  We study the statistical mechanics of classical self-gravitating systems
confined within a box of radius $R$ in general relativity. It has been found
that the caloric curve $T_{\infty}(E)$ has the form of a double spiral whose
shape depends on the compactness parameter $\nu=GNm/Rc^2$. The double spiral
shrinks as $\nu$ increases and finally disappears when $\nu_{\rm max}=0.1764$.
Therefore, general relativistic effects render the system more unstable. On the
other hand, the cold spiral and the hot spiral move away from each other as
$\nu$ decreases. Using a normalization $\Lambda=-ER/GN^2m^2$ and $\eta=GNm^2/R
k_B T_{\infty}$ appropriate to the nonrelativistic limit, and considering
$\nu\rightarrow 0$, the hot spiral goes to infinity and the caloric curve tends
towards a limit curve (determined by the Emden equation) exhibiting a single
cold spiral, as found in former works. Using another normalization ${\cal
M}=GM/Rc^2$ and ${\cal B}={Rc^4}/{GNk_B T_{\infty}}$ appropriate to the
ultrarelativistic limit, and considering $\nu\rightarrow 0$, the cold spiral
goes to infinity and the caloric curve tends towards a limit curve (determined
by the general relativistic Emden equation) exhibiting a single hot spiral.
This result is new. We discuss the analogies and the differences between this
asymptotic caloric curve and the caloric curve of the self-gravitating
black-body radiation. Finally, we compare box-confined isothermal models with
heavily truncated isothermal distributions in Newtonian gravity and general
relativity.
