聊聊SpringMVC(1)---SpringMVC之初始化過程

本文將分析SpringMVC自身的建立過程。

1.總體結構介紹

SpringMVC核心Servlet的繼承結構如圖

能夠看到Servlet的繼承機構中一共有5個類GenericServlet,HttpServlet在Java中，剩下的三個類HttpServlet,HttpServlet,HttpServlet是SpringMVC中的，中點介紹着三個類的建立過程。

這三個類直接實現三個接口EnvironmentCapable，EnvironmentAware，ApplicationContextAware其中XXXAware在Spring裏表示對XXX能夠感知，也就是你想要Spring的一下東西，能夠經過實現XXXAware接口告訴Spring，Spring看到後就會給你送過了，而接收的方式是經過實現接口的惟一方法setXXX。如FrameworkServlet實現了ApplicationContextAware接口，會自動調用以下方法：

@Override
public void setApplicationContext(ApplicationContext applicationContext) {
   if (this.webApplicationContext == null && applicationContext instanceof WebApplicationContext) {
      this.webApplicationContext = (WebApplicationContext) applicationContext;
      this.webApplicationContextInjected = true;
   }
}

EnvironmentCapable是指擁有Environment的能力，也就是能夠提供Environment，實現這個接口就是告訴Spring它能夠提供Environment，須要的時候就會調用getEnvironment

@Override
public ConfigurableEnvironment getEnvironment() {
   if (this.environment == null) {
      this.environment = createEnvironment();
   }
   return this.environment;
}

protected ConfigurableEnvironment createEnvironment() {
   return new StandardServletEnvironment();
}

2.HttpServletBean

@Override
public final void init() throws ServletException {
   if (logger.isTraceEnabled()) {
      logger.trace("Initializing servlet '" + getServletName() + "'");
   }
   // 將servlet中配置的參數封裝到pvs變量中，requiredProperties爲必須參數 若是沒有將報異常
   PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
   if (!pvs.isEmpty()) {
      try {
         BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
         ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
         bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
         //模板方法，能夠在子類調用，作一些初始化操做，bw表明DispatcherServlet
         initBeanWrapper(bw);
         //將配置的初始值設置到bw表明DispatcherServlet
         bw.setPropertyValues(pvs, true);
      }
      catch (BeansException ex) {
         if (logger.isErrorEnabled()) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
         }
         throw ex;
      }
   }

   // 模板方法，子類初始化入口
   initServletBean();

   if (logger.isTraceEnabled()) {
      logger.trace("Servlet '" + getServletName() + "' configured successfully");
   }
}

首先將Servlet中的配置參數使用BeanWarpper設置到DispatcherServlet的相關屬性，而後調用模板方法，子類經過這個方法初始化，BeanWarpper是Spring提供的一個用來操做JavaBean屬性的工具，使用它能夠直接修改一個對象的屬性。

3.FrameworkServlet

從HttpServletBean中可知，FrameworkServlet的初始化入口方法應該是initServletBean其代碼以下

@Override
protected final void initServletBean() throws ServletException {
   getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");
   if (logger.isInfoEnabled()) {
      logger.info("FrameworkServlet '" + getServletName() + "': initialization started");
   }
   long startTime = System.currentTimeMillis();

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

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

這裏核心代碼就兩句，一個是初始化WebApplicationContext，另外一句用於初始化FrameworkServlet，也是模板方法，子類覆蓋在這裏面作一些初始化操做。好像子類並無使用它，因此他的主要職責仍是初始化WebApplicationContext

protected WebApplicationContext initWebApplicationContext() {
   // 獲取rootContext
   WebApplicationContext rootContext =
         WebApplicationContextUtils.getWebApplicationContext(getServletContext());
   WebApplicationContext wac = null;
   // 1.若是已經經過構造方法設置了webApplicationContext,主要是Servlet3.0環境中
   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) {
      //2.查看容器是否已經存在了，當webApplicationContext已經存在ServletContext中時，
      // 經過配置在Servlet中的contextAttribute參數獲取
      wac = findWebApplicationContext();
   }
   if (wac == null) {
      // 3.若是webApplicationContext尚未建立，則建立一個
      wac = createWebApplicationContext(rootContext);
   }

   if (!this.refreshEventReceived) {
      //當ContextRefreshedEvent事件沒有觸發時調用次方法，模板方法，能夠在子類重寫
      onRefresh(wac);
   }

   if (this.publishContext) {
      // 將webApplicationContext保持到ServletContext中
      String attrName = getServletContextAttributeName();
      getServletContext().setAttribute(attrName, wac);
      if (logger.isTraceEnabled()) {
         logger.trace("Published WebApplicationContext of servlet '" + getServletName() +
               "' as ServletContext attribute [" + attrName + "]");
      }
   }

   return wac;
}

initWebApplicationContext主要作了三件事：

1.獲取spring的跟容器rootContext。

默認狀況下spring會將本身的容器設置成ServletContext的一個屬性，key定義以下

String ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE = WebApplicationContext.class.getName() + ".ROOT";

因此獲取跟容器代碼爲：

Object attr = sc.getAttribute(attrName);

2.設置webApplicationContext並根據狀況調用onRefresh方法。

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
   //建立類型 即XmlWebApplicationContext.class
   Class<?> contextClass = getContextClass();
   if (logger.isTraceEnabled()) {
      logger.trace("Servlet '" + getServletName() +
            "' will create custom WebApplicationContext context of class '" +
            contextClass.getName() + "'" + ", parent context [" + parent + "]");
   }
   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");
   }
   //具體建立實例化
   ConfigurableWebApplicationContext wac =
         (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);
   //設置環境
   wac.setEnvironment(getEnvironment());
   //設置父容器
   wac.setParent(parent);
   String configLocation = getContextConfigLocation();
   if (configLocation != null) {
      //將設置的configLocation參數傳給wac
      wac.setConfigLocation(configLocation);
   }
   configureAndRefreshWebApplicationContext(wac);

   return wac;
}

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());
   // 添加ContextRefreshEvent事件的監聽器
   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());
   }
   //後置處理器
   postProcessWebApplicationContext(wac);
   applyInitializers(wac);
   wac.refresh();
}

3.將webApplicationContext設置到ServletContext中。

4.DispatcherServlet

onRefresh是DispatcherServlet的入口方法，onRefresh調用了initStrategies，在initStrategies中調用9個初始化方法，就是初始化9個組件，後面會詳細介紹。

@Override
protected void onRefresh(ApplicationContext context) {
   initStrategies(context);
}

protected void initStrategies(ApplicationContext context) {
   initMultipartResolver(context);
   initLocaleResolver(context);
   initThemeResolver(context);
   initHandlerMappings(context);
   initHandlerAdapters(context);
   initHandlerExceptionResolvers(context);
   initRequestToViewNameTranslator(context);
   initViewResolvers(context);
   initFlashMapManager(context);

   if (logger.isDebugEnabled()) {
      if (this.disableLoggingRequestDetails) {
         logger.debug("Logging request parameters and headers is OFF.");
      }
      else {
         logger.warn("\n\n" +
               "!!!!!!!!!!!!!!!!!!!\n" +
               "Logging request parameters (DEBUG) and headers (TRACE) may show sensitive data.\n" +
               "If not in development, use the DispatcherServlet property \"disableLoggingRequestDetails=true\",\n" +
               "or lower the log level.\n" +
               "!!!!!!!!!!!!!!!!!!!\n");
      }
   }
}

5.小結：

SpringMVC的建立過程一個有三個層次，分別是HttpServletBean,FramworkSerlet和DispatcherServlet.

HttpServletBean的做用是將Servlet中的配置參數設置到相應的屬性；

FramworkSerlet的做用是初始化webApplicationContext；

DispatcherServlet的做用是初始化自身的9個組件

總的來講結構簡單實現複雜。