private static String getProperty(final String key, final String defaultValue) {
        return AccessController.doPrivileged((PrivilegedAction<String>) () -> System.getProperty(key, defaultValue));
    }
Adds to the list of {@link Closeable} resources to be managed by the client.
<p>
For internal use.
</p>
httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/H2AsyncClientBuilder.java