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  1. DZone
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  4. Handling ‘State’ in Java WebSocket Applications

Handling ‘State’ in Java WebSocket Applications

State is a concept dealt with in several facets of application development. With the newer paradigm of WebSockets, how do you handle state here?

Abhishek Gupta user avatar by
Abhishek Gupta
CORE ·
Apr. 29, 17 · Tutorial
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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);
}


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);
        }
    }
}


Another option is to encapsulate some of the common/global logic in a custom Configurator implementation, which can be accessed and 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
    }
}


Further Reading

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

Cheers!

WebSocket application Java (programming language)

Published at DZone with permission of Abhishek Gupta, DZone MVB. See the original article here.

Opinions expressed by DZone contributors are their own.

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