目录
1、SpringMVC 常用组件
-
DispatcherServlet：前端控制器，不需要工程师开发，由框架提供
作用：统一处理请求和响应，整个流程控制的中心，由它调用其它组件处理用户的请求 -
HandlerMapping：处理器映射器，不需要工程师开发，由框架提供
作用：根据请求的url、method等信息查找Handler（就是controller），即控制器方法 -
Handler：处理器，需要工程师开发
作用：在DispatcherServlet的控制下Handler对具体的用户请求进行处理 -
HandlerAdapter：处理器适配器，不需要工程师开发，由框架提供
作用：通过HandlerAdapter对处理器（控制器方法）进行执行 -
ViewResolver：视图解析器，不需要工程师开发，由框架提供
作用：进行视图解析，得到相应的视图，例如：ThymeleafView、InternalResourceView、RedirectView -
View：视图
作用：将模型数据通过页面展示给用户
2、DispatcherServlet 初始化过程
DispatcherServlet 本质上是一个 Servlet，所以天然的遵循 Servlet 的生命周期。所以宏观上是 Servlet 生命周期来进行调度。
a>初始化WebApplicationContext
所在类：org.springframework.web.servlet.FrameworkServlet，该类的 initServletBean 方法调用了 initWebApplicationContext 方法
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
// 第一次执行必为空，该if语句必然不会执行
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
// No context instance was injected at construction time -> see if one
// has been registered in the servlet context. If one exists, it is assumed
// that the parent context (if any) has already been set and that the
// user has performed any initialization such as setting the context id
// 第一次也找不到，wac还是为空
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
// 找不到则创建WebApplicationContext，这里可以先看 b>创建WebApplicationContext 再回来继续看
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// Either the context is not a ConfigurableApplicationContext with refresh
// support or the context injected at construction time had already been
// refreshed -> trigger initial onRefresh manually here.
synchronized (this.onRefreshMonitor) {
// 刷新WebApplicationContext，在这个类中onRefresh方法为空，用来给子类重写调用，这里可以看 c>DispatcherServlet初始化策略
onRefresh(wac);
}
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
// 获取属性名
String attrName = getServletContextAttributeName();
// 将wac共享到ServletContext对象中
getServletContext().setAttribute(attrName, wac);
}
return wac;
}b>创建WebApplicationContext
所在类：org.springframework.web.servlet.FrameworkServlet
protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
Class<?> contextClass = getContextClass();
if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
throw new ApplicationContextException(
"Fatal initialization error in servlet with name '" + getServletName() +
"': custom WebApplicationContext class [" + contextClass.getName() +
"] is not of type ConfigurableWebApplicationContext");
}
// 通过反射创建 IOC 容器对象
ConfigurableWebApplicationContext wac =
(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
// 设置环境
wac.setEnvironment(getEnvironment());
// 设置父容器，将Spring的IOC容器设置为SpringMVC的父容器
wac.setParent(parent);
String configLocation = getContextConfigLocation();
if (configLocation != null) {
wac.setConfigLocation(configLocation);
}
configureAndRefreshWebApplicationContext(wac);
return wac;
}c>DispatcherServlet初始化策略
所在类：org.springframework.web.servlet.DispatcherServlet
FrameworkServlet创建 WebApplicationContext 后，刷新容器，调用 onRefresh(wac)，此方法在DispatcherServlet中进行了重写，调用了initStrategies(context) 方法，初始化策略，即初始化DispatcherServlet的各个组件
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}3、DispatcherServlet调用组件处理请求
在 Servlet接口、GenericServlet类 的 service 方法都是抽象方法，没有具体内容
但是在 HttpServlet类 中对该方法进行了重写，主要是将形参的 ServletRequest 和 ServletResponse 转化为 HttpServletRequest 和 HttpServletResponse，再调用该方法自己的 service 方法，重写的方法如下
public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
if (req instanceof HttpServletRequest && res instanceof HttpServletResponse) {
HttpServletRequest request = (HttpServletRequest)req;
HttpServletResponse response = (HttpServletResponse)res;
this.service(request, response);
} else {
throw new ServletException("non-HTTP request or response");
}
}而 HttpServlet类 自己的 service 方法主要是针对不同的请求方式执行对应的方法而已，比如Post请求就执行doPost方法，Delete请求就执行doDelete方法
HttpServletBean类 没有对 service 方法进行重写，完全继承 HttpServlet类 的 service 方法
FrameworkServlet类 对 service 方法和 doXxx() 方法进行了重写，如下
/**
* Override the parent class implementation in order to intercept PATCH requests.
*/
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
// 若httpMethod为空或为PATCH，则执行processRequest方法(见下面的a>processRequest())，否则执行HttpServlet的service方法
if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
processRequest(request, response);
}
else {
// 由于FrameworkServlet重写了doXxx()方法，所以下面的代码实际上也会执行processRequest方法
super.service(request, response);
}
}a>processRequest()
FrameworkServlet重写HttpServlet中的service()和doXxx()，这些方法中调用了processRequest(request, response)
所在类：org.springframework.web.servlet.FrameworkServlet
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 {
// 执行服务，doService()是一个抽象方法，在DispatcherServlet中进行了重写，详见 b>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 {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
logResult(request, response, failureCause, asyncManager);
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}b>doService()
所在类：org.springframework.web.servlet.DispatcherServlet
@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);
}
RequestPath requestPath = null;
if (this.parseRequestPath && !ServletRequestPathUtils.hasParsedRequestPath(request)) {
requestPath = ServletRequestPathUtils.parseAndCache(request);
}
try {
// 处理请求和响应
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 (requestPath != null) {
ServletRequestPathUtils.clearParsedRequestPath(request);
}
}
}c>doDispatch()
所在类：org.springframework.web.servlet.DispatcherServlet
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);
// Determine handler for the current request.
/*
mappedHandler：调用链
包含handler、interceptorList、interceptorIndex
handler：浏览器发送的请求所匹配的控制器方法
interceptorList：处理控制器方法的所有拦截器集合
interceptorIndex：拦截器索引，控制拦截器afterCompletion()的执行
*/
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
// 通过控制器方法创建相应的处理器适配器，调用所对应的控制器方法
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;
}
}
// 调用拦截器的preHandle()
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
// 由处理器适配器调用具体的控制器方法，最终获得ModelAndView对象
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 调用拦截器的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);
}
// 后续处理：处理模型数据和渲染视图
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);
}
}
}
}d>processDispatchResult()
private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
@Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
@Nullable Exception exception) throws Exception {
boolean errorView = false;
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?
if (mv != null && !mv.wasCleared()) {
// 处理模型数据和渲染视图
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;
}
if (mappedHandler != null) {
// Exception (if any) is already handled..
// 调用拦截器的afterCompletion()
mappedHandler.triggerAfterCompletion(request, response, null);
}
}4、SpringMVC的执行流程
1、用户向服务器发送请求，请求被SpringMVC 前端控制器 DispatcherServlet捕获。
2、DispatcherServlet对请求URL进行解析，得到请求资源标识符（URI），判断请求URI对应的映射：
a) 不存在
i. 再判断是否配置了mvc:default-servlet-handler
ii. 如果没配置，则控制台报映射查找不到，客户端展示404错误
iii. 如果有配置，则访问目标资源（一般为静态资源，如：JS,CSS,HTML），找不到客户端也会展示404错误
b) 存在则执行下面的流程
① 根据该URI，调用HandlerMapping获得该Handler配置的所有相关的对象（包括Handler对象以及Handler对象对应的拦截器），最后以 HandlerExecutionChain执行链对象的形式返回。
② DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。
③ 如果成功获得HandlerAdapter，此时将开始执行拦截器的preHandler(…)方法【正向】
④ 提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)方法，处理请求。在填充Handler的入参过程中，根据你 的配置，Spring将帮你做一些额外的工作：
1) HttpMessageConveter： 将请求消息（如Json、xml等数据）转换成一个对象，将对象转换为指定的响应信息
2) 数据转换：对请求消息进行数据转换。如String转换成Integer、Double等
3) 数据格式化：对请求消息进行数据格式化。 如将字符串转换成格式化数字或格式化日期等
4) 数据验证： 验证数据的有效性（长度、格式等），验证结果存储到BindingResult或Error中
⑤ Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象。
⑥ 此时将开始执行拦截器的postHandle(...)方法【逆向】。
⑦ 根据返回的ModelAndView（此时会判断是否存在异常：如果存在异常，则执行HandlerExceptionResolver进行异常处理）选择一个适 合的ViewResolver进行视图解析，根据Model和View，来渲染视图。
⑧ 渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】。
⑨ 将渲染结果返回给客户端。
