public static Object getField(ConfigBuilder builder, String fieldName) {
        try {
            Field f = builder.getClass().getDeclaredField(fieldName);
            f.setAccessible(true);
            return f.get(builder);
        } catch (NoSuchFieldException | SecurityException | IllegalArgumentException | IllegalAccessException e) {
            throw new RuntimeException(e);
        }
    }
@Test
    public void test_require_private_getter() {
        FunctionTestConfig.Builder builder = createVariableAccessBuilder();
        assertEquals(ConfigInstanceUtil.getField(builder, "int_val"), 5);
        FunctionTestConfig conf = new FunctionTestConfig(builder);
        assertEquals(conf.int_val(), 5);
    }