Spring Security源码分析之SecurityContextPersistenceFilter

public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;

        if (request.getAttribute(FILTER_APPLIED) != null) {
            // 确保对于每个请求只应用一次过滤器
            chain.doFilter(request, response);
            return;
        }

        final boolean debug = logger.isDebugEnabled();

        request.setAttribute(FILTER_APPLIED, Boolean.TRUE);

        if (forceEagerSessionCreation) {
            HttpSession session = request.getSession();

            if (debug && session.isNew()) {
                logger.debug("Eagerly created session: " + session.getId());
            }
        }
        // 将 request/response 对象交给HttpRequestResponseHolder维持  
        HttpRequestResponseHolder holder = new HttpRequestResponseHolder(request,
                response);
        //通过SecurityContextRepository接口的实现类装载SecurityContext实例  
        //HttpSessionSecurityContextRepository将产生SecurityContext实例的任务交给SecurityContextHolder.createEmptyContext()  
        //SecurityContextHolder再根据策略模式的不同，  
        //把任务再交给相应策略类完成SecurityContext的创建  
        //如果没有配置策略名称，则默认为  
        //ThreadLocalSecurityContextHolderStrategy，  
        //该类直接通过new SecurityContextImpl()创建实例
        SecurityContext contextBeforeChainExecution = repo.loadContext(holder);

        try {
            //将产生的SecurityContext再通过SecurityContextHolder->  
            //ThreadLocalSecurityContextHolderStrategy设置到ThreadLocal 
            SecurityContextHolder.setContext(contextBeforeChainExecution);
            //继续把请求流向下一个过滤器执行  
            chain.doFilter(holder.getRequest(), holder.getResponse());

        }
        finally {
            //先从SecurityContextHolder获取SecurityContext实例  
            SecurityContext contextAfterChainExecution = SecurityContextHolder
                    .getContext();
            //关键性地除去SecurityContextHolder内容 - 在任何事情之前执行此操作
            //再把SecurityContext实例从SecurityContextHolder中清空
                        //若没有清空,会受到服务器的线程池机制的影响  
            SecurityContextHolder.clearContext();
            //将SecurityContext实例持久化到session中 
            repo.saveContext(contextAfterChainExecution, holder.getRequest(),
                    holder.getResponse());
            request.removeAttribute(FILTER_APPLIED);

            if (debug) {
                logger.debug("SecurityContextHolder now cleared, as request processing completed");
            }
        }
    }