Why surface tension is measure over a line and why its unit is j/sp.mtr?

It's because it's measured force per length, that is N/m

N, Newton: kg*m/s^2 = force

Surface tension = N/m = kg*m/s^2 / m

J= Joule = energy = N*m

but as you exchange N with J (kg*m^2/s^2) you are multiplaying another m, that's why you need divide with another m again:

N/m = kg*m/s^2 / m = kg*m^2/s^2 / m^2 = J/m^2

("^2" means square)