Handling ‘state’ in Java WebSocket applications

By and large, there are two kinds of states in a WebSocket application

• User/client specific: related to a connected user/Session e.g. user ID, list of subscriptions, last message received etc.
• Global: state which is relevant across your application and something which all connected users/Sessions might be able to use

User specific state

This can be handled using getUserProperties method on the Session object – this exposes a Map which you can use to store anything (Object type) using a String type key

	@OnOpen
	public void opened(@PathParam("userid") String id, Session peer){
	peer.getUserProperties().put("USER_ID" , id); //it's possible to store the ID as a member variable as well.. this is just an example
	}
	@OnMessage
	public void helloUser(Session peer, String message){
	String id = (String) peer.getUserProperties().get("USER_ID");
	peer.getBasicRemote().sendText("Hello "+ id + "! You sent "+ message);
	}

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using_session.txt

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Global state

There are multiple options here as well. Please note that these are scoped to a specific Endpoint

• getUserProperties in EndpointConfig – it exposes the same Map interface as the one in Session. Since the WebSocket runtime creates a single instance of an EndpointConfig object per Endpoint , it can be used a global state store

	private EndpointCondig epCfg;
	private static List<Session> peers = …;
	@OnOpen
	public void open(@PathParam("userid") String id, Session peer, EndpointConfig epCfg){
	this.epCfg = epCfg;
	peers.add(peer);
	this.epCfg.getUserProperties().put(peer.getId(), id); //store mapping of WebSocket Session ID to user ID
	}
	@OnMessage
	public void broadcast(Session from, String msg){
	String senderID = (String) this.epCfg.getUserProperties().get(from.getID()); //check the mapping
	for(Session peer : peers) { //loop over ALL connected clients
	if(peer.isOpen()){
	peer.getBasicRemote().sendText("Message from User "+ senderID + " – " + msg);
	}
	}
	}

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using_endpointconfig.txt

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• Another option is to encapsulate some of the common/global logic in a custom Configurator implementation which can be accessed & used within the endpoint logic

	//custom Configurator implementation which maps (authenticated) user name to a token (sent via HTTP header)
	public class TokenStore extends ServerEndpointConfig.Configurator {
	Map<String, String> userTokens;
	public TokenStore() {
	userTokens = new ConcurrentHashMap<>();
	}
	@Override
	public void modifyHandshake(ServerEndpointConfig sec, HandshakeRequest request, HandshakeResponse response) {
	String token = request.getHeaders().get("token").get(0);
	String name = request.getUserPrincipal().getName();
	userTokens.put(name, token);
	}
	public Map<String, String> getUserTokens(){
	return Collections.unmodifiableMap(userTokens);
	}
	}
	//the WebSocket (server) endpoint implementation which makes use of the token store
	@ServerEndpoint(value = "/service/{id}",configurator = TokenStore.class)
	public class BroadcastService {
	private String token;
	@OnOpen
	public void test(@PathParam("id") String id, EndpointConfig cfg) { //injeted config by runtime
	ServerEndpoint sCfg = (ServerEndpoint) cfg; //cast
	TokenStore store = sCfg.getConfigurator(); //get custom implementation instance
	token = cfgur.getUserTokens().get(id); //extract token and store as a member variable
	}
	}

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using_endpointconfig_configurator.txt

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	@OnOpen
	public void opened(@PathParam("userid") String id, Session peer){
	peer.getUserProperties().put("USER_ID" , id); //it's possible to store the ID as a member variable as well.. this is just an example
	}
	@OnMessage
	public void helloUser(Session peer, String message){
	String id = (String) peer.getUserProperties().get("USER_ID");
	peer.getBasicRemote().sendText("Hello "+ id + "! You sent "+ message);
	}

view raw

using_session.txt

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Further reading

• Java WebSocket API specification
• Java WebSocket API Handbook
• Other WebSocket blogs

Cheers!

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