acegi의 로그인 과정

	public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException,
			ServletException {
		if (!(request instanceof HttpServletRequest)) {
			throw new ServletException("Can only process HttpServletRequest");
		}

		if (!(response instanceof HttpServletResponse)) {
			throw new ServletException("Can only process HttpServletResponse");
		}

		HttpServletRequest httpRequest = (HttpServletRequest) request;
		HttpServletResponse httpResponse = (HttpServletResponse) response;

		if (requiresAuthentication(httpRequest, httpResponse)) {
			if (logger.isDebugEnabled()) {
				logger.debug("Request is to process authentication");
			}

			Authentication authResult;
//                
			try {
				onPreAuthentication(httpRequest, httpResponse);
				authResult = attemptAuthentication(httpRequest);//               dao                  
			}
			catch (AuthenticationException failed) {
				// Authentication failed
				unsuccessfulAuthentication(httpRequest, httpResponse, failed);

				return;
			}

			// Authentication success
			if (continueChainBeforeSuccessfulAuthentication) {
				chain.doFilter(request, response);
			}

			successfulAuthentication(httpRequest, httpResponse, authResult);

			return;
		}

		chain.doFilter(request, response);
	}

	protected void successfulAuthentication(HttpServletRequest request, HttpServletResponse response,
			Authentication authResult) throws IOException {
		if (logger.isDebugEnabled()) {
			logger.debug("Authentication success: " + authResult.toString());
		}

//        SecurityContextHolder 1980
SecurityContextHolder.getContext().setAuthentication(authResult);

		if (logger.isDebugEnabled()) {
			logger.debug("Updated SecurityContextHolder to contain the following Authentication: '" + authResult + "'");
		}
//              
		String targetUrl = determineTargetUrl(request);

		if (logger.isDebugEnabled()) {
			logger.debug("Redirecting to target URL from HTTP Session (or default): " + targetUrl);
		}

		onSuccessfulAuthentication(request, response, authResult);

		rememberMeServices.loginSuccess(request, response, authResult);

		// Fire event
		if (this.eventPublisher != null) {
			eventPublisher.publishEvent(new InteractiveAuthenticationSuccessEvent(authResult, this.getClass()));
		}

		sendRedirect(request, response, targetUrl);
	}