What causes BlockingOperationException in Netty 4?

Question

Recently, I find some BlockingOperationException in my netty4 project.
Some people said that when using the sync() method of start netty's ServerBootstrap can cause dead lock, because sync() will invoke await() method, and there is a method called 'checkDeadLock' in await().
But I don't think so. ServerBootstrap use the EventLoopGroup called boosGroup, and Channel use the workerGroup to operation IO, I don't think they will influence each other, they have different EventExecutor.
And in my practice, Deadlock exception doesn't appear in the Netty startup process, most of which occurs after the Channel of the await writeAndFlush.
Analysis source code, checkDeadLock, BlockingOperationException exception thrown is when the current thread and executor thread of execution is the same.
My project code is blow:

private void channelWrite(T message) {
    boolean success = true;
    boolean sent = true;
    int timeout = 60;
    try {
        ChannelFuture cf = cxt.write(message);
        cxt.flush();
        if (sent) {
            success = cf.await(timeout);
        }
        if (cf.isSuccess()) {
            logger.debug("send success.");
        }
        Throwable cause = cf.cause();
        if (cause != null) {
            this.fireError(new PushException(cause));
        }
    } catch (LostConnectException e) {
        this.fireError(new PushException(e));
    } catch (Exception e) {
        this.fireError(new PushException(e));
    } catch (Throwable e) {
        this.fireError(new PushException("Failed to send message“, e));
    }
    if (!success) {
        this.fireError(new PushException("Failed to send message"));
    }
}

I know Netty officials advise not to use sync() or await() method, but **I want to know what situation will causes deadlocks in process and the current thread and executor thread of execution is the same.
I change my project code**.

private void pushMessage0(T message) {
    try {
        ChannelFuture cf = cxt.writeAndFlush(message);
        cf.addListener(new ChannelFutureListener() {
            @Override
            public void operationComplete(ChannelFuture future) throws PushException {
                if (future.isSuccess()) {
                    logger.debug("send success.");
                } else {
                    throw new PushException("Failed to send message.");
                }
                Throwable cause = future.cause();
                if (cause != null) {
                    throw new PushException(cause);
                }
            }
        });
    } catch (LostConnectException e) {
        this.fireError(new PushException(e));
    } catch (Exception e) {
        this.fireError(new PushException(e));
    } catch (Throwable e) {
        this.fireError(new PushException(e));
    }
}

But I face a new problem, I can't get the pushException from the ChannelHandlerListener.