  Cerium, in which the 4$f$ valence electrons live at the brink between
localized and itinerant characters, exhibits varying crystal structures and
therefore anomalous physical properties with respect to temperature and
pressure. Understanding its electronic structure and related lattice properties
is one of the central topics in condensed matter theory. In the present work,
we employed the state-of-the-art first-principles many-body approach (i.e., the
density functional theory in combination with the single-site dynamical
mean-field theory) to study its electronic structure thoroughly. The
momentum-resolved spectral functions, total and $4f$ partial density of states,
optical conductivities, self-energy functions, and atomic eigenstate histograms
for cerium's four allotropes under ambient pressure were calculated and
analyzed carefully. The calculated results demonstrate that the 4$f$ electrons
in the $\alpha$, $\beta$, $\gamma$, and $\delta$ phases are all correlated with
heavily remormalized electron masses. In the $\alpha$ phase, the 4$f$ electrons
tend to be itinerant, which cause strong hybridization between the 4$f$ and
$spd$ bands and remarkable 4$f$ valence state fluctuation. While for the other
phases, the 4$f$ electrons are close to be localized. Our calculated results
support the Kondo volume collapse scenario for the cerium $\alpha-\gamma$
transition. Finally, we examined the site dependence of $4f$ electronic
structure in the $\beta$ phase. The calculated results suggest that it doesn't
exhibit a site selective 4$f$ localized state, contrary to previous prediction.
