The top quark mass is measured using a template method in the $tt¯\to$ lepton+jets channel (lepton is $e$ or $\mu$) using ATLAS data recorded in 2012 at the LHC. The data were taken at a proton-proton centre-of-mass energy of $s=8$ TeV and correspond to an integrated luminosity of 20.2 fb$\mathrm{<mrow\; data-mjx-texclass="ORD">-1</mrow>}$. The $tt¯\to$ lepton+jets channel is characterized by the presence of a charged lepton, a neutrino and four jets, two of which originate from bottom quarks ($b$). Exploiting a three-dimensional template technique, the top quark mass is determined together with a global jet energy scale factor and a relative $b$-to-light-jet energy scale factor. The mass of the top quark is measured to be $mtop=172.08\pm 0.39(stat)\pm 0.82(syst)$ GeV. A combination with previous ATLAS $mtop$ measurements gives $mtop=172.69\pm 0.25(stat)\pm 0.41(syst)$ GeV.