SpringMVC源碼解析-DispatcherServlet啓動流程和初始化

   在使用springmvc框架，會在web.xml文件配置一個DispatcherServlet,這正是web容器開始初始化，同時會在創建本身的上下文來持有SpringMVC的bean對象。

先從DispatcherServlet入手，從名字來看，它是一個Servlet。它的定義以下：

public class DispatcherServlet extends FrameworkServlet {

    它是繼承FrameworkServlet,來看一下整個的繼承關係。

從繼承關係來看，DispatcherServlet繼承FrameworkServlet和HttpServletBean而繼承HttpServlet,經過使用Servlet API 來對HTTP請求進行響應，成爲SpringMVC的前端處理器。

先看一個時序圖

注：做爲Servlet,DispatcherServlet的啓動和Servlet的啓動相關聯的。在Servlet初始化過程當中，Servlet的init方法會被調用，以進行初始化，然而DispatcherServlet的基類，因此從HttpServletBean中的初始化過程開始。      

        DispatcherServlet的工做分爲2部分，一部分是初始化（也就是圖的上半部分），有initServletBean()啓動，經過initWebApplicationContext()方法最終調用DispatcherServlet中的initStrategies()方法。另外一部分(也就是圖的下半部分),是對HTTP請求進行響應，做爲Servlet，Web容器會調用Servlet的doGet()和doPost()方法,在通過FrameworkServlet的processRequest()簡單處理後，會調用DispatcherServlet的doService方法，在這個方法調用中封裝了doDispatch(),繼續調用processDispatchResult方法返回調用信息。

接下來看看初始化的源碼的流程：

HttpServletBean.init方法

/**
	 * Map config parameters onto bean properties of this servlet, and
	 * invoke subclass initialization.
	 * @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() + "'");
		}
　　　　　　　　　　//獲取Servlet初始化參數
		// Set bean properties from init parameters.
		try {
			PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
			BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
			ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
			bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
			initBeanWrapper(bw);
			bw.setPropertyValues(pvs, true);
		}
		catch (BeansException ex) {
			logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
			throw ex;
		}
                //調用子類的方法
		// Let subclasses do whatever initialization they like.
		initServletBean();

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

　　例如web.xml中配置參數：

<servlet>  
    <servlet-name>TestServlet</servlet-name>  
    <servlet-class>com.lzyer.TestServlet</servlet-class>  
    <!--配置參數,能夠經過ServletConfig獲取參數-->
    <init-param>  
        <param-name>servlet-name</param-name>  
        <param-value>TestServlet</param-value>  
    </init-param>  
 </servlet>  
 <servlet-mapping>  
    <servlet-name>TestServlet</servlet-name>  
    <url-pattern>/TestServlet</url-pattern>  
  </servlet-mapping>

　　FrameworkServlet.initServletBean方法

/**
	 * Overridden method of {@link HttpServletBean}, invoked after any bean properties
	 * have been set. Creates this servlet's WebApplicationContext.
	 */
	@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 {
                         //建立Web應用上下文
			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");
		}
	}

　　FrameworkServlet.initWebApplicationContext()

/**
	 * 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;

		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
			wac = findWebApplicationContext();
		}
		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;
	}

　　先看WebApplicationContextUtils.getWebApplicationContext(getServletContext())，，主要是從ServletContext獲取WebApplicationContext

public static WebApplicationContext getWebApplicationContext(ServletContext sc, String attrName) {
		Assert.notNull(sc, "ServletContext must not be null");
                //從ServletContext中獲取
		Object attr = sc.getAttribute(attrName);
		if (attr == null) {
			return null;
		}
		if (attr instanceof RuntimeException) {
			throw (RuntimeException) attr;
		}
		if (attr instanceof Error) {
			throw (Error) attr;
		}
		if (attr instanceof Exception) {
			throw new IllegalStateException((Exception) attr);
		}
		if (!(attr instanceof WebApplicationContext)) {
			throw new IllegalStateException("Context attribute is not of type WebApplicationContext: " + attr);
		}
		return (WebApplicationContext) attr;
	}

　findWebApplicationContext和上面同樣從ContextServlet中查找，若是不存在就調用下面的createWebApplicationContext方法。

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
　　　　　　　　　　//獲取contextClass
		Class<?> contextClass = getContextClass();
		if (this.logger.isDebugEnabled()) {
			this.logger.debug("Servlet with name '" + getServletName() +
					"' will try to create custom WebApplicationContext context of class '" +
					contextClass.getName() + "'" + ", using 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");
		}
                //經過反射獲取實例
		ConfigurableWebApplicationContext wac =
				(ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

		wac.setEnvironment(getEnvironment());
                //設置雙親上下文
		wac.setParent(parent);
		wac.setConfigLocation(getContextConfigLocation());
                //配置和刷新應用
		configureAndRefreshWebApplicationContext(wac);

		return wac;
	}

　　看一下getContextClass()究竟是哪一個類？ XmlWebApplicationContext.class

最後configureAndRefreshWebApplicationContext調用refresh方法啓動容器。

回到initWebApplicationContext方法中

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);
		}

　　這個會觸發SpringMVC初始化策略

/**
	 * This implementation calls {@link #initStrategies}.
	 */
	@Override
	protected void onRefresh(ApplicationContext context) {
		initStrategies(context);
	}

	/**
	 * Initialize the strategy objects that this servlet uses.
	 * <p>May be overridden in subclasses in order to initialize further strategy objects.
	 */
	protected void initStrategies(ApplicationContext context) {
		initMultipartResolver(context);
		initLocaleResolver(context);
		initThemeResolver(context);
                //映射關係
		initHandlerMappings(context);
		initHandlerAdapters(context);
		initHandlerExceptionResolvers(context);
		initRequestToViewNameTranslator(context);
		initViewResolvers(context);
		initFlashMapManager(context);
	}

　　到此，SpringMVC的初始化的流程大概就是這樣，下篇就是SpringMVC請求流程。