【初】HTTP执行流程，SpringMVC执行流程，Java一次HTTP请求容器做了哪些事情，过滤器、拦截器、AOP执行流程
2022年7月25日大约 7 分钟
https://www.bilibili.com/video/BV1EG4y1i7FW
刚开始找工作的时候总有面试官问：Servlet的生命周期，SpringMVC的执行流程，我一直觉得这是个傻Ⅹ问题，都20年了，谁还不用SpringBoot了？还问这些古老的技术干嘛？（当然为了面试、吃饭，我还是每次面试前都背了这些）。
最近在机缘巧合下又研究了一下HTTP请求流程，这才发现它举足轻重的地位。
PS：当然现在虽然很清楚了，但面试还是得背，因为过一段时间还是会忘记。
一、流程图
下面是基于undertow 服务器画的流程图，你可以理解 undertow 和 tomcat 一样都是一个服务器，有兴趣的可以自行百度一下。（虽然我们都听过tomcat的大名，但你又真的理解什么是tomcat吗？所以不必纠结这个服务器的东西）
整个请求就是一个链式的，当用户发起一个请求的时候服务器就会分一个线程给当前请求，然后链式的去依次执行下面的链。
最重要的当属 org.springframework.web.servlet.DispatcherServlet#doDispatch 方法，它就是SpringMVC流程的核心了。
1-1、全流程图
1-2、SpringMVC 流程图
1-3、各种拦截时机图
二、方法详解
2-1、io.undertow.servlet.handlers.FilterHandler#handleRequest
这个方法就是产生 ServletRequest 、ServletResponse ，和Filter 链式调用的开始
public void handleRequest(final HttpServerExchange exchange) throws Exception {
final ServletRequestContext servletRequestContext = exchange.getAttachment(ServletRequestContext.ATTACHMENT_KEY);
// 获取 ServletRequest 和 ServletResponse
ServletRequest request = servletRequestContext.getServletRequest();
ServletResponse response = servletRequestContext.getServletResponse();
DispatcherType dispatcher = servletRequestContext.getDispatcherType();
Boolean supported = asyncSupported.get(dispatcher);
if(supported != null && ! supported) {
servletRequestContext.setAsyncSupported(false);
}
final List<ManagedFilter> filters = this.filters.get(dispatcher);
if(filters == null) {
// 如果filter为空就直接开始后面的执行
next.handleRequest(exchange);
} else {
// 开启filter链式调用
final FilterChainImpl filterChain = new FilterChainImpl(exchange, filters, next, allowNonStandardWrappers);
filterChain.doFilter(request, response);
}
}2-2、io.undertow.servlet.handlers.FilterHandler.FilterChainImpl#doFilter
这里就是链式调用的具体地方了，如果调用完后，也会从这里跳到下一步。 这里我只复制出精简的代码
public void doFilter(final ServletRequest request, final ServletResponse response) throws IOException, ServletException {
int index = location++;
if (index >= filters.size()) {
// 所有的filter执行完成了，下一步
next.handleRequest(exchange);
} else {
// 调用下一个 doFilter
filters.get(index).doFilter(request, response, this);
}
}2-3、io.undertow.servlet.handlers.ServletHandler#handleRequest
这里给一个建议，当我们从发现一个接口有很多实现类的时候，不知道具体会是哪一个实现类，我们可以挑选一个最为接近的然后断点看看。
这个方法里面主要就是去调用具体的 service 方法
public void handleRequest(final HttpServerExchange exchange) throws IOException, ServletException {
// ...
servlet.getInstance().service(request, response);
// ...
}2-4、javax.servlet.http.HttpServlet#service(ServletRequest, ServletResponse)
这个service 方法主要是把 ServletRequest 转成HttpServletRequest， ServletResponse 转成 HttpServletResponse
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
if (!(req instanceof HttpServletRequest && res instanceof HttpServletResponse)) {
throw new ServletException("non-HTTP request or response");
}
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
service(request, response);
}2-5、javax.servlet.http.HttpServlet#service(HttpServletRequest, HttpServletResponse)
这个 service 方法的意义是找到对应的 doGet、doPost ...
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 = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < lastModified) {
// 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 {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
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);
}
}2-6、org.springframework.web.servlet.FrameworkServlet#doPost
这个方法没有执行任何逻辑，只是执行下一步
protected final void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}2-7、org.springframework.web.servlet.FrameworkServlet#processRequest
这个方法做了一些初始化工作后就去执行 doService 方法
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
// ...
doService(request, response);
// ...
}2-8、org.springframework.web.servlet.DispatcherServlet#doService
这个方法给 request 里面封装了一些参数， 使框架对象可用于处理程序和视图对象。
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);
}
}
}
}2-9、org.springframework.web.servlet.DispatcherServlet#doDispatch
这就是 SpringMVC 的核心流程了
先来看一下官方的解释
处理实际分派给处理程序。 处理程序将通过按顺序应用servlet的HandlerMappings来获得。HandlerAdapter将通过查询servlet安装的HandlerAdapter来获得，以找到第一个支持处理程序类的HandlerAdapter。 所有HTTP方法都由该方法处理。由HandlerAdapter或处理程序自己决定哪些方法是可以接受的。
SpringMVC 执行流程图
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 {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// 通过 handlerMappings（处理器映射器） 找到对应的 handler
mappedHandler = getHandler(processedRequest);
// 找不到对应的 handler 就直接返回错误
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// 通过 handler 找到对应的 HandlerAdapter（处理器适配器）
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;
}
}
// 执行前置拦截器，如果被拦截那就结束了
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// 执行真正的handler并返回 ModelAndView
// 这里其实就说代理去执行我们真正的方法， AOP 就是在这里面去执行的
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 执行拦截器后置处理
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);
}
// 1、将ModelAndView传给ViewResolver视图解析器进行解析
// 2、ViewResolver（视图解析器）解析后返回具体View
// 3、渲染视图将mode 数据转成respone响应返回
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);
}
}
}
}2-10、org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter#handle
public final ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return handleInternal(request, response, (HandlerMethod) handler);
}后续方法的调用就是一些个处理，然后通过反射生成代理对象去执行，然后返回 ModelAndView 有兴趣的可以根据 全流程图 去打断点查看。
2-11、org.springframework.web.servlet.DispatcherServlet#processDispatchResult
这个方法会先去判断异常，如果没有异常就执行数据的渲染
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
// ...
if (mv != null && !mv.wasCleared()) {
render(mv, request, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
// ...
}2-12、org.springframework.web.servlet.DispatcherServlet#render
ViewResolver（视图解析器）解析后得到具体的 View
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// ...
View view;
// ...
view = mv.getView();
// ...
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
view.render(mv.getModelInternal(), request, response);
// ...
}2-13、org.springframework.web.servlet.view.AbstractView#render
渲染视图将mode 数据转成respone响应返回
public void render(@Nullable Map<String, ?> model, HttpServletRequest request,
HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
logger.debug("View " + formatViewName() +
", model " + (model != null ? model : Collections.emptyMap()) +
(this.staticAttributes.isEmpty() ? "" : ", static attributes " + this.staticAttributes));
}
Map<String, Object> mergedModel = createMergedOutputModel(model, request, response);
prepareResponse(request, response);
renderMergedOutputModel(mergedModel, getRequestToExpose(request), response);
}