Calculate with justification the rank of the following matrix $J \in \mathcal{M}_n(\mathbb{R})$: $$J = \left(\begin{array}{cccc} 1 & 1 & \cdots & 1 \\ 1 & 1 & \cdots & 1 \\ \vdots & \vdots & \ddots & \vdots \\ 1 & 1 & \cdots & 1 \end{array}\right)$$