Spring MVC文件请求处理MultipartResolver详解

目录

• 1 使用场景
• 2 MultipartResolver接口
• 2.1 MultipartResolver的功能
• 2.2 在DispatcherServlet中的使用
• 2.3 MultipartResolver实现类&配置方式
• 3 StandardServletMultipartResolver解析器
• 3.1 StandardServletMultipartResolver#isMultipart#
• 3.2 StandardServletMultipartResolver#resolveMultipart
• 3.3 StandardMultipartHttpServletRequest#parseRequest
• 3.4 HttpServletRequest#getParts
• 3.4.1 Tomcat实现
• 3.4.2 Undertow实现
• 3.4.2 Jetty实现
• 3.5 StandardServletMultipartResolver#cleanupMultipart
• 4 CommonsMultipartResolver解析器
• 4.1 CommonsMultipartResolver#isMultipart#
• 4.2 CommonsMultipartResolver#resolveMultipart
• 4.3 CommonsMultipartResolver#parseRequest
• 4.4 CommonsMultipartResolver#cleanupMultipart

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-Dispos编程ition: 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

DispatcherServle开发者_C学习t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

• 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Part API，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",  
           编程                                            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android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(HpythonttpServletRequest 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 resolveMul编程客栈tipart(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);  
      }  
   }  
}

到此这篇关于Spring MVC文件请求处理MultipartResolver详解的文章就介绍到这了,更多相关Spring MVC文件请求内容请搜索我们以前的文章或继续浏览下面的相关文章希望大家以后多多支持我们！