一、load-on-startup标签的作用

load-on-startup标签可以设置servlet的加载优先级别和容器是否在启动时加载该servlet，如下设置：

<load-on-startup>1</load-on-startup>

当值为0或者大于0时，代表容器启动时加载该servlet。正数的值越小，启动时加载该servlet的优先级越高。如果为负数，则容器启动时不会加载该servlet，只有该servlet被选择时才会加载。

二、分析有该标签DispatcherServlet的执行过程

web.xml文件：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>moon1</display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
	<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>

	<servlet-mapping>
		<servlet-name>DispatcherServlet</servlet-name>
		<url-pattern>*.do</url-pattern>
	</servlet-mapping>
	
	<filter>
		<filter-name>CharacterEncodingFilter</filter-name>
		<filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
		<init-param>
			<param-name>encoding</param-name>
			<param-value>UTF-8</param-value>
		</init-param>
		<init-param>
			<param-name>forceEncoding</param-name>
			<param-value>true</param-value>
		</init-param>
	</filter>
	<filter-mapping>
		<filter-name>CharacterEncodingFilter</filter-name>
		<url-pattern>/</url-pattern>
	</filter-mapping>
</web-app>

1、 Tomcat发布Web项目，由于web.xml文件配置org.springframework.web.servlet.DispatcherServlet时添加了<load-on-startup>1</load-on-startup>，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：

① 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);
	}
	catch (IOException ex) {
		throw new IllegalStateException("Could not load '" + DEFAULT_STRATEGIES_PATH + "': " + ex.getMessage());
	}
}

② 构造方法：

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

③ 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);
}