I have an ArcGIS Online API that I would like to use as an input/store in my GeoServer so that I can publish a service from GeoServer. The format available thorough the API is GeoJSON. I can see that there is an option of adding a WFS as a vector data source but the service I have does not have any WFS or WMS capabilities on and has only GeoJSON output format.
At present, probably no. While GeoTools does have a GeoJSON datastore that could be added to GeoServer it is currently unsupported, has no documentation and lacks the GeoServer wrapper that would provide you with a GUI element to allow you to set it up easily.
You can give it a try by adding the
gt-geojsonstore jar file (and dependencies) to the
WEB-INF/lib directory but I really don't know what would happen.
You could seek commercial support to finish up the work on the module and integrate it into GeoServer, or provide the needed code yourself.
Or you could prevail on the service provider to use an open standard (like WFS) to publish the data rather than a vendor encumbered one.