TooTallNate/Java-WebSocket · error · IllegalArgumentException

null as draft is permitted for `WebSocketServer` only!

Error message

null as draft is permitted for `WebSocketServer` only!

What it means

Constructor validation in WebSocketClient(URI, Draft, Map, int): the client requires an explicit non-null Draft because it must send a concrete handshake during connect. A null draft is only meaningful server-side, where WebSocketServer picks the best draft per handshake, hence the sentinel message.

Solutions

1. Pass a concrete draft, typically `new Draft_6455()`, to the WebSocketClient constructor.
2. If you want default behavior, use the single-argument constructor WebSocketClient(serverUri), which defaults to Draft_6455.
3. If draft selection should be dynamic, keep null drafts on the server side only.

When it happens

Trigger: Thrown at src/main/java/org/java_websocket/client/WebSocketClient.java:216 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of TooTallNate/Java-WebSocket@afeacbf8c0 (2026-09-09). Data as JSON: /api/errors/05b8e210f97f8e96. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/org/java_websocket/client/WebSocketClient.java:216

    this(serverUri, protocolDraft, httpHeaders, 0);
  }

  /**
   * Constructs a WebSocketClient instance and sets it to the connect to the specified URI. The
   * channel does not attampt to connect automatically. The connection will be established once you
   * call <var>connect</var>.
   *
   * @param serverUri      the server URI to connect to
   * @param protocolDraft  The draft which should be used for this connection
   * @param httpHeaders    Additional HTTP-Headers
   * @param connectTimeout The Timeout for the connection
   */
  public WebSocketClient(URI serverUri, Draft protocolDraft, Map<String, String> httpHeaders,
      int connectTimeout) {
    if (serverUri == null) {
      throw new IllegalArgumentException();
    } else if (protocolDraft == null) {
      throw new IllegalArgumentException("null as draft is permitted for `WebSocketServer` only!");
    }
    this.uri = serverUri;
    this.draft = protocolDraft;
    this.dnsResolver = new DnsResolver() {
      @Override
      public InetAddress resolve(URI uri) throws UnknownHostException {
        return InetAddress.getByName(uri.getHost());
      }
    };
    if (httpHeaders != null) {
      headers = new TreeMap<>(String.CASE_INSENSITIVE_ORDER);
      headers.putAll(httpHeaders);
    }
    this.connectTimeout = connectTimeout;
    setTcpNoDelay(false);
    setReuseAddr(false);
    this.engine = new WebSocketImpl(this, protocolDraft);
  }

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