This actually brings up an interesting point and a naive question. Would stealth fighters become AWACs killers? Please note that this is not specifically about the J-20, but stealth fighters in general. Since all AWACs tend to be slow, huge and unstealthy, would any stealthy fighter, which all should be equipped with advanced radars, be able to sneak up to an AWAC and shoot it down before being detected. How would AF's in the world protect their AWACs against such threat?
The Chinese military has always displayed a preference towards high impact assets that can majorly weaken the bulk of an enemy's forces and allow its own legacy forces an advantage, and the J20 should be a prime example of that.
I have said right from the start that the likes of the J20 will play an important role in hunting down and destroying opfor high value force multipliers like AWACS, tankers, military transports etc.
I think that is one of the main reasons why the J20 is clearly designed with long range in mind. They are designed to penetrate enemy defensive lines using stealth and raw performance, then haunt the enemy support lines killing the things that help the enemy fighters stay in the fight and in indeed, in the air itself.
Now, all stealth fighters will be able to do that to some extent, but I think the J20 should be able to do it the best.
If you are going to go hunting for enemy force multipliers, you need to penetrate deep behind the primary combat zones and operate deep behind enemy "lines" where they think their high value assets are safe.
That requires great range, as obviously if you are facing against an enemy with 5th gens as well, then their stealths will be looking to do the same to your support assets. That means that the side who has the 5th gen with the longer legs can afford to place their tankers further back, potentially out of range of opposing enemy 5th gens.
I think if you are serious about going AWACS hunting, you will also need to develop specialist tools. AAMs with the ability to home in passively, like the PL12, will be very useful, as any active homing AAM will instantly give the game away, and will probably be fairly vulnerable as I would imagine an AWACS can mount a fairly powerful and comprehensive electronic attack against active radar seekers.
Ideally you will want a stealthy EO and/or multi-seeker missile that can ignore the AWACS' powerful jamming while minimising enemy reaction times.
Super long range AAMs are unnecessary with stealths, as they are more for conventional fighters wishing to do the same job without being able to bypass enemy forward CAP.
In terms of defending against it, I think there are many things that can and will have to be done.
The easiest is fighter escorts for key assets, but I think that's the wrong approach. 5th gens will wipe the floor with legacy fighters, so you will either need lots of legacy fighters acting as escorts, or have 5th gens acting as escorts.
But neither method is fool proof, and to be honest, you really want your fighters out winning control of the skies instead of hiding in the backfield on babysitting duty.
Better, but more costly and long term solutions would be things like using UAVs to take over part of the mission of manned AWACs, so you can have large but ultimately expendable UAVs mounting powerful radars operating on the front lines, and datalinking their radar data back to manned AWACS further back, who can process it and directing fighters.
In effect, you are breaking your AWACS into two components, with the sensors deployed on unmanned, expendable assets close enough to the action to not compromise on radar coverage, while your manned analytical and command functions are moved even further back from the front lines, preferably outside of the reach of enemy stealths.
You will probably also see the development and deployment of cheap, disposable "tripwire" and/or sentinel UAVs.
The tripwire UAVs will use swarm logic to form an effective sensor net dense enough to detect stealths.
Sentinel UAVs will fly (or be docked) with high value assets and active as an ablative shield against enemy missile attacks by flying out and putting themselves between their asset and incoming enemy missiles. You can equip them with all sorts of tools to help them attract enemy missiles, such as radar reflectors, heat emitters to explosive chargers that they could set off in front of incoming missiles if all else fails.
Further down the line, when portable lasers become fieldable, I would expect AWACS and tankers to be amongst the first planes to be equipped with them.