  We study the effect of self-gravity on entropy by directly solving the 4D
semi-classical Einstein equation. In particular, we focus on whether the
Bekenstein-Hawking formula holds when self-gravity is extremely strong. As an
example, we consider a simple spherically symmetric static configuration
consisting of many quanta and construct a self-consistent non-perturbative
solution for $\hbar$ in which the entropy exactly follows the area law for many
local degrees of freedom of any kind. This can be a candidate for black holes
in quantum theory. It represents a compact dense configuration with
near-Planckian curvatures, and the interior typically behaves like a local
thermal state due to particle creation. Here, the information content is stored
in the interior bulk, and the self-gravity plays an essential role in changing
the entropy from the volume law to the area law. We finally discuss
implications to black holes in quantum gravity and a speculative view of
entropy as a gravitational charge.
