code: public void setPostFailureUrl(String postFailureUrl) {
  		AuthenticationFailureHandler failureHandler = getFailureHandler();
  
  		if (failureHandler instanceof SocialAuthenticationFailureHandler) {
  			failureHandler = ((SocialAuthenticationFailureHandler)failureHandler).getDelegate();
  		}
  
  		if (failureHandler instanceof SimpleUrlAuthenticationFailureHandler) {
  			SimpleUrlAuthenticationFailureHandler h = (SimpleUrlAuthenticationFailureHandler) failureHandler;
  			h.setDefaultFailureUrl(postFailureUrl);
  		} else {
  			throw new IllegalStateException("can't set postFailureUrl on unknown failureHandler, type is " + failureHandler.getClass().getName());
  		}
  	}
docs: The URL to redirect to if authentication fails or if authorization is denied by the user.
  @param postFailureUrl The failure URL. Defaults to "/signin" (relative to the servlet context).