\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\leftharpoonfill@{\arrowfill@\leftharpoonup\relbar\relbar}
\def\rightharpoonfill@{\arrowfill@\relbar\relbar\rightharpoonup}
\newcommand\rbjt{\mathpalette{\overarrow@\rightharpoonfill@}}
\newcommand\lbjt{\mathpalette{\overarrow@\leftharpoonfill@}}
\makeatother
\begin{document}
\[\rbjt a=\rbjt{AB}=\rbjt{ABCD}\]

\[\lbjt a=\lbjt{AB}=\lbjt{ABCD}\]
\end{document}