1. An image forming apparatus, comprising: a display device; a plurality of hardware resources configured to carry out image formation; a plurality of application programs performing respective processing of the plurality of application programs related to the image formation; a storage device storing rewritable shared data which is used by the plurality of application programs in common; and a shared-data control unit selecting one or more of the plurality of application programs as destinations of updating-start notification and notifying a start of updating of the shared data to the selected one or more application programs when acquisition and updating of the shared data is inhibited in response to a write-lock request received from one of the plurality of application programs, and the shared-data control unit notifying an end of the updating of the shared data to the selected application programs when the acquisition and updating of the shared data is allowed in response to an unlock request received from said one of the plurality of application programs, wherein the plurality of application programs include a first application program that is currently subject to updating of the shared data by the shared-data control unit and configured to display the shared data on the display device, a second application program that is not currently subject to updating of the shared data by the shared-data control unit and configured to display the shared data on the display device when acquisition of the shared data is not inhibited, and a third application program that is configured to not display the shared data on the display device, when updating of the shared data of the first application program is performed by the shared data control unit, an updating-start notification is sent to the second application program by the shared data control unit and no updating-start notification is sent to the third application program by the shared data control unit, and one of a screen of the shared data of the first application program and a screen of the shared data of the second application program is selectively displayed on the display device.