How to add headers in Java Websocket client

Builder configBuilder = ClientEndpointConfig.Builder.create();
configBuilder.configurator(new Configurator() {
    @Override
    public void beforeRequest(Map<String, List<String>> headers) {
    headers.put("Cookie", Arrays.asList("JSESSIONID=" + sessionID));
    }
});
ClientEndpointConfig clientConfig = configBuilder.build();
webSocketContainer.connectToServer(MyClientEndpoint.class, clientConfig, new URI(uri));

ClientEndpointConfig.Configurator.beforeRequest(Map<String,List<String>> headers) may be usable.

The JavaDoc about the argument headers says as follows:

the mutable map of handshake request headers the implementation is about to send to start the handshake interaction.

So, why don't you override beforeRequest method like below?

@Override
public void beforeRequest(Map<String,List<String>> headers)
{
    List<String> values = new ArrayList<String>();
    values.add("My Value");

    headers.put("X-My-Custom-Header", values);
}

You can pass ClientEndpointConfig to connectToServer(Class<? extends Endpoint> endpointClass, ClientEndpointConfig cec, URI path).

public class Config extends ClientEndpointConfig.Configurator{
    @Override
    public void beforeRequest(Map<String, List<String>> headers) {
        headers.put("Pragma", Arrays.asList("no-cache"));
        headers.put("Origin", Arrays.asList("https://www.bcex.ca"));
        headers.put("Accept-Encoding", Arrays.asList("gzip, deflate, br"));
        headers.put("Accept-Language", Arrays.asList("en-US,en;q=0.8,zh-CN;q=0.6,zh;q=0.4"));
        headers.put("User-Agent", Arrays.asList("Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36"));
        headers.put("Upgrade", Arrays.asList("websocket"));
        headers.put("Cache-Control", Arrays.asList("no-cache"));
        headers.put("Connection", Arrays.asList("Upgrade"));
        headers.put("Sec-WebSocket-Version", Arrays.asList("13"));
    }

    @Override
    public void afterResponse(HandshakeResponse hr) {
        Map<String, List<String>> headers = hr.getHeaders();
        log.info("headers -> "+headers);
    }
}