SpringMVC中的DispatcherServlet请求分析
作者：it_lihongmin
这篇文章主要介绍了SpringMVC中的DispatcherServlet请求分析, DispatcherServlet作为一个Servlet,那么当有请求到Tomcat等Servlet服务器时,会调用其service方法,再调用到其父类GenericServlet的service方法,需要的朋友可以参考下
一、service请求（servlet请求转换为Http请求）
DispatcherServlet作为一个Servlet，那么当有请求到Tomcat等Servlet服务器时，会调用其service方法。再调用到其父类GenericServlet的service方法，HttpServlet中实现，如下（开始请求的调用）：
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException(lStrings.getString("http.non_http"));
}
service(request, response);
}HttpServlet层：将request和response类型进行转换后，继续调用service方法：
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
ifModifiedSince = -1;
}
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}根据调用的Http请求的方式，调用具体的底层（FrameworkServlet层）方法，get、post请求等都会有相同的处理,比如doGet如下：
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response);
} catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
} catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
} finally {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
logResult(request, response, failureCause, asyncManager);
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}主要的核心逻辑为doService，但是Tomcat是使用线程池的方式接受来自客户端的请求的，当前请求中可能带有Locate（国际化参数信息），那么需要使用ThreadLocal在请求前记录参数信息，在请求之后finally中将参数恢复回去，不会影响到下一个请求。
Spring经常会这样进行处理，比如AopContext等处理Aop切面信息。
二、doService请求（request中添加SpringMVC初始化的九大件信息）
@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);
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}1、打印请求日志
2、请求中添加属性（WebApplicationContext容器，i18n解析器，主题解析器，主题，重定向属性处理）
3、核心方法 doDispatch
三、doDispatch
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
// 异步请求属性解析（重定向）
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// 如果是Multipart上传文件请求，则调用multipartResolver.resolveMultipart（上传文件解析器进行解析）
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// 确定Handler处理该请求（HandlerExecutionChain调用链，责任链模式）
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// 根据初始化时加载的适配器挨个匹配是否能适配该调用链
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// 对请求头中的last-modified进行处理
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;
}
}
// 对HandlerExecutionChain中的所有HandlerInterceptor调用preHandle方法
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 真正的请求调用
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
// 判断Controller返回的ModelAndView中是否有View，
// 没有则使用viewNameTranslator执行没有试图的解析规则
applyDefaultViewName(processedRequest, mv);
// 对HandlerExecutionChain中的所有HandlerInterceptor调用postHandle方法
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);
}
// 处理结果解析（ModelAndView或者Exception），保证最终会执行所以Interceptor的triggerAfterCompletion
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()) {
// 不知道什么情况下回进入相当于执行所以Interceptor的postHandle和afterCompletion方法
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// 清除当前文件上传请求的资源
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}这里是整个SpringMVC的核心，每个流程都可能会比较复杂，后续单独分析。流程如下（主要是标红的步骤）：
1、异步请求属性解析（重定向）
2、如果是Multipart上传文件请求，则调用multipartResolver.resolveMultipart（上传文件解析器进行解析）
4、确定Handler处理该请求（HandlerExecutionChain调用链，责任链模式）
5、根据初始化时加载的适配器挨个匹配是否能适配该调用链
6、对请求头中的last-modified进行处理
7、对HandlerExecutionChain中的所有HandlerInterceptor调用preHandle方法
8、真正的请求调用
9、没有试图的解析返回
10、对HandlerExecutionChain中的所有HandlerInterceptor调用postHandle方法
11、处理结果解析（ModelAndView或者Exception），保证最终会执行所以Interceptor的triggerAfterCompletion
12、清除当前文件上传请求的资源
HandlerInterceptor方法调用
当我们需要使用Spring的拦截器时，会集实现HandlerInterceptor的preHandle、postHandle、triggerAfterCompletion方法。
当第4步完成后我们获取到了HandlerExecutionChain调用链，其中包括需要执行的拦截器和真正调用的方法（后面专门分析专门获取的）。
但是在上面的步骤看到了对拦截器的三个方法的调用时机，其实每个调用的方法都差不多，就看preHandle方法即可，调用HandlerExecutionChain的applyPreHandle方法如下：
boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
HandlerInterceptor[] interceptors = getInterceptors();
if (!ObjectUtils.isEmpty(interceptors)) {
for (int i = 0; i < interceptors.length; i++) {
HandlerInterceptor interceptor = interceptors[i];
if (!interceptor.preHandle(request, response, this.handler)) {
triggerAfterCompletion(request, response, null);
return false;
}
this.interceptorIndex = i;
}
}
return true;
}由于preHandle方法允许返回false则不执行真实的Controller方法调用，所以需要每次判断。但是在执行preHandle和postHandle方法时，都允许最后调用一次triggerAfterCompletion方法。都是在拿到调用链中的所以有序的拦截器，轮训调用其对应的方法即可。
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