Spring MVC 源码分析 - MultipartResolver 组件

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Spring 版本:5.1.14.RELEASE

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MultipartResolver 组件,内容类型( Content-Type )为 multipart/* 的请求的解析器,主要解析文件上传的请求。例如,MultipartResolver 会将 HttpServletRequest 封装成 MultipartHttpServletRequest 对象,便于获取参数信息以及上传的文件

使用方式,可以参考《MyBatis 使用手册》中的 集成 Spring 模块下的 spring-mvc.xml 文件中配置 MultipartResolverCommonsMultipartResolver 实现类,然后在方法入参中用 MultipartFile 类型接收

关于在 SpringBoot 中如何使用文件上传可参考 Spring 官方文档

回顾

先来回顾一下在 DispatcherServlet 中处理请求的过程中哪里使用到 MultipartResolver 组件,可以回到《一个请求的旅行过程》中的 DispatcherServletdoDispatch 方法中看看,如下:

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    // ... 省略相关代码
    // <2> 检测请求是否为上传请求,如果是则通过 multipartResolver 将其封装成 MultipartHttpServletRequest 对象
    processedRequest = checkMultipart(request);
    // ... 省略相关代码
}

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
    // 如果该请求是一个涉及到 multipart (文件)的请求
    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 {
                // 将 HttpServletRequest 请求封装成 MultipartHttpServletRequest 对象,解析请求里面的参数以及文件
                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> 处,如果该请求是一个涉及到 multipart (文件)的请求,则通过 multipartResolverHttpServletRequest 请求封装成 MultipartHttpServletRequest 对象,解析请求里面的参数以及文件

MultipartResolver接口

org.springframework.web.multipart.MultipartResolver 接口,内容类型( Content-Type )为 multipart/* 的请求的解析器接口,代码如下:

public interface MultipartResolver {
	/**
	 * 是否为 multipart 请求
	 */
	boolean isMultipart(HttpServletRequest request);
	/**
	 * 将 HttpServletRequest 请求封装成 MultipartHttpServletRequest 对象
	 */
	MultipartHttpServletRequest resolveMultipart(HttpServletRequest request) throws MultipartException;

	/**
	 * 清理处理 multipart 产生的资源,例如临时文件
	 */
	void cleanupMultipart(MultipartHttpServletRequest request);
}

MultipartResolver 接口体系的结构如下:

一共有两块:

  • 上半部分,MultipartRequest 接口及其实现类
  • 下半部分,MultipartResolver 接口以及其实现类

初始化过程

DispatcherServletinitMultipartResolver(ApplicationContext context) 方法,初始化 MultipartResolver 组件,方法如下:

private void initMultipartResolver(ApplicationContext context) {
    try {
        // 从 Spring 上下文中获取名称为 "multipartResolver" ,类型为 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 默认为 null【注意】,需要自己配置,例如《MyBatis 使用手册》中的 集成 Spring 模块下的 spring-mvc.xml 文件中配置 MultipartResolver 为 CommonsMultipartResolver 实现类,也可以配置为 StandardServletMultipartResolver 实现类

  • 在 Spring Boot 中,multipartResolver 默认为 StandardServletMultipartResolver 实现类

目前 Spring 只提供上面两种实现类,接下来依次进行分析

StandardServletMultipartResolver

org.springframework.web.multipart.support.StandardServletMultipartResolver,实现 MultipartResolver 接口,基于 Servlet 3.0 标准的上传文件 API 的 MultipartResolver 实现类,代码如下:

public class StandardServletMultipartResolver implements MultipartResolver {

   /**
    * 是否延迟解析
    */
   private boolean resolveLazily = false;

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


   @Override
   public boolean isMultipart(HttpServletRequest request) {
      // 请求的 Content-type 必须 multipart/ 开头
      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 {
            // 删除临时的 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,请求的 Content-type 是否以 multipart/ 开头

  • resolveMultipart(HttpServletRequest request)方法,直接将 HttpServletRequest 转换成 StandardMultipartHttpServletRequest 对象

  • cleanupMultipart(MultipartHttpServletRequest request)方法,清理资源,删除临时的 javax.servlet.http.Part

StandardMultipartHttpServletRequest

org.springframework.web.multipart.support.StandardMultipartHttpServletRequest,继承 AbstractMultipartHttpServletRequest 抽象类,基于 Servlet 3.0 的 Multipart HttpServletRequest 实现类,包含了一个 javax.servlet.http.HttpServletRequest 对象和它的 javax.servlet.http.Part 对象们,其中 Part 对象会被封装成 StandardMultipartFile 对象

构造方法

public class StandardMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
	/**
	 * 普通参数名的集合
	 */
	@Nullable
	private Set<String> multipartParameterNames;

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

	public StandardMultipartHttpServletRequest(HttpServletRequest request, boolean lazyParsing) throws MultipartException {
		super(request);
		// 如果不需要延迟解析
		if (!lazyParsing) {
			// 解析请求
			parseRequest(request);
		}
	}
}
  • multipartParameterNames:普通参数名的集合,非上传文件的参数名
  • 如果不需要延迟解析,则调用 parseRequest(HttpServletRequest request) 方法,直接解析请求

parseRequest

parseRequest(HttpServletRequest request) 方法,解析请求,解析 HttpServletRequest 中的 Part 对象,如果是文件,则封装成 StandardMultipartFile 对象,否则就是普通参数,获取其名称,如下:

private void parseRequest(HttpServletRequest request) {
    try {
        // <1> 从 HttpServletRequest 中获取 Part 们
        Collection<Part> parts = request.getParts();
        this.multipartParameterNames = new LinkedHashSet<>(parts.size());
        MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
        // <2> 遍历 parts 数组
        for (Part part : parts) {
            // <2.1> 获得请求头中的 Content-Disposition 信息,MIME 协议的扩展
            String headerValue = part.getHeader(HttpHeaders.CONTENT_DISPOSITION);
            // <2.2> 对 Content-Disposition 信息进行解析,生成 ContentDisposition 对象
            // 包含请求参数信息,以面向“对象”的形式进行访问
            ContentDisposition disposition = ContentDisposition.parse(headerValue);
            // <2.3> 获得文件名
            String filename = disposition.getFilename();
            // <2.4> 情况一,文件名非空,说明是文件参数,则创建 StandardMultipartFile 对象
            if (filename != null) {
                if (filename.startsWith("=?") && filename.endsWith("?=")) {
                    filename = MimeDelegate.decode(filename);
                }
                files.add(part.getName(), new StandardMultipartFile(part, filename));
            }
            // <2.5> 情况二,文件名为空,说明是普通参数,则保存参数名称
            else {
                this.multipartParameterNames.add(part.getName());
            }
        }
        // <3> 将上面生成的 StandardMultipartFile 文件对象们,设置到父类的 multipartFiles 属性中
        setMultipartFiles(files);
    }
    catch (Throwable ex) {
        handleParseFailure(ex);
    }
}
  1. 从 HttpServletRequest 中获取 Part 们

  2. 遍历 parts 数组

    1. 从 Part 对象中获得请求头中的 Content-Disposition 信息,MIME 协议的扩展
    2. 对 Content-Disposition 信息进行解析,生成 ContentDisposition 对象,包含请求参数信息,以面向“对象”的形式进行访问
    3. ContentDisposition 对象中获得文件名
    4. 情况一,文件名非空,说明是文件参数,则创建 StandardMultipartFile 对象
    5. 情况二,文件名为空,说明是普通参数,则保存参数名称
  3. 将上面生成的 StandardMultipartFile 文件对象们,设置到父类的 multipartFiles 属性中

  4. 如果发生异常则抛出

其他方法

/** 初始化请求 */
@Override
protected void initializeMultipart() {
    parseRequest(getRequest());
}
/** 获取请求中的参数名称 */
@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);
}
/** 获取请求中的参数,参数名和参数值的映射 */
@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;
}
/** 获取请求的 Content-Type 内容类型 */
@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);
    }
}
/** 获取请求头信息 */
@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 的私有内部静态类,实现了 MultipartFile 接口和 Serializable 接口,内部封装了 javax.servlet.http.Part 对象和文件名称,代码如下:

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));
    }
}

这个类封装了 Servlet 3.0 的 Part 对象,也就是我们常用到的 MultipartFile 对象,支持对文件的操作,内部其实都是调用 javax.servlet.http.Part 的方法

AbstractMultipartHttpServletRequest

org.springframework.web.multipart.support.AbstractMultipartHttpServletRequest 抽象类,继承了 HttpServletRequestWrapper 类,实现了 MultipartHttpServletRequest接口

该类是 StandardMultipartHttpServletRequestDefaultMultipartHttpServletRequest 的父类,实现了一些公共的方法,代码如下:

public abstract class AbstractMultipartHttpServletRequest extends HttpServletRequestWrapper implements MultipartHttpServletRequest {
    /**
     * 请求中的文件信息
     */
	@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());
	}

    /** 获取请求头信息 */
	@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;
	}

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

    /** 获取指定文件名的单个文件 */
	@Override
	public MultipartFile getFile(String name) {
		return getMultipartFiles().getFirst(name);
	}

    /** 获取指定文件名的多个文件 */
	@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;
	}

	/** 交由子类实现 */
	protected void initializeMultipart() {
		throw new IllegalStateException("Multipart request not initialized");
	}
}

上面的方法都比较简单,用于获取请求中的文件对象

MultiValueMap<String, MultipartFile> multipartFiles属性,保存由子类解析出请求中的 Part 对象所封装成的 MultipartFile 对象

CommonsMultipartResolver

org.springframework.web.multipart.commons.CommonsMultipartResolver,实现 MultipartResolver、ServletContextAware 接口,继承 CommonsFileUploadSupport 抽象类,基于 Apache Commons FileUpload 的 MultipartResolver 实现类

如果需要使用这个 MultipartResolver 实现类,需要引入 commons-fileuploadcommons-iocommons-codec 组件,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,如果 Spring Boot 项目中需要使用 CommonsMultipartResolver,需要在 application.yml 中添加如下配置,排除其默认的配置,如下:

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

构造方法

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

	public CommonsMultipartResolver() {
		super();
	}

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

isMultipart

@Override
public boolean isMultipart(HttpServletRequest request) {
    // 必须是 POST 请求,且 Content-Type 为 multipart/ 开头
    return ServletFileUpload.isMultipartContent(request);
}

判断是否为 multipart 请求,必须是 POST 请求,且 Content-Type 为 multipart/ 开头

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() {
                // 解析请求,获取文件、参数信息
                MultipartParsingResult parsingResult = parseRequest(request);
                setMultipartFiles(parsingResult.getMultipartFiles());
                setMultipartParameters(parsingResult.getMultipartParameters());
                setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());
            }
        };
    }
    else {
        // 解析请求,获取文件、参数信息
        MultipartParsingResult parsingResult = parseRequest(request);
        return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),
                parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());
    }
}

将 HttpServletRequest 转换成 DefaultMultipartHttpServletRequest 对象

如果开启了延迟解析,则重写该对象的 initializeMultipart() 方法,用于解析请求

否则直接调用 parseRequest(HttpServletRequest request) 方法解析请求,返回 MultipartParsingResult 对象,包含 MultipartFile 对象和普通参数信息

parseRequest

parseRequest(HttpServletRequest request)方法,用于解析请求,返回 MultipartParsingResult 对象,包含 MultipartFile 对象、普通参数信息以及参数的 Content-Type 信息,方法如下:

protected MultipartParsingResult parseRequest(HttpServletRequest request) throws MultipartException {
    // <1> 获取请求中的编码
    String encoding = determineEncoding(request);
    // <2> 获取 ServletFileUpload 对象
    FileUpload fileUpload = prepareFileUpload(encoding);
    try {
        // <3> 获取请求中的流数据
        List<FileItem> fileItems = ((ServletFileUpload) fileUpload).parseRequest(request);
        // <4> 将这些流数据转换成 MultipartParsingResult,包含 CommonsMultipartFile、参数信息、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);
    }
}
  1. 获取请求中的编码

  2. 根据编码获取到 ServletFileUpload 对象( commons-fileupload 中的类),在 newFileUpload(FileItemFactory fileItemFactory) 方法中返回的就是 ServletFileUpload 对象,可以看到父类 CommonsFileUploadSupport 的构造方法,如下:

    // org.springframework.web.multipart.commons.CommonsFileUploadSupport.java
    
    public CommonsFileUploadSupport() {
        this.fileItemFactory = newFileItemFactory();
        // 由子类实现
        this.fileUpload = newFileUpload(getFileItemFactory());
    }
    

    具体细节就不讲述了

  3. 通过 ServletFileUpload 对象解析请求,返回流数据 List<FileItem> fileItems

  4. 调用父类 CommonsFileUploadSupport 的 parseFileItems(List<FileItem> fileItems, String encoding) 方法,将这些流数据转换成 MultipartParsingResult 对象

    // 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);
    }
    

    大致就是遍历 fileItems 集合,如果是一个简单的表单字段,那么就是一个普通的参数,将参数名和值保存起来

    否则就是文件,将其封装成 CommonsMultipartFile 保存起来

cleanupMultipart

cleanupMultipart(MultipartHttpServletRequest request)方法,清理文件产生的临时资源,如下:

// 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,继承 AbstractMultipartHttpServletRequest 抽象类,MultipartHttpServletRequest 的默认实现类,代码如下:

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;
	}
}

代码并不复杂,稍微阅读一下就理解了😈

总结

本文对 Spring MVC 处理请求的过程中使用到的 MultipartResolver 组件进行了分析,如果请求的 Content-Typemultipart/*,涉及到文件上传,所以处理请求的第一步需要通过 MultipartResolver 组件对请求进行转换处理。会将 HttpServletRequest 请求对象封装成 MultipartHttpServletRequest 对象,便于获取参数信息和操作上传的文件(MultipartFile 对象)。

MultipartResolver 组件的实现类有两种:

  • org.springframework.web.multipart.support.StandardServletMultipartResolver:实现 MultipartResolver 接口,基于 Servlet 3.0 标准的上传文件 API 的 MultipartResolver 实现类
  • org.springframework.web.multipart.commons.CommonsMultipartResolver:实现 MultipartResolver 接口,基于 Apache Commons FileUpload 的 MultipartResolver 实现类

两者的区别:

  • StandardServletMultipartResolver 会将 HttpServletRequest 封装成 StandardMultipartHttpServletRequest 对象,由 Servlet 3.0 提供 API 获取请求中的 javax.servlet.http.Part 对象,然后进行解析,文件会封装成 StandardMultipartFile 对象

  • CommonsMultipartResolver 会将 HttpServletRequest 封装成 DefaultMultipartHttpServletRequest 对象,由 Apache 的 Commons FileUpload 组件来实现,通过 org.apache.commons.fileupload.servlet.ServletFileUpload 对象获取请求中的 org.apache.commons.fileupload.FileItem 对象,然后进行解析,文件会封装成 CommonsMultipartFile 对象,如何使用可以参考上面的 CommonsMultipartResolver 小节

注意事项:

  • 在 Spring MVC 中,multipartResolver 默认为 null,需要自己配置,例如《MyBatis 使用手册》中的 集成 Spring 模块下的 spring-mvc.xml 文件中配置 MultipartResolver 为 CommonsMultipartResolver 实现类,也可以配置为 StandardServletMultipartResolver 实现类
  • 在 Spring Boot 中,multipartResolver 默认为 StandardServletMultipartResolver 实现类

参考文章:芋道源码《精尽 Spring MVC 源码分析》

posted @ 2020-12-15 10:01  月圆吖  阅读(1226)  评论(1编辑  收藏  举报