http请求流程
http请求的处理过程大致为：
那么服务器端web应用程序是如何进行处理的呢？下面让我们一起探索其中的原理，以下是一些个人理解，欢迎大家讨论和交流
1.Tomcat是Web应用服务器,是一种Servlet/JSP容器，因此，所有的请求都会从DispatcherServlet进行处理，首先请求会到达`doService`方法，该方法会调用doDispatch处理实际的请求。
@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;
// 确定给定的请求是否是包含请求，即不是来自外部的顶级HTTP请求。
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.
// 放入应用上下文对象
request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
// 放入LocaleResolver,作用是解析Request中的语言标志参数或head中的Accept-Language参数，并将解析的结果保存到指定的域中，此处放入的是AcceptHeaderLocaleResolver
request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
// 放入主题解析器
request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
// 放入主题资源
request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
// 解决请求重定向时参数获取的问题
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);
}
RequestPath previousRequestPath = null;
if (this.parseRequestPath) {
previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
ServletRequestPathUtils.parseAndCache(request);
}
try {
// 实际处理请求的入口
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);
}
}
if (this.parseRequestPath) {
ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
}
}
} 2.doDispatch方法，先尝试获取处理当前请求的请求处理链，如果没有获取，则抛出异常；否则，执行请求处理链，ConversionServiceExposingInterceptor->ResourceUrlProviderExposingInterceptor
此处
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
// 如果请求是一个文件上传请求，则对请求进行封装解析出相关参数；否则，什么也不做，返回原请求参数
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
// 获取能够处理该请求的请求处理链
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
// 获取处理请求的适配器，RequestMappingHandlerAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = HttpMethod.GET.matches(method);
if (isGet || HttpMethod.HEAD.matches(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
// 请求前置处理，请求处理链中的前置方法，如果预处理认为可以终止，则后续请求链不在执行
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
// 执行请求处理，调用AbstractHandlerMethodAdapter类中的handle方法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 请求后置处理，在请求处理完成后的处理， 注册的请求处理链中的后置处理方法
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 ServletException("Handler dispatch failed: " + err, err);
}
// 处理modelAndView,如果出现异常，则处理异常，抛出；否则，反向执行请求处理链中的afterCompletion方法
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new ServletException("Handler processing failed: " + err, 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) {
// 如果请求时上传文件请求，则清理文件上传使用的资源
cleanupMultipart(processedRequest);
}
}
}
} 3.AbstractHandlerMethodAdapter#handleInternal方法。handle方法会调用内部的handleInternal方法
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ModelAndView mav;
// 校验请求类型和会话是否合法，如果没有设定，则不校验
checkRequest(request);
// Execute invokeHandlerMethod in synchronized block if required.
if (this.synchronizeOnSession) { // 同步阻塞请求
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No HttpSession available -> no mutex necessary
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No synchronization on session demanded at all...
// 非同步阻塞请求，实际调用此处
mav = invokeHandlerMethod(request, response, handlerMethod);
}
if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
}
else {
// 对响应内容进行预处理
prepareResponse(response);
}
}
return mav;
} 4.调用ServletInvocableHandlerMethod#invokeHandlerMethod方法
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
// 封装请求
ServletWebRequest webRequest = new ServletWebRequest(request, response);
// 数据绑定工厂 用于请求参数与controller的参数（实体类）类型和参数转换
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
// 设置请求参数解析器，默认有27个
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
// 设置返回值处理器，默认15个
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
// 设置参数名称解析器
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
// 获取请求重定向前的参数 DispatcherServlet#doService方法中设置的
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
// 设置默认模型不允许在重定向场景中使用
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);
asyncManager.registerCallableInterceptors(this.callableInterceptors);
asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
if (asyncManager.hasConcurrentResult()) {
Object result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult();
LogFormatUtils.traceDebug(logger, traceOn -> {
String formatted = LogFormatUtils.formatValue(result, !traceOn);
return "Resume with async result [" + formatted + "]";
});
invocableMethod = invocableMethod.wrapConcurrentResult(result);
}
// 调用目标方法进行业务逻辑处理，并对返回值进行处理
invocableMethod.invokeAndHandle(webRequest, mavContainer);
if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}
// 返回结果 invocableMethod.invokeAndHandle方法中标记了请求处理完成
return getModelAndView(mavContainer, modelFactory, webRequest);
} 5.ServletInvocableHandlerMethod#invokeAndHandle方法，该方法完成业务方法的调用，以及对业务方法返回结果的处理
public void invokeAndHandle(ServletWebRequest webRequest, ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 调用业务方法，并返回执行结果
Object returnValue = invokeForRequest(webRequest, mavContainer, providedArgs);
setResponseStatus(webRequest);
if (returnValue == null) {
if (isRequestNotModified(webRequest) || getResponseStatus() != null || mavContainer.isRequestHandled()) {
disableContentCachingIfNecessary(webRequest);
mavContainer.setRequestHandled(true);
return;
}
}
else if (StringUtils.hasText(getResponseStatusReason())) {
mavContainer.setRequestHandled(true);
return;
}
mavContainer.setRequestHandled(false);
Assert.state(this.returnValueHandlers != null, "No return value handlers");
try {
// 对业务方法的返回结果进行处理
this.returnValueHandlers.handleReturnValue(
returnValue, getReturnValueType(returnValue), mavContainer, webRequest);
}
catch (Exception ex) {
if (logger.isTraceEnabled()) {
logger.trace(formatErrorForReturnValue(returnValue), ex);
}
throw ex;
}
} 6.调用ServletInvocableHandlerMethod#invokeAndHandle方法，该方法中会调用invokeForRequest方法，此方法中调用doInvoke方法，最终完成业务方法调用，并将业务方法的执行结果返回
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
// 参数解析
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Arguments: " + Arrays.toString(args));
}
// 调用业务方法
return doInvoke(args);
} 7.调用HandlerMethodReturnValueHandlerComposite#handleReturnValue对返回结果进行处理,该方法中会调用RequestResponseBodyMethodProcessor#handleReturnValue方法处理结果（controller层方法标注有@ResponseBody注解的方法）
public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
ModelAndViewContainer mavContainer, NativeWebRequest webRequest)
throws IOException, HttpMediaTypeNotAcceptableException, HttpMessageNotWritableException {
// 设置标记，说明请求已经处理完成
mavContainer.setRequestHandled(true);
ServletServerHttpRequest inputMessage = createInputMessage(webRequest);
ServletServerHttpResponse outputMessage = createOutputMessage(webRequest);
if (returnValue instanceof ProblemDetail detail) {
outputMessage.setStatusCode(HttpStatusCode.valueOf(detail.getStatus()));
if (detail.getInstance() == null) {
URI path = URI.create(inputMessage.getServletRequest().getRequestURI());
detail.setInstance(path);
}
}
// Try even with null return value. ResponseBodyAdvice could get involved.
// 此处完成业务方法返回值->响应体内容的转换，也是实现自已返回格式的输出位置
writeWithMessageConverters(returnValue, returnType, inputMessage, outputMessage);
} 8.上述方法执行完成后，返回到RequestMappingHandlerAdapter#handleInternal方法
.....
else {
// No synchronization on session demanded at all...
// 非同步阻塞请求，实际调用此处
mav = invokeHandlerMethod(request, response, handlerMethod);
}
if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
}
else {
// 对响应内容进行预处理,如对响应内容设置缓存时间等，缓存有效期内，浏览器不会在发起请求
prepareResponse(response);
}
}
return mav; 8.返回到DispatcherServlet#doServlet方法,执行完ha.handle调用后。首先，执行请求处理链中的postHandle方法，然后执行processDispatchResult检测是否发生异常，没有异常，则反向执行请求链中的afterCompletion方法（默认方法空，没有实际的逻辑），到此doDispatch方法分析完成。
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
......
// Actually invoke the handler.
// 执行请求处理，调用AbstractHandlerMethodAdapter类中的handle方法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 请求后置处理，在请求处理完成后的处理， 注册的请求处理链中的后置处理方法
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 ServletException("Handler dispatch failed: " + err, err);
}
// 处理modelAndView,如果出现异常，则处理异常，抛出；否则，反向执行请求处理链中的afterCompletion方法
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new ServletException("Handler processing failed: " + err, 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) {
// 如果请求时上传文件请求，则清理文件上传使用的资源
cleanupMultipart(processedRequest);
}
}
}
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