If you open any cell phone, you can see similar filters in the RF section. The ceramic-coaxial resonator is made as a ceramic tube (outer shape is square) metallized on all surfaces except of the "live" end and coupling tabs. In filters the resonators are preadjusted by grounding to length, then soldered together to form coupled resonators.
I tested samples I obtained at an exposition; I made one and two-resonator filters and also a nice reject filter by grounding coupling tabs and loops on the external surface. The ceramic material is designed to be stable over temperature, and the filters are good and much smaller than with an air-filled cavity.