static CloseableHttpClient GET_ASYNC_CLIENT() {
        final CloseableHttpClient client = ASYNC_CLIENT;
        if (client != null) {
            return client;
        }
        LOCK.lock();
        try {
            if (ASYNC_CLIENT == null) {
                ASYNC_CLIENT = new ClassicToAsyncAdaptor(HttpAsyncClientBuilder.create()
                        .setConnectionManager(PoolingAsyncClientConnectionManagerBuilder.create()
                                .useSystemProperties()
                                .setMaxConnPerRoute(100)
                                .setMaxConnTotal(200)
                                .setMessageMultiplexing(true)
                                .setDefaultConnectionConfig(ConnectionConfig.custom()
                                        .setValidateAfterInactivity(TimeValue.ofSeconds(10))
                                        .build())
                                .build())
                        .useSystemProperties()
                        .evictExpiredConnections()
                        .evictIdleConnections(TimeValue.ofMinutes(1))
                        .build(), Timeout.ofMinutes(5));
            }
            return ASYNC_CLIENT;
        } finally {
            LOCK.unlock();
        }
    }
Executor for {@link Request}s.
<p>
A connection pool with maximum 100 connections per route and
a total maximum of 200 connections is used internally.

@since 4.2
httpclient5-fluent/src/main/java/org/apache/hc/client5/http/fluent/Executor.java