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：

1. 判断当前HttpServletRequest请求是否是文件请求
   1. 是：将当前HttpServletRequest请求的数据（文件和普通参数）封装成MultipartHttpServletRequest对象
   2. 不是：不处理
2. DispatcherServlet对原始HttpServletRequest或MultipartHttpServletRequest对象进行业务处理
3. 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：

1. 使用Servlet 3.0的Part API，获取Part集合
2. 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：

1. 创建ServletFileUpload文件上传对象
2. 解析文件请求
3. 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：

1. 解析文件请求，封装FormData对象
2. 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();
    // 文件删除配置：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：

1. 请求方法为POST
2. 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：
3. 请求方法在supportedMethods列表中
4. 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：

1. 解析文件请求
2. 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);
      }
   }
}