I derive the current phase relation for a clean SNS junction featuring conventional
superconductors. Starting from an energy state description of current, I show how
spectral flow leads to the current phase relation of the system in different limits.
One advantage of this approach is that we can keep track of the contributions of
Andreev bound states and continuum states separately, which allows for a more intuitive
understanding of the current phase relation and the way it changes between different limits.