SpringMVC源码分析--文件上传



SpringMVC提供了文件上传的功能，接下来我们就简单了解一下SpringMVC文件上传的开发及大致过程。

首先需要在springMVC的配置文件中配置文件上传解析器

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1. <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver"/>    

<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。

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1. <dependency>  
2.             <groupId>commons-fileupload</groupId>  
3.             <artifactId>commons-fileupload</artifactId>  
4.             <version>1.3.2</version>  
5.         </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：

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1. <form id='fForm' class="form-actions form-horizontal" action="/file/upload.action"   
2.               encType="multipart/form-data" target="uploadf" method="post">  
3.                  <div class="control-group">  
4.                     <label class="control-label">上传文件:</label>  
5.                     <div class="controls">  
6.                         <input type="file"  name="file" style="width:550">  
7.                               
8.                     </div>  
9.                 </div>  
10.                  <div class="control-group">  
11.                     <div class="controls">  
12.                     <button type="button" id="subbut" class="btn">submit</button>  
13.                     </div>  
14.                 </div>  
15.         </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

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1. @Controller  
2. @RequestMapping("/file")  
3. public class FileUpload {  
4.   
5.     @RequestMapping("/upload")  
6.     @ResponseBody  
7.     public String upload(@RequestParam(value = "file", required = false)MultipartFile... files ){  
8.           
9.         for (MultipartFile f : files) {  
10.             if (f.getSize() > 0) {  
11.                 File targetFile = new File("1.jpg");  
12.                 try {  
13.                     f.transferTo(targetFile);  
14.                 } catch (IllegalStateException | IOException e) {  
15.                     e.printStackTrace();  
16.                 }  
17.             }  
18.         }  
19.           
20.         return null;  
21.     }  
22.       
23.     @RequestMapping("/uploads")  
24.     public String uploads(){  
25.         return "fileupload";  
26.     }  
27. }  

@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。

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1. processedRequest = checkMultipart(request);  

processedRequest = checkMultipart(request);

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1. protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {  
2.         //首先判断请求是否是文件上传  
3.         if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {  
4.             if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {  
5.                 logger.debug("Request is already a MultipartHttpServletRequest - if not in a forward, " +  
6.                         "this typically results from an additional MultipartFilter in web.xml");  
7.             }  
8.             else if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) instanceof MultipartException) {  
9.                 logger.debug("Multipart resolution failed for current request before - " +  
10.                         "skipping re-resolution for undisturbed error rendering");  
11.             }  
12.             else {  
13.                 return this.multipartResolver.resolveMultipart(request);  
14.             }  
15.         }  
16.         //如果不是文件上传则直接返回请求  
17.         // If not returned before: return original request.  
18.         return request;  
19.     }  

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

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1. @Override  
2.     public boolean isMultipart(HttpServletRequest request) {  
3.         return (request != null && ServletFileUpload.isMultipartContent(request));  
4.     }  

@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

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1. @Override  
2.     public MultipartHttpServletRequest resolveMultipart(final HttpServletRequest request) throws MultipartException {  
3.         Assert.notNull(request, "Request must not be null");  
4.         //this.resolveLazily默认值是false  
5.         if (this.resolveLazily) {  
6.             return new DefaultMultipartHttpServletRequest(request) {  
7.                 @Override  
8.                 protected void initializeMultipart() {  
9.                     MultipartParsingResult parsingResult = parseRequest(request);  
10.                     setMultipartFiles(parsingResult.getMultipartFiles());  
11.                     setMultipartParameters(parsingResult.getMultipartParameters());  
12.                     setMultipartParameterContentTypes(parsingResult.getMultipartParameterContentTypes());  
13.                 }  
14.             };  
15.         }  
16.         else {  
17.             MultipartParsingResult parsingResult = parseRequest(request);  
18.             return new DefaultMultipartHttpServletRequest(request, parsingResult.getMultipartFiles(),  
19.                     parsingResult.getMultipartParameters(), parsingResult.getMultipartParameterContentTypes());  
20.         }  
21.     }  

@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。

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1. public String upload(@RequestParam(value = "file", required = false)MultipartFile... files ){  
2.           
3.         for (MultipartFile f : files) {  
4.             if (f.getSize() > 0) {  
5.                 File targetFile = new File("1.jpg");  
6.                 try {  
7.                     f.transferTo(targetFile);  
8.                 } catch (IllegalStateException | IOException e) {  
9.                     e.printStackTrace();  
10.                 }  
11.             }  
12.         }  
13.           
14.         return null;  
15.     }