Does the Paho Java client has any callback function called when a new client connection is established? Within that function I want to validate user and if failed refuse connection.
When a new client connect from javascript or any client I should be able to allow or refuse new client.
This is not possible to do from a client, the MQTT protocol has no support for anything other than passing authentication information from a single client to the broker, not the other way round.
You have to do it from within the broker. Different brokers all implement authentication differently so it will depend entirely on which broker you are using.
Mosquitto has an API to write authentication plugins. e.g. https://github.com/jpmens/mosquitto-auth-plug
HiveMQ Also has an API http://www.hivemq.com/blog/mqtt-security-fundamentals-advanced-authentication-mechanisms
Mosca - https://github.com/mcollina/mosca/wiki/Authentication-&-Authorization