java fern doesnt like very strong light. I would suggest hiding it to get less light if its getting direct sunlight. Java fern can turn brown due to too much ferts, or not enough co2 . I dont know your tank is set up and how the chemistry in the water is, but try little fixes here and there. Ill give you a hopefully insightful observation
My main tank has java fern which is doing wonderfully. It has ferts, co2, "excel" co2 booster dosing, and LED lights with just a bit of direct sunlight for 1 hour a day.
My side tank which houses left over cuttings gets some direct sun as well but no lights over head, and no co2 injection. the Java fern in this tank is dying. Also the water chemistry is not as good due to my laziness