spring securty源码分析login page

看慕课网的课程，发现需要设置一个logginPage，但并不清楚为什么要设置

public void configure(HttpSecurity http) throws Exception {
		http.formLogin()
			.loginPage("/authentication/require")//control拦截这个url
			.loginProcessingUrl(SecurityConstants.DEFAULT_SIGN_IN_PROCESSING_URL_FORM)//登陆页面中的表单提交url
			.successHandler(imoocAuthenticationSuccessHandler)
			.failureHandler(imoocAuthenticationFailureHandler);
	}

从字面意思是log in 的登陆页面，但是很明显不是，一般登陆页面都是一个html页面或者其它什么的

1. any request in browser
2. analyse the process
   org.springframework.security.web.authentication.AnonymousAuthenticationFilter#doFilter
   ->org.springframework.security.web.session.SessionManagementFilter#doFilter
   ->org.springframework.security.web.access.ExceptionTranslationFilter#doFilter
   ->org.springframework.security.web.access.intercept.FilterSecurityInterceptor#invoke
   in this method, invoke the
   org.springframework.security.access.intercept.AbstractSecurityInterceptor#beforeInvocation

in this step, will throw exception

try {
			this.accessDecisionManager.decide(authenticated, object, attributes);
		}
		catch (AccessDeniedException accessDeniedException) {
			publishEvent(new AuthorizationFailureEvent(object, attributes, authenticated,
					accessDeniedException));

			throw accessDeniedException;
		}

process the exception in the ExceptionTranslationFilter

if (ase != null) {
				handleSpringSecurityException(request, response, chain, ase);
			}

if the exception is AccessDeniedException and is Anonymous

private void handleSpringSecurityException(HttpServletRequest request,
			HttpServletResponse response, FilterChain chain, RuntimeException exception)
			throws IOException, ServletException {
		if (exception instanceof AuthenticationException) {
			

			sendStartAuthentication(request, response, chain,
					(AuthenticationException) exception);
		}
		else if (exception instanceof AccessDeniedException) {
			Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
			if (authenticationTrustResolver.isAnonymous(authentication) || authenticationTrustResolver.isRememberMe(authentication)) {
				
				sendStartAuthentication(
						request,
						response,
						chain,
						new InsufficientAuthenticationException(
								"Full authentication is required to access this resource"));
			}

go into the sendStartAuthentication

SecurityContextHolder.getContext().setAuthentication(null);
		requestCache.saveRequest(request, response);
		logger.debug("Calling Authentication entry point.");
		authenticationEntryPoint.commence(request, response, reason);

in the commence method:

1. redirectUrl = buildRedirectUrlToLoginPage(request, response, authException)
2.	redirectStrategy.sendRedirect(request, response, redirectUrl);

in the above step1, get the loginForm url