第三阶段|SpringMAC之初识——详述load-on-startup标签的作用第三阶段|#SpringMAC

一、load-on-startup 标签作用：用来控制容器启动时，是否加载Servlet。

1

load-on-startup 标签，为负数或不配置时，容器启动时，不加载Servlet。（调用时加载）
load-on-startup 标签，为正数，容器启动时，加载Servlet。且顺序优先级： 0 > 1>2>...>n
二、有该标签DispatcherServlet的执行过程
web.xml配置如下：

cassini index.jsp DispatcherServlet org.springframework.web.servlet.DispatcherServlet contextConfigLocationclasspath:application.xml 1 DispatcherServlet *.do CharacterEncodingFilter org.springframework.web.filter.CharacterEncodingFilter encodingUTF-8 【第三阶段|SpringMAC之初识——详述load-on-startup标签的作用】forceEncodingtrue CharacterEncodingFilter /

1、 Tomcat发布Web项目，由于web.xml文件配置org.springframework.web.servlet.DispatcherServlet时添加了1，所以依次执行DispatcherServlet类静态代码块、构造方法和init方法，如下代码：
静态代码块

static { // Load default strategy implementations from properties file. // This is currently strictly internal and not meant to be customized // by application developers. try { ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, DispatcherServlet.class); defaultStrategies = PropertiesLoaderUtils.loadProperties(resource); //strategy [?str?t?d?i] n. 策略 } catch (IOException ex) { throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage()); } }

构造方法

public DispatcherServlet() { super(); setDispatchOptionsRequest(true); }

init方法：该方法继承自HttpServletBean抽象类

@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"); } }

2.执行DispatcherServlet类initServletBean()方法，该方法继承自FrameworkServlet抽象类；

@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"); } }

3.执行DispatcherServlet类initWebApplicationContext()方法，该方法继承自FrameworkServlet抽象类；

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) {// refreshEventReceived全局变量为false // 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; }

4.执行DispatcherServlet类onRefresh(ApplicationContext context)方法；

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

5.执行DispatcherServlet类initStrategies(ApplicationContext context)方法；

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