The Klein-Gordon equation Green's function.

Question

How would I go about solving the following integral? \begin{equation} G_\text{ret}(x-x')=i\theta(t-t')\int\frac{d^3\mathbf{p} }{(2\pi)^32E}\left[e^{-iE(t-t')+i\mathbf{p}\cdot(\mathbf{r}-\mathbf{r}')}-e^{iE(t-t')+i\mathbf{p}\cdot(\mathbf{r}-\mathbf{r}')}\right], \end{equation} where $E=+\sqrt{\mathbf{p}^2+m^2}$. Apparently the answer is \begin{equation} G_\text{ret}(x-x')=\theta(t-t')\left[\frac{1}{2\pi}\delta((x-x')\cdot(x-x'))-\frac{m}{4\pi}J_1(m(x-x)\cdot(x-x'))\right], \end{equation} where $J_1$ is a Bessel function of the first kind.