  The jet energy scale, jet energy resolution, and their systematic
uncertainties are measured for jets reconstructed with the ATLAS detector in
2012 using proton-proton data produced at a centre-of-mass energy of 8 TeV with
an integrated luminosity of 20 fb$^{-1}$. Jets are reconstructed from clusters
of energy depositions in the ATLAS calorimeters using the anti-$k_t$ algorithm.
A jet calibration scheme is applied in multiple steps, each addressing specific
effects including mitigation of contributions from additional proton-proton
collisions, loss of energy in dead material, calorimeter non-compensation,
angular biases and other global jet effects. The final calibration step uses
several \textit{in situ} techniques and corrects for residual effects not
captured by the initial calibration. These analyses measure both the jet energy
scale and resolution by exploiting the transverse momentum balance in
$\gamma$+jet, $Z$+jet, dijet, and multijet events. A statistical combination of
these measurements is performed. In the central detector region, the derived
calibration has a precision better than 1% for jets with transverse momentum
150 GeV $< p_{\mathrm{T}}<$ 1500 GeV, and the relative energy resolution is
$(8.4\pm 0.6)$% for $p_{\mathrm{T}}=$ 100 GeV and $(23\pm 2)$% for
$p_{\mathrm{T}}=$ 20 GeV. The calibration scheme for jets with radius parameter
$R=1.0$, for which jets receive a dedicated calibration of the jet mass, is
also discussed.
