SpringMVC源码分析--文件上传

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver"/>  springMVC的文件上传需要commons-fileupload 包的支持，需要引入。
[html] view plain copy print?<dependency>              <groupId>commons-fileupload</groupId>              <artifactId>commons-fileupload</artifactId>              <version>1.3.2</version>          </dependency>  
<dependency>		    <groupId>commons-fileupload</groupId>		    <artifactId>commons-fileupload</artifactId>		    <version>1.3.2</version>		</dependency>文件上传的jsp表单页面页面，文件名称为file：
[html] view plain copy print?<form id='fForm' class="form-actions form-horizontal" action="/file/upload.action"                 encType="multipart/form-data" target="uploadf" method="post">                   <div class="control-group">                      <label class="control-label">上传文件:</label>                      <div class="controls">                          <input type="file"  name="file" style="width:550">                                                    </div>                  </div>                   <div class="control-group">                      <div class="controls">                      <button type="button" id="subbut" class="btn">submit</button>                      </div>                  </div>          </form>  
<form id='fForm' class="form-actions form-horizontal" action="/file/upload.action" 		      encType="multipart/form-data" target="uploadf" method="post">				 <div class="control-group">					<label class="control-label">上传文件:</label>					<div class="controls">						<input type="file"  name="file" style="width:550">												</div>				</div>				 <div class="control-group">					<div class="controls">					<button type="button" id="subbut" class="btn">submit</button>					</div>				</div>		</form>文件上传的后台处理页面
[java] view plain copy print?@Controller  @RequestMapping("/file")  public class FileUpload {        @RequestMapping("/upload")      @ResponseBody      public String upload(@RequestParam(value = "file", required = false)MultipartFile... files ){                    for (MultipartFile f : files) {              if (f.getSize() > 0) {                  File targetFile = new File("1.jpg");                  try {                      f.transferTo(targetFile);                  } catch (IllegalStateException | IOException e) {                      e.printStackTrace();                  }              }          }                    return null;      }            @RequestMapping("/uploads")      public String uploads(){          return "fileupload";      }  }  
@Controller@RequestMapping("/file")public class FileUpload {	@RequestMapping("/upload")	@ResponseBody	public String upload(@RequestParam(value = "file", required = false)MultipartFile... files ){				for (MultipartFile f : files) {			if (f.getSize() > 0) {				File targetFile = new File("1.jpg");				try {					f.transferTo(targetFile);				} catch (IllegalStateException | IOException e) {					e.printStackTrace();				}			}		}				return null;	}		@RequestMapping("/uploads")	public String uploads(){		return "fileupload";	}}这样就可以在jsp页面中提交一个图片，后台将图片保存到工程目录下，页面如下：
接下来我们了解一下文件上传到达处理的Controller时做的预处理
在请求到达DispatcherServelet时，对文件上传处理的操作是在doDispatch方法中，用于判断这个请求是否是文件上传操作，具体的文件上传相关的操作都在这个方法中实现了。
[java]%20view%20plain%20copy%20print?processedRequest = checkMultipart(request);  
processedRequest = checkMultipart(request);[java] view plain copy print?protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {          //首先判断请求是否是文件上传          if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {              if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {                  logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +                          "this typically results from an additional MultipartFilter in web.xml");              }              else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {                  logger.debug("Multipart resolution failed for current request before - " +                          "skipping re-resolution for undisturbed error rendering");              }              else {                  return this.multipartResolver.resolveMultipart(request);              }          }          //如果不是文件上传则直接返回请求          // If not returned before: return original request.          return request;      }  
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {		//首先判断请求是否是文件上传		if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {			if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {				logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +						"this typically results from an additional MultipartFilter in web.xml");			}			else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {				logger.debug("Multipart resolution failed for current request before - " +						"skipping re-resolution for undisturbed error rendering");			}			else {				return this.multipartResolver.resolveMultipart(request);			}		}		//如果不是文件上传则直接返回请求		// If not returned before: return original request.		return request;	}this.multipartResolver.isMultipart(request)这个方法操作用来判断请求是否是文件上传操作，具体实现是在我们配置的multipartResolver中
[java] view plain copy print?@Override      public boolean isMultipart(HttpServletRequest request) {          return (request != null && ServletFileUpload.isMultipartContent(request));      }  
@Override	public boolean isMultipart(HttpServletRequest request) {		return (request != null && ServletFileUpload.isMultipartContent(request));	}ServletFileUpload.isMultipart方法判断请求是否是文件上传的实现是在commons-fileupload包中了，这就是我们为什么需要引入commons-fileupload的原因
当我们已经判断请求是文件上传的请求时，接下来的操作是在this.multipartResolver.resolveMultipart(request);中了。
CommonsMultipartResolver类中的resolveMultipart方法
[java] view plain copy print?@Override      public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {          Assert.notNull(request, "Request must not be null");          //this.resolveLazily默认值是false          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());          }      }  
@Override	public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {		Assert.notNull(request, "Request must not be null");		//this.resolveLazily默认值是false		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());		}	}具体的解析过程我们就不分析了，最终结果是生成一个DefaultMultipartHttpServletRequest，其主要的实现就是将我们在前端页面上传的文件file，生成MultipartFile对象，这就是为什么在Controller中，我们file参数的类型是MultipartFile，如下，这样我们就可以通过MultipartFile来进行编程来实现文件的一系列操作了。
[java] view plain copy print?public String upload(@RequestParam(value = "file", required = false)MultipartFile... files ){                    for (MultipartFile f : files) {              if (f.getSize() > 0) {                  File targetFile = new File("1.jpg");                  try {                      f.transferTo(targetFile);                  } catch (IllegalStateException | IOException e) {                      e.printStackTrace();                  }              }          }                    return null;      }