Defining a centered shadowboxed listings environment

Here is a proposal using tcolorbox. Colors, shadows, etc. can be adjusted to your liking. You may replace style=tcblatex by any listings setting you want to have as default for your environment.

\documentclass[twocolumn]{article}

\usepackage{lipsum}
\usepackage[most]{tcolorbox}    

\newenvironment{CenteredShadowboxListing}[1][]{%
  \tcbset{listing options={style=tcblatex,#1}}\tcbwritetemp}%
  {\endtcbwritetemp%
  \tcbox[enhanced,arc=0pt,outer arc=0pt,top=1mm,bottom=1mm,left=1mm,right=1mm,
  boxrule=0.6pt,drop fuzzy shadow,before=\begin{center},after=\end{center}]%
  {\tcbusetemplisting}}

\begin{document}
\lipsum[4]
\begin{CenteredShadowboxListing}
line of code
line of code
\end{CenteredShadowboxListing}
\lipsum[4]
\end{document}

UPDATE: With tcolorbox version 2.41 of 2013/07/23, the code to typset the example above can be written more compact with the same result:

\documentclass[twocolumn]{article}

\usepackage{lipsum}
\usepackage[most]{tcolorbox}% version 2.41 or newer

\newtcblisting{CenteredShadowboxListing}[1][]{%
  listing options={style=tcblatex,#1},hbox,listing only,
  enhanced,arc=0pt,outer arc=0pt,top=1mm,bottom=1mm,left=1mm,right=1mm,
  boxrule=0.6pt,drop fuzzy shadow,before=\begin{center},after=\end{center}}

\begin{document}
\lipsum[4]
\begin{CenteredShadowboxListing}
line of code
line of code
\end{CenteredShadowboxListing}
\lipsum[4]
\end{document}