JMS AUTO_ACKNOWLEDGE when is it acknowledged?

I have tried to google this, but have not been successful. If I am using AUTO_ACKNOWLEDGE, and I have a consumer client written in Java, when is the message acknowledged? I am using a MessageListener which contains an onMessage method. Is the acknowledgement sent back to the server before onMessage or after onMessage completes or at some other point? Thanks in advance for any help anyone is able to provide!