1、问题产生
在写一个基于SpringMVC的前后端分离的项目中，前端通过Ajax的POST方法将文件上传到后端，前端上传的除了文件 还有其它字段，于是使用FormData对象来存储并post到后端； 后端的controller中通过request.getParameter(“key”)来获取前端提交的数据，但是遇到问题就是获取的值一直为NULL。
前端请求的代码：
var formData = new FormData();
var shopImg = $('#shop-img')[0].files[0]; // 这里是文件
formData.append('shopImg', shopImg);
formData.append('username', 'jae');
formData.append('password', '123456');
$.ajax({
url : url,
type : 'POST',
data : formData,
enctype: 'multipart/form-data',
processData: false, // 不对数据进行处理
contentType: false,
cache: false,
success : function(data) {
if (data.success) {
$.toast('提交成功！');
}
else{
$.toast('提交失败！' + data.errMsg);
}
}
});
后端获取参数方式：
request.getParameter("username");
null
2、调试思路
2.1 什么是FormData
FormData对象用来将数据编译成键值对，以便使用XMLHttpRequest来发送数据，其主要用于发送表单数据，但是也可以用来发送带键的数据，而 独立于表单的使用。
下面是前端发送的请求信息
Provisional headers are shown
Accept: */*
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryBevlA3nAb5XKi0XB
Origin: http://127.0.0.1:8080
Referer: http://127.0.0.1:8080/shopadmin/shopoperation
User-Agent: Mozilla/5.0 (Linux; Android 5.0; SM-G900P Build/LRX21T) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.62 Mobile Safari/537.36
X-Requested-With: XMLHttpRequest
\_: 1552878374830
------WebKitFormBoundaryBevlA3nAb5XKi0XB
Content-Disposition: form-data; name="shopImg"
undefined
------WebKitFormBoundaryBevlA3nAb5XKi0XB
Content-Disposition: form-data; name="shopStr"
{"shopName":"12","shopAddr":"12","phone":"12","shopDesc":"","shopCategory":{"shopCategoryId":2},"area":{"areaId":2}}
------WebKitFormBoundaryBevlA3nAb5XKi0XB--
2.2 SpringMVC中如何处理前端的请求
1）用户发送请求到前端控制器DispatcherServlet
2）DispatcherServlet收到请求调用处理器映射器HandlerMapping
3）HandlerMapping根据请求的URL找到具体的处理器，生成处理器执行链HandlerExecutionChain返回给DispatcherServlet
4）DispatcherServlet根据处理器Handler获取处理器适配器Handler Adapter执行HandlerAdapter处理一系列操作，如参数封装，数据格式转换
数据验证等
5）执行处理器Handler（我们自己写的Controller）
下面略
2.3 DEBUG跟踪请求
<1> 当前端请求到达SpringMVC的DispatcherServlet时，首先会到达它的doService()方法。代码如下
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
logRequest(request);
// Keep a snapshot of the request attributes in case of an include,
// to be able to restore the original attributes after the include.
Map<String, Object> attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
attributesSnapshot.put(attrName, request.getAttribute(attrName));
}
}
}
// Make framework objects available to handlers and view objects.
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
if (this.flashMapManager != null) {
FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
if (inputFlashMap != null) {
request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
}
request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
}
try {
doDispatch(request, response); // 接下来调用该函数--<2>
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
<2> 接着会调用doDispatch(request, response)
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request; // 已经处理的请求
HandlerExecutionChain mappedHandler = null; // 处理器映射器
boolean multipartRequestParsed = false; // 是否已经解析了multipartRquest
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null; // 模型视图
Exception dispatchException = null;
try {
processedRequest = checkMultipart(request); // 首先检查该请求是否是MultipartRequest 我们再跟进去。----<3>
// 这里对比两次request是否为相同，如果相同说明就不是multipartRequest
// 于是将request作为HttpServletRequest对象处理，这时候在我们的controller中
// 通过requet.getParademeter("key")是获取到的key值为null
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
<3> 跟进到checkMultipart(request)中
protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
// 这里首先判断this.multipartResolver是否为null我们在该文件开始部分的注释中找到这样的说明
<li>The dispatcher's strategy for resolving multipart requests is determined by a
{@link org.springframework.web.multipart.MultipartResolver} implementation.
Implementations for Apache Commons FileUpload and Servlet 3 are included; the typical
choice is {@linkorg.springframework.web.multipart.commons.CommonsMultipartResolver}.
The MultipartResolver bean name is "multipartResolver"; default is none.
// 从文档中可以看到，对于multipart requests默认是没有提供解析器的，所以下面的if判断就为false
最后返回的request和入参的request一样，再回到上面的代码
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 {
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; 返回到----<2>
}
3、产生原因
产生的原因就是我们没有提供org.springframework.web.multipart.MultipartResolver，导致我们的multipart/formdata请求无法被正确的解析，进而导致我们在controller中无法获取到参数。
4、解决方法
在Spring-mvc.xml文件中配置相应的bean，这样在checkMultipart(request)就可以将HttpServletRequest进行包装
<bean id="multipartResolver"
class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
<property name="maxUploadSize" value="10485760"/>
<property name="maxInMemorySize" value="40960" />
</bean>
5、收获
再遇到前端请求后端无法获取预期的参数时，最好的调试方法就是根据SpringMVC请求的过程从源头跟进，这样能大大节省解决问题的时间。
