Gravitational microlensing provides a unique probe to study the population of lens objects and the emission region of the innermost parts of quasar accretion disks. We show that the current X-ray observations of microlensing signatures of emission close to the innermost stable orbit are able to constrain the population of lens objects down to planet mass scales. In particular, we found a planet mass fraction to be larger than 0.0001 of the halo mass, which is equivalent to 2,000 unbound objects ranging from Moon to Jupiter mass, free floating between stars, per main sequence star in the lens galaxy of RXJ1131-1231. In addition, two microlensing effects can be used to constrain black hole spins and X-ray reflection regions for high redshift quasars. The first effect is the excess iron line equivalent widths of lensed quasars compared to normal AGN, and the second is the distribution of iron line peak energies of lensed quasars. A microlensing analysis of the iron line equivalent widths prefers high spin values and very steep iron line emissivity profiles for quasars at z~2. I will also discuss the prospect of these sciences in the era of the next generation of X-ray telescopes.