剖析Spring MVC,介绍Spring MVC是如何处理请求的。
上一篇文章中，介绍了Spirng MVC的结构与创建过程，然后我们来看Spring MVC是如何处理请求的。
从Servlet中知道，处理请求是HttpServlet中service，HttpServlet中doPost,doGet等方法，从service路由到各个方法。详见HttpServlet代码。
在HttpServletBean中并没有重写这些方法。
1. service
在FrameworkServlet中重写了HttpServlet中service，HttpServlet中doPost,doGet等方法。
public enum HttpMethod {
GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
}service：添加PATCH，然后调用processRequest方法,其余则由父类service方法做处理。doGet,doPost,doPut,doDelete：直接调用processRequest。doOptions,doTrace：可有参数配置调用processRequest还是交给父类处理。- 没有重写
doHead方法。
2. processRequest
来看processRequest方法
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//获取原来保存的LocaleContext
//private static final ThreadLocal<LocaleContext> localeContextHolder =
// new NamedThreadLocal<>("LocaleContext");
//
// private static final ThreadLocal<LocaleContext> inheritableLocaleContextHolder =
// new NamedInheritableThreadLocal<>("LocaleContext");
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//根据request中locale创建当前LocaleContext
//new SimpleLocaleContext(request.getLocale())
LocaleContext localeContext = buildLocaleContext(request);
//获取原来保存的的RequestAttributes
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
//根据request, response创建当前的ServletRequestAttributes
//new ServletRequestAttributes(request, response);
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
//异步请求
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
//设置Interceptor
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//初始化ContextHolders,设置当前的LocaleContext，ServletRequestAttributes到Holder中
//if (localeContext != null) {
// LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
//}
//if (requestAttributes != null) {
// RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
//}
initContextHolders(request, localeContext, requestAttributes);
try {
//调用doService
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
//request还原原始的LocaleContext，RequestAttributes
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
//日志
logResult(request, response, failureCause, asyncManager);
//发布时间消息
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}- 可以通过
LocaleContextHolder,RequestContextHolder获取到LocaleContext,RequestAttributes。 publishRequestHandledEvent,可以实现ApplicationListener来使用这个监听：@Component public class RequestHandlerListener implements ApplicationListener<ServletRequestHandledEvent> { private final Logger log = LoggerFactory.getLogger(ServletRequestHandledEvent.class); @Override public void onApplicationEvent(ServletRequestHandledEvent event) { log.info("request event: {}",event); } }
3. doService
可以看到交由doService请求：
protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
throws Exception;这是一个抽象方法，来看子类DispatcherServlet中实现：
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
//日志
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.
//当include请求时，将Attribute保存到备份attributesSnapshot中
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.
//设置WebApplicationContext，localeResolver属性
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());
//对Redirect参数传递
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 {
//交给doDispatch处理
doDispatch(request, response);
}
finally {
//还原，将attributesSnapshot设置回去
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}4. doDispatch
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
//异步请求
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
//检查是否为上传请求，如果是则返回MultipartHttpServletRequest，不是则原样返回
processedRequest = checkMultipart(request);
//true:上传请求 false:飞上传请求
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//从HandlerMapping中获取Handler
//if (this.handlerMappings != null) {
// for (HandlerMapping mapping : this.handlerMappings) {
// HandlerExecutionChain handler = mapping.getHandler(request);
// if (handler != null) {
// return handler;
// }
// }
//}
mappedHandler = getHandler(processedRequest);
//未获取到抛出异常，SC_NOT_FOUND=404
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//获取handler adapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
// 处理GET、HEAD请求的Last-Modified
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
//PRECONDITION_FAILED=412
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
//执行相应的Interceptor的preHandle
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
//调用handler处理请求
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
//异步直接返回
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
//if (mv != null && !mv.hasView()) {
// String defaultViewName = getDefaultViewName(request);
// if (defaultViewName != null) {
// mv.setViewName(defaultViewName);
// }
//}
//如果view为空，根据processedRequest设置默认view
applyDefaultViewName(processedRequest, mv);
//执行相应Interceptor的postHandle
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
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 {
//异步
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) {
//删除资源
cleanupMultipart(processedRequest);
}
}
}
}5. processDispatchResult
Handler,HandlerMapping,HandlerAdapter在后面的文章中将详细描述，先看processDispatchResult:
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
//如果有异常,设置到view
if (exception != null) {
if (exception instanceof ModelAndViewDefiningException) {
logger.debug("ModelAndViewDefiningException encountered", exception);
mv = ((ModelAndViewDefiningException) exception).getModelAndView();
}
else {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(request, response, handler, exception);
errorView = (mv != null);
}
}
// Did the handler return a view to render?
////设置local,构建view,渲染
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("No view rendering, null ModelAndView returned.");
}
}
if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Concurrent handling started during a forward
return;
}
//请求处理完，触发Interceptor的afterCompletion,释放。
if (mappedHandler != null) {
mappedHandler.triggerAfterCompletion(request, response, null);
}
}到此，也就是Spring MVC处理请求流程简介。具体HandlerMapping，HandlerAdapter如何工作在后文中详细描述。