Difficulty converting python web socket code to java

I'm trying to convert some python web socket code to Java (code that talks to a consumer device:)

        ssl_context = ssl.create_default_context()
        ssl_context.check_hostname = False
        ssl_context.verify_mode = ssl.CERT_NONE
        ws_connection_local = connect(f"wss://{target_ip}:{WEBSOCKET_SSL_PORT}", ssl_context=ssl_context)

my Java .. (actually groovy) replacement is this...

CountDownLatch latch = new CountDownLatch(1);

String url = "wss://$host:$port"
WebSocket ws = HttpClient
        .newHttpClient()
        .newWebSocketBuilder()
        .buildAsync(URI.create(url), new WebSocketClient(latch))

whereupon I get the error:

java.util.concurrent.CompletionException: javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Caused by: javax.net.ssl.SSLHandshakeException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Caused by: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target
Caused by: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

On the theory that the problem is something to do with SSL certificates or host verification, I followed this advice prior to opening the connection: How to disable SSL verification?

and set trust all certificates with the recommended no-op HttpsURLConnection.setDefaultSSLSocketFactory etc and HttpsURLConnection.setDefaultHostnameVerifier etc. However that didn't help, I get the same error. I also set debugger breakpoints in the TrustManager and HostnameVerifier to see if they were invoked, and they were not.