/*
 *
 *  * Copyright 2002-2017 the original author or authors.
 *  *
 *  * Licensed under the Apache License, Version 2.0 (the "License");
 *  * you may not use this file except in compliance with the License.
 *  * You may obtain a copy of the License at
 *  *
 *  *      http://www.apache.org/licenses/LICENSE-2.0
 *  *
 *  * Unless required by applicable law or agreed to in writing, software
 *  * distributed under the License is distributed on an "AS IS" BASIS,
 *  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  * See the License for the specific language governing permissions and
 *  * limitations under the License.
 *
 */

package org.springframework.security.web.server.ui;

import org.springframework.core.io.buffer.DataBuffer;
import org.springframework.core.io.buffer.DataBufferFactory;
import org.springframework.core.io.buffer.DataBufferUtils;
import org.springframework.http.HttpMethod;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.server.reactive.ServerHttpResponse;
import org.springframework.security.web.server.util.matcher.ServerWebExchangeMatcher;
import org.springframework.security.web.server.util.matcher.ServerWebExchangeMatchers;
import org.springframework.web.server.ServerWebExchange;
import org.springframework.web.server.WebFilter;
import org.springframework.web.server.WebFilterChain;
import reactor.core.publisher.Mono;

import java.nio.charset.Charset;

/**
 * @author Rob Winch
 * @since 5.0
 */
public class LoginPageGeneratingWebFilter implements WebFilter {
	private ServerWebExchangeMatcher matcher = ServerWebExchangeMatchers
		.pathMatchers(HttpMethod.GET, "/login");

	@Override public Mono<Void> filter(ServerWebExchange exchange, WebFilterChain chain) {
		return this.matcher.matches(exchange)
			.filter(ServerWebExchangeMatcher.MatchResult::isMatch)
			.switchIfEmpty(chain.filter(exchange).then(Mono.empty()))
			.flatMap(matchResult -> render(exchange));
	}

	private Mono<Void> render(ServerWebExchange exchange) {
		boolean isError = exchange.getRequest().getQueryParams().containsKey("error");
		ServerHttpResponse result = exchange.getResponse();
		result.setStatusCode(HttpStatus.FOUND);
		result.getHeaders().setContentType(MediaType.TEXT_HTML);
		byte[] bytes = createPage(isError);
		DataBufferFactory bufferFactory = exchange.getResponse().bufferFactory();
		DataBuffer buffer = bufferFactory.wrap(bytes);
		return result.writeWith(Mono.just(buffer))
			.doOnError( error -> DataBufferUtils.release(buffer));
	}

	private static byte[] createPage(boolean isError) {
		String page =  "<!DOCTYPE html>\n"
			+ "<html lang=\"en\">\n"
			+ "  <head>\n"
			+ "    <meta charset=\"utf-8\">\n"
			+ "    <meta name=\"viewport\" content=\"width=device-width, initial-scale=1, shrink-to-fit=no\">\n"
			+ "    <meta name=\"description\" content=\"\">\n"
			+ "    <meta name=\"author\" content=\"\">\n"
			+ "    <title>Please sign in</title>\n"
			+ "    <link href=\"https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-beta/css/bootstrap.min.css\" rel=\"stylesheet\" integrity=\"sha384-/Y6pD6FV/Vv2HJnA6t+vslU6fwYXjCFtcEpHbNJ0lyAFsXTsjBbfaDjzALeQsN6M\" crossorigin=\"anonymous\">\n"
			+ "    <link href=\"http://getbootstrap.com/docs/4.0/examples/signin/signin.css\" rel=\"stylesheet\" crossorigin=\"anonymous\"/>\n"
			+ "  </head>\n"
			+ "  <body>\n"
			+ "     <div class=\"container\">\n"
			+ "      <form class=\"form-signin\" method=\"post\" action=\"/login\">\n"
			+ "        <h2 class=\"form-signin-heading\">Please sign in</h2>\n"
			+ createError(isError)
			+ "        <p>\n"
			+ "          <label for=\"username\" class=\"sr-only\">Username</label>\n"
			+ "          <input type=\"text\" id=\"username\" name=\"username\" class=\"form-control\" placeholder=\"Username\" required autofocus>\n"
			+ "        </p>\n"
			+ "        <p>\n"
			+ "          <label for=\"password\" class=\"sr-only\">Password</label>\n"
			+ "          <input type=\"password\" id=\"password\" name=\"password\" class=\"form-control\" placeholder=\"Password\" required>\n"
			+ "        </p>\n"
			+ "        <button class=\"btn btn-lg btn-primary btn-block\" type=\"submit\">Sign in</button>\n"
			+ "      </form>\n"
			+ "    </div>\n"
			+ "  </body>\n"
			+ "</html>";

		return page.getBytes(Charset.defaultCharset());
	}

	private static String createError(boolean isError) {
		return isError ? "<div class=\"alert alert-danger\" role=\"alert\">Invalid credentials</div>" : "";
	}
}

Add LoginPageGeneratingWebFilter