Did you generate self signed certificate and trying to use it?
Then import your self signed certificate to new keystore and use that keystore as a trust store on your client side.
For a tyrus websocket client, I use like this:
String keyStorePath = StompClientTest.class.getResource("/myapp.keystore").getPath();
System.getProperties().put("javax.net.debug", "all"); // debug your certificate checking
System.getProperties().put(SslContextConfigurator.KEY_STORE_FILE, keyStorePath);
System.getProperties().put(SslContextConfigurator.TRUST_STORE_FILE, keyStorePath);
System.getProperties().put(SslContextConfigurator.KEY_STORE_PASSWORD, "secret");
System.getProperties().put(SslContextConfigurator.TRUST_STORE_PASSWORD, "secret");
final SslContextConfigurator defaultConfig = new SslContextConfigurator();
defaultConfig.retrieve(System.getProperties());
SslEngineConfigurator sslEngineConfigurator = new SslEngineConfigurator(defaultConfig);
sslEngineConfigurator.setHostVerificationEnabled(false);
StandardWebSocketClient webSocketClient = new StandardWebSocketClient();
webSocketClient.getUserProperties().put(ClientProperties.SSL_ENGINE_CONFIGURATOR, sslEngineConfigurator);
For tomcat read answer in following question: https://stackoverflow.com/a/32205864/386213