I tried to modify the repair amount of mending using the setRepairAmount method.
But as soon i use this method e.g. setRepairAmount(1) the PlayerItemMendEvent will be called more then 500 times (which results that the item is instantly repaired).

Output: https://paste.md-5.net/ajirexulic.md

In Class A:

@EventHandler
    public void mending(PlayerItemMendEvent event) {
        System.out.println("MENDING EVENT CALLED");
    }

In Class B:

@EventHandler
    public void mending(PlayerItemMendEvent event) {
        event.setRepairAmount(1);
    }

(Two event listeners to be sure if the event will be called multiple times or if the debug message will be outputed multiple times)