  Hadron masses can be decomposed as a sum of components which are defined
through hadronic matrix elements of QCD operators. The components consist of
the quark mass term, the quark energy term, the glue energy term and the trace
anomaly term. We calculate these components of mesons with lattice QCD for the
first time. The calculation is carried out with overlap fermion on $2+1$ flavor
domain-wall fermion gauge configurations. We confirm that $\sim 50\%$ of the
light pion mass comes from the quark mass and $\sim 10\%$ comes from the quark
energy, whereas, the contributions are found to be the other way around for the
$\rho$ mass. The combined glue components contribute $\sim 40 - 50\%$ for both
mesons. It is interesting to observe that the quark mass contribution to the
mass of the vector meson is almost linear in quark mass over a large quark mass
region below the charm quark mass. For heavy mesons, the quark mass term
dominates the masses, while the contribution from the glue components is about
$400\sim500$ MeV for the heavy pseudoscalar and vector mesons. The charmonium
hyperfine splitting is found to be dominated by the quark energy term which is
consistent with the quark potential model.
