How to set snmp trap threshold

Question

I want to monitor some firewall devices by snmp.
Now, I can get cpu info of device by snmp4j.
But I have no idea how to use snmp trap to get specified warning and error messages from device.
How to set CPU threshold so that a trap message will be sent according to that.
For example, CPU usage above 70%, a trap message should be sent to manager PC.
Should I set this threshold on every firewall device by command or implement it by snmp4j?
I am confused, please give me some suggesions.

Thanks in advance.