public S loadDefaultInstance() {
        load();

        if (defaultClass == null) {
            return null;
        }

        return createInstance(defaultClass);
    }
@Test
    public void testLoadDefaultInstance() {
        SlotChainBuilder slotChainBuilder = SpiLoader.of(SlotChainBuilder.class).loadDefaultInstance();
        assertNotNull(slotChainBuilder);
        assertTrue(slotChainBuilder instanceof DefaultSlotChainBuilder);
    }