Plums have maggots

Hey there! My plums are magnificent but many of them have one maggot living inside, leaving brown trails (excrement i hear). I read that they leave the fruit end of summer and enter the soil, where they hibernate and become moths that come back in spring and lay eggs on the fruit.
Has anyone had any success in keeping these maggots out of the fruits?
I'm in Northern Germany, near Berlin.

Thank you!
Best
Lewis