Regarding teeth is broken due to cavity.

If you are asking that,will it pain while doing root canal treatment, then the answer will be no as ur dentist will give us local anaesthesia which will last for about 2-3 hours after that he/she will prescribe u some painkiller medicines to alleviate ur pain secondly it will not pain in future if the root canal treatment is done successfully. Answer to your second question is again no,if it is other than wisdom tooth or very badly broken tooth Third question's answer is that composite is one of the best material in dentistry,so if ur dentist does it successfully and if it is a standard quality, company composite then it will last almost for many many years. Now coming to the care,u get the filling done after having intraoral x ray done and following all the postoperative instructions given by your dentist. Regards!

Get the X ray done to decide outcome of the treatment.

Take a regular follow up of your dentist.

After Local anaesthesia , pain is not a worrying factor! Root canal treatment gives pain relief too and saves your natural teeth.so go for it! Composite fillings need replacement when they show signs of discolouration and expansion.

Usually the dentist will do the root canal treatment under local anaesthesia which will stay for 2-3 hrs,after that pain killers r prescribed.so it won't be painful, And composite filling,if it is done using good quality material it will last for years,u need to replace the filling when it is discoloured ,and follow the instructions given by dentist