web/src/main/java/org/springframework/security/web/authentication/preauth/AbstractPreAuthenticatedProcessingFilter.java
public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {

		if (logger.isDebugEnabled()) {
			logger.debug("Checking secure context token: "
					+ SecurityContextHolder.getContext().getAuthentication());
		}

		if (requiresAuthentication((HttpServletRequest) request)) {
			doAuthenticate((HttpServletRequest) request, (HttpServletResponse) response);
		}

		chain.doFilter(request, response);
	}
public void doFilter(ServletRequest request, ServletResponse response,
			FilterChain chain) throws IOException, ServletException {

		if (logger.isDebugEnabled()) {
			logger.debug("Checking secure context token: "
					+ SecurityContextHolder.getContext().getAuthentication());
		}

		if (requiresAuthentication((HttpServletRequest) request)) {
			doAuthenticate((HttpServletRequest) request, (HttpServletResponse) response);
		}

		chain.doFilter(request, response);
	}
Try to authenticate a pre-authenticated user with Spring Security if the user has
not yet been authenticated.