一.DispatcherServlet的初始化
在我们第一次学Servlet编程，学java web的时候，还没有那么多框架。我们开发一个简单的功能要做的事情很简单，就是继承HttpServlet，根据需要重写一下doGet，doPost方法，跳转到我们定义好的jsp页面。Servlet类编写完之后在web.xml里注册这个Servlet类。
除此之外，没有其他了。我们启动web服务器，在浏览器中输入地址，就可以看到浏览器上输出我们写好的页面。为了更好的理解上面这个过程，你需要学习关于Servlet生命周期的三个阶段，就是所谓的“init-service-destroy”。
以上的知识，我觉得对于你理解SpringMVC的设计思想，已经足够了。SpringMVC当然可以称得上是一个复杂的框架，但是同时它又遵循Servlet世界里最简单的法则，那就是“init-service-destroy”。我们要分析SpringMVC的初始化流程，其实就是分析DispatcherServlet类的init()方法，让我们带着这种单纯的观点，打开DispatcherServlet的源码一窥究竟吧。
1.DispacherServlet的初始化
1.<init-param>配置元素读取
用Eclipse IDE打开DispatcherServlet类的源码，ctrl+T看一下。
DispatcherServlet类的初始化入口方法init()定义在HttpServletBean这个父类中，HttpServletBean类作为一个直接继承于HttpServlet类的类，覆写了HttpServlet类的init()方法，实现了自己的初始化行为。
@Override public final void init() throws ServletException { if (logger.isDebugEnabled()) { logger.debug("Initializing servlet ‘" + getServletName() + "‘"); }
// Set bean properties from init parameters. try { //解析init-param并封装到pvs中,并进行验证 PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties); //将当前的Servelt类转换成BeanWrapper,从而能够以Spring的方式来对init-param进行注入 BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this); ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext()); //注册一个字符串到资源文件的编辑器 bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, this.environment)); //留给子类覆盖 initBeanWrapper(bw); //<init-param>配置元素利用JavaBean的方式,进行属性注入 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. //留给子类扩展 initServletBean();
if (logger.isDebugEnabled()) { logger.debug("Servlet ‘" + getServletName() + "‘ configured successfully"); } }?
步骤:
1.封装以验证初始化参数
2.将当前的Servelt类转换成BeanWrapper
3.注册相对于Resource的属性编辑器
4.属性注入
5.servletBean的初始化
我在web.xml中注册的DispatcherServlet配置如下：
<!-- springMVC配置开始 --> <servlet> <servlet-name>appServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:spring/spring-servlet.xml</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <servlet-mapping> <servlet-name>appServlet</servlet-name> <url-pattern>/</url-pattern> </servlet-mapping> <!-- springMVC配置结束 -->?
可以看到，我注册了一个名为contextConfigLocation的<init-param>元素，其值为“classpath:spring/spring-servlet.xml”，这也是大家常常用来指定SpringMVC配置文件路径的方法。上面那段try,catch块包裹的代码发挥的作用，一个是将“classpath:spring/spring-servlet.xml”这段字符串转换成classpath路径下的一个资源文件，供框架初始化读取配置元素。在我的工程中是在spring文件夹下面的配置文件spring-servlet.xml。
另外一个作用，就是将contextConfigLocation的值读取出来，然后通过setContextConfigLocation()方法设置到DispatcherServlet中，这个setContextConfigLocation()方法是在FrameworkServlet类中定义的，也就是上面继承类图中DispatcherServlet的直接父类。
我们在setContextConfigLocation()方法上面打上一个断点，启动web工程，可以看到下面的调试结果。
在HttpServletBean这个类的设计中，运用了依赖注入思想完成了<init-param>配置元素的读取。他抽离出HttpServletBean这个类的目的也在于此，就是“以依赖注入的方式来读取Servlet类的<init-param>配置信息”，而且这里很明显是一种setter注入。
明白了HttpServletBean类的设计思想，我们也就知道可以如何从中获益。具体来说，我们继承HttpServletBean类（就像DispatcherServlet做的那样），在类中定义一个属性，为这个属性加上setter方法后，我们就可以在<init-param>元素中为其定义值。在类被初始化后，值就会被注入进来，我们可以直接使用它，避免了样板式的getInitParameter()方法的使用，而且还免费享有Spring中资源编辑器的功能，可以在web.xml中，通过“classpath:”直接指定类路径下的资源文件。
注意，虽然SpringMVC本身为了后面初始化上下文的方便，使用了字符串来声明和设置contextConfigLocation参数，但是将其声明为Resource类型，同样能够成功获取。鼓励读者们自己继承HttpServletBean写一个测试用的Servlet类，并设置一个参数来调试一下，这样能够帮助你更好的理解获取配置参数的过程。
2.WebApplicaitonContext的建立
SpringMVC使用了Spring容器来容纳自己的配置元素，拥有自己的bean容器上下文。在SpringMVC初始化的过程中，非常关键的一步就是要建立起这个容器上下文，而这个建立上下文的过程，发生在FrameworkServlet类中，由上面init()方法中的initServletBean()方法触发。
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 { //初始化WebApplicationContext容器 this.webApplicationContext = initWebApplicationContext(); //初始化FrameworkServlet initFrameworkServlet(); } catch (ServletException | 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"); } }
initWebApplicationContext()
对sevlet功能所使用的变量进行初始化
protected WebApplicationContext initWebApplicationContext() { WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(getServletContext()); WebApplicationContext wac = null;? if (this.webApplicationContext != null) { // A context instance was injected at construction time -> use it //1.context实例在构造函数中就已经被注入,因为servlet只能被实例化一次 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); } //刷新上下文环境 configureAndRefreshWebApplicationContext(cwac); } } } if (wac == null) { //2.根据contextAtrribute属性加载WebApplicaitonContext wac = findWebApplicationContext(); } if (wac == null) { // No context instance is defined for this servlet -> create a local one //3.重新创建WebApplicationContext 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; }?
步骤:
1.寻找或者创建对应的WebApplicationContext
(1)通过构造函数的注入进行初始化
(2)通过contextAtrribute进行初始化
(3)重新创建WebApplicationContext实例
默认创建的是XMLWebApplcaitonContext(org.springframework.web.context.support.XmlWebApplicationContext)
总结:
initWebApplicationContext()方法，封装了建立Spring容器上下文的整个过程，方法内的逻辑如下：
- 获取由ContextLoaderListener初始化并注册在ServletContext中的RootContext
- 如果webApplicationContext已经不为空，表示这个Servlet类是通过编程式注册到容器中的（Servlet 3.0+中的ServletContext.addServlet() ），Context也由编程式传入。若这个传入的Context还没被初始化，将RootContext上设置为它的父Context，然后将其初始化，否则直接使用。
- 通过wac变量的引用是否为null，通过第2步的判断来完成Context的设置（即Context是否已经用编程式方式传入），如果wac==null成立，说明该Servlet不是由编程式注册到容器中的。此时以contextAttribute属性的值为key，在ServletContext中查找Context，查找得到，说明Context已经以别的方式初始化并注册在contextAttribute下，直接使用。
- 检查wac变量的引用是否为null，如果wac==null成立，说明2、3两步中的上下文初始化策略都没成功，此时调用createWebApplicationContext(rootContext)，建立一个全新的以rootContext为父上下文的上下文，作为SpringMVC配置元素的容器上下文。大多数情况下我们所使用的上下文，就是这个新建的上下文。
- 以上三种初始化上下文的策略，都会回调onRefresh(ApplicationContext context)方法（回调的方式根据不同策略有不同），onRefresh方法在DispatcherServlet类中被覆写，以上面得到的上下文为依托，完成SpringMVC中默认实现类的初始化。
- 最后，将这个上下文发布到ServletContext中，也就是将上下文以一个和Servlet类在web.xml中注册名字有关的值为键，设置为ServletContext的一个属性。你可以通过改变publishContext的值来决定是否发布到ServletContext中，默认为true。
2.configureAndRefreshWebApplciaitonContext
构造函数或者单独创建都会调用该方法,对已经创建好的WebApplicationContext配置和刷新
protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) { if (ObjectUtils.identityToString(wac).equals(wac.getId())) { // The application context id is still set to its original default value // -> assign a more useful id based on available information if (this.contextId != null) { wac.setId(this.contextId); } else { // Generate default id... wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(getServletContext().getContextPath()) + ‘/‘ + getServletName()); } }? wac.setServletContext(getServletContext()); wac.setServletConfig(getServletConfig()); wac.setNamespace(getNamespace()); wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));? // The wac environment‘s #initPropertySources will be called in any case when the context // is refreshed; do it eagerly here to ensure servlet property sources are in place for // use in any post-processing or initialization that occurs below prior to #refresh ConfigurableEnvironment env = wac.getEnvironment(); if (env instanceof ConfigurableWebEnvironment) { ((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig()); }? postProcessWebApplicationContext(wac); applyInitializers(wac); //刷新,调用的是父类abstractApplicationContext的方法 wac.refresh(); }
<1>设置contextId
<2>设置ServletContext
<3>设置ServletConfig
<4>设置Namespace
<5>添加ApplicationListener
3.刷新
DispatcherServlet类覆写了父类FrameworkServlet中的onRefresh(ApplicationContext context)方法
onRefresh()
protected void onRefresh(ApplicationContext context) { initStrategies(context); } ------------------------------------------- protected void initStrategies(ApplicationContext context) { //初始化DispatcherServelt的九大组件 initMultipartResolver(context); initLocaleResolver(context); initThemeResolver(context); initHandlerMappings(context); initHandlerAdapters(context); initHandlerExceptionResolvers(context); initRequestToViewNameTranslator(context); initViewResolvers(context); initFlashMapManager(context); }
1.初始化MultipartResolver(用于处理文件上传,通过getBean来获取)
2.初始化LocalResolver(国际化配置3种)
<1>基于URL参数的国际化配置(AcceptHeaderLocalResolver)
<2>基于Session的国际化配置(SessionLocalResolver)
<3>基于Cookie的国际化配置
3.初始化ThemeResolver(主题资源解析器)
4.初始化HandleMapping
initHandlerMappings()
private void initHandlerMappings(ApplicationContext context) { this.handlerMappings = null; //判断WebApplicaitonContext中是否HandleMapping //detectAllHandlerMapping设置为true if (this.detectAllHandlerMappings) { // Find all HandlerMappings in the ApplicationContext, including ancestor contexts. //在WebApplicaitonContext中找出所有的HandleMapping Map<String, HandlerMapping> matchingBeans = BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false); if (!matchingBeans.isEmpty()) { this.handlerMappings = new ArrayList<>(matchingBeans.values()); // We keep HandlerMappings in sorted order. //如果设置了优先级对HandleMapping进行排序 AnnotationAwareOrderComparator.sort(this.handlerMappings); } } else { try { ////detectAllHandlerMapping设置为false,根据名字获取 HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class); this.handlerMappings = Collections.singletonList(hm); } catch (NoSuchBeanDefinitionException ex) { // Ignore, we‘ll add a default HandlerMapping later. } }
// Ensure we have at least one HandlerMapping, by registering // a default HandlerMapping if no other mappings are found. //还是没有就根据默认策略,创建HandleMapping if (this.handlerMappings == null) { this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class); if (logger.isTraceEnabled()) { logger.trace("No HandlerMappings declared for servlet ‘" + getServletName() + "‘: using default strategies from DispatcherServlet.properties"); } } }
1.detectAllHandlerMappings变量默认为true，所以在初始化HandlerMapping接口默认实现类的时候，会把WebApplicaitonContext中所有HandlerMapping类型的Bean都注册在HandlerMappings这个List变量中。
2.如果你手动设置为false，那么将尝试获取id为handlerMapping的Bean，新建一个只有一个元素的List，将其赋给handlerMappings。
3.如果经过上面的过程，handlerMappings变量仍为空，那么说明你没有在WebApplicaitonContext中提供自己HandlerMapping类型的Bean定义。此时，SpringMVC将采用默认初始化策略(通过从DispatcherServlet.properties 所配置键值对 找到对应的默认HandlerMapping )来初始化handlerMappings。
getDefaultStrategies ()
@SuppressWarnings("unchecked") protected <T> List<T> getDefaultStrategies(ApplicationContext context, Class<T> strategyInterface) { //通过策越接口获取到key String key = strategyInterface.getName(); //根据key获取到对应的value String value = defaultStrategies.getProperty(key); if (value != null) { //将value转换成数组 String[] classNames = StringUtils.commaDelimitedListToStringArray(value); List<T> strategies = new ArrayList<T>(classNames.length); //遍历数组 for (String className : classNames) { try { //通过反射来获取默认组件(如:HandleMapping.......) Class<?> clazz = ClassUtils.forName(className, DispatcherServlet.class.getClassLoader()); Object strategy = createDefaultStrategy(context, clazz); //将获取到的默认组件添加到list集合中 strategies.add((T) strategy); } catch (ClassNotFoundException ex) { throw new BeanInitializationException( "Could not find DispatcherServlet‘s default strategy class [" + className + "] for interface [" + key + "]", ex); } catch (LinkageError err) { throw new BeanInitializationException( "Error loading DispatcherServlet‘s default strategy class [" + className + "] for interface [" + key + "]: problem with class file or dependent class", err); } } return strategies; } else { return new LinkedList<T>(); } }?
DispatcherServlet.properties 的默认策越配置
# Default implementation classes for DispatcherServlet‘s strategy interfaces. # Used as fallback when no matching beans are found in the DispatcherServlet context. # Not meant to be customized by application developers. org.springframework.web.servlet.LocaleResolver=org.springframework.web.servlet.i18n.AcceptHeaderLocaleResolver org.springframework.web.servlet.ThemeResolver=org.springframework.web.servlet.theme.FixedThemeResolver org.springframework.web.servlet.HandlerMapping=org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping, org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping org.springframework.web.servlet.HandlerAdapter=org.springframework.web.servlet.mvc.HttpRequestHandlerAdapter, org.springframework.web.servlet.mvc.SimpleControllerHandlerAdapter, org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter org.springframework.web.servlet.HandlerExceptionResolver=org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver, org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver, org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver org.springframework.web.servlet.RequestToViewNameTranslator=org.springframework.web.servlet.view.DefaultRequestToViewNameTranslator org.springframework.web.servlet.ViewResolver=org.springframework.web.servlet.view.InternalResourceViewResolver org.springframework.web.servlet.FlashMapManager=org.springframework.web.servlet.support.SessionFlashMapManager
5.初始化HandlerAdapter
DiapatcherServelt通过HandlerMapping找到相对应Handler后,会轮询HandlerAdapter模块,查找能够处理当前HTTP请求的HandlerAdapter的实现,HandlerAdapter模块根据HandlerMapping返回相对应的Handler类型,
<1>HttpRequestHandleAdapter
仅仅支持对HTTPRequestHandle的适配,它简单地将HTTP请求对象和响应对象传递给HttpRequestHandle的实现,它并不需要返回值,它主要基于HTTP的远程调用的实现上
<2>SimpleControllerHandlerAdapter
将HTTP请求适配到一个控制器的实现处理
<3>AnnotationMethodHandlerAdapter
6.初始化HandlerExceptionResolvers
作用:可以捕获到不同的异常,根据异常的类型来判断返回不同的ModeAndView,从而可以自定义异常的页面
7.初始化RequestToViewNameTranslator
8.初始化ViewResolvers
当Controller将请求处理结果放入到ModeAndView中以后,DispatcherServlet会根据ModeAndView选择合适的View进行渲染.
9.初始化FlashMapManager
SpringMVC的Flash attribures提供了一个请求存储属性,可供其他属性使用.在使用重定向时候非常有必要,比如:Post/Redirect/Get模式.Flash attributes在重定向之前暂存(就像存在session中以便重定向之后还能使用.并立即删除)
FlashMap 用保持Flash attributes,FlashMapManager用于存储,检索,管理FlashMap实例
Flash attributes默认开启,而且不会导致HTTP Session的创建 这两个实例都可以通过RequestContextUtils来获取
3.总结
回顾整个SpringMVC的初始化流程，我们看到，通过HttpServletBean、FrameworkServlet、DispatcherServlet三个不同的类层次，SpringMVC的设计者将三种不同的职责分别抽象，运用模版方法设计模式分别固定在三个类层次中。其中HttpServletBean完成的是<init-param>配置元素的依赖注入，FrameworkServlet完成的是容器上下文的建立，DispatcherServlet完成的是SpringMVC具体编程元素的初始化策略
二、请求转发流程详解
SpringMVC完成初始化流程之后，就进入Servlet标准生命周期的第二个阶段，即“service”阶段。在“service”阶段中，每一次Http请求到来，容器都会启动一个请求线程，通过service()方法，委派到doGet()或者doPost()这些方法，完成Http请求的处理。
在初始化流程中，SpringMVC巧妙的运用依赖注入读取参数，并最终建立一个与容器上下文相关联的Spring子上下文。这个子上下文，为接下来的Http处理流程中各种编程元素提供了容身之所。如果说将Spring上下文关联到Servlet容器中，是SpringMVC框架的第一个亮点，那么在请求转发流程中，SpringMVC对各种处理环节编程元素的抽象，就是另外一个独具匠心的亮点。
Struts2采取的是一种完全和Web容器隔离和解耦的事件机制。诸如Action对象、Result对象、Interceptor对象，这些都是完全脱离Servlet容器的编程元素。Struts2将数据流和事件处理完全剥离开来，从Http请求中读取数据后，下面的事件处理流程就只依赖于这些数据，而完全不知道有Web环境的存在。
反观SpringMVC，无论HandlerMapping对象、HandlerAdapter对象还是View对象，这些核心的接口所定义的方法中，HttpServletRequest和HttpServletResponse对象都是直接作为方法的参数出现的。这也就意味着，框架的设计者，直接将SpringMVC框架和容器绑定到了一起。或者说，整个SpringMVC框架，都是依托着Servlet容器元素来设计的。下面就来看一下，源码中是如何体现这一点的。
1.请求转发的入口
就像任何一个注册在容器中的Servlet一样，DispatcherServlet也是通过自己的service()方法来接收和转发Http请求到具体的doGet()或doPost()这些方法的。以一次典型的GET请求为例，经过HttpServlet基类中service()方法的委派，请求会被转发到doGet()方法中。doGet()方法，在DispatcherServlet的父类FrameworkServlet类中被覆写。
FrameworkServlet
doGet()
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
可以看到，这里只是简单的转发到processRequest()这个方法。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
//获取当前的LocaleContext
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
//创建LocaleContext
LocaleContext localeContext = buildLocaleContext(request);
//获取当前的RequestAttributes
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
//创建RequestAttributes
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
//将创建的localeContext和requestAttributes绑定到当前线程
initContextHolders(request, localeContext, requestAttributes);
try {
//核心
doService(request, response);
}
catch (ServletException | IOException ex) {
failureCause = ex;
throw ex;
}
catch (Throwable ex) {
failureCause = ex;
throw new NestedServletException("Request processing failed", ex);
}
finally {
//请求处理后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");
}
}
}
//无论成功与否都发布事件通知
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
步骤:
(1)获取当前的LocaleContext和RequestAttributes,保证在请求前后都能够被获取
(2).根据当前Request创建相对应的LocaleContext和RequestAttributes,并绑定到当前线程
(3).委托doService()做进一步的处理
(4).请求处理后恢复线程到原始状态
(5).请求处理结束后无论成功与否都发布事件通知
FrameworkServlet类中的doService()方法。
又是一个抽象方法，这也是SpringMVC类设计中的惯用伎俩：父类抽象处理流程，子类给予具体的实现。真正的实现是在DispatcherServlet类中。
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<>();
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 {
//核心
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);
}
}
}
}
调用request.setAttribute()方法注入组件(将前面在初始化流程中实例化的对象设置到http请求的属性中，供下一步处理使用，其中有容器的上下文对象、本地化解析器等SpringMVC特有的编程元素)
不同于Struts2中的ValueStack，SpringMVC的数据并没有从HttpServletRequest对象中抽离出来再存进另外一个编程元素，这也跟SpringMVC的设计思想有关。因为从一开始，SpringMVC的设计者就认为，不应该将请求处理过程和Web容器完全隔离。
所以，你可以看到，真正发生请求转发的方法doDispatch()中，它的参数是HttpServletRequest和HttpServletResponse对象。这给我们传递的意思也很明确，从request中能获取到一切请求的数据，从response中，我们又可以往服务器端输出任何响应，Http请求的处理，就应该围绕这两个对象来设计。我们不妨可以将SpringMVC这种设计方案，是从Struts2的过度设计中吸取教训，而向Servlet编程的一种回归和简化。
总结:封装Request对象
2.请求转发的抽象描述
接下来让我们看看doDispatch()这个整个请求转发流程中最核心的方法。DispatcherServlet所接收的Http请求，经过层层转发，最终都是汇总到这个方法中来进行最后的请求分发和处理。doDispatch()这个方法的内容，就是SpringMVC整个框架的精华所在。它通过高度抽象的接口，描述出了一个MVC（Model-View-Controller）设计模式的实现方案。Model、View、Controller三种层次的编程元素，在SpringMVC中都有大量的实现类，各种处理细节也是千差万别。但是，它们最后都是由，也都能由doDispatch()方法来统一描述，这就是接口和抽象的威力，万变不离其宗。
DispatcherServlet
doDispatch()
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 {
//如果是MultipartContent类型的request则转换为MultipartHttpServletReqeust类型的Request
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
//根据Request信息寻找到相对应的Handler
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
//没有找到对应的Handle,通过Response反馈错误信息
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
//根据当前的Handler获取到相对应HandelrAdapter
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
//判断当前的Handler是否支持last-modifie头处理
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;
}
}
// Apply preHandle methods of registered interceptors.
//拦截器链的perHandler的调用
HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
if (interceptors != null) {
for (int i = 0; i < interceptors.length; i++) {
HandlerInterceptor interceptor = interceptors[i];
if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) {
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
return;
}
interceptorIndex = i;
}
}
// Actually invoke the handler.
//真正激活handle并返回view,调用Handle的方法
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
// Do we need view name translation?
//视图名称转换应用于需要添加前缀后缀的情况(添加前后缀)
if (mv != null && !mv.hasView()) {
mv.setViewName(getDefaultViewName(request));
}
// Apply postHandle methods of registered interceptors.
//应用所有拦截器的postHandle方法
if (interceptors != null) {
for (int i = interceptors.length - 1; i >= 0; i--) {
HandlerInterceptor interceptor = interceptors[i];
interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
}
}
}
catch (ModelAndViewDefiningException ex) {
logger.debug("ModelAndViewDefiningException encountered", ex);
mv = ex.getModelAndView();
}
catch (Exception ex) {
Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
mv = processHandlerException(processedRequest, response, handler, ex);
errorView = (mv != null);
}
// Did the handler return a view to render?
//如果在Handler实例的处理中返回了View,那么需要做页面跳转处理
if (mv != null && !mv.wasCleared()) {
//根据视图进行页面跳转
render(mv, processedRequest, response);
if (errorView) {
WebUtils.clearErrorRequestAttributes(request);
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Null ModelAndView returned to DispatcherServlet with name ‘" + getServletName() +
"‘: assuming HandlerAdapter completed request handling");
}
}
// Trigger after-completion for successful outcome.
//完成处理激活触发器
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
}
catch (Exception ex) {
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
}
catch (Error err) {
ServletException ex = new NestedServletException("Handler processing failed", err);
// Trigger after-completion for thrown exception.
triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
throw ex;
}
finally {
// Clean up any resources used by a multipart request.
if (processedRequest != request) {
cleanupMultipart(processedRequest);
}
}
}
步骤:
doDispatch()
(1).MultipartContent类型的Reqeust的处理
Request请求如果是MultipartContent类型的request则转换为MultipartHttpServletReqeust类型
(2).根据Request信息寻找到相对应的Handler
getHandler()
DispatcherServlet
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
//遍历handlerMappings
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name ‘" + getServletName() + "‘");
}
//根据当前的Request获取HandlerExecutionChain
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
--------------------------------------------------------------------------------------
AbstractHandleMapping
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
//根据Request获取相对应的Handler
Object handler = getHandlerInternal(request);
if (handler == null) {
//没有就使用默认的
handler = getDefaultHandler();
}
//如果在没有,就直接返回Null
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = obtainApplicationContext().getBean(handlerName);
}
//将配置的对应拦截器加入到执行链中,以保证这些拦截器可以有效地作用到目标方法
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (CorsUtils.isCorsRequest(request)) {
CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
根据Request寻找到对应的Handler,无非就是找到对应的Controller(都是继承于AbstractController),
getHandlerInternal()
protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
//截取用于匹配的URL有效路径
String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
//根据路径寻找到对应的Handler
Object handler = lookupHandler(lookupPath, request);
//根据URL没有匹配到Handler
if (handler == null) {
// We need to care for the default handler directly, since we need to
// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
Object rawHandler = null;
if ("/".equals(lookupPath)) {
//如果请求的路径是‘/‘,使用RootHandelr来处理
rawHandler = getRootHandler();
}
if (rawHandler == null) {
//获取默认的Handler
rawHandler = getDefaultHandler();
}
if (rawHandler != null) {
// Bean name or resolved handler?
//判断handler是否为String
if (rawHandler instanceof String) {
String handlerName = (String) rawHandler;
rawHandler = obtainApplicationContext().getBean(handlerName);
}
validateHandler(rawHandler, request);
handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
}
}
return handler;
}
1.截取用于匹配的URL路径
2.根据路径寻找handler
->直接匹配情况的处理 ->通配符匹配的处理
3.如果请求路径仅仅是"/",那么使用RootHandler进行处理
4.无法找到使用默认的Handler
5.如果返回的Handler为Stirng类型,意味着返回的是配置的BeanName,则需要根据BeanName获取对应的Bean
6.最后通过getHandlerExecutionChain方法,对返回的Handler封装,目的是将配置中对应的拦截器加入到执行链中,以保证这些拦截器可以有效地作用目标对象
(3).没有找到对应的Handler的错误处理
(4).根据当前的Handler寻找相对应的HandlerAdapter
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
//遍历所有的handlerAdapters
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
//判断某个handlerAdapter是否适用于当前的handler
if (ha.supports(handler)) {
return ha;
}
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
--------------------------------------------------------------------------------
SimpleControllerHandlerAdapter
@Override
public boolean supports(Object handler) {
return (handler instanceof Controller);
}
遍历所有的handlerAdapters,在遍历中判断某个handlerAdapter是否适用于当前的handler,比如说:SimpleControllerHandlerAdapter,这个判断的依据是当前的handler是否属于Controller接口的实现类
(5)缓存处理
(6)HandlerInterceptor的处理
preHander() -> 该方法在进入Handler方法执行之前执行此方法 应用场景：如身份认证，身份授权。
postHandle -> 该方法在进入Handler方法之后，返回ModelAndView之前执行 应用场景：如 统一制定视图
afterCompletion ->应用场景：统一日志处理，统一异常处理
(7)逻辑处理
通过适配器中转调用Handle方法并返回视图
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
-----------------------------------------------------------------
public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
throws Exception {
return ((Controller) handler).handleRequest(request, response);
}
-------------------------------------------------------------------
public ModelAndView handleRequest(HttpServletRequest request, HttpServletResponse response)
throws Exception {
if (HttpMethod.OPTIONS.matches(request.getMethod())) {
response.setHeader("Allow", getAllowHeader());
return null;
}
// Delegate to WebContentGenerator for checking and preparing.
//委派WebContentGenerator进行检查request和准备response
checkRequest(request);
prepareResponse(response);
// Execute handleRequestInternal in synchronized block if required.
//如果需要在session内同步执行
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
//调用用户的逻辑,返回ModeAndView对象
return handleRequestInternal(request, response);
}
}
}
//调用用户的逻辑,返回ModeAndView对象
return handleRequestInternal(request, response);
}
(8)异常视图处理
(9)根据视图跳转页面
protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
// Determine locale for request and apply it to the response.
Locale locale =
(this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
response.setLocale(locale);
View view;
String viewName = mv.getViewName();
if (viewName != null) {
// We need to resolve the view name.
//解析视图的名字,获取到对应的View,如ThymleafView.......
view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
if (view == null) {
throw new ServletException("Could not resolve view with name ‘" + mv.getViewName() +
"‘ in servlet with name ‘" + getServletName() + "‘");
}
}
else {
// No need to lookup: the ModelAndView object contains the actual View object.
view = mv.getView();
if (view == null) {
throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
"View object in servlet with name ‘" + getServletName() + "‘");
}
}
// Delegate to the View object for rendering.
if (logger.isDebugEnabled()) {
logger.debug("Rendering view [" + view + "] in DispatcherServlet with name ‘" + getServletName() + "‘");
}
try {
if (mv.getStatus() != null) {
response.setStatus(mv.getStatus().value());
}
//根据获取到的View进行页面跳转
view.render(mv.getModelInternal(), request, response);
}
catch (Exception ex) {
if (logger.isDebugEnabled()) {
logger.debug("Error rendering view [" + view + "] in DispatcherServlet with name ‘" +
getServletName() + "‘", ex);
}
throw ex;
}
}
步骤:
1.解析视图名字
InternalResourceViewResolver
@Nullable
protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
Locale locale, HttpServletRequest request) throws Exception {
if (this.viewResolvers != null) {
//遍历所有的视图解析器
for (ViewResolver viewResolver : this.viewResolvers) {
//根据视图的名字找出对应的视图
View view = viewResolver.resolveViewName(viewName, locale);
if (view != null) {
return view;
}
}
}
return null;
}
----------------------------------------------------------
AbstractCachingViewResolver
public View resolveViewName(String viewName, Locale locale) throws Exception {
if (!isCache()) {
//不存在缓存的情况下直接创建视图
return createView(viewName, locale);
}
else {
//存在->直接从缓存中提取
Object cacheKey = getCacheKey(viewName, locale);
View view = this.view AccessCache.get(cacheKey);
if (view == null) {
synchronized (this.viewCreationCache) {
view = this.viewCreationCache.get(cacheKey);
if (view == null) {
// Ask the subclass to create the View object.
view = createView(viewName, locale);
if (view == null && this.cacheUnresolved) {
view = UNRESOLVED_VIEW;
}
if (view != null) {
this.viewAccessCache.put(cacheKey, view);
this.viewCreationCache.put(cacheKey, view);
if (logger.isTraceEnabled()) {
logger.trace("Cached view [" + cacheKey + "]");
}
}
}
}
}
return (view != UNRESOLVED_VIEW ? view : null);
}
}
UrlBasedViewResolver
createView()
protected View createView(String viewName, Locale locale) throws Exception {
// If this resolver is not supposed to handle the given view,
// return null to pass on to the next resolver in the chain.
if (!canHandle(viewName, locale)) {
return null;
}
// Check for special "redirect:" prefix.
//处理redirect
if (viewName.startsWith(REDIRECT_URL_PREFIX)) {
String redirectUrl = viewName.substring(REDIRECT_URL_PREFIX.length());
RedirectView view = new RedirectView(redirectUrl, isRedirectContextRelative(), isRedirectHttp10Compatible());
String[] hosts = getRedirectHosts();
if (hosts != null) {
view.setHosts(hosts);
}
return applyLifecycleMethods(viewName, view);
}
// Check for special "forward:" prefix.
//处理forward
if (viewName.startsWith(FORWARD_URL_PREFIX)) {
String forwardUrl = viewName.substring(FORWARD_URL_PREFIX.length());
return new InternalResourceView(forwardUrl);
}
// Else fall back to superclass implementation: calling loadView.
return super.createView(viewName, locale);
}
----------------------------------------------------------------------------
protected View createView(String viewName, Locale locale) throws Exception {
return loadView(viewName, locale);
}
------------------------------------------------------
protected View loadView(String viewName, Locale locale) throws Exception {
AbstractUrlBasedView view = buildView(viewName);
View result = applyLifecycleMethods(viewName, view);
return (view.checkResource(locale) ? result : null);
}
---------------------------------------------------
protected AbstractUrlBasedView buildView(String viewName) throws Exception {
AbstractTemplateView view = (AbstractTemplateView) super.buildView(viewName);
view.setExposeRequestAttributes(this.exposeRequestAttributes);
view.setAllowRequestOverride(this.allowRequestOverride);
view.setExposeSessionAttributes(this.exposeSessionAttributes);
view.setAllowSessionOverride(this.allowSessionOverride);
view.setExposeSpringMacroHelpers(this.exposeSpringMacroHelpers);
return view;
}
2.页面跳转
通过ViewName解析到相对应的View,加上前缀和后缀,并将Model中的数据以属性的方式设置到request中,最后,进行跳转到相对应的页面
步骤:
1.解析视图名字
遍历所有的视图解析器,根据视图的名字找出对应的视图,不存在缓存的情况下会根据返回的字符串来创建不同视图,以"redirect"开头创建RedirectView,以"forward"开头创建ForwardView存在的话,存在的话就直接从缓存中提取,最后进行相关设置,并添加数据返回
2.根据获取到的View进行页面跳转
先解析需要使用的属性,并添加到modeMap中,然后将modeMap中的数据以属性的方式设置到Request中,最后决定调用RequestDispatcher的include还是forward方法到指定的JSP页面
原文地址：https://www.cnblogs.com/itxiaok/p/10357795.html