Spring MVC檔案請求處理詳解：MultipartResolver

Xianuii發表於2022-11-28
SpringMVC

org.springframework.web.multipart.MultipartResolver是Spring-Web針對RFC1867實現的多檔案上傳解決策略。

1 使用場景

前端上傳檔案時，無論是使用比較傳統的表單，還是使用FormData物件，其本質都是傳送一個multipart/form-data請求。
例如，前端模擬上傳程式碼如下：

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));

實際會傳送如下HTTP請求：

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

在後端可以透過MultipartHttpServletRequest接收檔案：

@RestController  
@RequestMapping("file")  
public class FileUploadController {   
    @RequestMapping("/upload")  
    public String upload(MultipartHttpServletRequest request) {  
        // 獲取非檔案引數  
        String value1 = request.getParameter("key1");  
        System.out.println(value1); // value1  
        String value2 = request.getParameter("key2");  
        System.out.println(value2); // value2  
        // 獲取檔案  
        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介面

2.1 MultipartResolver的功能

org.springframework.web.multipart.MultipartResolver是Spring-Web根據RFC1867規範實現的多檔案上傳的策略介面。
同時，MultipartResolver是Spring對檔案上傳處理流程在介面層次的抽象。
也就是說，當涉及到檔案上傳時，Spring都會使用MultipartResolver介面進行處理，而不涉及具體實現類。 MultipartResolver`介面原始碼如下：

public interface MultipartResolver {  
	/**
	* 判斷當前HttpServletRequest請求是否是檔案請求
	*/
    boolean isMultipart(HttpServletRequest request);  
	/**
	*  將當前HttpServletRequest請求的資料（檔案和普通引數）封裝成MultipartHttpServletRequest物件
	*/
    MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;  
	/**
	*  清除檔案上傳產生的臨時資源（如伺服器本地臨時檔案）
	*/
    void cleanupMultipart(MultipartHttpServletRequest request);  
}

2.2 在DispatcherServlet中的使用

DispatcherServlet中持有MultipartResolver成員變數：

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在初始化時，會從Spring容器中獲取名為multipartResolver的物件（該物件是MultipartResolver實現類），作為檔案上傳解析器：

/**  
 * 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容器中不存在名為multipartResolver的物件，DispatcherServlet並不會額外指定預設的檔案解析器。此時，DispatcherServlet不會對檔案上傳請求進行處理。也就是說，儘管當前請求是檔案請求，也不會被處理成MultipartHttpServletRequest，如果我們在控制層進行強制型別轉換，會拋異常。

DispatcherServlet在處理業務時，會按照順序分別呼叫這些方法進行檔案上傳處理，相關核心原始碼如下：

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {  
   HttpServletRequest processedRequest = request;
   boolean multipartRequestParsed = false;
   try {
		// 判斷&封裝檔案請求
         processedRequest = checkMultipart(request);  
         multipartRequestParsed = (processedRequest != request); 
         // 請求處理……
   }  
   finally {   
         // 清除檔案上傳產生的臨時資源
         if (multipartRequestParsed) {  
            cleanupMultipart(processedRequest);  
         }  
   }  
}

在checkMultipart()方法中，會進行判斷、封裝檔案請求：

/**  
 * 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;  
}

總的來說，DispatcherServlet處理檔案請求會經過以下步驟：

  1. 判斷當前HttpServletRequest請求是否是檔案請求
    1. 是：將當前HttpServletRequest請求的資料（檔案和普通引數）封裝成MultipartHttpServletRequest物件
    2. 不是：不處理
  2. DispatcherServlet對原始HttpServletRequest或MultipartHttpServletRequest物件進行業務處理
  3. 業務處理完成，清除檔案上傳產生的臨時資源

2.3 MultipartResolver實現類&配置方式

Spring提供了兩個MultipartResolver實現類：

  • org.springframework.web.multipart.support.StandardServletMultipartResolver：根據Servlet 3.0+ Part Api實現
  • org.springframework.web.multipart.commons.CommonsMultipartResolver：根據Apache Commons FileUpload實現

在Spring Boot 2.0+中，預設會在org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration中建立StandardServletMultipartResolver作為預設檔案解析器：

@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;  
   }  
}

當需要指定其他檔案解析器時，只需要引入相關依賴，然後配置一個名為multipartResolver的bean物件：

@Bean  
public MultipartResolver multipartResolver() {  
    MultipartResolver multipartResolver = ...;  
    return multipartResolver;  
}

接下來，我們分別詳細介紹兩種實現類的使用和原理。

3 StandardServletMultipartResolver解析器

3.1 StandardServletMultipartResolver#isMultipart

StandardServletMultipartResolver解析器的透過判斷請求的Content-Type來判斷是否是檔案請求：

public boolean isMultipart(HttpServletRequest request) {  
   return StringUtils.startsWithIgnoreCase(request.getContentType(),  
         (this.strictServletCompliance ? "multipart/form-data" : "multipart/"));  
}

其中，strictServletCompliance是StandardServletMultipartResolver的成員變數，預設false，表示是否嚴格遵守Servlet 3.0規範。簡單來說就是對Content-Type校驗的嚴格程度。如果strictServletCompliance為false，請求頭以multipart/開頭就滿足檔案請求條件；如果strictServletCompliance為true，則需要請求頭以multipart/form-data開頭。

3.2 StandardServletMultipartResolver#resolveMultipart

StandardServletMultipartResolver在解析檔案請求時，會將原始請求封裝成StandardMultipartHttpServletRequest物件：

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

需要注意的是，這裡傳入this.resolveLazily成員變數，表示是否延遲解析。我們可以來看對應建構函式原始碼：

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

如果需要修改resolveLazily成員變數的值，需要在初始化StandardServletMultipartResolver時指定值。
在Spring Boot 2.0+中，預設會在org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration中建立StandardServletMultipartResolver作為預設檔案解析器，此時會從MultipartProperties中讀取resolveLazily值。因此，如果是使用Spring Boot 2.0+預設配置的檔案解析器，可以在properties或.yml檔案中指定resolveLazily值：

spring.servlet.multipart.resolve-lazily=true

如果是使用自定義配置的方式配置StandardServletMultipartResolver，則可以在初始化的手動賦值：

@Bean  
public MultipartResolver multipartResolver() {  
    StandardServletMultipartResolver multipartResolver = new StandardServletMultipartResolver();  
    multipartResolver.setResolveLazily(true);  
    return multipartResolver;  
}

3.3 StandardMultipartHttpServletRequest#parseRequest

當resolveLazily為true時，會馬上呼叫parseRequest()方法會對請求進行實際解析，該方法會完成兩件事情：

  1. 使用Servlet 3.0的Part API，獲取Part集合
  2. 解析Part物件，封裝表單引數和表單檔案
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);  
   }  
}

經過parseRequest()方法處理，我們在業務處理時，直接呼叫StandardMultipartHttpServletRequest介面的getXxx()方法就可以獲取表單引數或表單檔案資訊。

當resolveLazily為false時，在MultipartResolver#resolveMultipart()階段並不會進行檔案請求解析。也就是說，此時StandardMultipartHttpServletRequest物件的成員變數都是空值。那麼，resolveLazily為false時檔案請求解析是在什麼時候完成的呢？
實際上，在呼叫StandardMultipartHttpServletRequest介面的getXxx()方法時，內部會判斷是否已經完成檔案請求解析。如果未解析，就會呼叫partRequest()方法進行解析，例如：

@Override  
public Enumeration<String> getParameterNames() {  
   if (this.multipartParameterNames == null) {  
      initializeMultipart();  // parseRequest(getRequest());
   }  
   // 業務處理……
}

3.4 HttpServletRequest#getParts

根據StandardMultipartHttpServletRequest#parseRequest原始碼可以發現，StandardServletMultipartResolver解析檔案請求依靠的是HttpServletRequest#getParts方法。
這是StandardServletMultipartResolver是根據標準Servlet 3.0實現的核心體現。
在Servlet 3.0中定義了javax.servlet.http.Part，用來表示multipart/form-data請求體中的表單資料或檔案：

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，提供了獲取multipart/form-data請求體各個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;  
}

所有實現標準Servlet 3.0規範的Web伺服器，都必須實現getPart()/getParts()方法。也就是說，這些Web伺服器在解析請求時，會將multipart/form-data請求體中的表單資料或檔案解析成Part物件集合。透過HttpServletRequest的getPart()/getParts()方法，可以獲取這些Part物件，進而獲取multipart/form-data請求體中的表單資料或檔案。
每個Web伺服器對Servlet 3.0規範都有自己的實現方式。對於Spring Boot來說，通常使用的是Tomcat/Undertow/Jetty內嵌Web伺服器。通常只需要瞭解這三種伺服器的實現方式即可。

3.4.1 Tomcat實現

Tomcat是Spring Boot預設使用的內嵌Web伺服器，只需要引入如下依賴：

<dependency>  
   <groupId>org.springframework.boot</groupId>  
   <artifactId>spring-boot-starter-web</artifactId>  
</dependency>

會預設引入Tomcat依賴：

<dependency>  
   <groupId>org.springframework.boot</groupId>  
   <artifactId>spring-boot-starter-tomcat</artifactId>  
</dependency>

Tomcat解析檔案請求的核心在於org.apache.catalina.connector.Request#parseParts方法，核心程式碼如下：

// 1、建立ServletFileUpload檔案上傳物件
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、解析檔案請求
    List<FileItem> items =  
            upload.parseRequest(new ServletRequestContext(this));
    // 3、封裝Part物件
    for (FileItem item : items) {  
        ApplicationPart part = new ApplicationPart(item, location);  
        this.parts.add(part);  
        }  
    }  
    success = true;  
}

核心步驟如下：

  1. 建立ServletFileUpload檔案上傳物件
  2. 解析檔案請求
  3. 封裝Part物件
    org.apache.tomcat.util.http.fileupload.FileUploadBase#parseRequest會進行實際解析檔案請求：
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;  
    }
}

簡單來說，Tomcat會使用java.io.InputStream和java.io.OutputStream（傳統IO流）將multipart請求中的表單引數和檔案儲存到伺服器本地臨時檔案，然後將本地臨時檔案資訊封裝成Part物件返回。
也就是說，我們在業務中獲取到的檔案實際上都來自伺服器本地臨時檔案。

3.4.2 Undertow實現

為了使用Undertow伺服器，需要引入如下依賴：

<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解析檔案請求的核心在於io.undertow.servlet.spec.HttpServletRequestImpl#loadParts方法，核心程式碼如下

final List<Part> parts = new ArrayList<>();  
String mimeType = exchange.getRequestHeaders().getFirst(Headers.CONTENT_TYPE);  
if (mimeType != null && mimeType.startsWith(MultiPartParserDefinition.MULTIPART_FORM_DATA)) {  
	// 1、解析檔案請求，封裝FormData物件
    FormData formData = parseFormData();  
    // 2、封裝Part物件
    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;

核心步驟如下：

  1. 解析檔案請求，封裝FormData物件
  2. 封裝Part物件
    io.undertow.servlet.spec.HttpServletRequestImpl#parseFormData方法會進行實際解析檔案請求，核心程式碼如下：
final FormDataParser parser = originalServlet.getFormParserFactory().createParser(exchange) 
try {  
    return parsedFormData = parser.parseBlocking();
}

io.undertow.server.handlers.form.MultiPartParserDefinition.MultiPartUploadHandler#parseBlocking核心程式碼如下：

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);

在這個過程中，Undertow會使用java.io.InputStream和java.io.OutputStream（傳統IO流），結合java.nio.ByteBuffer將multipart請求中的表單引數和檔案儲存到伺服器本地臨時檔案，然後將本地臨時檔案資訊封裝成Part物件返回（具體細節可以繼續深入閱讀相關原始碼）。
也就是說，我們在業務中獲取到的檔案實際上都來自伺服器本地臨時檔案。

3.4.2 Jetty實現

為了使用Jetty伺服器，需要引入如下依賴：

<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解析檔案請求的核心在於org.eclipse.jetty.server.Request#getParts方法，核心程式碼如下

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會建立檔案解析器：

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則會進行檔案請求解析：

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;  
}

在這個過程中，Jetty會使用java.io.InputStream和java.io.OutputStream（傳統IO流），結合java.nio.ByteBuffer將multipart請求中的表單引數和檔案儲存到伺服器本地臨時檔案，然後將本地臨時檔案資訊封裝成Part物件返回。
也就是說，我們在業務中獲取到的檔案實際上都來自伺服器本地臨時檔案。

3.5 StandardServletMultipartResolver#cleanupMultipart

StandardServletMultipartResolver#cleanupMultipart方法會將臨時檔案刪除：

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

為了使用CommonsMultipartResolver解析器，除了基礎的spring-boot-starter-web，還需要額外引入如下依賴：

<dependency>  
   <groupId>commons-fileupload</groupId>  
   <artifactId>commons-fileupload</artifactId>  
   <version>1.4</version>  
</dependency>

然後，配置名為multipartResolver的bean（此時Spring Boot不會新增預設檔案解析器）：

@Bean  
public MultipartResolver multipartResolver() {  
    CommonsMultipartResolver multipartResolver = new CommonsMultipartResolver();  
    // 檔案刪除配置：multipartResolver.setXxx()  
    multipartResolver.setResolveLazily(true);  
    return multipartResolver;  
}

4.1 CommonsMultipartResolver#isMultipart

CommonsMultipartResolver解析器會根據請求方法和請求頭來判斷檔案請求，原始碼如下：

public boolean isMultipart(HttpServletRequest request) {  
   return (this.supportedMethods != null ?  
         this.supportedMethods.contains(request.getMethod()) &&  
               FileUploadBase.isMultipartContent(new ServletRequestContext(request)) :  
         ServletFileUpload.isMultipartContent(request));  
}

supportedMethods成員變數表示支援的請求方法，預設為null，可以在初始化時指定。
當supportedMethods為null時，即在預設情況下，會呼叫ServletFileUpload.isMultipartContent()方法進行判斷。此時檔案請求的滿足條件為：

  1. 請求方法為POST
  2. 請求頭Content-Type為以multipart/開頭
    當supportedMethods不為null時，檔案請求滿足條件為：
  3. 請求方法在supportedMethods列表中
  4. 請求頭Content-Type為以multipart/開頭

4.2 CommonsMultipartResolver#resolveMultipart

CommonsMultipartResolver在解析檔案請求時，會將原始請求封裝成DefaultMultipartHttpServletRequest物件：

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());  
   }  
}

與StandardServletMultipartResolver相同，CommonsMultipartResolver的resolveLazily成員變數也表示是否會馬上解析檔案。
當resolveLazily為false時，即預設情況下，不會立即解析檔案，只是會將原始請求進行簡單封裝。只有在呼叫DefaultMultipartHttpServletRequest#getXxx方法時，會判斷檔案是否已經解析。如果沒有解析，會呼叫DefaultMultipartHttpServletRequest#initializeMultipart進行解析。
當resolveLazily為true時，會立即呼叫CommonsMultipartResolver#parseRequest方法進行檔案解析。

4.3 CommonsMultipartResolver#parseRequest

CommonsMultipartResolver#parseRequest方法會進行檔案請求解析，總的來說包括兩個步驟：

  1. 解析檔案請求
  2. 封裝響應
List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);  
return parseFileItems(fileItems, encoding);

深入閱讀原始碼可以發現，在解析檔案請求時，會採用與StandardServletMultipartResolver+Tomcat相同的方式儲存臨時檔案：

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方法會將臨時檔案刪除：

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);  
      }  
   }  
}

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