\documentclass{article}

\makeatletter
\def\thickhline{%
  \noalign{\ifnum0=`}\fi\hrule \@height \thickarrayrulewidth \futurelet
   \reserved@a\@xthickhline}
\def\@xthickhline{\ifx\reserved@a\thickhline
               \vskip\doublerulesep
               \vskip-\thickarrayrulewidth
             \fi
      \ifnum0=`{\fi}}
\makeatother

\newlength{\thickarrayrulewidth}
\setlength{\thickarrayrulewidth}{2\arrayrulewidth}

\begin{document}

\begin{tabular}{ l | c | r }
  \hline
  1 & 2 & 3 \\
  \hline
  4 & 5 & 6 \\
  \hline
  7 & 8 & 9 \\
  \hline
\end{tabular}
\qquad
\begin{tabular}{ l | c | r }
  \thickhline
  1 & 2 & 3 \\
  \hline
  4 & 5 & 6 \\
  \hline
  7 & 8 & 9 \\
  \thickhline
\end{tabular}

\bigskip

\setlength{\thickarrayrulewidth}{\arrayrulewidth}
\setlength{\arrayrulewidth}{0.1pt}

\begin{tabular}{ l | c | r }
  \hline
  1 & 2 & 3 \\
  \hline
  4 & 5 & 6 \\
  \hline
  7 & 8 & 9 \\
  \hline
\end{tabular}
\qquad
\begin{tabular}{ l | c | r }
  \thickhline
  1 & 2 & 3 \\
  \hline
  4 & 5 & 6 \\
  \hline
  7 & 8 & 9 \\
  \thickhline
\end{tabular}

\end{document}