再看一遍SpringMVC的工作流程

high level overview 工作流程图

初始化DispatcherServlet的过程:

因为在web.xml文件中配置了DispatcherServlet:

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<servlet>
<servlet-name>spring</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>classpath:config/esf/spring-servlet.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>

<servlet-mapping>
<servlet-name>spring</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>

容器启动的时候会调用Servlet的init()方法对Servlet进行初始化操作.

根据继承关系找到init()方法的定义:

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/**
* 将配置的参数映射到servlet中,并调用由子类覆盖的抽象初始方法
* @throws ServletException if bean properties are invalid (or required
* properties are missing), or if subclass initialization fails.
*/
@Override
public final void init() throws ServletException {
if (logger.isDebugEnabled()) {
logger.debug("Initializing servlet '" + getServletName() + "'");
}

// Set bean properties from init parameters.
try {
// 封装及验证初始化参数
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
// 将当前servlet实例转化成BeanWrapper实例
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
//注册相对于Resource的属性编辑器
ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
initBeanWrapper(bw);
// 属性注入
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
throw ex;
}

// Let subclasses do whatever initialization they like.
initServletBean();

if (logger.isDebugEnabled()) {
logger.debug("Servlet '" + getServletName() + "' configured successfully");
}
}

通过覆盖initServletBean(),继续执行DispatcherServlet的初始化逻辑:

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/**
* Overridden method of {@link HttpServletBean}, invoked after any bean properties
* have been set. Creates this servlet's WebApplicationContext.
* 在获取了所有初始化参数后调用,创建WebApplicationContext上下文
*/
@Override
protected final void initServletBean() throws ServletException {
getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
if (this.logger.isInfoEnabled()) {
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
}
long startTime = System.currentTimeMillis();

try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}
catch (RuntimeException ex) {
this.logger.error("Context initialization failed", ex);
throw ex;
}

if (this.logger.isInfoEnabled()) {
long elapsedTime = System.currentTimeMillis() - startTime;
this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +
elapsedTime + " ms");
}
}

initServletBean()函数添加了一个计时器统计初始化执行的耗时,主要的逻辑委托给initWebApplicationContext():

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/**
* Initialize and publish the WebApplicationContext for this servlet.
* <p>Delegates to {@link #createWebApplicationContext} for actual creation
* of the context. Can be overridden in subclasses.
* @return the WebApplicationContext instance
* @see #FrameworkServlet(WebApplicationContext)
* @see #setContextClass
* @see #setContextConfigLocation
*/
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;

// 寻找或者创建WebApplicationContext实例
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);
}
// 配置文件装载
configureAndRefreshWebApplicationContext(cwac);
}
}
}
// 通过在web.xml中配置的contextAttribute进行初始化
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
wac = findWebApplicationContext();
}
// 重新创建WebApplicationContext的实例
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
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.
onRefresh(wac);
}

if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
if (this.logger.isDebugEnabled()) {
this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
"' as ServletContext attribute with name [" + attrName + "]");
}
}

return wac;
}

创建了WebApplicationContext的实例后,通过onRefresh()进行配置文件的装载:

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protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
protected void initStrategies(ApplicationContext context) {
// 初始化multipartResolver用于处理文件上传
initMultipartResolver(context);
// 国际化配置
initLocaleResolver(context);
// 主题解析器
initThemeResolver(context);
// handlerMaping负责接收request请求,根据webapplicationContext的配置返回对应的controller
initHandlerMappings(context);
// DispatcherServlet通过这个适配器获取所有的handler来处理request
initHandlerAdapters(context);
// 异常处理
initHandlerExceptionResolvers(context);
// 当controller方法没有返回一个ModelAndView对象或者逻辑试图名称,并且该方法中没有直接网response的 输出流中写数据的时候,spring就会采用约定好的方式提供一个 逻辑 视图名称。
initRequestToViewNameTranslator(context);
// 根据controller返回的ModelAndView选择合适 的试图进行渲染
initViewResolvers(context);
initFlashMapManager(context);
}

至此,DispatcherServlet的初始化算是结束了,接下来看一下如何处理请求 。

doGet和 doPost都 将处理委托给processRequest

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/**
* Process this request, publishing an event regardless of the outcome.
* <p>The actual event handling is performed by the abstract
* {@link #doService} template method.
*/
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

long startTime = System.currentTimeMillis();
Throwable failureCause = null;

// 提取 当前线程的LocaleContext和RequestAttributes属性,放入previous*中便于恢复,并创建对应的 LocaleContext和RequestAttributes 绑定到当前线程
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);

RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

// The central class for managing asynchronous request processing, mainly intended
// as an SPI and not typically used directly by application classes.
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

initContextHolders(request, localeContext, requestAttributes);

try {
doService(request, response);
}
catch (ServletException ex) {
failureCause = ex;
throw ex;
}
catch (IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}

finally {
// 恢复线程到原始状态
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}

if (logger.isDebugEnabled()) {
if (failureCause != null) {
this.logger.debug("Could not complete request", failureCause);
}
else {
if (asyncManager.isConcurrentHandlingStarted()) {
logger.debug("Leaving response open for concurrent processing");
}
else {
this.logger.debug("Successfully completed request");
}
}
}

// 无论成功与否发布事件通知
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}

继续移交给doService进行后续处理:

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/**
* Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
* for the actual dispatching.
*/
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
" processing " + request.getMethod() + " request for [" + getRequestUri(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<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
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());

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);
}
}
}
}

继续 移交给doDispatch进行后续 处理:

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/**
* Process the actual dispatching to the handler.
* <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
* The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
* to find the first that supports the handler class.
* <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
* themselves to decide which methods are acceptable.
* @param request current HTTP request
* @param response current HTTP response
* @throws Exception in case of any kind of processing failure
*/
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 {
// 首先处理文件的上传请求,转换该请求为MultipartHttpServletRequest类型的请求
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);

// Determine handler for the current request.
// 遍历 当前加载 的所有handlerMapping,调用 该 handlerMapping的getHandler方法根据 请求链接获取对应的Method方法,与该链接对应的拦截器intercepter一并放入执行chain中
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null || mappedHandler.getHandler() == null) {
// 如果找不到对应 的handler,log中打印 No mapping found for HTTP request with URI [/adfaf] in DispatcherServlet with name 'DispatcherServlet'
// SPR-10481添加了可配置参数throwExceptionIfNoHandlerFound,若在web.xml中dispatcherServlet的init-param中配置其为true,这里同时还能抛出一个NoHandlerFoundException异常,若为默认的false,则返回404
noHandlerFound(processedRequest, response);
return;
}

// Determine handler adapter for the current request.
// 遍历所有的handler适配器,根据适配器的support返回值判断是否支持该handler
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 (logger.isDebugEnabled()) {
logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}

if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}

// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

if (asyncManager.isConcurrentHandlingStarted()) {
return;
}

applyDefaultViewName(request, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Error err) {
triggerAfterCompletionWithError(processedRequest, response, mappedHandler, 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);
}
}
}
}