Check whether this message handler is currently running.
<p>Note that even when this message handler is running the
{@link #isBrokerAvailable()} flag may still independently alternate between
being on and off depending on the concrete sub-class implementation.
Check whether this message handler is currently running. <p>Note that even when this message handler is running the {@link #isBrokerAvailable()} flag may still independently alternate between being on and off depending on the concrete sub-class implementation.