The check

        if (position + size < 0)
            throw new IllegalArgumentException("Negative position + size");

in the FileLock constructor guarantees that position() + size() does not overflow a long. Therefore the use of Math.addExact()

        // Test whether this is below that
        try {
            if (Math.addExact(this.position, this.size) <= position)
                return false;
        } catch (ArithmeticException ignored) {
            // the sum of this.position and this.size overflows the range of
            // long hence their mathematical sum is greater than position
        }

in overlaps() is not necessary.