Spring Security3源码分析（4）-FilterChainProxy执行过程分析

通过FilterChainProxy的初始化、自定义标签的分析后，Spring Security需要的运行环境已经准备好了。 

这样当用户访问应用时，过滤器就开始工作了。web.xml配置的Filter：org.springframework.web.filter.DelegatingFilterProxy就不介绍了，该类仅仅是初始化一个FilterChainProxy，然后把所有拦截的请求交给FilterChainProxy处理。 

FilterChainProxy的doFilter方法如下 

 public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
         throws IOException, ServletException {
      //FilterInvocation类封装了request、response、chain
     //在方法中传递
      FilterInvocation fi = new FilterInvocation(request, response, chain);
     //获取http标签中创建的所有Filter
     List filters = getFilters(fi.getRequestUrl());
     if (filters == null || filters.size() == 0) {
         if (logger.isDebugEnabled()) {
             logger.debug(fi.getRequestUrl() +
                     filters == null ? " has no matching filters" : " has an empty filter list");
         }

         chain.doFilter(request, response);

         return;
     }
     //把实际doFilter任务交给VirtualFilterChain处理
     VirtualFilterChain virtualFilterChain = new VirtualFilterChain(fi, filters);
     virtualFilterChain.doFilter(fi.getRequest(), fi.getResponse());
 }

现在来分析VirtualFilterChain的doFilter方法： 

 private static class VirtualFilterChain implements FilterChain {
     private FilterInvocation fi;
     private List additionalFilters;
     private int currentPosition = 0;

     private VirtualFilterChain(FilterInvocation filterInvocation, List additionalFilters) {
         this.fi = filterInvocation;
         this.additionalFilters = additionalFilters;
     }

     public void doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException {
         //判断过滤器的个数是否与当前位置相等
         if (currentPosition == additionalFilters.size()) {
             if (logger.isDebugEnabled()) {
                 logger.debug(fi.getRequestUrl()
                     + " reached end of additional filter chain; proceeding with original chain");
             }
             fi.getChain().doFilter(request, response);
         } else {
             //当前位置加一
             currentPosition++;
             //根据当前位置从过滤器列表中取出一个Filter
             Filter nextFilter = additionalFilters.get(currentPosition - 1);
             if (logger.isDebugEnabled()) {
                 logger.debug(fi.getRequestUrl() + " at position " + currentPosition + " of "
                     + additionalFilters.size() + " in additional filter chain; firing Filter: '"
                     + nextFilter + "'");
             }
            //执行取出Filter的doFilter方法
            nextFilter.doFilter(request, response, this);
         }
     }
 }

注意这里 

 nextFilter.doFilter(request, response, this);

VirtualFilterChain把自身作为参数传递给doFilter方法，这样doFilter方法最后会调用VirtualFilterChain的doFilter方法，这样控制就又回到了VirtualFilterChain，于是VirtualFilterChain又将当前位置currentPosition前移，调用下一个Filter的doFilter方法。当additionalFilters中所有元素的doFilter都执行完毕，VirtualFilterChain执行fi.getChain().doFilter，而fi.getChain()的值就是FilterChainProxy的doFilter方法中的参数chain的值。  
这样我们就理解了FilterChainProxy是如何处理整个Filter链的了。 

接下来，就要一一分析每个Filter的功能与作用了。