心体现。
在Servlet 3.0中定义了javax.servlet.http.Part，用来表示multipart/form-data请求体中的表单数据或文件：
public interface Part {
public InputStream getInputStream() throws IOException;
public String getContentType();
public String getName();
public String getSubmittedFileName();
public long getSize();
public void write(String fileName) throws IOException;
public void delete() throws IOException;
public String getHeader(String name);
public Collection<String> getHeaders(String name);
public Collection<String> getHeaderNames();
}
在javax.servlet.http.HttpServletRequest，提供了获取multipart/form-data请求体各个part的方法：
public interface HttpServletRequest extends ServletRequest {
/**
* Return a collection of all uploaded Parts.
*
* @return A collection of all uploaded Parts.
* @throws IOException
* if an I/O error occurs
* @throws IllegalStateException
* if size limits are exceeded or no multipart configuration is
* provided
* @throws ServletException
* if the request is not multipart/form-data
* @since Servlet 3.0
*/
public Collection<Part> getParts() throws IOException, ServletException;
/**
* Gets the named Part or null if the Part does not exist. Triggers upload
* of all Parts.
*
* @param name The name of the Part to obtain
*
* @return The named Part or null if the Part does not exist
* @throws IOException
* if an I/O error occurs
* @throws IllegalStateException
* if size limits are exceeded
* @throws ServletException
* if the request is not multipart/form-data
* @since Servlet 3.0
*/
public Part getPart(String name) throws IOException, ServletException;
}
所有实现标准Servlet 3.0规范的Web服务器，都必须实现getPart()/getParts()方法。也就是说，这些Web服务器在解析请求时，会将multipart/form-data请求体中的表单数据或文件解析成Part对象集合。通过HttpServletRequest的getPart()/getParts()方法，可以获取这些Part对象，进而获取multipart/form-data请求体中的表单数据或文件。
每个Web服务器对Servlet 3.0规范都有自己的实现方式。对于Spring Boot来说，通常使用的是Tomcat/Undertow/Jetty内嵌Web服务器。通常只需要了解这三种服务器的实现方式即可。
3.4.1 Tomcat实现
Tomcat是Spring Boot默认使用的内嵌Web服务器，只需要引入如下依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
会默认引入Tomcat依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</dependency>
Tomcat解析文件请求的核心在于org.apache.catalina.connector.Request#parseParts方法，核心代码如下：
// 1、创建ServletFileUpload文件上传对象
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());
this.parts = new ArrayList<>();
try {
// 2、解析文件请求
List<FileItem> items =
upload.parseRequest(new ServletRequestContext(this));
// 3、封装Part对象
for (FileItem item : items) {
ApplicationPart part = new ApplicationPart(item, location);
this.parts.add(part);
}
}
success = true;
}
核心步骤如下：
- 创建ServletFileUpload文件上传对象
- 解析文件请求
- 封装
Part对象
org.apache.tomcat.util.http.fileupload.FileUploadBase#parseRequest会进行实际解析文件请求：
public List<FileItem> parseRequest(final RequestContext ctx) throws FileUploadException {
final List<FileItem> items = new ArrayList<>();
boolean successful = false;
try {
final FileItemIterator iter = getItemIterator(ctx);
final FileItemFactory fileItemFactory = Objects.requireNonNull(getFileItemFactory(),
"No FileItemFactory has been set.");
final byte[] buffer = new byte[Streams.DEFAULT_BUFFER_SIZE];
while (iter.hasNext()) {
final FileItemStream item = iter.next();
// Don't use getName() here to prevent an InvalidFileNameException.
final String fileName = item.getName();
final FileItem fileItem = fileItemFactory.createItem(item.getFieldName(), item.getContentType(),
item.isFormField(), fileName);
items.add(fileItem);
try {
Streams.copy(item.openStream(), fileItem.getOutputStream(), true, buffer);
} catch (final FileUploadIOException e) {
throw (FileUploadException) e.getCause();
} catch (final 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;
}
}
简单来说，Tomcat会使用java.io.InputStream和java.io.OutputStream（传统IO流）将multipart请求中的表单参数和文件保存到服务器本地临时文件，然后将本地临时文件信息封装成Part对象返回。
也就是说，我们在业务中获取到的文件实际上都来自服务器本地临时文件。
3.4.2 Undertow实现
为了使用Undertow服务器，需要引入如下依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-undertow</artifactId>
</dependency>
Undertow解析文件请求的核心在于io.undertow.servlet.spec.HttpServletRequestImpl#loadParts方法，核心代码如下
final List<Part> parts = new ArrayList<>();
String mimeType = exchange.getRequestHeaders().getFirst(Headers.CONTENT_TYPE);
if (mimeType != null && mimeType.startsWith(MultiPartParserDefinition.MULTIPART_FORM_DATA)) {
// 1、解析文件请求，封装FormData对象
FormData formData = parseFormData();
// 2、封装Part对象
if(formData != null) {
for (final String namedPart : formData) {
for (FormData.FormValue part : formData.get(namedPart)) {
parts.add(new PartImpl(namedPart,
part,
requestContext.getOriginalServletPathMatch().getServletChain().getManagedServlet().getMultipartConfig(),
servletContext, this));
}
}
}
} else {
throw UndertowServletMessages.MESSAGES.notAMultiPartRequest();
}
this.parts = parts;
核心步骤如下：
- 解析文件请求，封装FormData对象
- 封装
Part对象
io.undertow.servlet.spec.HttpServletRequestImpl#parseFormData方法会进行实际解析文件请求，核心代码如下：
final FormDataParser parser = originalServlet.getFormParserFactory().createParser(exchange)
try {
return parsedFormData = parser.parseBlocking();
}
io.undertow.server.handlers.form.MultiPartParserDefinition.MultiPartUploadHandler#parseBlocking核心代码如下：
InputStream inputStream = exchange.getInputStream();
try (PooledByteBuffer pooled = exchange.getConnection().getByteBufferPool().getArrayBackedPool().allocate()){
ByteBuffer buf = pooled.getBuffer();
while (true) {
buf.clear();
int c = inputStream.read(buf.array(), buf.arrayOffset(), buf.remaining());
if (c == -1) {
if (parser.isComplete()) {
break;
} else {
throw UndertowMessages.MESSAGES.connectionTerminatedReadingMultiPartData();
}
} else if (c != 0) {
buf.limit(c);
parser.parse(buf);
}
}
exchange.putAttachment(FORM_DATA, data);
}
return exchange.getAttachment(FORM_DATA);
在这个过程中，Undertow会使用java.io.InputStream和java.io.OutputStream（传统IO流），结合java.nio.ByteBuffer将multipart请求中的表单参数和文件保存到服务器本地临时文件，然后将本地临时文件信息封装成Part对象返回（具体细节可以继续深入阅读相关源码）。
也就是说，我们在业务中获取到的文件实际上都来自服务器本地临时文件。
3.4.2 Jetty实现
为了使用Jetty服务器，需要引入如下依赖：
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-jetty</artifactId>
</dependency>
Jetty解析文件请求的核心在于org.eclipse.jetty.server.Request#getParts方法，核心代码如下
MultipartConfigElement config = (MultipartConfigElement)this.getAttribute("org.eclipse.jetty.multipartConfig");
this._multiParts = this.newMultiParts(config);
// 省略……
return this._multiParts.getParts();
org.eclipse.jetty.server.Request#newMultiParts会创建文件解析器：
private MultiParts newMultiParts(MultipartConfigElement config) throws IOException {
MultiPartFormDataCompliance compliance = this.getHttpChannel().getHttpConfiguration().getMultipartFormDataCompliance();
switch(compliance) {
case RFC7578:
return new MultiPartsHttpParser(this.getInputStream(), this.getContentType(), config, this._context != null ? (File)this._context.getAttribute("javax.servlet.context.tempdir") : null, this);
case LEGACY:
default:
return new MultiPartsUtilParser(this.getInputStream(), this.getContentType(), config, this._context != null ? (File)this._context.getAttribute("javax.servlet.context.tempdir") : null, this);
}
}
org.eclipse.jetty.server.MultiParts.MultiPartsHttpParser#getParts或org.eclipse.jetty.server.MultiParts.MultiPartsUtilParser#getParts则会进行文件请求解析：
public Collection<Part> getParts() throws IOException {
Collection<Part> parts = this._httpParser.getParts();
this.setNonComplianceViolationsOnRequest();
return parts;
}
public Collection<Part> getParts() throws IOException {
Collection<Part> parts = this._utilParser.getParts();
this.setNonComplianceViolationsOnRequest();
return parts;
}
在这个过程中，Jetty会使用java.io.InputStream和java.io.OutputStream（传统IO流），结合java.nio.ByteBuffer将multipart请求中的表单参数和文件保存到服务器本地临时文件，然后将本地临时文件信息封装成Part对象返回。
也就是说，我们在业务中获取到的文件实际上都来自服务器本地临时文件。
3.5 StandardServletMultipartResolver#cleanupMultipart
StandardServletMultipartResolver#cleanupMultipart方法会将临时文件删除：
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 {
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);
}
}
}
4 CommonsMultipartResolver解析器
为了使用CommonsMultipartResolver解析器，除了基础的spring-boot-starter-web，还需要额外引入如下依赖：
<dependency>
<groupId>commons-fileupload</groupId>
<artifactId>commons-fileupload</artifactId>
<version>1.4</version>
</dependency>
然后，配置名为multipartResolver的bean（此时Spring Boot不会添加默认文件解析器）：
@Bean
public MultipartResolver multipartResolver() {
CommonsMultipartResolver multipartResolver = new CommonsMultipartResolver();
// 文件请求解析配置：multipartResolver.setXxx()
multipartResolver.setResolveLazily(true);
return multipartResolver;
}
4.1 CommonsMultipartResolver#isMultipart
CommonsMultipartResolver解析器会根据请求方法和请求头来判断文件请求，源码如下：
public boolean isMultipart(HttpServletRequest request) {
return (this.supportedMethods != null ?
this.supportedMethods.contains(request.getMethod()) &&
FileUploadBase.isMultipartContent(new ServletRequestContext(request)) :
ServletFileUpload.isMultipartContent(request));
}
supportedMethods成员变量表示支持的请求方法，默认为null，可以在初始化时指定。
当supportedMethods为null时，即在默认情况下，会调用ServletFileUpload.isMultipartContent()方法进行判断。此时文件请求的满足条件为：
- 请求方法为
POST - 请求头
Content-Type为以multipart/开头
当supportedMethods不为null时，文件请求满足条件为：
- 请求方法在
supportedMethods列表中 - 请求头
Content-Type为以multipart/开头
4.2 CommonsMultipartResolver#resolveMultipart
CommonsMultipartResolver在解析文件请求时，会将原始请求封装成DefaultMultipartHttpServletRequest对象：
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() {
MultipartParsingResult parsingResult = parseRequest(request);
setMultipartFiles(parsingResult.getMultipartFiles());
setMultipartParameters(parsingResult.getMultipartParameters());
setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());
}
};
}
else {
MultipartParsingResult parsingResult = parseRequest(request);
return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
}
}
与StandardServletMultipartResolver相同，CommonsMultipartResolver的resolveLazily成员变量也表示是否会马上解析文件。
当resolveLazily为false时，即默认情况下，不会立即解析文件，只是会将原始请求进行简单封装。只有在调用DefaultMultipartHttpServletRequest#getXxx方法时，会判断文件是否已经解析。如果没有解析，会调用DefaultMultipartHttpServletRequest#initializeMultipart进行解析。
当resolveLazily为true时，会立即调用CommonsMultipartResolver#parseRequest方法进行文件解析。
4.3 CommonsMultipartResolver#parseRequest
CommonsMultipartResolver#parseRequest方法会进行文件请求解析，总的来说包括两个步骤：
- 解析文件请求
- 封装响应
List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
return parseFileItems(fileItems, encoding);
深入阅读源码可以发现，在解析文件请求时，会采用与StandardServletMultipartResolver+Tomcat相同的方式保存临时文件：
public List<FileItem> parseRequest(RequestContext ctx)
throws FileUploadException {
List<FileItem> items = new ArrayList<FileItem>();
boolean successful = false;
try {
FileItemIterator iter = getItemIterator(ctx);
FileItemFactory fac = getFileItemFactory();
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 = ((FileItemIteratorImpl.FileItemStreamImpl) item).name;
FileItem fileItem = fac.createItem(item.getFieldName(), item.getContentType(),
item.isFormField(), fileName);
items.add(fileItem);
try {
Streams.copy(item.openStream(), fileItem.getOutputStream(), true);
} catch (FileUploadIOException e) {
throw (FileUploadException) e.getCause();
} catch (IOException e) {
throw new IOFileUploadException(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?
}
}
}
}
}
4.4 CommonsMultipartResolver#cleanupMultipart
CommonsMultipartResolver#cleanupMultipart方法会将临时文件删除：
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);
}
}
}
