@UnstableApi
    public WebSocketClient webSocketClient() {
        final WebSocketClient webSocketClient = this.webSocketClient.get();
        if (webSocketClient != null) {
            return webSocketClient;
        }

        final WebSocketClient newWebSocketClient = webSocketClientBuilder().build();
        if (this.webSocketClient.compareAndSet(null, newWebSocketClient)) {
            return newWebSocketClient;
        } else {
            final WebSocketClient oldWebClient = this.webSocketClient.get();
            assert oldWebClient != null;
            return oldWebClient;
        }
    }
Returns the {@link WebSocketClient} configured
by {@link #configureWebSocketClient(WebSocketClientBuilder)}.
junit5/src/main/java/com/linecorp/armeria/internal/testing/ServerRuleDelegate.java