TooTallNate/Java-WebSocket · error · IllegalArgumentException

This draft does not support Sec-WebSocket-Protocol

Error message

This draft does not support Sec-WebSocket-Protocol

What it means

Thrown by WebSocketImpl.getProtocol() when the connection's negotiated draft is not Draft_6455. Protocol (subprotocol) negotiation via the Sec-WebSocket-Protocol header only exists in the RFC 6455 draft; older drafts (Draft_75/76/Hybi) carry no protocols, so querying getProtocol() on such a connection is a programming error rather than returning null. Fires only for non-6455 drafts with a non-null draft.

Solutions

1. Check the negotiated draft before calling getProtocol(), e.g. `if (conn.getDraft() instanceof Draft_6455)`.
2. Always use Draft_6455 (the default) on client and server so subprotocols are available.
3. If no protocols may exist, treat a null return from getProtocol() (draft == null case) as 'unknown' instead of forcing a query on an unsupported draft.

When it happens

Trigger: Thrown at src/main/java/org/java_websocket/WebSocketImpl.java:888 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/68aeeaf3632e83d4. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/org/java_websocket/WebSocketImpl.java:888

    return channel instanceof ISSLChannel;
  }

  @Override
  public SSLSession getSSLSession() {
    if (!hasSSLSupport()) {
      throw new IllegalArgumentException(
          "This websocket uses ws instead of wss. No SSLSession available.");
    }
    return ((ISSLChannel) channel).getSSLEngine().getSession();
  }

  @Override
  public IProtocol getProtocol() {
    if (draft == null) {
      return null;
    }
    if (!(draft instanceof Draft_6455)) {
      throw new IllegalArgumentException("This draft does not support Sec-WebSocket-Protocol");
    }
    return ((Draft_6455) draft).getProtocol();
  }

  @Override
  public <T> void setAttachment(T attachment) {
    this.attachment = attachment;
  }

  public ByteChannel getChannel() {
    return channel;
  }

  public void setChannel(ByteChannel channel) {
    this.channel = channel;
  }

  public WebSocketWorker getWorkerThread() {

View on GitHub (pinned to afeacbf8c0)