a) Is it possible through some means of configuration to always make a few number of subscribers to be automatically subscribed to a specific topic when the broker goes online? Or it is something that can only be handled by code?You should use the JMS API to create a durable subscription. Note that you only need to do this once. A durable subscription will survive a broker restart.
Is there any API to find the subscribers that are subscribed to a topic currently?Not using the JMS API.
What is the solution if a subscriber subscribes to a topic after a message has been published and he wants to see the message?Use a queue instead?