Implement maintenance mode system

Classes can register reasons for maintenance. While at least one reason
is set, all requests are blocked and responded to with a 503 status
code.

src/main/java/de/thm/arsnova/services/StatusService.java

+/*
+ * This file is part of ARSnova Backend.
+ * Copyright (C) 2012-2018 The ARSnova Team
+ *
+ * ARSnova Backend is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ARSnova Backend is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package de.thm.arsnova.services;
+
+import java.util.Map;
+
+public interface StatusService {
+	void putMaintenanceReason(Class<?> type, String reason);
+	void removeMaintenanceReason(Class<?> type);
+	Map<Class<?>, String> getMaintenanceReasons();
+	boolean isMaintenanceActive();
+}

src/main/java/de/thm/arsnova/services/StatusServiceImpl.java

+/*
+ * This file is part of ARSnova Backend.
+ * Copyright (C) 2012-2018 The ARSnova Team
+ *
+ * ARSnova Backend is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ARSnova Backend is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package de.thm.arsnova.services;
+
+import org.springframework.stereotype.Service;
+
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Keeps track of maintenance reasons registered by other components. While at least one maintenance reason is active,
+ * API access is blocked by {@link de.thm.arsnova.web.MaintenanceModeFilter}.
+ *
+ * @author Daniel Gerhardt
+ */
+@Service
+public class StatusServiceImpl implements StatusService {
+	private final Map<Class<?>, String> maintenanceReasons = new HashMap<>();
+
+	@Override
+	public void putMaintenanceReason(final Class<?> type, final String reason) {
+		maintenanceReasons.put(type, reason);
+	}
+
+	@Override
+	public void removeMaintenanceReason(final Class<?> type) {
+		maintenanceReasons.remove(type);
+	}
+
+	@Override
+	public Map<Class<?>, String> getMaintenanceReasons() {
+		return maintenanceReasons;
+	}
+
+	@Override
+	public boolean isMaintenanceActive() {
+		return !maintenanceReasons.isEmpty();
+	}
+}

src/main/java/de/thm/arsnova/web/MaintenanceModeFilter.java

+/*
+ * This file is part of ARSnova Backend.
+ * Copyright (C) 2012-2018 The ARSnova Team
+ *
+ * ARSnova Backend is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * ARSnova Backend is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+package de.thm.arsnova.web;
+
+import de.thm.arsnova.services.StatusService;
+import org.springframework.stereotype.Component;
+import org.springframework.web.filter.OncePerRequestFilter;
+
+import javax.servlet.FilterChain;
+import javax.servlet.ServletException;
+import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.HttpServletResponse;
+import java.io.IOException;
+
+/**
+ * Blocks API requests while maintenance reasons are active.
+ *
+ * @author Daniel Gerhardt
+ */
+@Component
+public class MaintenanceModeFilter extends OncePerRequestFilter {
+	private StatusService statusService;
+
+	public MaintenanceModeFilter(final StatusService statusService) {
+		this.statusService = statusService;
+	}
+
+	@Override
+	protected void doFilterInternal(final HttpServletRequest httpServletRequest, final HttpServletResponse httpServletResponse, final FilterChain filterChain) throws ServletException, IOException {
+		if (statusService.isMaintenanceActive()) {
+			httpServletResponse.setStatus(503);
+			return;
+		}
+		filterChain.doFilter(httpServletRequest, httpServletResponse);
+	}
+}

src/main/webapp/WEB-INF/web.xml

@@ -79,6 +79,16 @@
 		<url-pattern>/*</url-pattern>
 	</filter-mapping>
 
+	<filter>
+		<filter-name>maintenanceModeFilter</filter-name>
+		<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
+		<async-supported>true</async-supported>
+	</filter>
+	<filter-mapping>
+		<filter-name>maintenanceModeFilter</filter-name>
+		<url-pattern>/*</url-pattern>
+	</filter-mapping>
+
 	<session-config>
 		<tracking-mode>COOKIE</tracking-mode>
 	</session-config>