java.nio.file.WatchEvent gives me only relative path. How can I get the absolute path of the modified file?

You need to get the parent directory from the WatchKey to resolve the full path

WatchKey key;
WatchEvent<Path> event;

Path dir = (Path)key.watchable();
Path fullPath = dir.resolve(event.context());

This piece of code reads like it needs accompanying documentation to be grasped, it makes little sense on its own. What were their intentions with this particular API design?

And this is only the beginning of possibly unintuitive usage. Java's file watcher API is subjectively inferior to alternative libraries.

Granted that you'll want to watch multiple directories (e.g. monitoring a file tree for change) storing the registered WatchKey and it's associated Path in a Map<WatchKey, Path> would also be a viable solution.

When an event is triggered the Map could be asked for the associated Path with the given WatchKey and then the Path of the changed file could be resolved with the help of the Path the WatchKey is associated with.