精盡Spring MVC源碼分析 - MultipartResolver 組件

時間 2020-12-21

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原文原文鏈接

該系列文檔是本人在學習 Spring MVC 的源碼過程當中總結下來的，可能對讀者不太友好，請結合個人源碼註釋 Spring MVC 源碼分析 GitHub 地址進行閱讀html

Spring 版本：5.2.4.RELEASEjava

該系列其餘文檔請查看：《精盡 Spring MVC 源碼分析 - 文章導讀》git

MultipartResolver 組件，內容類型( Content-Type )爲 multipart/* 的請求的解析器，主要解析文件上傳的請求。例如，MultipartResolver 會將 HttpServletRequest 封裝成 MultipartHttpServletRequest 對象，便於獲取參數信息以及上傳的文件github

使用方式，能夠參考《MyBatis 使用手冊》中的 集成 Spring 模塊下的 spring-mvc.xml 文件中配置 MultipartResolver 爲 CommonsMultipartResolver 實現類，而後在方法入參中用 MultipartFile 類型接收web

關於在 SpringBoot 中如何使用文件上傳可參考 Spring 官方文檔spring

回顧

先來回顧一下在 DispatcherServlet 中處理請求的過程當中哪裏使用到 MultipartResolver 組件，能夠回到《一個請求的旅行過程》中的 DispatcherServlet 的 doDispatch 方法中看看，以下：apache

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    // ... 省略相關代碼
    // <2> 檢測請求是否爲上傳請求，若是是則經過 multipartResolver 將其封裝成 MultipartHttpServletRequest 對象
    processedRequest = checkMultipart(request);
    // ... 省略相關代碼
}

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
    // 若是該請求是一個涉及到 multipart （文件）的請求
    if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
        if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
            if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
                logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
            }
        }
        else if (hasMultipartException(request)) {
            logger.debug("Multipart resolution previously failed for current request - " +
                    "skipping re-resolution for undisturbed error rendering");
        }
        else {
            try {
                // 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 對象，解析請求裏面的參數以及文件
                return this.multipartResolver.resolveMultipart(request);
            }
            catch (MultipartException ex) {
                if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                    logger.debug("Multipart resolution failed for error dispatch", ex);
                    // Keep processing error dispatch with regular request handle below
                }
                else {
                    throw ex;
                }
            }
        }
    }
    // If not returned before: return original request.
    return request;
}

<2> 處，若是該請求是一個涉及到 multipart （文件）的請求，則經過 multipartResolver 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 對象，解析請求裏面的參數以及文件數組

MultipartResolver接口

org.springframework.web.multipart.MultipartResolver 接口，內容類型( Content-Type )爲 multipart/* 的請求的解析器接口，代碼以下：spring-mvc

public interface MultipartResolver {
	/**
	 * 是否爲 multipart 請求
	 */
	boolean isMultipart(HttpServletRequest request);
	/**
	 * 將 HttpServletRequest 請求封裝成 MultipartHttpServletRequest 對象
	 */
	MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;

	/**
	 * 清理處理 multipart 產生的資源，例如臨時文件
	 */
	void cleanupMultipart(MultipartHttpServletRequest request);
}

MultipartResolver 接口體系的結構以下：mvc

一共有兩塊：

上半部分，MultipartRequest 接口及其實現類
下半部分，MultipartResolver 接口以及其實現類

初始化過程

在 DispatcherServlet 的 initMultipartResolver(ApplicationContext context) 方法，初始化 MultipartResolver 組件，方法以下：

private void initMultipartResolver(ApplicationContext context) {
    try {
        // 從 Spring 上下文中獲取名稱爲 "multipartResolver" ，類型爲 MultipartResolver 的 Bean
        this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
        if (logger.isTraceEnabled()) {
            logger.trace("Detected " + this.multipartResolver);
        }
        else if (logger.isDebugEnabled()) {
            logger.debug("Detected " + this.multipartResolver.getClass().getSimpleName());
        }
    }
    catch (NoSuchBeanDefinitionException ex) {
        // Default is no multipart resolver.
        this.multipartResolver = null;
        if (logger.isTraceEnabled()) {
            logger.trace("No MultipartResolver '" + MULTIPART_RESOLVER_BEAN_NAME + "' declared");
        }
    }
}

在 Spring MVC 中，multipartResolver 默認爲 null【注意】，須要本身配置，例如《MyBatis 使用手冊》中的 集成 Spring 模塊下的 spring-mvc.xml 文件中配置 MultipartResolver 爲 CommonsMultipartResolver 實現類，也能夠配置爲 StandardServletMultipartResolver 實現類
在 Spring Boot 中，multipartResolver 默認爲 StandardServletMultipartResolver 實現類

目前 Spring 只提供上面兩種實現類，接下來依次進行分析

StandardServletMultipartResolver

org.springframework.web.multipart.support.StandardServletMultipartResolver，實現 MultipartResolver 接口，基於 Servlet 3.0 標準的上傳文件 API 的 MultipartResolver 實現類，代碼以下：

public class StandardServletMultipartResolver implements MultipartResolver {

   /**
    * 是否延遲解析
    */
   private boolean resolveLazily = false;

   public void setResolveLazily(boolean resolveLazily) {
      this.resolveLazily = resolveLazily;
   }


   @Override
   public boolean isMultipart(HttpServletRequest request) {
      // 請求的 Content-type 必須 multipart/ 開頭
      return StringUtils.startsWithIgnoreCase(request.getContentType(), "multipart/");
   }

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

   @Override
   public void cleanupMultipart(MultipartHttpServletRequest request) {
      if (!(request instanceof AbstractMultipartHttpServletRequest) || ((AbstractMultipartHttpServletRequest) request).isResolved()) {
         // To be on the safe side: explicitly delete the parts,
         // but only actual file parts (for Resin compatibility)
         try {
            // 刪除臨時的 Part
            for (Part part : request.getParts()) {
               if (request.getFile(part.getName()) != null) {
                  part.delete();
               }
            }
         }
         catch (Throwable ex) {
            LogFactory.getLog(getClass()).warn("Failed to perform cleanup of multipart items", ex);
         }
      }
   }

}

isMultipart(HttpServletRequest request)方法，請求的 Content-type 是否以 multipart/ 開頭
resolveMultipart(HttpServletRequest request)方法，直接將 HttpServletRequest 轉換成 StandardMultipartHttpServletRequest 對象
cleanupMultipart(MultipartHttpServletRequest request)方法，清理資源，刪除臨時的 javax.servlet.http.Part 們

StandardMultipartHttpServletRequest

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest，繼承 AbstractMultipartHttpServletRequest 抽象類，基於 Servlet 3.0 的 Multipart HttpServletRequest 實現類，包含了一個 javax.servlet.http.HttpServletRequest 對象和它的 javax.servlet.http.Part 對象們，其中 Part 對象會被封裝成 StandardMultipartFile 對象

構造方法

public class StandardMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
	/**
	 * 普通參數名的集合
	 */
	@Nullable
	private Set<String> multipartParameterNames;

	public StandardMultipartHttpServletRequest(HttpServletRequest request) throws MultipartException {
		this(request, false);
	}

	public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing) throws MultipartException {
		super(request);
		// 若是不須要延遲解析
		if (!lazyParsing) {
			// 解析請求
			parseRequest(request);
		}
	}
}

multipartParameterNames：普通參數名的集合，非上傳文件的參數名
若是不須要延遲解析，則調用 parseRequest(HttpServletRequest request) 方法，直接解析請求

parseRequest

parseRequest(HttpServletRequest request) 方法，解析請求，解析 HttpServletRequest 中的 Part 對象，若是是文件，則封裝成 StandardMultipartFile 對象，不然就是普通參數，獲取其名稱，以下：

private void parseRequest(HttpServletRequest request) {
    try {
        // <1> 從 HttpServletRequest 中獲取 Part 們
        Collection<Part> parts = request.getParts();
        this.multipartParameterNames = new LinkedHashSet<>(parts.size());
        MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
        // <2> 遍歷 parts 數組
        for (Part part : parts) {
            // <2.1> 得到請求頭中的 Content-Disposition 信息，MIME 協議的擴展
            String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
            // <2.2> 對 Content-Disposition 信息進行解析，生成 ContentDisposition 對象
            // 包含請求參數信息，以面向「對象」的形式進行訪問
            ContentDisposition disposition = ContentDisposition.parse(headerValue);
            // <2.3> 得到文件名
            String filename = disposition.getFilename();
            // <2.4> 狀況一，文件名非空，說明是文件參數，則建立 StandardMultipartFile 對象
            if (filename != null) {
                if (filename.startsWith("=?") && filename.endsWith("?=")) {
                    filename = MimeDelegate.decode(filename);
                }
                files.add(part.getName(), new StandardMultipartFile(part, filename));
            }
            // <2.5> 狀況二，文件名爲空，說明是普通參數，則保存參數名稱
            else {
                this.multipartParameterNames.add(part.getName());
            }
        }
        // <3> 將上面生成的 StandardMultipartFile 文件對象們，設置到父類的 multipartFiles 屬性中
        setMultipartFiles(files);
    }
    catch (Throwable ex) {
        handleParseFailure(ex);
    }
}

從 HttpServletRequest 中獲取 Part 們
遍歷 parts 數組
1. 從 Part 對象中得到請求頭中的 Content-Disposition 信息，MIME 協議的擴展
2. 對 Content-Disposition 信息進行解析，生成 ContentDisposition 對象，包含請求參數信息，以面向「對象」的形式進行訪問
3. 從ContentDisposition 對象中得到文件名
4. 狀況一，文件名非空，說明是文件參數，則建立 StandardMultipartFile 對象
5. 狀況二，文件名爲空，說明是普通參數，則保存參數名稱
將上面生成的 StandardMultipartFile 文件對象們，設置到父類的 multipartFiles 屬性中
若是發生異常則拋出

其餘方法

/** 初始化請求 */
@Override
protected void initializeMultipart() {
    parseRequest(getRequest());
}
/** 獲取請求中的參數名稱 */
@Override
public Enumeration<String> getParameterNames() {
    if (this.multipartParameterNames == null) {
        initializeMultipart();
    }
    if (this.multipartParameterNames.isEmpty()) {
        return super.getParameterNames();
    }

    // Servlet 3.0 getParameterNames() not guaranteed to include multipart form items
    // (e.g. on WebLogic 12) -> need to merge them here to be on the safe side
    Set<String> paramNames = new LinkedHashSet<>();
    Enumeration<String> paramEnum = super.getParameterNames();
    while (paramEnum.hasMoreElements()) {
        paramNames.add(paramEnum.nextElement());
    }
    paramNames.addAll(this.multipartParameterNames);
    return Collections.enumeration(paramNames);
}
/** 獲取請求中的參數，參數名和參數值的映射 */
@Override
public Map<String, String[]> getParameterMap() {
    if (this.multipartParameterNames == null) {
        initializeMultipart();
    }
    if (this.multipartParameterNames.isEmpty()) {
        return super.getParameterMap();
    }
    // Servlet 3.0 getParameterMap() not guaranteed to include multipart form items
    // (e.g. on WebLogic 12) -> need to merge them here to be on the safe side
    Map<String, String[]> paramMap = new LinkedHashMap<>(super.getParameterMap());
    for (String paramName : this.multipartParameterNames) {
        if (!paramMap.containsKey(paramName)) {
            paramMap.put(paramName, getParameterValues(paramName));
        }
    }
    return paramMap;
}
/** 獲取請求的 Content-Type 內容類型 */
@Override
public String getMultipartContentType(String paramOrFileName) {
    try {
        Part part = getPart(paramOrFileName);
        return (part != null ? part.getContentType() : null);
    }
    catch (Throwable ex) {
        throw new MultipartException("Could not access multipart servlet request", ex);
    }
}
/** 獲取請求頭信息 */
@Override
public HttpHeaders getMultipartHeaders(String paramOrFileName) {
    try {
        Part part = getPart(paramOrFileName);
        if (part != null) {
            HttpHeaders headers = new HttpHeaders();
            for (String headerName : part.getHeaderNames()) {
                headers.put(headerName, new ArrayList<>(part.getHeaders(headerName)));
            }
            return headers;
        }
        else {
            return null;
        }
    }
    catch (Throwable ex) {
        throw new MultipartException("Could not access multipart servlet request", ex);
    }
}

StandardMultipartFile

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest 的私有內部靜態類，實現了 MultipartFile 接口和 Serializable 接口，內部封裝了 javax.servlet.http.Part 對象和文件名稱，代碼以下：

private static class StandardMultipartFile implements MultipartFile, Serializable {

    private final Part part;

    private final String filename;

    public StandardMultipartFile(Part part, String filename) {
        this.part = part;
        this.filename = filename;
    }

    @Override
    public String getName() {
        return this.part.getName();
    }

    @Override
    public String getOriginalFilename() {
        return this.filename;
    }

    @Override
    public String getContentType() {
        return this.part.getContentType();
    }

    @Override
    public boolean isEmpty() {
        return (this.part.getSize() == 0);
    }

    @Override
    public long getSize() {
        return this.part.getSize();
    }

    @Override
    public byte[] getBytes() throws IOException {
        return FileCopyUtils.copyToByteArray(this.part.getInputStream());
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return this.part.getInputStream();
    }

    @Override
    public void transferTo(File dest) throws IOException, IllegalStateException {
        this.part.write(dest.getPath());
        if (dest.isAbsolute() && !dest.exists()) {
            // Servlet 3.0 Part.write is not guaranteed to support absolute file paths:
            // may translate the given path to a relative location within a temp dir
            // (e.g. on Jetty whereas Tomcat and Undertow detect absolute paths).
            // At least we offloaded the file from memory storage; it'll get deleted
            // from the temp dir eventually in any case. And for our user's purposes,
            // we can manually copy it to the requested location as a fallback.
            FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest.toPath()));
        }
    }

    @Override
    public void transferTo(Path dest) throws IOException, IllegalStateException {
        FileCopyUtils.copy(this.part.getInputStream(), Files.newOutputStream(dest));
    }
}

這個類封裝了 Servlet 3.0 的 Part 對象，也就是咱們經常使用到的 MultipartFile 對象，支持對文件的操做，內部其實都是調用 javax.servlet.http.Part 的方法

AbstractMultipartHttpServletRequest

org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest 抽象類，繼承了 HttpServletRequestWrapper 類，實現了 MultipartHttpServletRequest接口

該類是 StandardMultipartHttpServletRequest 和 DefaultMultipartHttpServletRequest 的父類，實現了一些公共的方法，代碼以下：

public abstract class AbstractMultipartHttpServletRequest extends HttpServletRequestWrapper implements MultipartHttpServletRequest {
    /**
     * 請求中的文件信息
     */
	@Nullable
	private MultiValueMap<String, MultipartFile> multipartFiles;

	protected AbstractMultipartHttpServletRequest(HttpServletRequest request) {
		super(request);
	}

	@Override
	public HttpServletRequest getRequest() {
		return (HttpServletRequest) super.getRequest();
	}

	@Override
	public HttpMethod getRequestMethod() {
		return HttpMethod.resolve(getRequest().getMethod());
	}

    /** 獲取請求頭信息 */
	@Override
	public HttpHeaders getRequestHeaders() {
		HttpHeaders headers = new HttpHeaders();
		Enumeration<String> headerNames = getHeaderNames();
		while (headerNames.hasMoreElements()) {
			String headerName = headerNames.nextElement();
			headers.put(headerName, Collections.list(getHeaders(headerName)));
		}
		return headers;
	}

    /** 獲取文件名稱列表 */
	@Override
	public Iterator<String> getFileNames() {
		return getMultipartFiles().keySet().iterator();
	}

    /** 獲取指定文件名的單個文件 */
	@Override
	public MultipartFile getFile(String name) {
		return getMultipartFiles().getFirst(name);
	}

    /** 獲取指定文件名的多個文件 */
	@Override
	public List<MultipartFile> getFiles(String name) {
		List<MultipartFile> multipartFiles = getMultipartFiles().get(name);
		if (multipartFiles != null) {
			return multipartFiles;
		}
		else {
			return Collections.emptyList();
		}
	}

	@Override
	public Map<String, MultipartFile> getFileMap() {
		return getMultipartFiles().toSingleValueMap();
	}

	@Override
	public MultiValueMap<String, MultipartFile> getMultiFileMap() {
		return getMultipartFiles();
	}

	public boolean isResolved() {
		return (this.multipartFiles != null);
	}

	protected final void setMultipartFiles(MultiValueMap<String, MultipartFile> multipartFiles) {
		this.multipartFiles = new LinkedMultiValueMap<>(Collections.unmodifiableMap(multipartFiles));
	}

	protected MultiValueMap<String, MultipartFile> getMultipartFiles() {
		if (this.multipartFiles == null) {
			initializeMultipart();
		}
		return this.multipartFiles;
	}

	/** 交由子類實現 */
	protected void initializeMultipart() {
		throw new IllegalStateException("Multipart request not initialized");
	}
}

上面的方法都比較簡單，用於獲取請求中的文件對象

MultiValueMap<String, MultipartFile> multipartFiles屬性，保存由子類解析出請求中的 Part 對象所封裝成的 MultipartFile 對象

CommonsMultipartResolver

org.springframework.web.multipart.commons.CommonsMultipartResolver，實現 MultipartResolver、ServletContextAware 接口，繼承 CommonsFileUploadSupport 抽象類，基於 Apache Commons FileUpload 的 MultipartResolver 實現類

若是須要使用這個 MultipartResolver 實現類，須要引入 commons-fileupload、commons-io 和 commons-codec 組件，例如：

<dependencies>
    <dependency>
        <groupId>commons-fileupload</groupId>
        <artifactId>commons-fileupload</artifactId>
        <version>1.4</version>
    </dependency>
    <dependency>
        <groupId>commons-io</groupId>
        <artifactId>commons-io</artifactId>
        <version>2.8.0</version>
    </dependency>
    <dependency>
        <groupId>commons-codec</groupId>
        <artifactId>commons-codec</artifactId>
        <version>1.15</version>
    </dependency>
</dependencies>

注意，若是 Spring Boot 項目中須要使用 CommonsMultipartResolver，須要在 application.yml 中添加以下配置，排除其默認的配置，以下：

spring:
  autoconfigure:
    exclude: org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration

構造方法

public class CommonsMultipartResolver extends CommonsFileUploadSupport implements MultipartResolver, ServletContextAware {
	/**
	 * 是否延遲解析
	 */
	private boolean resolveLazily = false;

	public CommonsMultipartResolver() {
		super();
	}

	public CommonsMultipartResolver(ServletContext servletContext) {
		this();
		setServletContext(servletContext);
	}
}

isMultipart

@Override
public boolean isMultipart(HttpServletRequest request) {
    // 必須是 POST 請求，且 Content-Type 爲 multipart/ 開頭
    return ServletFileUpload.isMultipartContent(request);
}

判斷是否爲 multipart 請求，必須是 POST 請求，且 Content-Type 爲 multipart/ 開頭

resolveMultipart

@Override
public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {
    Assert.notNull(request, "Request must not be null");
    if (this.resolveLazily) {
        return new DefaultMultipartHttpServletRequest(request) {
            @Override
            protected void initializeMultipart() {
                // 解析請求，獲取文件、參數信息
                MultipartParsingResult parsingResult = parseRequest(request);
                setMultipartFiles(parsingResult.getMultipartFiles());
                setMultipartParameters(parsingResult.getMultipartParameters());
                setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());
            }
        };
    }
    else {
        // 解析請求，獲取文件、參數信息
        MultipartParsingResult parsingResult = parseRequest(request);
        return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
                parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
    }
}

將 HttpServletRequest 轉換成 DefaultMultipartHttpServletRequest 對象

若是開啓了延遲解析，則重寫該對象的 initializeMultipart() 方法，用於解析請求

不然直接調用 parseRequest(HttpServletRequest request) 方法解析請求，返回 MultipartParsingResult 對象，包含 MultipartFile 對象和普通參數信息

parseRequest

parseRequest(HttpServletRequest request)方法，用於解析請求，返回 MultipartParsingResult 對象，包含 MultipartFile 對象、普通參數信息以及參數的 Content-Type 信息，方法以下：

protected MultipartParsingResult parseRequest(HttpServletRequest request) throws MultipartException {
    // <1> 獲取請求中的編碼
    String encoding = determineEncoding(request);
    // <2> 獲取 ServletFileUpload 對象
    FileUpload fileUpload = prepareFileUpload(encoding);
    try {
        // <3> 獲取請求中的流數據
        List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
        // <4> 將這些流數據轉換成 MultipartParsingResult，包含 CommonsMultipartFile、參數信息、Content-type
        return parseFileItems(fileItems, encoding);
    }
    catch (FileUploadBase.SizeLimitExceededException ex) {
        throw new MaxUploadSizeExceededException(fileUpload.getSizeMax(), ex);
    }
    catch (FileUploadBase.FileSizeLimitExceededException ex) {
        throw new MaxUploadSizeExceededException(fileUpload.getFileSizeMax(), ex);
    }
    catch (FileUploadException ex) {
        throw new MultipartException("Failed to parse multipart servlet request", ex);
    }
}

獲取請求中的編碼
根據編碼獲取到 ServletFileUpload 對象（ commons-fileupload 中的類），在 newFileUpload(FileItemFactory fileItemFactory) 方法中返回的就是 ServletFileUpload 對象，能夠看到父類 CommonsFileUploadSupport 的構造方法，以下：
```
// org.springframework.web.multipart.commons.CommonsFileUploadSupport.java

public CommonsFileUploadSupport() {
    this.fileItemFactory = newFileItemFactory();
    // 由子類實現
    this.fileUpload = newFileUpload(getFileItemFactory());
}
```
具體細節就不講述了
經過 ServletFileUpload 對象解析請求，返回流數據 List<FileItem> fileItems

調用父類 CommonsFileUploadSupport 的 parseFileItems(List<FileItem> fileItems, String encoding) 方法，將這些流數據轉換成 MultipartParsingResult 對象

// org.springframework.web.multipart.commons.CommonsFileUploadSupport.java

protected MultipartParsingResult parseFileItems(List<FileItem> fileItems, String encoding) {
    MultiValueMap<String, MultipartFile> multipartFiles = new LinkedMultiValueMap<>();
    Map<String, String[]> multipartParameters = new HashMap<>();
    Map<String, String> multipartParameterContentTypes = new HashMap<>();

    // Extract multipart files and multipart parameters.
    for (FileItem fileItem : fileItems) {
        if (fileItem.isFormField()) {
            String value;
            String partEncoding = determineEncoding(fileItem.getContentType(), encoding);
            try {
                value = fileItem.getString(partEncoding);
            }
            catch (UnsupportedEncodingException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Could not decode multipart item '" + fileItem.getFieldName() +
                            "' with encoding '" + partEncoding + "': using platform default");
                }
                value = fileItem.getString();
            }
            String[] curParam = multipartParameters.get(fileItem.getFieldName());
            if (curParam == null) {
                // simple form field
                multipartParameters.put(fileItem.getFieldName(), new String[] {value});
            }
            else {
                // array of simple form fields
                String[] newParam = StringUtils.addStringToArray(curParam, value);
                multipartParameters.put(fileItem.getFieldName(), newParam);
            }
            multipartParameterContentTypes.put(fileItem.getFieldName(), fileItem.getContentType());
        }
        else {
            // multipart file field
            CommonsMultipartFile file = createMultipartFile(fileItem);
            multipartFiles.add(file.getName(), file);
            LogFormatUtils.traceDebug(logger, traceOn ->
                    "Part '" + file.getName() + "', size " + file.getSize() +
                            " bytes, filename='" + file.getOriginalFilename() + "'" +
                            (traceOn ? ", storage=" + file.getStorageDescription() : "")
            );
        }
    }
    return new MultipartParsingResult(multipartFiles, multipartParameters, multipartParameterContentTypes);
}

大體就是遍歷 fileItems 集合，若是是一個簡單的表單字段，那麼就是一個普通的參數，將參數名和值保存起來

不然就是文件，將其封裝成 CommonsMultipartFile 保存起來

cleanupMultipart

cleanupMultipart(MultipartHttpServletRequest request)方法，清理文件產生的臨時資源，以下：

// CommonsMultipartResolver.java
@Override
public void cleanupMultipart(MultipartHttpServletRequest request) {
    if (!(request instanceof AbstractMultipartHttpServletRequest) ||
            ((AbstractMultipartHttpServletRequest) request).isResolved()) {
        try {
            cleanupFileItems(request.getMultiFileMap());
        }
        catch (Throwable ex) {
            logger.warn("Failed to perform multipart cleanup for servlet request", ex);
        }
    }
}
// CommonsFileUploadSupport.java
protected void cleanupFileItems(MultiValueMap<String, MultipartFile> multipartFiles) {
    for (List<MultipartFile> files : multipartFiles.values()) {
        for (MultipartFile file : files) {
            if (file instanceof CommonsMultipartFile) {
                CommonsMultipartFile cmf = (CommonsMultipartFile) file;
                cmf.getFileItem().delete();
                LogFormatUtils.traceDebug(logger, traceOn -> "Cleaning up part '..."));
            }
        }
    }
}

DefaultMultipartHttpServletRequest

org.springframework.web.multipart.support.DefaultMultipartHttpServletRequest，繼承 AbstractMultipartHttpServletRequest 抽象類，MultipartHttpServletRequest 的默認實現類，代碼以下：

public class DefaultMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {

	private static final String CONTENT_TYPE = "Content-Type";

	@Nullable
	private Map<String, String[]> multipartParameters;

	@Nullable
	private Map<String, String> multipartParameterContentTypes;

	public DefaultMultipartHttpServletRequest(HttpServletRequest request, MultiValueMap<String, MultipartFile> mpFiles,
			Map<String, String[]> mpParams, Map<String, String> mpParamContentTypes) {

		super(request);
		setMultipartFiles(mpFiles);
		setMultipartParameters(mpParams);
		setMultipartParameterContentTypes(mpParamContentTypes);
	}

	public DefaultMultipartHttpServletRequest(HttpServletRequest request) {
		super(request);
	}

	@Override
	@Nullable
	public String getParameter(String name) {
		String[] values = getMultipartParameters().get(name);
		if (values != null) {
			return (values.length > 0 ? values[0] : null);
		}
		return super.getParameter(name);
	}

	@Override
	public String[] getParameterValues(String name) {
		String[] parameterValues = super.getParameterValues(name);
		String[] mpValues = getMultipartParameters().get(name);
		if (mpValues == null) {
			return parameterValues;
		}
		if (parameterValues == null || getQueryString() == null) {
			return mpValues;
		}
		else {
			String[] result = new String[mpValues.length + parameterValues.length];
			System.arraycopy(mpValues, 0, result, 0, mpValues.length);
			System.arraycopy(parameterValues, 0, result, mpValues.length, parameterValues.length);
			return result;
		}
	}

	@Override
	public Enumeration<String> getParameterNames() {
		Map<String, String[]> multipartParameters = getMultipartParameters();
		if (multipartParameters.isEmpty()) {
			return super.getParameterNames();
		}

		Set<String> paramNames = new LinkedHashSet<>();
		paramNames.addAll(Collections.list(super.getParameterNames()));
		paramNames.addAll(multipartParameters.keySet());
		return Collections.enumeration(paramNames);
	}

	@Override
	public Map<String, String[]> getParameterMap() {
		Map<String, String[]> result = new LinkedHashMap<>();
		Enumeration<String> names = getParameterNames();
		while (names.hasMoreElements()) {
			String name = names.nextElement();
			result.put(name, getParameterValues(name));
		}
		return result;
	}

	@Override
	public String getMultipartContentType(String paramOrFileName) {
		MultipartFile file = getFile(paramOrFileName);
		if (file != null) {
			return file.getContentType();
		}
		else {
			return getMultipartParameterContentTypes().get(paramOrFileName);
		}
	}

	@Override
	public HttpHeaders getMultipartHeaders(String paramOrFileName) {
		String contentType = getMultipartContentType(paramOrFileName);
		if (contentType != null) {
			HttpHeaders headers = new HttpHeaders();
			headers.add(CONTENT_TYPE, contentType);
			return headers;
		}
		else {
			return null;
		}
	}

	protected final void setMultipartParameters(Map<String, String[]> multipartParameters) {
		this.multipartParameters = multipartParameters;
	}

	protected Map<String, String[]> getMultipartParameters() {
		if (this.multipartParameters == null) {
			initializeMultipart();
		}
		return this.multipartParameters;
	}

	protected final void setMultipartParameterContentTypes(Map<String, String> multipartParameterContentTypes) {
		this.multipartParameterContentTypes = multipartParameterContentTypes;
	}

	protected Map<String, String> getMultipartParameterContentTypes() {
		if (this.multipartParameterContentTypes == null) {
			initializeMultipart();
		}
		return this.multipartParameterContentTypes;
	}
}

代碼並不複雜，稍微閱讀一下就理解了😈

總結

本文對 Spring MVC 處理請求的過程當中使用到的 MultipartResolver 組件進行了分析，若是請求的 Content-Type 爲 multipart/*，涉及到文件上傳，因此處理請求的第一步須要經過 MultipartResolver 組件對請求進行轉換處理。會將 HttpServletRequest 請求對象封裝成 MultipartHttpServletRequest 對象，便於獲取參數信息和操做上傳的文件（MultipartFile 對象）。

MultipartResolver 組件的實現類有兩種：

org.springframework.web.multipart.support.StandardServletMultipartResolver：實現 MultipartResolver 接口，基於 Servlet 3.0 標準的上傳文件 API 的 MultipartResolver 實現類
org.springframework.web.multipart.commons.CommonsMultipartResolver：實現 MultipartResolver 接口，基於 Apache Commons FileUpload 的 MultipartResolver 實現類

二者的區別：

StandardServletMultipartResolver 會將 HttpServletRequest 封裝成 StandardMultipartHttpServletRequest 對象，由 Servlet 3.0 提供 API 獲取請求中的 javax.servlet.http.Part 對象，而後進行解析，文件會封裝成 StandardMultipartFile 對象
CommonsMultipartResolver 會將 HttpServletRequest 封裝成 DefaultMultipartHttpServletRequest 對象，由 Apache 的 Commons FileUpload 組件來實現，經過 org.apache.commons.fileupload.servlet.ServletFileUpload 對象獲取請求中的 org.apache.commons.fileupload.FileItem 對象，而後進行解析，文件會封裝成 CommonsMultipartFile 對象，如何使用能夠參考上面的 CommonsMultipartResolver 小節

注意事項：

在 Spring MVC 中，multipartResolver 默認爲 null，須要本身配置，例如《MyBatis 使用手冊》中的 集成 Spring 模塊下的 spring-mvc.xml 文件中配置 MultipartResolver 爲 CommonsMultipartResolver 實現類，也能夠配置爲 StandardServletMultipartResolver 實現類
在 Spring Boot 中，multipartResolver 默認爲 StandardServletMultipartResolver 實現類