小析web.xml中context-param,Listener,Filter,Servlet处理顺序

web.xml中四种配置参数的执行顺序是:context-param ---> Listener ---> Filter ---> Servlet;

1. web容器首先会读取位于web.xml中的两个节点<context-param>和<listener>;

2. 然后去创建一个ServletContext上下文,被所有的组件共享;

3. 将web.xml中配置的<context-param>转化为键值对,存放于servletContext中;

4. web容器负责实例化监听器并注册监听器以用来监听各种事件,由于可以配置多个监听器,所以监听器本身的注册及执行也是有顺序的:

web容器是根据监听器实现的接口以及在web.xml中的顺序注册的,在web应用程序执行的时候,监听器执行的顺序和它们的注册顺序是一致的;在容器关闭的时候,监听器是按照和注册顺序相反的顺序被通知调用;监听器可以按照如下进行分类：

Object

Event

Listener Interface and Event Class

Web context	Initialization and destruction	javax.servlet.ServletContextListener and ServletContextEvent
Attribute added, removed, or replaced	javax.servlet.ServletContextAttributeListener and ServletContextAttributeEvent
Session	Creation, invalidation, activation, passivation, and timeout	javax.servlet.http.HttpSessionListener , javax.servlet.http.HttpSessionActivationListener , and HttpSessionEvent
Attribute added, removed, or replaced	javax.servlet.http.HttpSessionAttributeListener and HttpSessionBindingEvent
Request	A servlet request has started being processed by web components	javax.servlet.ServletRequestListener and ServletRequestEvent
Attribute added, removed, or replaced	javax.servlet.ServletRequestAttributeListener and ServletRequestAttributeEvent

监听器是设计模式中观察者模式的一种应用,容器维护了对某种事件的一个监听器列表,当事件被触发的时侯,响应的监听器就会被按一定的顺序调用. Spring中的ContextLoaderListener实现了ServletContextListener,在ServletContext初始化的时候,就会去初始化webapplicationcontext,从而完成spring IOC中bean的注册;

5. web容器通过调用Filter的init方法初始化Filter,Filter主要用于在请求被处理之前和之后拦截并作出响应;Filter可以通过<filter-mapping>对特定的资源进行处理;当一个请求到来时,针对某种资源的filter会被组织成一个filterChain,filter可以通过调用filterChanin.dofilter()调用后面的filter;如果在一个filter的doFilter方法中调用filterChain.doFilter(),那么filterChian.doFilter()之后的代码将会在response返回的时候才会被执行;如下段代码,当程序进入到filterChain.doFilter(request, response)时,下一个filter的doFilter方法会被调用,如果下一个filter的doFilter方法中也有filterChain.doFilter(request, response),将会继续对下下一个filter的doFilter方法,直到针对当前请求的最后一个filter,然后进入Servlet进行应用逻辑的处理,待处理结束后,会按照刚才调用filter相反的顺序逐一执行filterChain.doFilter(request, response)之后的代码.

public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain filterChain) throws IOException, ServletException {
		HttpServletRequest req = (HttpServletRequest)request;		
		if(!INITIAL_PAGE.equals(req.getServletPath()) && !LOGIN_ACTION.equals(req.getServletPath())){
			Object user = req.getSession().getAttribute(WebConstants.USER_INFO);
			if(user == null){				
				req.getRequestDispatcher(LOGIN_PAGE_PATH).forward(request, response);
			}else{
				log.info("pre filterChain doFiler in LoginCheckFilter");
				filterChain.doFilter(request, response);
				log.info("post filterChain doFiler in LoginCheckFilter");
			}
		}else{
			log.info("pre filterChain doFiler in LoginCheckFilter");
			filterChain.doFilter(request, response);
			log.info("post filterChain doFiler in LoginCheckFilter");
		}
	}

以下是在我的应用程序中的输出：

2010-08-15 14:22:29,948 [http-8080-Processor25] INFO  org.mywork.web.utils.LoginCheckFilter - pre filterChain doFiler in LoginCheckFilter
2010-08-15 14:22:29,948 [http-8080-Processor25] INFO  org.mywork.web.actions.LoginAction
2010-08-15 14:22:30,212 [http-8080-Processor25] WARN  org.apache.struts2.components.Form - No configuration found for the specified action: 'userQueryAction.action' in namespace: '/basic'. Form action defaulting to 'action' attribute's literal value.
2010-08-15 14:22:30,252 [http-8080-Processor25] WARN  org.apache.struts2.components.Form - No configuration found for the specified action: 'userQueryAction.action' in namespace: '/basic'. Form action defaulting to 'action' attribute's literal value.
2010-08-15 14:22:30,299 [http-8080-Processor25] INFO  org.mywork.web.utils.LoginCheckFilter - post filterChain doFiler in LoginCheckFilter

6. 在filterChain中的filter方法执行完毕的时候,相应的servlet会被调用,sevlet可以通过<servlet-mapping>进行配置使其对特定的url进行处理;在sevlet处理完成后,应用程序可能会再次进入filter,执行某个filter内doFilter()方法中filterChain.doFilter()之后的代码.