Servlet中的Filter在Struts2中的执行流程

1.Servlet中filter的执行流程可以依照web.xml中的filter配置进行顺序执行。我们知道web.xml中元素执行的顺序 context-param->listener->filter->servlet。

且无论这类元素摆放位置如何，都是遵循这个原则的。

下面看个例子

web.xml的配置：

	<!-- 编码过滤器 -->
 	<filter>
 		<filter-name>setCharacterEncoding</filter-name>
 		<filter-class>com.company.strutstudy.web.servletstudy.filter.EncodingFilter</filter-class>
 		<init-param>
 			<param-name>encoding</param-name>
 			<param-value>utf-8</param-value>
 		</init-param>
 	</filter>
 	<filter-mapping>
 		<filter-name>setCharacterEncoding</filter-name>
 		<url-pattern>/*</url-pattern>
 	</filter-mapping>
 	<!-- 一个简单的sevlet读取init参数的配置 -->
 	<servlet>
 		<servlet-name>helloworld</servlet-name>
 		<servlet-class>com.company.strutstudy.web.servletstudy.servlet.HelloWorldServlet</servlet-class>
 		<init-param>
 			<param-name>encoding</param-name>
 			<param-value>UTF-8</param-value>
 		</init-param>
 	</servlet>
 	<servlet-mapping>
 		<servlet-name>helloworld</servlet-name>
 		<url-pattern>/hello</url-pattern>
 	</servlet-mapping>
 	<!--禁用浏览器缓存的过滤器 -->
 	<filter>
 		<filter-name>nocachefilter</filter-name>
 		<filter-class>com.company.strutstudy.web.servletstudy.filter.NoCacheFilter</filter-class>
 	</filter>
 	<filter-mapping>
 		<filter-name>nocachefilter</filter-name>
 		<url-pattern>/*</url-pattern>
 	</filter-mapping>

执行结果：

before encoding utf-8 filter！
cache do the before!
doservice...UTF-8
doget...
dopost...
cache do the after!
after encoding utf-8 filter！

ps：servlet在filter配置之前，那filter也会进行执行，且在servlet之前。
2.在struts2中配置执行filter，经过debug源码发现，一旦这个filter配置在struts2的filterDispatcher后面，在filterDispatcher中的dofilter中会自动过滤掉该filter。

web.xml的配置：

<!-- 编码过滤器 -->
 	<filter>
 		<filter-name>setCharacterEncoding</filter-name>
 		<filter-class>com.company.strutstudy.web.servletstudy.filter.EncodingFilter</filter-class>
 		<init-param>
 			<param-name>encoding</param-name>
 			<param-value>utf-8</param-value>
 		</init-param>
 	</filter>
 	<filter-mapping>
 		<filter-name>setCharacterEncoding</filter-name>
 		<url-pattern>/*</url-pattern>
 	</filter-mapping> 
 	<!-- 加载struts2核心 -->
 	<filter>
 		<filter-name>StrutsFilter</filter-name>
 		<filter-class>org.apache.struts2.dispatcher.FilterDispatcher</filter-class>
 		<init-param>
 			<param-name>packages</param-name>
 			<param-value>com.company.strutstudy.web.action</param-value>
 		</init-param>
 	</filter>
 	<filter-mapping>
 		<filter-name>StrutsFilter</filter-name>
 		<url-pattern>/*</url-pattern>
 	</filter-mapping>
 	<!--禁用浏览器缓存的过滤器 -->
 	<filter>
 		<filter-name>nocachefilter</filter-name>
 		<filter-class>com.company.strutstudy.web.servletstudy.filter.NoCacheFilter</filter-class>
 	</filter>
 	<filter-mapping>
 		<filter-name>nocachefilter</filter-name>
 		<url-pattern>/*</url-pattern>
 	</filter-mapping>

EncodingFilter源码：

public class EncodingFilter implements Filter {
 	private String encoding;
 	private Map<String, String> params = new HashMap<String, String>();
 	// 项目结束时就已经进行销毁
 	public void destroy() {
 		System.out.println("end do the encoding filter!");
 		params=null;
 		encoding=null;
 	}
 	public void doFilter(ServletRequest req, ServletResponse resp,
 			FilterChain chain) throws IOException, ServletException {
 		//UtilTimerStack.push("EncodingFilter_doFilter:");
 		System.out.println("before encoding " + encoding + " filter！");
 		req.setCharacterEncoding(encoding);
 		// resp.setCharacterEncoding(encoding);
 		// resp.setContentType("text/html;charset="+encoding);
 		chain.doFilter(req, resp);
 		System.out.println("after encoding " + encoding + " filter！");
 		System.err.println("----------------------------------------");
 		//UtilTimerStack.pop("EncodingFilter_doFilter:");
 	}
 
 	// 项目启动时就已经进行读取
 	public void init(FilterConfig config) throws ServletException {
 		System.out.println("begin do the encoding filter!");
 		encoding = config.getInitParameter("encoding");
 		for (Enumeration e = config.getInitParameterNames(); e
 				.hasMoreElements();) {
 			String name = (String) e.nextElement();
 			String value = config.getInitParameter(name);
 			params.put(name, value);
 		}
 	}
 }

Struts2中的FilterDipatcher中的doFilter方法源码：

      /**
      * 当执行完编码Filter后，filter链表对象filterchain会执行其后的配置的filterDispatcher，随后执行该filter的dofilter方法。
      */ 
     public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
         HttpServletRequest request = (HttpServletRequest) req;
         HttpServletResponse response = (HttpServletResponse) res;
         ServletContext servletContext = getServletContext();
         String timerKey = "FilterDispatcher_doFilter: ";
         try {
             UtilTimerStack.push(timerKey);
             request = prepareDispatcherAndWrapRequest(request, response);
             ActionMapping mapping;
             try {
             	//得到存储Action信息的ActionMapping对象，actionMapper被注入，如何被注入待研究
                 mapping = actionMapper.getMapping(request, dispatcher.getConfigurationManager());
             } catch (Exception ex) {
                 LOG.error("error getting ActionMapping", ex);
                 dispatcher.sendError(request, response, servletContext, HttpServletResponse.SC_INTERNAL_SERVER_ERROR, ex);
                 return;
             }
             if (mapping == null) {//如果这个请求是action请求，那么这里得到的mapping不会是null
                 // there is no action in this request, should we look for a static resource?
                 String resourcePath = RequestUtils.getServletPath(request);
 
                 if ("".equals(resourcePath) && null != request.getPathInfo()) {
                     resourcePath = request.getPathInfo();
                 }
               //如果请求的资源以/struts开头，则当作静态资源处里
                 if (serveStatic && resourcePath.startsWith("/struts")) {
                     findStaticResource(resourcePath, findAndCheckResources(resourcePath), request, response);
                 } else {
                 	//否则继续往下传，知道没有filter为止
                     // this is a normal request, let it pass through
                     chain.doFilter(request, response);
                 }
                 // The framework did its job here
                 return;
             }
            //得到存储Action信息的ActionMapping对象
             dispatcher.serviceAction(request, response, servletContext, mapping);
 
         } finally {
             try {
                 ActionContextCleanUp.cleanUp(req);
             } finally {
                 UtilTimerStack.pop(timerKey);
             }
         }
     }
 

NoCacheFilter的源码：

public class NoCacheFilter implements Filter{
 
 	public void destroy() {
 		
 	}
 
 	public void doFilter(ServletRequest req,
 			ServletResponse res, FilterChain chain)
 			throws IOException, ServletException {
 		/*
 		          并不是所有的浏览器都能完全支持上面的三个响应头，因此最好是同时使用上面的三个响应头。
 			Expires数据头：值为GMT时间值，为-1指浏览器不要缓存页面
 			Cache-Control响应头有两个常用值： no-cache指浏览器不要缓存当前页面；
 			ax-age:xxx指浏览器缓存页面xxx秒。
 		 */
 		HttpServletResponse hres=(HttpServletResponse) res;
 		hres.setDateHeader("Expires",-1);
 		hres.setHeader("Cache-Control","no-cache"); 
 		hres.setHeader("Pragma","no-cache");
 		System.out.println("cache do the before!");
 		chain.doFilter(req, res);
 		System.out.println("cache do the after!");
 	}
 	public void init(FilterConfig config) throws ServletException {	
 	}
 }

执行结果：

before encoding utf-8 filter！
 do action! 
 after encoding utf-8 filter！