@Internal
    protected void addCloseable(final Closeable closeable) {
        if (closeable == null) {
            return;
        }
        if (closeables == null) {
            closeables = new ArrayList<>();
        }
        closeables.add(closeable);
    }
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