public boolean isClosed() {
        return this.closed.get();
    }
Method that can be called to determine whether the connection manager has been shut down and
is closed or not.

@return {@code true} if the connection manager has been shut down and is closed, otherwise
return {@code false}.
@since 5.4
httpclient5/src/main/java/org/apache/hc/client5/http/impl/nio/PoolingAsyncClientConnectionManager.java