  A comprehensive study on the nature of the $Z_c(3900)$ resonant structure is
carried out in this work. By constructing the pertinent effective Lagrangians
and considering the important final-state-interaction effects, we first give a
unified description to all the relevant experimental data available, including
the $J/\psi\pi$ and $\pi\pi$ invariant mass distributions from the $e^+e^-\to
J/\psi\pi\pi$ process, the $h_c\pi$ distribution from $e^+e^-\to h_c\pi\pi$ and
also the $D\bar D^{*}$ spectrum in the $e^+e^-\to D\bar D^{*}\pi$ process.
After fitting the unknown parameters to the previous data, we search the pole
in the complex energy plane and find only one pole in the nearby energy region
in different Riemann sheets. Therefore we conclude that $Z_c(3900)$ is of
$D\bar D^*$ molecular nature, according to the pole counting rule
method~[Nucl.~Phys.~A543, 632 (1992); Phys.~Rev.~D 35,~1633 (1987)]. We
emphasize that the conclusion based upon the pole counting method is not
trivial, since both the $D\bar D^{*}$ contact interactions and the explicit
$Z_c$ exchanges are introduced in our analyses and they lead to the same
conclusion.
