add websocket support to immediately refresh tree when new entries are available

commafeed-server/src/main/java/com/commafeed/frontend/ws/WebSocketConfigurator.java

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+package com.commafeed.frontend.ws;
+
+import java.util.Optional;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+import javax.servlet.http.HttpSession;
+import javax.websocket.HandshakeResponse;
+import javax.websocket.server.HandshakeRequest;
+import javax.websocket.server.ServerEndpointConfig;
+import javax.websocket.server.ServerEndpointConfig.Configurator;
+
+import com.commafeed.backend.model.User;
+import com.commafeed.frontend.session.SessionHelper;
+
+import lombok.RequiredArgsConstructor;
+
+@Singleton
+@RequiredArgsConstructor(onConstructor = @__({ @Inject }))
+public class WebSocketConfigurator extends Configurator {
+
+	public static final String SESSIONKEY_USERID = "userId";
+
+	private final WebSocketSessions webSocketSessions;
+
+	@Override
+	public void modifyHandshake(ServerEndpointConfig config, HandshakeRequest request, HandshakeResponse response) {
+		HttpSession httpSession = (HttpSession) request.getHttpSession();
+		if (httpSession != null) {
+			Optional<User> user = SessionHelper.getLoggedInUser(httpSession);
+			if (user.isPresent()) {
+				config.getUserProperties().put(SESSIONKEY_USERID, user.get().getId());
+			}
+		}
+	}
+
+	@SuppressWarnings("unchecked")
+	@Override
+	public <T> T getEndpointInstance(Class<T> endpointClass) throws InstantiationException {
+		return (T) new WebSocketEndpoint(webSocketSessions);
+	}
+}

commafeed-server/src/main/java/com/commafeed/frontend/ws/WebSocketEndpoint.java

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+package com.commafeed.frontend.ws;
+
+import java.io.IOException;
+
+import javax.inject.Inject;
+import javax.inject.Singleton;
+import javax.websocket.CloseReason;
+import javax.websocket.CloseReason.CloseCodes;
+import javax.websocket.Endpoint;
+import javax.websocket.EndpointConfig;
+import javax.websocket.MessageHandler;
+import javax.websocket.Session;
+
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+
+@Slf4j
+@Singleton
+@RequiredArgsConstructor(onConstructor = @__({ @Inject }))
+public class WebSocketEndpoint extends Endpoint {
+
+	private final WebSocketSessions sessions;
+
+	@Override
+	public void onOpen(Session session, EndpointConfig config) {
+		Long userId = (Long) config.getUserProperties().get(WebSocketConfigurator.SESSIONKEY_USERID);
+		if (userId == null) {
+			reject(session);
+		} else {
+			log.debug("created websocket session for user {}", userId);
+			sessions.add(userId, session);
+		}
+
+		// converting this anonymous inner class to a lambda causes the following error when a message is sent from the client
+		// Unable to find decoder for type <javax.websocket.MessageHandler$Whole>
+		// this error is only visible when registering a listener to ws.onclose on the client
+		session.addMessageHandler(new MessageHandler.Whole<String>() {
+			@Override
+			public void onMessage(String message) {
+				if ("ping".equals(message)) {
+					session.getAsyncRemote().sendText("pong");
+				}
+			}
+		});
+	}
+
+	private void reject(Session session) {
+		try {
+			session.close(new CloseReason(CloseCodes.VIOLATED_POLICY, "unauthorized"));
+		} catch (IOException e) {
+			throw new RuntimeException(e);
+		}
+	}
+
+	@Override
+	public void onClose(Session session, CloseReason reason) {
+		sessions.remove(session);
+
+	}
+
+}

commafeed-server/src/main/java/com/commafeed/frontend/ws/WebSocketMessageBuilder.java

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+package com.commafeed.frontend.ws;
+
+import com.commafeed.backend.model.FeedSubscription;
+
+import lombok.experimental.UtilityClass;
+
+@UtilityClass
+public class WebSocketMessageBuilder {
+
+	public static final String newFeedEntries(FeedSubscription subscription) {
+		return String.format("%s:%s", "new-feed-entries", subscription.getId());
+	}
+
+}

commafeed-server/src/main/java/com/commafeed/frontend/ws/WebSocketSessions.java

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+package com.commafeed.frontend.ws;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.stream.Collectors;
+
+import javax.inject.Singleton;
+import javax.websocket.Session;
+
+import com.commafeed.backend.model.User;
+
+import lombok.extern.slf4j.Slf4j;
+
+@Singleton
+@Slf4j
+public class WebSocketSessions {
+
+	// a user may have multiple sessions (two tabs, on mobile, ...)
+	private final Map<Long, Set<Session>> sessions = new ConcurrentHashMap<>();
+
+	public void add(Long userId, Session session) {
+		sessions.computeIfAbsent(userId, v -> ConcurrentHashMap.newKeySet()).add(session);
+	}
+
+	public void remove(Session session) {
+		sessions.values().forEach(v -> v.removeIf(e -> e.equals(session)));
+	}
+
+	public void sendMessage(User user, String text) {
+		Set<Session> userSessions = sessions.entrySet()
+				.stream()
+				.filter(e -> e.getKey().equals(user.getId()))
+				.flatMap(e -> e.getValue().stream())
+				.collect(Collectors.toSet());
+
+		log.debug("sending '{}' to {} users via websocket", text, userSessions.size());
+		for (Session userSession : userSessions) {
+			if (userSession.isOpen()) {
+				userSession.getAsyncRemote().sendText(text);
+			}
+		}
+	}
+}