分析配置DispatcherServlet类时load-on-startup标签作用
<servlet> <servlet-name>DispatcherServlet</servlet-name> <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>classpath:application.xml</param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet>
SpringMVC中DispatcherServlet是该框架的核心，所有的请求处理及返回都要经过该Servlet，于是我们必须在web.xml里面配置该Servlet，大多时候我们在配置该Servlet的时候都会顺手配置一下：<load-on-startup>数值</load-on-startup>元素。起初就是以为加载DispatcherServlet的，我们来详细研究一下关于load-on-startup的作用及其过程。
作用：load-on-startup元素标记容器表示是否在启动的时候就加载这个servlet(实例化并调用其init()方法)，而<load-on-startup>x</load-on-startup>中x的取值1，2，3，4，5代表的是优先级，而非启动延迟时间。
过程：
1、因为该servlet是实例化并调用init()方法的，我们先进入该类寻找init()方法
从该类中可看出其定义的几个resolver都是由final修饰，表示该属性一旦被初始化便不可改变,这里不可改变的意思对基本类型来说是其值不可变,而对对象属性来说其引用不可再变。
所以我们在配置这些的时候不能随意定值，必须和其一致，如下示例：
<!--application.xml中的multipartResolver的id是个定值，如果写错了就会报错--> <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver" p:defaultEncoding="UTF-8" p:maxInMemorySize="1024000"></bean>
经查找并未在DispatcherServlet中找到init()方法，我们往上一级找其父类（FrameworkServlet）未果，再往上一级（HttpServletBean）通过查找在HttpServletBean中找到了该方法
/** * 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() + "'"); } // Set bean properties from init parameters. 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())); initBeanWrapper(bw); bw.setPropertyValues(pvs, true); } catch (BeansException ex) { if (logger.isErrorEnabled()) { 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"); } }
由其介绍可知其作用大概是，map配置参数到这个servlet的bean属性,并调用子类初始化，其子类初始化的处理就在于initServletBean();的作用，我们去寻找此方法的来源
（HttpServletBean）中找到了，但是个空方法，我们往回继续找在（FrameworkServlet）中找到了
/** * 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 { 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"); } }
我们打开initWebApplicationContext()方法：
/** * 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; }
整个过程就是为了给wac赋值，并返回该值，最后执行了onRefresh()方法：
@Override protected void onRefresh(ApplicationContext context) { initStrategies(context); }
然后执行initStrategies方法：
/** * 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) { //用于处理上传请求。处理方法是将普通的request包装成MultipartHttpServletRequest，后者可以直接调用getFile方法获取File. initMultipartResolver(context); //SpringMVC主要有两个地方用到了Locale：一是ViewResolver视图解析的时候；二是用到国际化资源或者主题的时候。 initLocaleResolver(context); //用于解析主题。SpringMVC中一个主题对应一个properties文件，里面存放着跟当前主题相关的所有资源、 //如图片、css样式等。SpringMVC的主题也支持国际化， initThemeResolver(context); //用来查找Handler的。 initHandlerMappings(context); //从名字上看，它就是一个适配器。Servlet需要的处理方法的结构却是固定的，都是以request和response为参数的方法。 //如何让固定的Servlet处理方法调用灵活的Handler来进行处理呢？这就是HandlerAdapter要做的事情 initHandlerAdapters(context); //其它组件都是用来干活的。在干活的过程中难免会出现问题，出问题后怎么办呢？ //这就需要有一个专门的角色对异常情况进行处理，在SpringMVC中就是HandlerExceptionResolver。 initHandlerExceptionResolvers(context); //有的Handler处理完后并没有设置View也没有设置ViewName，这时就需要从request获取ViewName了， //如何从request中获取ViewName就是RequestToViewNameTranslator要做的事情了。 initRequestToViewNameTranslator(context); //ViewResolver用来将String类型的视图名和Locale解析为View类型的视图。 //View是用来渲染页面的，也就是将程序返回的参数填入模板里，生成html（也可能是其它类型）文件。 initViewResolvers(context); //用来管理FlashMap的，FlashMap主要用在redirect重定向中传递参数。 initFlashMapManager(context); }
至此就完成了整个初始化过程了。
