We detail here the general principle of a self-adaptive oscillator in which the intertwined operation of a 100-m-long active optical resonator and a standard semiconductor laser mutually coupled by stimulated Brillouin scattering offers an ultimate high spectral purity. Single frequency operation of this self-adaptive photonic oscillator is achieved without any servo locking or stabilization electronics. In free running operation, this principle leads to a Lorentzian linewidth of 40 mHz and a Flicker noise linewidth of 200 Hz for 0.01 s integration time. The long-term drift of the optical frequency without absolute frequency locking is in the range of 10 MHz over hours. This principle applies to any wavelength attainable by laser diodes which opens tremendous opportunities, in particular in applications where atomic or molecular transitions require precise wavelengths.